US1939113A - Hydraulic drive - Google Patents

Description

Dec. 12, 1933.

HYDRAULI C DRIVE J. P. FERRIS I 1,939,113

Filed June 4, 1931 3 Sheets-Sheet 1 JOHN 1 .F'ERRlS.

Dec. 12, 1933. v J FERRls 1,939,113

HYDRAULIC muvn Filed June 4, 1951 3 Sheets-Sheet 2 Jul-4N 1 .F ERHlS.

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J. P. FERRIS HYDRAULIC DRIVE Dec. 12-, 1933.

Filed June' 4, 1931 3 Sheets-Sheet 3 M u. mm

F PW N H DW J u Qw Y Patented Dec. 12, 1933 PATENT OFFICE HYDRAULIC DRIVE John P. Ferris, Whitefish Bay, Wis., assignor to The Oilgear Company, Milwaukee, Wis., a corporation of Wisconsin Application'June 4, 1931. Serial No. 542,066

10 Claims. This invention relates to drives for machines or mills having two or more rolls or stands of A paper mill may be driven at various speeds and the speeds of the individual rolls may be varied relatively to each other to produce certain kinds of paper or to meet certain operating conditions but, when once regulated, the speed of each roll should remain absolutely proportional to the speed of the other rolls until again regulated.

The prior drives did. not maintain the speed of the several rolls absolutely proportional to each other and required constant regulation.

The invention has as .an object to provide a drive which will maintain the speed of one roll or the other driven member uniformly proportional to the speed of another roll or other driven member.

Another object is to provide a drive of this character which is operated hydraulically.

Another object is to provide a drive which will maintain constant the relative speeds of a plurality of rolls or other driven members and which may be adjusted to produce closely graduated variations in speed.

Another object is to provide a drive of this character which is positive and precise in operation.

v According to the invention in a general aspect, one of the rolls functions as a master roll and is driven by a hydraulic transmission having the relative displacements of its'pump and motor preset or fixed, each of the other rolls is driven to by a variable displacementhydraulic transmission, and the speeds of the several rolls are maintained exactly proportional to each other by stroke changing mechanism which varies the ratio between the displacements of the pump and the motor of each variable speed transmission in response to a variation between the speed 01' the master roll and the speed of the roll driven by that transmission.

The invention is illustrated in the accompanying drawings in which the views are as follows:

Fig. 1 is a circuit diagram illustrating one method of, applying the invention.

Fig. 2 is a circuit diagram illustrating another method of applying the invention. Fig. 3 is a vertical sectional. view showing mechanism for changing the stroke of a pump or motor.

Fig. 4 is a detail plan view taken on the line 44 of Fig. 3.

For the purpose of illustration, the invention is 00 shown applied to a paper mill but only the dr-iving roll of each of three stands of rolls has been shown as such machines or mills are well known.

One standof rolls has a drivingroll 1 which drives the other rolls in that stand and functions as a controlling or master roll, and each of the other stands of rolls has a driving roll 2 which drives the other rolls in its stand and has its speed controlled from the master roll.

The master roll 1 is driven by a hydraulic 7o transmission which has its motor 3 connected to the roll 1 through a gear transmission or reduction gear 4 and its pump 5 connected to the motor 3 by pipes 6 and 7.

Each of the rolls 2 is driven by a hydraulic transmission which has its motor 8 connected to the roll 2 through a gear transmission or reduction gear 9 and its pump 10 connected to the motor 8 by pipes 11 and 12.

The gear transmissions 4 and 9 have not been illustrated in detail nor described herein as they are parts of the paper mill and of standard construction.

The motors 3 and 8 and the pumps 5 and 10 may be of the type shown in Patent No. 1,558,002 which was issued October 20, 1925 to Walter Ferris. Such a pump has the characteristic of operating as a pump when driven mechanically and of operating as a hydraulic motor when supplied with driving liquid.

The pumps 5 and 10 are driven in unison at speeds which preferably remain constant and are proportional to each other. For the purpose of illustration, the pumps are shown as being driven by chains 13 from a line shaft 14 which is driven by a steam turbine 15 such as is ordinarily employed for driving the several rolls of a. paper mill.

If the ratio between the displacements of the pump and motor of .each hydraulic transmission remains constant and the shaft 14 is rotated at a constant speed, the speeds of the rolls will remain constant and the speed of each roll 2 will remain proportional to the speed of the master roll 1.

If any of the hydraulic transmissions has the ratio between the displacements of its pump and motor.varied, the speed of the roll driven by that transmission will vary, and it is immaterial whether the stroke of the pump or the stroke of the motor is changed to change this ratio. For the purpose of illustration, the pumps are shown as having variable displacements and the motors as having constant displacements, but the motors may as readily have variable displacements and the pumps constant displacements.

The motor 3 and the pump 5 may both have constant displacements and the speed of the roll 1 be governed solely by regulating the speed of the shaft 14, but the pump 5 is shown provided with stroke changing mechanism 16 which may be operated manually to vary the speed of the roll 1 independently of the speed of the shaft 14.

Each pump has a circular driver 17 which is driven by the chain 13 and carries the pistons and cylinders of the pump around a pintle 18 through which liquid flows to and'from the cylinders.

The pintle 18 is carried by a pendulum 19 which has its lower end pivoted upon a stationary shaft 20 and its'upper end connected to stroke changing mechanism for swinging the pendulum upon the shaft 20 to move the axis of the pintle toward and from the axis of the driver 17.

