US2718949A

US2718949A - Crane attachment

Info

Publication number: US2718949A
Application number: US350916A
Authority: US
Inventors: Walter D Taylor
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-04-24
Filing date: 1953-04-24
Publication date: 1955-09-27
Anticipated expiration: 1972-09-27

Description

Sept. 27, 1955 W. D. TAYLO R CRANE ATTACHMENT 3 Sheets-Sheet 1 Filed April 24, 1955 INVENTOR.

6 m abz Sept. 27, 1955 w. D. TAYLOR 2,713,949

CRANE ATTACHMENT Filed April 24, 1953 '3 Sheets-Sheet 2 I INVEN TOR. WA 729? .0. TAJ Z 0/? Sept. 27, 1955 w. D. TAYLOR 2,718,949

CRANE ATTACHMENT Filed April 24, 1953 a Sheets-Sheet s INVENTOR. W417i)? 0, 7797101? Patented Sept. 27, 1955 CRANE ATTACHMENT Walter D. Taylor, New Boston, Mich. Application April 24, 1953, Serial No.'350,916

Claims. (Cl. 192-47) This invention relates to hoisting mechanism for cranes and particularly to a device which will enable the hoisting machinery to act as a brake when a load is lowered.

In certain types of cranes, but in particular the T and T truck cranes made by The Browning Crane and Shovel Co. of Cleveland, Ohio, the only way a load can be lowered is by disengaging the load cable drum from the hoisting machinery and regulating the descent of the load by means of the load drum brake. This mode of operation is extremely hard on the brake when the load is heavy and if, for some reason, the brake should fail, the load would plummet to the ground. However, if the cable drum could be connected to the various reduction gears and shafting comprising the hoisting machine, and then the weight of the load permitted to drive the machinery backwards, there is enough inherent resistance in the machinery by virtue of the gear reduction factor and friction to act as a brake and in conjunction with the conventional brake to permit the load to descend at a reasonable speed without wear to the conventional brake. The crane is also safer because once the load is driving the machinery there is very little if any possibility that this machinery will break and permit the load to drop too rapidly. Therefore, it is an object of this invention to provide a device which may be installed in a crane of the type mentioned to permit the load carrying cable to drive the cable drum and the hoisting machinery which in turn acts as a brake and permits the load to descend at a uniform rate without such use of the conventional brake as causes excessive wear.

A second object of the invention is to provide a device which may be installed in cranes of the type mentioned, and which will allow the weight of the load to drive the hoisting machinery during the descent of the load, thereby assisting the conventional braking mechanism and reducing the strain thereon.

In the Browning crane of the type mentioned, the boom of the crane is raised or lowered by a cable wound around a drum which forms a part of the hoisting machinery. 'A second cable passing over the end of the boom is attached to the load at one end and wound around a drum at the other end. This latter drum is connected to the boom hoist drum by various gearing, chain and sprocket drives, and clutches. However, the connection between the two drums is such that if the load is permitted .to drive the hoisting machinery during the descent of the load, the drum carrying the boom hoist cable will wind up the cable and raise the boom. Therefore, as the cable supporting the load pays out, the cable supporting the boom is wound in, and the two act in opposition toeach other. Because of such opposed movement it is impossible to lower the.

load by having it drive the hoisting machinery and still retain the boom in a fixed position. Therefore, a third object of the invention'is to provide a device which may be installed in a crane of the type mentioned, and which will permit the load while descending to drive the hoisting machinery and yet not affect the position of the boom.

Yet another object of the invention is to'permit in a crane of the type mentioned the lowering of the load in such a manner as to drive the hoisting machinery backwards and at the same time permit the lowering of the crane boom. Asexplained above, without the use of this invention, when the load was lowered in such manner as to permit the weight of the load to drive the hoisting machinery, the movement of the machinery was such that the cable drum operating the boom would wind up the boom cable and raise the boom. By employing the present in-,

vention this result does not occur, but rather the opposite is made possible. The load and boom may be lowered simultaneously even though the load is driving the hoisting machinery.

