CA1171069A - Safety mechanism for hoisting drums - Google Patents

Abstract

SAFETY MECHANISM FOR HOISTING DRUMS

Abstract The safety system in a hoist having a motor with a motor shaft, a power transmission main drive, a drum, a safety brake drivingly coupled on an operating element on or close to the drum in which a mechanical out-of-sync detector produces a bidirectional or, preferably, a unidirectional brake-setting output from inputs from the drum and the motor shaft,the unidirectional brake-setting rotational output for setting the brake. In a preferred embodiment, an error correction is made into the detector for obtaining a first unidirectional output during normal operation, and the brake-setting unidirectional rotation is in the opposite direction for setting the brake.
One form of detector is a mechanical differential assembly and another form of detector is a set of coaxial shafts that measure differential rotation between the drum and motor input shafts.
A unique brake actuator is provided which uses a large spring force which is controlled by a trigger mechanism and is easily reset remotely after the brake has been set.

Description

Description SAFETY MECHANISM FOR HOISTING DRUMS

Technical Field This invention pertains to hoisting equipment and, more particularly, to safety systems for such hoisting equip~ent.

Background Art United States of America Patents 4,175,727 and 4,177,973 are directed to safety systems for automatically setting a drum brake or other type of emergency holding device directly on the drum or on a shaft drivingly con-nected with the drum and in close proximity to the drum so that the system makes the hoist essentially single failure-proof. This means that should a failure occur in any loca-tion in the input drive or should there be a load hang-up or two-blocking condition, this hazard or failure would immedi-ately be detected and the brake set. This type of system is intended to serve as a substitute for the conventional re-dundant drive systems utilized for safety in cranes.
U.S. Patent 4,177,973 discloses, lnter alia, detecting when the drum and motor are no longer running in a predetermined synchronous condition and uses this out-of-sync determination to set the safety brake~
One embodiment covered by said earlier patents is an embodiment which uses compressed air to hold the brake open against a spring-applied force, and the detection of the failure or hazard condition electrically or electro-mechanically releases the air to allow the brake to be applied. In many installations, air supplies and electrical controls for setting valves are not readily available or desirable. Thus, while the earlier patents contemplated improved detectors and brake actuators, this application is also directed to an improved brake setting device.

' , The purpose of all of these safety system concepts is to enable an instantaneous application of a large-capa-city safety brake to be applied to the drum or on the ele-menk closely drivingly coupled to the drum so that the brake S can be set in the even-t of a failure or hazard condition or by remote command and brought to rest quickly beEore it has accelerated to an uncontrollable speed.

Disclosure of Invention This invention is directed to improved concepts relating to such safety systems as are described in said earlier patents. Thus it is an object of this invention to provide an improved method and apparatus for detecting a failure or other hazard in a hoisting device and setting a safety brake in response thereto.
It is another object of this invention to provide a totally mechanical detector and brake-applying system for a hoisting device.
It is still another object of one form of the in-vention to provide a mechanical out~of~sync detector whichgenerates its own force for applying the brake in an out-of-sync condition.
Basically, these objects are obtained in their broadest sense by providing a mechanical differential, out-of-sync detector, the output of which is generated when mechanical inputs from khe motor shaft and from the drum or related shaft to be monitored change their relative veloci-ties to one another, or when one of the drive line inputs to the detector is otherwise interrupted due to drive line com-ponent ~ailure, system overspeed, or loss of electrical pow-er, causing the differential output shaft to rotate via a differential gear set and trigger some form of brake-actuat-ing device to apply the brake.
In one form of the invention, the out-of-sync detector itselE provides the force or muscle necessary to apply the brake without intervening air or electrical ele-ments. Since the air and electrical elements are frequently not available or are susceptible to failure themselves in ~ ~'7~

the frequently dusty or dirty environment around a hoisting device, the benefit of a purely mechanical system is very advanta~eous. This sys-tem, in eEfect, stands alone such that any failure within the crane or any hazard condition, such as two-blocking or load hang-up, will immediately be sensed and directly converted to stopping the motor and set-ting the emergency brake to grab the drum hefore it reaches an appreciable dangerous velocity.
In another embodiment, the out-of-sync detector signals a triggering mechanism to release a cocked high-force spring to set the brake.
It is another object oE this invention to provide a mechanical detector which produces a unidirectional output under normal conditions but produces an opposite directional output in a failure or hazard condition to set the brake.
It is still another object to provide a method and apparatus for correcting for internal slippage in a hoisting system of the type in which an energy absorption device which could cause slippage between the motor and the drum is an integral part of the drive system for the drum. The purpose of this object is to avoid inadvertent or "nuisance"
trips or settings of the safety brake due to the inherent slippage of the energy absorption device.
It is still another object of the preferred embodiment to monitor the rotational velocities of the motor shaft and drum and to produce a predetermined unidirectional output rotation from a detector and to change the direction of such predetermined unidirectional output upon the occur-rence of a failure or a hazard condikion which changed the relative velocities and/or directions between the motor shaft and drum.
Basically, these objects, in a preferred embodi-ment, are obtained in their broadest sense by providing a mechanical detector which monitors the differential between khe input velocities of the drum and motor to produce a known or predetermined unidirectional output rotation from such detector. Upon a hazard or failure condition, one of the input velocities will be affected to change the differential input velocities from the drum and motor and, in response thereto, change the unidirectional output rota-tion to the opposite direction. This opposite direction of the output rotation then preferably mechanically releases the safety brake to bring the drum to a halt. In one em-bodiment, this detector is a differential assembly in ~hich input shafts Erom the drum and motor provide the input, with the differential assembly having an output shaft whose direction will either be unidirectional or have a zero velo-city durin~ normal operating conditions, but which willrotate in one direction (opposite to said normal direction) upon a failure or hazard condition. In another embodiment, the inputs are two coaxially aligned rotational members which produce either a zero velocity output or a known uni-directional rotational output. With a failure or hazardcondition, the inputs to one of the coaxial members will be varied and produce an o~tput from zero velocity in a desired rotational direction or, where the output was unidirection-al, will change the direction of rotation of the output to the opposite direction. This desired or opposite direction-al rotatiorl is then detected and sets the brake.
In anv of the above embodiments, a nulling or correctiGn device can be added which will produce a continu-ous, predetermined, unidirectional differential output from the detector in excess of any differen-tial movement between the drum and motor shaft caused by slippa~e or inexact drive train ratios. With such a correction device, a failure or hazard condition will vary the inputs such that the direc-tion o~ the OlltpUt will alwa~s go to the opposite direction to set the brake.
In a preferred embodiment, the bra~e is set by a triggering device, but other brake-setting techniques may be employed.
An advantage of these embodiments which produce a zero or unidirectional output under normal operating condi-tions is that it allows the operational condition between the motor shaft and drum to be visually monitored, and, after the occurrence of a failure or hazard, which produces ~ ~'7~