Other objects, advantages and meritorious features will more fully appear in the following claims, specification and drawings, wherein:

Fig. 1 is a schematic view of the hoisting machinery of a crane in which the invention is adapted to be installed;

Fig. 2 is a schematic front view of the right hand end of the machinery shown in Fig. 1;

Fig. 3 is a cross sectional view taken on line 33 of Fig. 2 and showing the chain sprocket and the invention installed therein;

Fig. 4 is a cross sectional view taken on line 44 of Fig. 3;

Fig. 5 is a cross sectional view taken on the line 55 of Fig. 4;

Fig. 6 is a cross sectional view taken on the line 66 of Fig. 4; and

Hoisting machinery While the crane boom has been omitted from the drawings, it will be understood that the boom is pivotally mounted upon the front of a rotatable carriage. The hoisting machinery shown in Fig. 1 is mounted upon the carriage and cable drum 10 is provided with windings of a cable 12 which is attached to the boom near its outer end. By rotation of the drum the boom may be either raised or lowered.

Disposed in the outer end of the boom is a sheave over which cable 14 runs. One end of the cable is adapted to be attached to a load which the crane will pick up, and the other end is wound around the load drum 16, in a reverse direction to the windings of cable 12 around the boom drum.

The hoisting machinery illustrated in Fig. 1 further comprises an engine 18 with a master clutch 20 mounted on the drive shaft thereof. The driven half of the clutch is connected by a rotatable shaft to a series of reduction gears including a pinion gear 22 which is disposed in driving engagement with a spur 24. A pinion gear 26 mounted to rotate with gear 24 drives a spur gear 28:

36 which extends entirely across the hoisting machinery. z

A load drum clutch 34, hereinafter referred to as the load clutch is mounted on shaft 36 so that the driving por- 3 tion of the clutch is fixed to rotate with the shaft. The driven half of the clutch is rotatably mounted on the load shaft.

Rotatably mounted upon the shaft 36 and in driving engagement with the driven half of the load clutch is the load drum 16 heretofore mentioned. Connected to the load drum'and encircling the shaft 36 is a strap type brake 33 which is adapted to control the movement of the load drum when the latter is disconnected, by virtue of the load clutch, from driving engagement with shaft 36. This brake only controls the movement of the load drum and does not brake the rotation of the shaft except if the latter is coupled with the drum.

Mounted upon the right hand end of shaft 36 is the driving half of a clutch 40, hereinafter described as the upper clutch. The driven half of this clutch is mounted for rotation about shaft 36. It comprisesa clutch shoe engaging flange 42 spaced radially from shaft 36 by a web 44. The web is centrally provided with an axial bearing surface 46 whereby the driven portion of the clutch is journaled upon sleeve 43 mounted upon the shaft 36. Rotation of sleeve 48 about the shaft is prevented by a pair of keys 50 suitably mounted on the shaft.

The periphery of the flange 42, at the right of web 44 I (see Fig. 4-), exhibits a chain sprocket 52 provided with teeth 54 adapted to engage a drive chain 56. The chain runs downwardly and forwardly to a second chain sprocket 58 mounted upon the driven half of clutch 60. The driven half is journaled upon shaft 62 and connected to a hollow shaft 64 also journaled upon shaft 62. The driving half of clutch 60 is mounted upon the shaft 62 to rotate therewith. Spur gear is mounted upon the left end of shaft 62.

Journaled upon shaft 62 is the boom hoist cable drum 10. Connected to this drum is a strap brake 66 adapted to control the movement of the drum when the latter is either free to rotate about shaft 62 or is not in driving engagement with the engine. Hollow shaft 64 is connected at its left end with the drum.

Referring now to Figs. 2, 3 and 4, the shaft 36 is shown as supported at the right hand end by a web 68, provided with bearing head 70 through which shaft 36 passes. Spaced from the head and mounted upon the shaft as previously described is sleeve 48. As shown in Figs. 3 and 4 the sleeve is provided at the end adjacent the bearing head 70 of the shaft supporting web, with a radially extending triangular spider 72. Adjacent each of the vertices of this triangular spider is an offset arm 74, forming with the spider a bifurcation. Three of such bifurcations are equally spaced around the perimeter of the sleeve.

Mounted upon pin 76 for pivotal movement within each bifurcation is a sprocket dog 78 shaped as shown in Fig. 3. The dog is adapted to engage teeth 80 formed on the under side of flange 42, to the right of web 44, as shown in Fig. 4. Offset toward the nose of each dog from radial alignment with the pivot pin, when the dogs are shown in the disengaged position as in Fig. 3, is a spring loaded friction shoe 82 adapted to slidably bear against the web 44. A tension screw 84 is adapted to adjust the frictional drag of each shoe against the web. When the dogs are in engagement with the teeth 80, the friction shoes lie substantially in radial alignment with the pivot pins 76.

The foregoing is generally descriptive of the hoisting machinery to which the invention is applied. To best illustrate the need for and objects of the invention, a description of the operation of the hoisting machinery without the application of the invention is desirable.

Operation of hoisting machinery To raise the boom when all the clutches are disengaged and all the brakes released, the master clutch 20 is first engaged. The

shafts

36 and 62 will rotate in opposition with shaft 62 turning in a clockwise direction. Clutch 60 is next engaged which will cause hollow shaft 64 to turn and thereby rotate the boom drum winding up the cable 12 attached to the boom. When shaft 64 rotates so too do sprocket 58 and chain 56. As the chain moves it turns sprocket 52 in opposition to the rotation of shaft 36. In Fig. 3 the sprocket 52 would be seen as rotating in a counterclockwise direction and shaft 36 in a clockwise direction. When the sprocket is moving counterclockwise the friction shoes dragging over web 44 will tend to keep the dogs from striking the teeth of the sprocket.

To lower the boom, clutch 60 is disengaged. The weight of the boom then reverses the previous rotation of the boom drum, sprocket 58, and chain 56. Referring to Fig. 3, the sprocket 52 is now moving in a clockwise direction. As the speed of descent of the boom increases, a speed is reached wherein sprockets 52 begin to rotate faster than the shaft 36. At this point the friction shoes riding over web 44 force the nose of the dogs outwardly and into engagement with teeth 80 of the sprocket. Consequently the speed of rotation of the sprocket 52 cannot exceed the speed of rotation of the shaft 36 when the boom is being lowered. This is the function of the dogs 78, to control the speed of descent of the boom by not permitting the sprocket 52 to exceed the speed of rotation of shaft 36 To raise a load being held with the load brake, and with the engine running, and with all the clutches disengaged, the master clutch 20 is first engaged. Now shafts 36' and 62 rotate in opposition in respectively counter clockwise and clockwise directions (Fig. 1). Load clutch 34 is engaged and load brake 38 released. This rotates the load drum winding up cable 14 and consequently raises the load.

To lower the load the clutch 34 is disengaged and the load brake 38 is set. By gradual release of the brake the load drum is allowed to revolve by virtue of the weight of the load. If the load is heavy, considerable wear of the brake 38 results, in addition to substantial strain on the brake strap.

As is obvious to one skilled in the operation of a crane, it is desirable when lowering a heavy load to assist the load brake by coupling the load drum to the hoisting machinery so that'the" inherent drag of the latter will take part of the strain. Yet when this is attempted in a crane of the type described above, the following result obtains. With the load clutch engaged, the master clutch must be disengaged so that the engine will not be acting upon the reduction gears in opposition to the direction that the machinery will have to turn if the weight of the load is to revolve it. Then shaft 36 begins to rotate backwards or clockwise in Fig. 1, counterclockwise in Fig. 3. As it rotates, so the spider 72 carrying the sprocket dogs 78 will also begin to rotate, dragging the friction shoes .83 across the surface of the web 44. The action of the friction shoes and the web will pivot the dogs into engagement with teeth 80 of the sprocket. When this occurs, despite the fact that the upper clutch 44) is disengaged, the driven half of the clutch will begin to revolve, driving chain 56, which in turn will rotate sprocket 58. Upon rotation of sprocket 58, the boom hoist drum will revolve in a clockwise direction winding up the cable and raising the boom. Therefore, as the load descends the boom ascends with the result that the load in fact is not lowered. Consequently it becomes apparent that the only method of lowering the load is solely by use of the load brake and with the load drum disconnected from the hoisting machinery.

Load lowering control mechanism This invention was conceived in order to permit the load drum to drive shaft 36 without engagement of the dogs with the sprocket teeth. This is accomplished by providing a circular plate 86 exhibiting three cars 88, and an upstanding annular shoulder 90. The shoulder is spaced outwardly concentrically from an axial opening through the plate which permits the plate to lie flat against the spider 72 and rotatably seat upon a small shoulder 92 formed in the face of the spider. The plate is held in this position against axial movement by a washer 94 encircling the shaft 36 and abutting the face of the bearing head 70.