a desired directional output to set the brake, the cause of the Eailure or hazard is more easily traced. Resetting of the brake is also advantageously achieved. The detector in the preferred embodiment will detect overspeed as well as other failure conditions.
In a preferred embodimentl the brake would normal-ly be set by using a low~speed, high-force reset mechanism, such as alr cylinders or electric worm drives, for example, to cock a high-force spring. The high-force spring will be retained in a cocked position by a triggering mechanism which is controlled by a relatively low force. It is an advantage of the preferred embodiments that the resetting of the spring and trigger mechanism can be controlled remotely by an operator and, also advantageously, the trigger can be released and the brake set also remotely by an operator.
The apparatus of the preferred embodiments is relatively inexpensive to manufacture and is essentially single-failure-proof. That is, the detector will produce a desired brake-setting output in the event of any failure in the drive train between the motor and the drum, a two-block-ing or load hang-up condition, and, if desired, an overspeed condition of the type in which the motor controller may erroneously signal the drum to be driven in a dangerous overspeed condition.
A further advantage is the elimination of nuisance trips~ particularly if there is slippage in the drive between the motor and drum. If a unique energy absorption device of the type shown in U.S. Patent 4,175,727 is em-ployed in the drive train, slippage could occur. The energy absorption is intended to absorb the high-velocity kinetic energy of the drive train in the event of a failure. Since the energy absorption device normally will allow some rela~
tive slippage between the drum and motor, over a long opera-tional span, this slippage could accumulate in any detector and must therefore be nulled out or otherwise compensated for.
When no correction or nulling device is provided, the differential relative velocities between the motor shaft :~.1'7~

and drum can be zero iE equal speed reductions are employed to the detector between the drum and motor. If the speed reductions/ although fixed, are different, there will be a known unidirectional output from the detector. When an energy absorption device is provided, there may be bidirec-tional slippage between the inputs to the detector from the drum and the motor; but with the correction device of this invention, these bidirectional differentials can be con-verted to a continuous, known unidirectional output of the 10 detectorO
It should also be understood that while a total system and variations thereof will be illustrated and described, various components themselves are unique and have utility apart from a total system.

Brief Description of the Drawings Fig. 1 is a schematic plan view of a holsting mechanism employing the safety system of this invention.
Fig. 2 is a section along the line 2-2 of Fig. 1.
Fig. 3 is a fragmentary section take along line 3-3 of Fig. l of a differential detection system embodying the principles of the invention.
Fig. 3A is a schematic fragmentary section of another embodiment of detection device employed in one form of safety system, Fig. 4 is a side elevation of the hoisting device and safety system of Fig. 1.
Fig. 5 is a fragmentary section of an overspeed clutch forming a part of an embodiment of the invention.
Fig. 6 is a load-sensitive control for the over-speed clutch of Fig. 5.
Fig. 7 is a schematic elevation of another embodi~
ment of brake-setting apparatus.
Fig. ~ is similar to ~ig. 7 but illustrates a total mechanical brake-setting apparatus.
Fig. 9 is a schematic plan of a safety system embodying the p~inciples of the invention.

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FigO 10 is a fragmentary enlarged section o-E a portion of a brake-setting trigger employed in the apparatus of Fig. 9.
Fig. 11 is a schematic taken generally along the line 11-11 of Fig. 12, illustrating a differential assembly embodiment of the system shown in Fig. 9.
Fig. 12 is a side elevation of a portion of the apparatus shown in Fig. 9.
Fig. 13 is an isometric of a portion of the 10 System.
Fig. 14 is a section taken generally along the line 14-14 of Fig. 11.
Figu 15 is a section taken generally along the line 15-15 of Fig.12, illustrating the slip correction device embodying the principles of the invention.
Fig. 16 is a schematic fragmentary side elevation of another embodiment of a detector embodying the principles o~ the inventi~n.
Fig. 17 is a fragmentary p:Lan of the device shown in Fig. 16.

Best Mode for Carr~inq Out the Invention As best shown in Fig. 1, a hoist system includes an operating brake 2 coupled to a motor shaft 2a which is powered by a motor 3. A coupling 4 couples the motor to a conventional gear reduction unit 5, such as a 500:1 reduc-tion, which has an output shaft 7 rotatably carried in a pillow block 8. A drum pinion 9 meshes with the drum gear 10 .
A drum 11 is rotated by the drum gear on a shaft lla which is rotatably supported in a pair of spaced pillow hlocks 13. As described in earlier patent 4,175,727, a unique feature of the hoist system is that it is provided with a second brake, such as a band brake 14, wrapped on a brake drum 12. As will be described in more detail below, a brake-applying assembly or brake actuator 15 will set the brake in response to a detected failure or other hazard condition.

In the preferred embodiment of the invention, a torque limiter assembly 6 of ~he type shown in Patent No.
4,175,727 is provided to limit the torque which would be imposed from high-speed rotational kinetic energy of the motor and high-speed drive elements of the gear reduction and motor drive if a load hang-up, overload or two-blocking occurs.
It is a unique feature of this invention that a mechanical differential, out-of-sync detector 20 is provided for detecting the ~ailure or hazard condition. In one em bodiment, the detector also provides the mechanical force Eor applying the band brake 14. In pre~erred embodiments, the detector merely signals the out-of-sync detection and a sep-arate brake actuator sets the brake, as in Figs. 7 and 8.
The out-of-sync detector 20 includes an input shaft 30 which, in the embodiment illustrated, is coupled to the motor shaft 2a by a conventional right angle drive 19 having a gear reduction equivalent to that of the total gear reduction between the motor and the drum. A conventional right angle drive 1~ also couples the drum sha~t lla to an input shaft 31 to the detector 20. The purpose of the gear reduction in right angle drive 19 is to bring the two input shafts enter-ing the dectector to approximately the same speed. Exact speed e~uality is desirable, but if suitable nulling is provided, as will be described, exact speed equality is not essential. Other forms of speed reduction can also be provided.
Each of the input shafts 30,31, as is best shown in Fig. 3, is keyed to a drive gear which meshes with a side gear 26. The side gears 26 are keyed to pinion gears 27 that ~esh with a planetary gear 27a of a planet carrier 28.
As is well understood, equal and opposite rotational velo-cities of the drum input shaft 30 and motor input shaft 31 will cause the pinion gears 27 to rotate the gear 27a about a planet carrier post 28a that is fixed by a pin 46 to an output shaEt 29 carried in bearings 25. Should the rota-tional velocity on either of the input shafts vary, an angu-lar velocity will be created in the output shaft 29, the L'7~

speed of which will depend on the relative variation between the veloclties of the two input shafts. Thus, for example, if one of the input shafts should stop completely, the rota-tional output of the output shaft 29 will immediately reach a maximum speed. It is this rotation of the output shaft which triggers the brake actuation and, in one embodiment, creates or generates the force necessary for applying the brake on the brake drum 12. In this regard, while the pre-ferred location for the brake is directly on the drum, it should be understood that it is also possible to place this brake on a downstream pinion shaft in close proximity to the drum rather than directly on the drum. The purpose of this brake is to apply a stopping force on the drum as close to the drum as is practicable so that no substantial risk oc-curs from a failure of some drive element between the brakeand the drum. Furthermore~ the input shaft 31 to the detec-tor could also be from any location in the drive train be-tween the motor and drum, which location is at the desired point to be monitored. Preferably, this location, however, will be at or close to the drum.
One form of brake actuator mechanism is sbown as 15 in Fig. 1 and includes a lever 16a keyed to an extension 29a of shaft 29. The lever is coupled to the free end of the brake band 14 such that rotation of the lever 16a in either direction will set the band brake in a conventional manner. The lever 16a is provided with a notch 16b in which is inserted a spring-centered cog 17. A conventional clutch 32 couples shaft 29 to its extension 29a. A clutch throw out bar 33 ~ollows a cam 34 on the drum to decouple the shaft extension 29a once each rotation of the drum. If the shaft 29 rotates through some predetermined maximum angle, as, for example~ because of differences in the speed reduc-tions between the motor and drum and the speed reductions between the motor and drum and detector 20 or because of creep in the drive train, lever 16a will rotate toward the maximum angle, but the clutch 32 will decouple the lever 16a before it rotates far enough for the cog to leave the notch 16b. Thus each time the clutch is decoupled at each ?~

complete revolution of the drum, the lever is returned to its centered position by the cog 17. In the event of a failure or load hang-up, etc., however, the shaft 29 will rotate at a high velocity and the drum will be stopped before one or perhaps at the most two more revolutions of the drum are possible.
It should also be understood that in those instances where there is an exact match between the drum, motor, and detector speed reductions and no torque-limiting device such as 6 which could cause creep, a nulling device such as clutch 32 is not necessary.
The torque-limiting device 6 generally has a driven gear 40 which is driven by one of the upper stages of the gear reduction in the gear case 5. The gear 40 is provided with clutch facing 39 which is splined, as at 41, to a shaft 42. A spring 37 pushes a pressure plate 38 against the clutch facing, thus releasably holding the gear 40 in driving engagement with the shaft 42. ~ pinion gear 44 is fixed to the shaft 42 by a key 45. As is well under~
stood, by adjusting the position of the spring holder 36, a desired torque can be carried between the clutch facing and the driven gear 40. If an overload occurs, such as by ex-cessive load, load hang-up, two-blocking, a jamming in the drive train, or the like, the high-speed kinetic energy up-stream of the driven gear 50 will be dissipated as heat inthe clutch facing and the downstream drive components from pinion gear 44 to the drum will be stopped. ~ecause of this safety feature of the torque-limiting device, however, there may be a certain minimum amount of creep or relative rota-tional movement between the gear 40 and the shaft 36 so thatthere may be slight variations between the input shaft 30 and the drum input shaft 31 in the differential detector 20, as earlier described.
In one embodiment, a centrifugal clutch ~7 (Fig.5) is provided to decouple input shaft 30 Erom motor shaft 2a when the motor shaft rotates at an overspeed above some above some pretermined percentage of its normal driven speed. That is, if some failure occurs which causes the mo-7~

An overspeed clutch 47 is provided in a preferredtor to rotate beyond its set speed, the shaft 30 will become decoupled and stop, thus providing a variation between the relative velocities of shaft 30 and shaft 31 to provide a rotational output to output shaft 29 and`set the brake. It is important in the differential assembly that the differen-tial not back drive the input shafts, and thus, in a prefer-red embodiment, there are provided drag mechanisms on each of the input shafts to assure that the output shaft is ro-tated when one of the input shafts changes its velocityrelative to the other to provide a variation between the relative velocities of the input shafts. Continuous fric-tion drags could be provided on each of the shafts for this purpose, or the inputs could be through worm gear drives;
but in the preferred embodiment, the shafts are broken into two sections, namely, an external section 31e and an inter-nal section 31i and an external section 30e and an internal section 30i. The internal and external sections of each shaft are joined by a conventional one-way clutch or "NO
sAK" clutching device 21 of the type manufactured by Ann Arbor ~earing and Manufacturing Company, Ann Arbor, ~ichi-gan. These types of devices are well known, and in the in-vention here described, are uniquely positioned so that the external section 30er when driving in either direction, will freely rotate the internal section 30i. I,ikewise, the clutch on the drum input shaft i5 positioned so that when external section 31e is providing a driving input, the in-ternal section 31i will freely rotate. The converse is not true, however. That is, if at any time, one of the internal sections tries to drive the external section of that input shaft, the clutch will lock up so that the internal section cannot rotate. This provides a unique and more positive cIutching or drag device for the input shafts to assure that when there is a change in velocity relative to the other input shaft, ~his change cannot be transmitted backwards to rotation of the other input shaft, but rather must be con-verted immediately and in all cases to a rotational output of the output shaft 29.