The plate is further provided with an arcuate slot 96 through which protrudes a pin 98 mounted on the face of the triangularly shaped spider 72. The relative position of the pin and the arcuate slot is such that if the plate is rotated clockwise (Fig. 3) through the length of the slot, the ends of the ears 88 of the plate will abut pins 100 mounted upon the dogs, and hold the dogs in a position such that the noses thereof are spaced inwardly from the sprocket teeth. In Fig. 6 the ears 88 of the plate are shown as rounded on one corner to enable the ears to cam outwardly the pins 100 upon rotation of the plate.

The plate 86 normally rotates with the shaft 36, in a position such that the ears 88 do not abut the pins 100, as shown in Figs. 3 or 6, but rather the leading edge of the ears juxtapose the pins. With the shaft 36 rotating in a clockwise direction (Fig. 3), which is the direction of rotation of such shaft when the engine is driving the machinery, the pin 98 abuts the leading end of the arcuate slot and thereby the plate 86 rotates with the shaft. During the rotation of shaft 36 in this direction, the ears do not earn outwardly the pins 100. However, when the master clutch is disengaged and it is sought to lower the load using the hoisting machinery as a brake, the shaft 36 will rotate in the opposite or counterclockwise direction. The plate 86 will of course also rotate in this direction, but because the drag of the friction shoes tends to force the noses of the dogs outwardly and the pins 100 inwardly, plate 86 cannot rotate upon the shaft 36 to a position such that pin 98 abuts the leading end of arcuate slot 96. In other words, the plate will be carried in rotation with shaft 36 by virtue of the abutment of the pins 100 with the trailing edge of the ears 88. V

Therefore, to force the plate to rotate upon shaft 36 and cam the pins 100 outwardly, a brake band 102 encircles the annular shoulder 90. The band is fastened at one end to the web 68 by a bracket 104. The other end of the band exhibits a rod 106 slidably projecting through a bracket 108 which is mounted on one of the bearing caps of the bearing head 70. Encircling the rod and abutting the bracket 108 is a coil spring 110. A nut 112, threadedly mounted on the rod is adapted to adjust the tension of the spring. The brake band tends to retard the rotation of the plate 86 as it turns with the shaft 36 in a counterclockwise direction (Fig. 3). The dragging effect of the brake band upon the plate is adjusted by the nut 112 until the drag overcomes the resistance t0 outward movement of the pins 100 as they are acted upon by the ears. When such resistance is overcome the plate will rotate upon the shaft 36 through the length of arcuate slot 96 and in so rotating will cam outwardly the pins 100 mounted on the dogs with the result that the noses of the dogs will pivot inwardly toward shaft 36. The drag on plate 86 is sustained as long as shaft 36 continues to rotate in the counterclockwise direction and consequently insures that the cars will hold the dogs in a position of disengagement with the sprocket teeth. Because the dogs are in such position the weight of the load will revolve the load drum, shaft 36, and the reduction gears, and yet the boom drum will remain stationary.

It is also apparentthat because the dogs cannot engage the sprocket teeth as long as shaft 36 continues to rotate as above described, it is possible to lower the boom drum by releasing the brake 66. The weight of the boom will unwind the cable 12 from the boom drum thereby revolving hollow shaft 64, sprocket 58, and sprocket 52. The sprocket 52 and the shaft 36 are now the shaft is rotating and pin 98 is moving through the length of slot 96, the dogs and pins 100 will be carried off the outward end of the ears 88 of the plate. The drag of the friction shoes across web 44 will hold the dogs inwardly from the sprocket teeth and the dogs and sprocket teeth will assume the relationship and function first above described.

What I claim is:

1. In crane hoisting machinery having a shaft supported at one end by a bearing hub with a spider mounted on the shaft adjacent the hub and exhibiting three pivotally mounted dogs each provided with a nose adapted to engage asprocket and turn the sprocket withthe shaft when the shaft revolves in one direction; a circular plate limitedly rotatably encircling the shaft mounted in juxtaposition upon a face of the spider and exhibiting three radially extending ears defining camlike surfaces; said plate cut away to provide an arcuate slot radially spaced from the shaft; an upstanding pin mounted on the spider and extending through the slot to limit the rotation of the plate relative to the spider; an axially outwardly extending pin mounted on each dog opposite the nose end thereof and adapted to abut said ears; an annular axially extending shoulder integral with said plate and concentric with the shaft; a brake band partially encircling said shoulder and supported at each end upon the bearing hub, one end of said band exhibiting a rod slidably received through a bracket offset upon the hub, a coil spring encircling the rod and abutting said bracket to urge the band yieldingly against the shoulder, said band operable against the shoulder to retard movement of the plate relative to the movement of the shaft and cam said means outwardly over the ears to pivot the nose portion of the dogs inwardly toward the shaft when the shaft rotates in said one direction, and operable to permit free rotation of the plate when the shaft rotates in the opposite direction.