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embodiment. Any type of conventional overspeed device can be employed, but it is an advantageous feature oE one embod-iment of this invention to employ a mechanical clutch having a clutch friction plate 48 keyed to shaft 2a and an opposed friction and pressure plate 49 keyed to a separate stub shaft 50 which drives the right angle gear box l9. A spring 52 is compressed b~ centrifugal governor weights 54 to hold the friction plates in driving engagement. When shaft 2a rotates rapidly, as in an overspeed condition, the weigh-ts 54 swing outwardly and spring 52 is released, thereby allow-ing plates 48 and 49 to slip relative to one another. Once shaft 50 is released from shaft 2a, the detector signals the out-of-sync condition and the brake 14 is set.
Control systems for high-performance hoists are sometimes des~gned to sense the lifted load and to command motor 3 to operate at higher than full-load rated speed when handling a lighter load. This may be as hlgh as 300 percent when operating under such no-load conditlon. Conventional overspeed drives are generally set in this no-load case to cut out at more than 3Q0 percent full-load speed. This reduces the safety when handling a full load.
Thus, if desired, the clutch 47 can also be made load-sensitive. To release the friction discs 48 and ~9 sooner or at a lower speed, a bell crank 56 is threaded in a nut 58 such that when screwed away from spring 52, the weights 54 will have less pressure on them and will open to release the discs 48 and 49 sooner or at a lower overspeed.
If the bell crank is screwed in the opposite direction, higher overspeed can occur before the clutch plates are separated.
Motion of the bell crank 56 is provided by a line 60 coupled to a pivoted arm 62 that is balanced by a cali-brated spring 64. The drum line 70 is reaved about a trav-eling block 72 and thence to a sheave 73 on arm 62. As the load is increased, arm 62 is lowered, thus moving bell crank 56q While the detector 20 can signal an electrical shutoff or brake-setting device, it advantageously prefer-13ably signals or triggers a mechanical brake actuator. In the embodiment of Fig. 1, the detector can itself apply the brake. Two forms of triggering devices for setting the brake 14 are illustrated in Figs. 7 and 8. It is common to both these triggering devices that a large spring force can be applied to set the brake, but a small trigger release force is all that is necessary to release the spring. This allows an inexpensive, trouble-free, manual or powered reset mechanism to again set the large spring force using a slower but highly leveraged resetting force.
In Fig. 7, the brake band 14 is set by a spring 74 having a large spring force, as is necessary for high-load capacity drums. A lever 75 is engaged by a trigger 76 which holds the spring in a cocked position. The trigger 7~ is anchored or locked by a conventional trigger release cam 78.
A solenoid 80 having an extendible arm 81 pivotally mounts one end of the cam 78. The cam is also pivoted at 83 and has an end 84 that abuts the trigger 76. A spring 89 urges the cam 78 into the phantom-line position to disengage from the trigger 76. When solenoid 80 is energized, the trigger release cam is in the solid-line position. Thus it is ap-parent that the small, easily controlled spring 89 is all that must be overcome to hold the large spring 74 in the cocked position.
To reset the trigger and spring 74, a relatively slow-speed rotary screw drive 90 moves the trigger, sole-noid, and trigger release to the left. The trigger strikes a cam 92 that rotates the trigger counterclockwise, and the solenoid is energized to again hold the trigger in the cocked position. Movement of the screw to shift the trigger to the right then reengages the lever 75 and recompresses the spring 74. Since the spring can be compressed slowly, the highly leveraged screw drive is easily able to overcome very large spring forces.
Fig. 8 illustrates a mechanical trigger release.
In this embodiment, the crane can be electrically de-ener-gized without having to set the brake 14, which is a disad-vantage in the embodiment of Fig. 7. In this preferred em-bodiment, the lever 16a (Fig. 1~, rather than being coupled directly to the brake band 14, is coupled to an elongated cable 94 that i~ connected to the trigger release cam 78 by a lost-motion slot 95. As the lever 16a rotates in an out-of-sync condition, the cable 94 is pulled, pivotin~ trigger release cam 78 into the phantom-line position to release trigyer 76 in the same manner as in Fig. 7. Resetting of the spring 74, trigger 76, and cam 78 is similar to the above description of Fig. 7 Fig. 3~ illustrates a modification of the detector 20 capable of providing a signal for setting a brake actua-tor. In this embodiment, the detector output shaft 29 is provided with a flyball governor 97 that meshes with a rack 98 slidably mounted in the shaft 29. As the ball levers swing out from an out-of-sync condition, teeth on the levers meshing with the rack extend the rac]~. The rack engages a normally closed switch 99 to open the switch and de-energize solenoid 80, for example, to set the brake.
If desired, a normally energized electric clutch 100 can be added to any of the embodlments to decouple the motor shaft from the detector for setting the brake automat-ically if an electrical power failure occurs. Furthermore, this clutch or the overspeed clutch could also be placed on the drum side of the input to the differential detector.
The operation of the various embodiments of the safety system will now be described. During normal opera-tion, such as with the motor shaft 2a being rotated at ap-proximately 1200 rpm, the drum speed will be reduced to approximately 2O4 rpm at the drum shaft lla. The motor shaEt at its 1200 rpm is then coupled through the centrifu-gal clutch 47 and right angle/gear reducer drive 19 to the differential detector assembly 20 via the input shaft 30.
Similarly, the 2.~ rpm rotation of the drum shaft is coupled via right angle drive 18 to provide the same rpm input to the input shaft 31. It should be understood that these gear reductions do not have to be exact so long as they are proportionate, and the gear reduction, which is approxi-mately 2:1 within the differential drive assembly, is sized .

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accordingly. The desired result is that shaft 31 and shaft 30, when the hoist is operating either in the lowering or hoisting mode, provide substantially the same angular velo-cities to the differential year 27a so that output shaft ~9 rotates not at all or perhaps rotates one way or another at a very low rate, depending on the amount of slippage, vari-ance in gear reductions, or creep in the system. If there is an overload, a load hang-up, a two-blocking or any fail-ure in a drive component such that the drum tries to stop, the torque-limiting device, if provided, will dissipate the high speed kinetic energy in the motor and upstream drive components, and the input shaft 31 will slow down or stop immediately, thus providing a variation between the angular velocities of the input shaft 31 and the input shaft 30.
The motor input shaft will then cause the output shaft 29 to rotate rapidly; for example, at about 600 rpm., since no slippage or back rotation can occur due to the clutch or drag 21 on the drum input shaftO Rotation of the output shaft 29 will immediately rotate the ball governor 97 or rotate the lever 16a, and the force applied by this rotation will be used either as a signaling device, as in Figs. 7 and 8I to set the brake, or, in a totally mechanical system, as in Fig. 1, to directly tighten the band brake. Should the motor shaft 2a rotate above its rated speed f such as where the controller may fail and allow the motor to drive the hoist too rapidly, the clutch 47 will disengage the motor shaft from the detector, stopping the input shaft 30 and providing an out-of-sync rotatio~ of shaft 29. Similarly, i~ either the input shaft 30 or the input shaft 31 of the differential assemhly should fail or any component in these inputs to the differential assembly should fail, the shaft 29 again will be rotated to set the brake. There is in essence no type of single failure that is not detected and the brake actuated, resulting in an extremely safe, rela-tively ine~pensive detection and brake actuating systemfor the hoist mechanism. Furthermore, any combination of overspeed and electrical clutches, as described, can be used with the detector, dependin~ upon the re~uirements for a particular hoist.

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As best shown in Fig. 9, a pre~erred form of the invention includes a motor M is drivingly coupled to a drum D by a conventional power transmission main drive train in-cluding a gear reduction unit 10. In the preferred embodi-ment, the gear reduction unit is of the type shown in Patent 4,175,727 and includes a passive energy absorption devices 12.
The opposite end of the motor is connected through a secondary drive train to an input shaft 214 of an out-of-sync detector 216. In the embodiment of Fig. 9, the out-of-sync detector is a mechanical differential assembly, as best shown in Fig. 11. The motor is joined to the input shaft 214 by an electric clutch 218 and conventional right-angle drive element 219. The motor ~ is provided with a conven-tional electrically controlled operational brake 220. Thistype of brake is generally set when electrically de-ener-gized, i.e., in the absence of electricity. The electrical clutch 218 is employed between the motor and the motor input shaft 214 for overspeed protection. For this purpose, the clutch 218, which may be a conventional electric clutch, is drivingly coupled between the input shaft 214 and the motor when the electric motor is being energiæed, but becomes decoupled when the electricity is removed from the motor or during a total electrical blackout. The clutch also will become decoupled when the drum's rotational velocity exceeds a predetermined value, as in a hazard condition of the type in which the motor controller directs the motor to run at an unsafe overspeed condition. "Overspeed," as used herein, is a well known term in the art to signify a condition when, for the particular hoisting system, the motor is running at an excessive speed, for example, 150 and 200 percent of its normal operating speed. A suitable clutch is available from Warner Electric, Inc., of Beloit, Wisconsin. Similar mech-anically decoupled overspeed clutches are also suitable.
A second input shaft 222 to the detector 216 is from the drum D. The drum is also provided with a safety , ':' ' :' " ''' ' : ~ :

brake 224, which, preferably, is a conventional heavy-duty ban~ brake but which may also be any other suitable brake, such as the ca]iper brake of the type shown in Patent 4,175,727~ The brake is set by a brake actuator 226 which receives a signal Erom the detector 216 of a failure or hazard condition, which signal is, in a preferred embodi-ment, transmitted via a mechanical cable 227.
The detector 216 is provided with a gear reduction 28 which couples the drum shaft 222 to a conventional dif-ferential gear assembly 230 via a NO-BAK'~ coupling 229. As mentioned ear`lier, NO-BARI~ clutching devices are essentially conventional clutches which will drivingly couple or trans-mit motion from the input shaft 222 into the differential 230 in either rotational direction, but will lock up and not transmit motion in the reverse direction, that is, from the differential assembly 230 back to the input shaft 222.
The differential assembly 230 is of a conventional construction having a set of bevel gears 236-239 which are freely rotatable on spindles that are keyed to a housing 240. The housing is keyed to a spindle 242, whereas gears 236 and 23~ are freely rotatable on spindle 242. As is well understood with differential assemblies, clockwise rotation in the direction of arrow 243 of the gear 236, with the housing held stationary, will result in counterclockwise rotation of gear 238 in the direction of arrow 244. As is also well understood, if the entire housing rotates in a clockwise direction, as shown by arrow 245, then gear 238 will rotate in a clockwise direction. (The rotational directions for purposes of this description will be viewed arbitrarily as looking in the direction of arrows 14-14 of Fig. 11, and also are arbitrarily shown as in the load lowering mode).
Gear 23~ is keyed to an output shaft 249 on which is mounted a one-~ay sprag clutch 250. The shaft 249 will freewheel in the counterclockwise direction of rotation in the sprag clutch 250, but in the opposite direction will lock up with the clutch, causing rotation of the clutch.

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~s earlier stated, the input to the differential housing 240 is through the NO-BAK~ 229 and thence to the drum. The second input to the differen-tial assembly is through worm gear 251, driven by a screw shaft 252. Worm year 257 is keyed to gear 236. Shaft 214 from motor M is connected to shaEt 252 through a slip correction device 25~
(Fig. 15). The slip correction device includes a first set of gears 256 and 257 having diametrical ratios which produce a slower rotational velocity in shaft 252 than is in shaft 214. Gear 257 is coupled to shaft 252 through a friction slip clutch 258. A second set of gears 260 and 261 result in a rotational ve].ocity of shaft 252 which is greater than the rotational velocity of shaft 214. Gear 260 is coupled to shaft 214 through a one-way sprag clutch 263. The result of this gearing and clutching arrangement is that when shaft 214 is turning counterclockwise (looking left in Fig. 15), ~or example, the drive will be through gears 256 and 257 to reduce the velocity of shaft 252. Gear 260 will freewheel on shaft 214. When shaft 214 is rotated clockwise, sprag clutch 263 locks gear 260, resulting in shaft 252 being rotated at a higher velocity than shaft 214, with gear 257 slipping in clutch 258. The purpose of this slip correction device is to provide an input direction and velocity to the differential 230 which are different than those from the input from the NO-BAK'~ 229 from the drum (assuming that the gear reductions between the drum and its dif~erential input and the motor and the differentlal input through gear 251 are at or about th~ same reductions) such that gear 238 will always result in a net unidirectional rotational output in the counterclockwise direction. If there was no slip cor-rection device and the gear reductions between the drum and motor to the differential assembly were identical, then the gear 238 would be stationary under normal operating condi-tions. With the slip correction device, the counterclock-wise component added to the gear 238 allows the output shaft 249 to rotate in a freewheeling condition; and should lim-ited slip occur between the motor and the drum due to a passive energy absorption device, such as device 212, then ' ' . ' .
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the slip will be less than that required to trip the brake and the added component of velocity added to the gear ~38 and will null out the slip during operation of the hoistO
As a further example, assume thta the motor is rotated in a 5 direction to rotate the drum in a raising mode. For this description, assume that when raising, shaft 252 will be made to turn slower than shaft 214. Also assume the teeth angles between screw 252 and year 251 are such that the in-put to the differential through worm gear 251 will be faster 10 but in the same direction as the input to the differential through NO-BAK~ 229. Assume these directions are counter-clockwise. The net output will result in rotation of shaft