2. In crane hoisting machinery having a shaft with a spider mounted thereon exhibiting three pivotal dogs each provided with a nose adapted to engage a sprocket and turn the sprocket with the shaft when the shaft revolves in one direction; a plate limitedly rotatably mounted upon the shaft juxtaposing the spider and exhibiting three radially extending ears; said plate cut away to provide an arcuate slot radially spaced from the shaft; an upstanding pin mounted on the spider and extending through said slot to position the ears of the plate adjacent the dogs; an axially outwardly extending pin mounted on each dog opposite the nose end thereof and adapted to abut said ears; an annular axially extending shoulder integral with said plate and concentric with the shaft; and a spring loaded brake band encircling the shoulder and tensioned to effect a drag upon the plate as it rotates with the shaft in said one direction to thereby retard the rotation of the plate and move the ears thereof against the pins mounted on the dogs to pivot the dogs inwardly and away from the sprocket, said brake band operable when the shaft revolves in the opposite direction to permit free rotation of the plate.

3. In crane hoisting machinery having a shaft with a spider mounted thereon exhibiting three pivotal dogs each provided with a nose portion adapted to engage a sprocket to rotate the sprocket with the shaft when the shaft revolves in one direction; a fiat circular plate provided with a central opening therethrough and adapted to be rotatably mounted on the shaft to juxtaposition with the spider; three radially extending camming ears mounted on said plate about its periphery; an axially extending shoulder integral with said plate and concentrically positioned with respect to said opening; a pin mounted on the spider in engagement with the plate and adapted to position the plate adjacent the dogs; means mounted on the dogs in camlike engagement with the ears; and brake mechanism associated with the said shoulder adapted to retard the rotation of the plate as the load shaft revolves in said one direction and hold the ears against the movement of said second mentioned means to cause said means to cam outwardly over the ears and pivot the nose portion of the dogs inwardly toward the load shaft.

4; In crane hoisting machinery having a shaft supported at one end within a bearing hub and a spider mounted on the shaft exhibiting three pivotal dogs each tending to engage a sprocket rotatably mounted upon the shaft when the shaft revolves in one direction; a plate rotatably mounted upon the shaft adjacent the spider and exhibiting three radially extending ears, an annular shoulder mounted upon the plate and concentrically spaced from the shaft; said dogs provided with a part opposite the nose end thereof adapted to engage said ears; means mounted on the spider and abutting the plate for limiting the amount of rotatable movement therebetween; a brake band encircling said annular shoulder and pivotally mounted at one end upon said bearing hub offset therefrom, said band exhibiting at the opposite end a tangentially extending rod portion, said rod slidably received in a bracket mounted upon and offset from said bearing hub, a coil spring encircling the rod and abutting at one end said bracket, tension adjusting means mounted upon the end of the rod and abutting the other end of said spring, said band yieldingly tensioned against the shoulder to retard the rotation of said plate as the shaft revolves in said one direction but permitting free rotation of the plate as the shaft revolves in the opposite direction.

5. In crane hoisting mechanism having a shaft with a spider mounted thereon exhibiting three pivotal dogs each provided with a nose portion adapted to engage a sprocket to rotate the sprocket with the shaft when the shaft revolves in one direction; a plate rotatably mounted upon the shaft juxtaposing said spider and exhibiting three radially extending ears, an annulus mounted on the plate concentrically spaced from the shaft, said plate cut away to provide an arcuate slot spaced radially outwardly from the annulus; a pin mounted upon the spider spaced radially outwardly from the shaft and adapted to extend through the slot in said plate and limit the rotation of the plate relative to the spider; said dogs exhibiting a raised portion opposite the nose end thereof adapted to engage the ears to be cammed outwardly thereby; and brake mechanism encircling the annulus and adapted to retard the rotation of the plate upon the shaft when the shaft rotates in said one direction to permit the raised portion of the dogs to ride outwardly over the ears and pivot the nose end of the dogs inwardly toward the shaft.