2~9 in the counterclockwise direction.
When lowering the load, sprag clutch 263 becomes 15 engaged and shaft 252 is rotated at a yreater speed than shaft 214. This results in an input from worm gear 251 to the difEerential assembly, which is greater than the input from NO-E~AK'U 229. Since the direction of rotation of NO-BAK"' 229 is now in the opposite or clockwise direction, 20 as shown by arrow 243, however, the net result will again produce an output rotation of sha~t 249 again in the counterclockwise, freewheeling direction.
This unidirectional freewheeling rotation of output shaft 249 is uniquely employed with limi-ted slip 25 operation. With the absence of a passive energy absorption device, the shaft will be at zero velocity. The important point, however, is that the sprag clutch 250 will produce a rotational output whenever the direction of shaft 249 is in the clockwise direction, either from zero velocity, where 30 there is no clip correction, or from a counterclockwise direction, where there is slip correction. This output is used to signal the hazard or failure condition to set the brake.
Various examples of the operation for detection of 35 a hazard or failure will now be given. As a first example, consider the drive between the motor and the drum via gear reduction 210 is broken while the drurn is being rotated in a raising mode. The drum will imrnediately reverse to a lowering mode, which will reverse the direction of the NO-BAK~ 229 but will not reverse the motor input direction.
This will reverse the direction of the housing 240, immedi-ately causing a clockwise rotational direction to shaft ~49 to produce a brake set signal.
As a second example, consider two-blocking or load hang-up while the load is being raised. In this n~ode, the drum stops rotating; this stops rotation of the differential housing 240. During normal operation in the raising direc-tion, worm gear 251 was being rotated counterclockwise at aspeed slower than the housing 240, resulting in a net coun-terclockwise rotation of shaft 249. However, when the NO-~AK~ 229 becomes stopped, then gear 238 is driven clock-wise, which causes shaft 249 to engage with clutch 250 and produce a brake-set output.
As another example, consider a failure in the detector itself 9 for example, between the NO-BAKTN 229 and the input shaft 222 while raising a load. Since a NO-BAK~
transmits motion only when powerecl by the shaft 222, the NO-BAK~ will lock up upon failure of that drive~ The gear 238 will be moved in a clockwise direction, locking up clutch 250 to produce a brake-set~
As another type of failure, consider a failure in the detector itself when raising the load, such as by a failure of shaft 252. As is well known, a worm gear cannot backdrive the screw shaft 252. This locks up the input from the motor side of the differential, and the gear 238 will be driven in a clockwise direction to produce a brake-set sig-nal as soon as the drum is reversed to the lowering mode.
In the case of overspeed in the down ~r lowering direction (overspeed is not a problem in the raising direc-tion), the clutch 21~ will decouple, again stopping the worm gear 251. The drum input will be in the direction of arrow 245, causing a clockwise direction of shaft 249, and will set the brake.
Thus, as is readily apparent, the detector does develop a brake-setting output signal in generally all conceivable types of failures or hazard conditions.

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, The signal or clockwise movement of sprag clutch 250 is uniquely provided to trigger release of the brake in a manner which allows the trigger to be reset without manual intervention. This is an advantage when testing an install-ed system as well as to reset the brake during inadvertenttrips or when the brake has been set intentionally by a failure or by the operator of the hoist. This brake-actuat-ing mechanism is best shown in Figs. 9, 10 and 11. In the embodiment illustrated, the band brake 224 is connected at its free end to a bell crank 270. ~his bell crank is con-nected to a trigger mechanism 272 of a type similar to that shown in Figs. 7 and 8. The trigger mechanism 272 employs a catch 274 having a cam surface 276 and a lower cam sur-face 278. A latch 2B0 has an abutment end 282. The brake is set by a large force-applying compression spring, shown schemat~
ically as 284. The brake is retracted or reset by a conven-tional air bag or cylinder and piston 286. The air bag is such that it is energized to compress -the spring 284 and loosen the brake band. The end 282 of the latch 280 engages the cam surface 276. The latch is raised by engagement of a boss 2~8 on the backside of the catch. The arrangement of the cams 276 and 278 are such that they will try to rotate latch 280 clockwise in a brake-releasing condition, but so long as the latch 280 is held in the upright condition, the catch 274 cannot move. Thus, once latch 280 is held in the raised position, catch 274 cannot move and the air can be vented from the air bags 28~. As a typical example, the force by the spring 284 will cause approximately one hundred fifty pounds of orce to be required to hold the latch 2~0 in the raised position. ~his one hundred fifty-pound force is to be contrastecl with the cocking force of several thou sand pounds (6,000 lbs., for example) that will be ~laced in the spring 284 by the air bags as is commonly necessary for setting this type of band brake. The advantage, of course, as with the embodiment of Figs. 1-8, is that now a very quick acting, s~all restraining force can be used to trigger or release a much larger brake-applying force.

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The upper end of latch 280 is coupled by the cable 227 to a slide 292. The slide is held in a raised position by a smaller force compression spring 29~ of perhaps twenty or thirty pounds force . The slide 292 is carried in a track 295. The slide is held up bv a roller 296 whieh is carried on a wedge ring 29$. The wedge ring is rotated eounterclockwise by a reset cable 300, which is coupled to a tension spring 302~ Due to the downward pull by cable 227, wedges 306 on the wedge ring loek into a wedge carrier 307 that is fixed to the sprag cluteh 250 in the position shown in solid lines in Fig. 14 (the operational position). In this eondition, of course, the output shaft 249 from the differential will either be stationary or rotating in the counterclockwise direetion so that the wedge ring freely rotates on the shaf-t 249.
When an output signal to set the brake is produced by clockwise rotation of the output shaft 249, the sprag clutch 250 locks up and rotates eloekwise. This elockwise rotation of the sprag clutch rotates the wedge ring 98 in the cloekwise direction into the phantom line position of Fig. 14. This removes roller 296 from under the slide 292, allowing the one hundred fifty-pound force in cable 290 to be released. Latch 280 is rotated cloekwise, releasing catch 2i4, allowing spring 284 to set the brake.
To reset the bra]~e, air is applied to the air bags 286, moving the cateh 274 to the left. The eompression spring 294 raises slide 292. The reset spring 302 pulls wedge ring 298 eounterelockwise to reset the roller beneath the slide. Wedge ring 298 is slightly oversized on its shaft such that its wedges 306 release from the wedge ear-rier 307 when the downward load, eaused by cable 227, is released from the wedge ring. This allows the wedge ring to be rotated into its reset condition or a counterclockwise direetion. The resetting of the slide 292 and the wedge 35 ring occurs prior to the time the air bags have retracted eateh 27~ to its further left reset position. Thus the wedge ring remains free on eluteh 250 until the lateh 2$0 is eaused to rotate slightly eloekwise by the spring 284 after the air ba$s have been vented. Then the one hundred fifty-pound force is again applied to the slide to hold the wedge ring tight against the carrier and thus the sprag clutch 250.
Figs~ 16 and 17 illustrate a simplified embodiment usable with or without a slip correction device 254. Assum-ing the slip correction device is employed, the differential detector of FigO 16 employs an equivalent motor input shaft 252 from the motor and an equivalent hollow input shaft 242 from the drum. If the slip correction device is not em-ployed, then the motor input shaft would be equivalent to shaft 214. A collar 320 is pivotally connected to a bell crank 221. A spring 322 holds the bell crank in a counter-clockwise direction and is equivalent to the spring 294 of Fig. 14. Cable 227 is attached to the right-hand end of the bell crank. The collar 320 is keyed within an axial slot 324 in the shaft 252. A roller 326 is rotatably mounted on the collar. The rGller is held against a cam surface 330 which forms part of a cam block 332. The cam block is joined to shaft 242 by a friction clutch 333 and a one-way clutch 338, positioned by needle thrust bearings 334 and radial bearings 337. The cam bloc]s Ereewheels on the shaft 2~2 in one direction of rotation, but is joined to the shaft 242 in locked arrangement by the one-way sprag clutch 338 and friction clutch 333. The cam block moves with the shaft 214 and is held in the direction of the arrow 340 by a coil spring 342. The cable 227 produces approximately a one hun-dred fifty-pound pull on the collar, urging it axially to the right in Fig. 16, energizing the friction clutch 333.
It can be seen that as the cam block 332 is moved in the direction opposite to the arrow 340, the roller moves off the cam surface 330 and is pulled by the force in cable 227 into a slot 344. This allows the bell crank to rotate clockwise, releasing the trigger to set the brake in the manner shown for the embodiment of Figs. 9-15. The friction clutch is also de-energized, allowing cam block 332 to free-ly rotate with shaft 152. Resetting of the brake is essen-tially the same as in the preferred embodiment, with the . .

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spring 222 moving the collar back to the left. This move-ment withdraws the roller 326 from the slot 344 and allows spring 3~2 to reset the cam block in the direction of the arrow 340. The roller 326 then is precluded from moving to 5 the right in Fig. 16, which then holds the bell crank in a fixed position when the one hundred Eifty~pound load is again placed on the cable 227 by the trigger mechanism.
In the embodiment of Figs. 16 and 17, as in the earlier embodiment, the device senses a differential move-10 ment between the shaft 242 and the shaft 214 or 252 in apredetermined direction to cause the sprag clutch 338 to rotate clockwise in the embodiment illustrated to rotate the cam block and release the collar 320. The normal operating rotations will produce either a zero differential rotation 15 between shafts 252 or 214 and 242, if there are perfectly matched gear reductions between the motor and drum and the detector and if there is no relative slippage in the drive trains~ If these conditions do not exist, then a unidirec-cional rotational differential exists. In either case, a 20 failure or hazard condition will cause an output rotation in a predetermined direction and opposite said unidirectional rotation, if such is in effect, to set the safety brake.
Various examples of operation of the embodiment of Figs. 16 and 17 will now be explained. In a first example, 25 assuming the motor input shaft 252 is turning clockwise and the drum input shaft 242 is also turning clockwise~ but because of the slip correction, the shaft 242 will be trav-eling clos~kwise slower than will shaft 252. In this condi-tion, the sprag clutch is freewheeling. The shaft 242 runs 30 slower because we are assuming that the slip correction device is being employed and thus the shaft is equivalent to shaft 252 of Fiy. 15. 'rhere are also provided a NO-BAKI~ 350 locking shaft 242 to the frame and a NO-BAKm 360 locking shaft 252 to the frame. Thus NO-BAKI~ 350 will allow shaft 35 242 to drive clockwise or counterclockwise, but will not allow shaft 242 to be driven by shaft 252. Similarly, NO-BAKI" 360 will allow shaft 214 or 252 to be driven from the motor but will not allow shaft 214 or 252 to backdrive through NO-BAK~ 360. Now assume that the motor shaft fails, creating a failure condition. NO-~AK'~ 360 locks shaft 252, shaft 242 thus turns clockwise faster than shaft 252, clutch 338 engages, and the cam block 332 gets rotated clockwise, causing roller 326 to enter slot 344 and set the brake.
As a second example, consider raising the load, with the motor input shaft 252 turning counterclockwise and the drum shaft 242 turning counterclockwise. The slip cor-rection device, however, will cause the shaft 242 to rotate faster than shaft 252 in the counterclockwise direction such that the sprag clutch still freewheels. Again assume that there is a discontinuity in the main drive between the motor and the drurn. Shaft 252 will lock up. The drum, since it was raising a load until the drive failure, will immediately reverse direction and begin to lower the load. Thus the shaft 242 will reverse to a clockwise direction, causing the cam block 332 to move clockwise and set the brake.
A ~ur~her advantage is that whenever the opera-tional brake is set, any slippage or failure in the opera-tional brake will also be detected and set the safety brake.This overcomes a major problem in hoists because the brakes can otherwise wear severely without detection by the opera-tor and allow the load to slip.
As another example, assume a two-blocking hazard during raising. Shaft 252 will be turning counterclockwise;
shaft 242 will be turning counterclockwise. Shaft 242 will stop when the cable sheaves become two-blocked ("two-block-ing" occurs when the traveling blocks engage the stationary blocks). Since shaft 242 is stopped and the shaft 252 con-tinues to turn counterclockwise, the roller 326 moves offthe cam surface 330 into the slot and sets the brake.