6. In a crane hoisting machinery having a shaft supported at one end by a bearing hub with a spider mounted on the shaft exhibiting three pivotally mounted dogs each provided with a nose portion tending to engage a sprocket to rotate the sprocket with the shaft when the shaft revolves in one direction; a flat circular plate provided with a central opening therethrough and adapted to be rotatably mounted on the shaft in juxtaposition with the spider; three radially extending camlike ears mounted on said plate about its periphery; an axially extending shoulder integral with said plate and concentrically positioned with respect to said opening; a pin mounted on the spider in engagement with the plate and adapted to position the plate adjacent the dogs; a cam follower mounted on the dogs in engagement with the ears; a brake bank partially encircling said shoulder and pivotally supported at one end upon the bearing hub and yieldingly slidably supported at the other end thereof, said band operable against the shoulder to retard movement of the plate relative to the movement of the shaft and cam said means outwardly over the ears and pivot the nose portion of the dogs inwardly toward the shaft when the shaft rotates in said one direction, and operable to permit free rotation of the plate when the shaft rotates in the opposite direction.

7. In crane hoisting machinery having a load shaft and a boom shaft coupled together by a chain drive operating over a sprocket fixed to the boom shaft and over a sprocket rotatably mounted on the load shaft with three dogs mounted on the load shaft tending to engage the sprocket mounted thereon when the load shaft revolves in one direction; a plate rotatably mounted on the load shaft adjacent the dogs and provided with three outwardly projecting camming ears adapted to cam inwardly said dogs, a brake associated with the plate and operable to effect a drag upon the plate when the load shaft revolves in said one direction and reduce the speed of rotation of the plate relative to the dogs thereby camming the dogs inwardly from the sprocket.

8. An invention of the character described comprising, in combination, a shaft rotatably supported adjacent one end by a bearing hub, a spider mounted on the shaft exhibiting radially extending arms, a dog pivotally supported by each of said arms and exhibiting a sprocket engaging nose portion, a sprocket web rotatably mounted upon the shaft for rotation thereabout and exhibiting teeth adapted to mesh with said dogs, means associated with each dog tending to pivot the dogs into locking engagement with said teeth when the shaft is rotating in one direction, a plate mounted on the shaft juxtaposing the web and exhibiting radially extending ears each shaped to define a cam surface, means mounted on the spider and abutting the plate to position the ears thereof in limited rotatable relation to said dogs, a pin mounted upon each dog in engagement with said ears, said plate exhibiting an axially extending shoulder concentrically spaced from the shaft, a brake band partially encircling said shoulder and supported at each end upon the bearing hub, one end of said band exhibiting a rod slidably received through a bracket offset upon the hub, a coil spring encircling the rod and abutting said bracket to urge the band yieldingly against the shoulder, said band operable against the shoulder to retard movement of the plate relative to the movement of the shaft and cam said pins outwardly over the ears to pivot the nose portion of the dogs inwardly toward the shaft when the shaft rotates in said one direction and operable to permit free rotation of the plate when the shaft rotates in the opposite direction.

9. An invention of the character described comprising: a. load shaft; a boom shaft; a chain sprocket rigidly mounted on the boom shaft; a chain sprocket rotatably mounted on the load shaft; a chain drive coupling the sprockets together; a spider mounted on the load shaft adjacent the sprocket thereon and provided with three sprocket engaging dogs tending to engage the sprocket when the shaft turns in one direction and drive the boom shaft; a plate rotatably mounted on the load shaft adjacent the dogs and provided with three outwardly projecting camming ears adapted to cam inwardly said dogs; a brake band associated with the plate and operable when the plate and shaft revolve in said one direction to effect a drag upon the plate and reduce the speed of rotation thereof relative to the dogs thereby camming the dogs over said ears inwardly toward the load shaft and preventing engagement of the sprocket therewith, and operable when the load shaft revolves in the opposite direction to permit free rotation of the plate.

10. An invention of the character described comprising two shafts rotatably coupled together by a chain and sprocket drive, the chain sprocket on one shaft rotatable about the shaft, a spider mounted on said one shaft and provided with pivoted dogs tending to couple the shaft and sprocket together upon rotation of the shaft in one References Cited in the file of this patent UNITED STATES PATENTS Lindgren Dec. 15, 1925 Worthington May 22, 1928 Wehinger Mar. 31, 1931 Jensen May 16, 1933 Osgood Apr. 10, 1951