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Claims (15)

Claims

1. A safety system in a load-carrying hoist in which there is defined a last upstream load-carrying component and a last downstream load-carrying component and a drive motor with a motor shaft, a power transmission main drive operatively coupled to the motor, an operating brake operatively associated with the motor to hold the load when the motor is de-energized, a drum and an emergency safety brake drivingly coupled on an operating element on or close to the drum but independent of the main drive so as to provide emergency holding of the load in the event of a main drive failure, a safety brake actuator respon-sive to an output from an out-of-sync detector for applying said safety brake, characterized by:
a mechanical out-of-sync detector, said detector including a monitoring secondary drive train having a first input shaft drivingly coupled to the last upstream load-carrying component, a second input shaft drivingly coupled to the last downstream load-carrying component, means for detecting a pre-determined variation in relative speed or direction between said two input shafts and producing an emergency brake-setting rota-tion output to set said emergency safety brake.

2. The safety system of claim 1, further characterized by said emergency safety brake-setting rotational output being unidirectional, said means for detecting a predetermined rela-tive speed or direction between said two shafts producing said unidirectional emergency brake-setting rotational output, said power transmission main drive having minor slippage producing an accumulative error between the relative rotations of the last upstream and last downstream load-carrying components, error compensating means for producing a predetermined, limited, dif-ferential rotational direction and velocity between said first and second input shafts to generate a unidirectional rotational output in a predetermined direction opposite said emergency safety brake-setting unidirectional brake-setting rotational output and of a velocity greater than said relative rotation accumulative error for compensating for such relative rotation accumulative error.

3. The safety system of claim 2, further characterized by said out-of-sync detector including a differential assembly having meshing differential gears and a differential carrier, one of said input shafts drivingly coupled to one of the differ-ential gears, another of said input shafts being coupled to another of the differential gears, an emergency safety brake-actuating output shaft being drivingly coupled from said differ-ential assembly, whereby said input shafts produce said rota-tions in said emergency safety brake-actuating output shaft, and one-way clutch means drivingly coupling said emergency brake-actuating output shaft to said emergency safety brake actuator only when said output shaft rotates in said unidirectional emergency safety brake-setting output rotational direction.

4. The safety system of claim 2, further characterized by said detector including coaxially aligned first and second input shafts drivingly coupled to said last upstream and last downstream load-carry.ing components, an axially movable collar on said first input shaft movable between an emergency safety brake-set position and an emergency brake-off position, linkage means coupling said collar to said safety brake actuator, stop means on said second input shaft and operative in one unidirec-tional differential rotation between said input shafts to hold said collar in said brake-off position but in the opposite dif-ferential direction of rotation, releasing said collar to move into said emergency safety brake-set position and cause the linkage means to produce said emergency safety brake-setting rotation output to trigger the brake actuator and set the safety brake.

5. The saety system of claim 3, further characterized by said brake actuator including a unidirectional, rotation transmitting, one way clutch on said brake-actuating output shaft, a wedge ring releasably, drivingly coupled to said one-way clutch, means biasing said ring in an emergency safety brake-hold position, means on said ring for holding the safety brake, said safety brake including trigger means having a latch and a catch, said catch having a large force biasing the catch into a brake-set position, said latch having sufficient leverage to hold said catch against a large force by a small force when in a brake-off position but movable upon release of said small force to move in a safety brake-set position, releasing said catch to move into its large force safety brake-set position, and linkage means coupling said latch to said wedge ring brake-holding means for holding said latch in said brake-off position, whereby rotation of said detector output shaft in said unidirec-tional emergency safety brake-setting output rotation rotates said wedge ring to release said linkage means and set the safety brake G

6. The system of claim 2, further characterized by said accumulative error being a variation in speed reductions in the main drive train between the last upstream and last down-stream load-carrying components and the secondary drive train that also couples the last upstream and last downstream load-carrying components and the detector, said compensating means including means for producing said second unidirectional rota-tion opposite the unidirectional safety brake-setting rotational direction in the output in excess of such speed reduction variation.

7. The system of claim 2, further characterized by said main drive having a member which will cause rotational slippage error between the drum and the motor shaft, said com-pensating means producing said second unidirectional rotation in the output in excess of such rotational slippage error.

8. The system of claim 1, further characterized by said detecting and said safety brake actuator means being com-bined and including force-applying means for tightening the safety brake on said drum or drum-operating element, and the rotational output of said output shaft caused by said variation in relative direction or speed of said input shafts providing the force for moving said force-applying means for tightening the safety brake.

9. The system of claim 1, further characterized by said first and second input shafts having minor and major relative angular velocity differences and including nulling means for periodically restoring the output shaft to an in-sync condition to compensate for minor relative angular velocity differences between said input shafts.

10. The system of claim 9, further characterized by said output shaft having inner and outer ends, said nulling means including a disconnect clutch on said output shaft oper-able to disconnect the outer end from said output shaft inner end, means for centering said outer end of said output shaft when disconnected from said inner end, and means responsive to rotation of said driving element for periodically disconnecting said clutch.

11. The system of claim 1, further characterized by including an overspeed detection clutch for disconnecting said one of said input shafts during an excessive threshold overspeed condition to initiate said relative angular velocity difference between said first and second input shafts.

12. The system of claim 11, further characterized by including load magnitude responsive means to correlate load magnitude to motor speed for varying the threshold excessive overspeed condition dependent upon load magnitude.

13. The system of claim 1, further characterized by including an electrically activated driving clutch operable when de-energized to disconnect an input shaft from its respective last upstream or last downstream load-carrying component to initiate said relative angular velocity difference between said first and second input shafts when an electrical failure occurs.

14. The system of claim 1, further characterized by said safety brake actuator including a high-force holding device and a low-force anchor releasably holding said high-force hold-ing device in a cocked condition, said safety brake actuator including a large force spring urging said safety brake in an engaged position, means coupled to said high-force holding device for holding said spring in said cocked condition against movement of the safety brake into an engaged condition, means coupled to said high-force holding device for holding said spring in said cocked condition against movement of the safety brake into an engaged condition, means responsive to a hazard condition for moving said low-force anchor to release said high-force holding device for setting said safety brake, and means for restoring said spring, low-force anchor and high-force holding device to their initial cocked condition and release said safety brake.

15. The system of claim 1, further characterized by each of said input shafts including means for restricting move-ment of said input shaft when attempted to be back-driven by said means for detecting said variation in relative speed or direction between said two input shafts, and thus the output is always caused to rotate during differential angular velocities between said input shafts rather than back-driving an input shaft.