When the axes of the pintle and the driver coincide, the pump is at zero stroke and no liquid will be delivered thereby.

As the aids of the pintle moves away from the axis of the driver, the stroke and delivery of the pump increases until the pendulum reaches the limit of its movement at which time the pump is at full stroke and delivering maximum volume.

The stroke of the pump 5 is controlled manually by the stroke changing mechanism 16 which may be of standard construction but the stroke of each pump 10 is controlled automatically by mechanism such as that shown in Fig. 3.

This mechanism includes a control rod 21 which extends through the upper end of the pendulum 19 and has its front end threaded and its rear end arranged in a guide 22 carried by the casing 23 of the pump.

The rod 21 has two half-bearings 24 and 25 arranged thereon upon opposite sides of two trunnions 26 which extend from opposite sides of I the pendulum 19 in the plane of the rod 21.

The half-bearing 24 is held against rearward movement upon the rod 21 by a nut 27 threaded thereon, and the half-bearing 25 is urged against the trunnions 26 to urge them against the halfbearing 24 by a helical compression spring 28 which encircles the rod 21 between the halfbearing 25 and a thrust plate 29 arranged in an opening 30 in the front wall of the pump casing 23.

The thrust plate 29 is held against outwar movement by a guide bearing 31 which is secured to the outside of the casing 23 and has the rear end of a hollow nut 32 arranged therein.

The nut 32 is threaded upon the threaded end of the control rod 21 and has an annular flange 33 formed thereon and engaging an anti-friction bearing 34 which encircles the nut 32 between the flange 33 and the guide bearing 31 and takes the thrust of the spring 28.

When the nut 32 is rotated in one direction, it draws the rod 21 forward and increases the stroke of the pump and, when it is rotated in the opposite direction, the spring 28 moves the rod 21 in the opposite direction and decreases the stroke of the pump; the rod 21 being prevented from rotating by a set screw threaded through the half-bearing 24.

The nut 32 has a shaft 35 secured to its front end and journaled in a bearing 36 carried by the housing 3'7 of a gear transmission or differential gear 38 which is supported upon a pedestal 39.

The differential gear 38 includes a gear case 40 which is arranged within the housing 37 and has two tubular hubs 41 and 42 journaled in the side walls thereof.

The hub 41 has a pinion 43 secured thereon in mesh with a gear 44 which is arranged upon the shaft 35 and secured to the end of the nut 32.

A shaft 45 is journaled in the hub 41 and has a bevel gear 46 secured to its inner end and its outer end connected by a coupling 47 to a constant speed motor 48 carried by the pedestal 39.

A shaft 49 is journaled in the hub 42 and has a bevel gear 50 secured to its inner end and its outer end connected by a coupling 51 to a variable speed motor 52 which is carried by the pedestal 39 and operates in the opposite direction from the constant speed motor 48.

The gears 46 and 50 both mesh with two bevel pinions or spider gears 53 which are arranged diametrically opposite each other between the gears 46 and 50 and are each journaled upon a stub shaft 54 carried by the gear case 40.

When the motors 48 and 52 drive the bevel gears 46 and 50 at the same speed but in opposite directions the pinions 53 simply rotate upon the shafts 54 and the gear case 40 remains stationary. Consequently, no movement is imparted to the gear 44 and the stroke of the pump remains unchanged.

When the bevel gears 46 and 50 are driven at unequal speeds in opposite directions, the faster moving bevel gear carries the pinions 53 around the slower moving-bevel gear and rotates the gear case 40 to impart rotation to the nut 32 through the pinion 43 and the gear 44. Consequently, a variation in the speed of one electric motor causes a change in pump displacement.

The constant speed electric motor 48 may be connected to either end of the differential gear 38 and the variable speed motor 52 may be connected to the other end thereof. In Figs. 1 and 3, the difierential gear is shown with the motor 48 connected to the end thereof which carries the pinion 43 and with the motor 52 connected to the other end and, in Fig. 2, the positions of the motors are shown reversed.

In the application of the invention shown in Fig. l, the motors 48 are synchronous motors which are connected to an alternating current power line 55, and the motors 52 are variable speed motors which have one terminal of each connected to a direct current power line 56.

The other terminal of each motor 52 is connected to the other side of the power line 56 through 'a rheostat 57 and also through a thermionic tube 58 which is connected in parallel with the rheostat 57.

The filament of each tube 58 is heated by cur- .rent from a suitable source, such as a battery 59,

and the filament current is controlled by a rheostat 60.

The grid of each tube 58 is connected to one terminal of a self-opening switch 61 which is actuated by rotation of the roll 2 and has its other terminal connected to the corresponding terminal of the other switches 61 and also to one terminal of a self-opening switch 62 which is similar to the switch 61 and is actuated by rotation of the master roll 1.

The other terminal of the switch 62 is connected to one terminal of a battery 63 which has 5 its other terminal connected to the filament of each tube 58.

When either the switch 62 or a switch 61 is open, the grid circuit of the tube 58 controlled by that switch 61 is open and the motor 52 connected to that tube 58 receives its entire supply of current through the rheostat 57 but, when both of these switches are closed, the grid circuit of that tube is closed and additional current may flow from the power line 56 through that tube to the motor 52 to increase its speed.

The switch 62 is opened and closed intermittently by an interrupteror cam wheel 64 which carries a bevel gear 65 and is fixed upon a shaft 66 journaled in suitable bearings (not shown).

The bevel gear 65 meshes with a bevel pinion 67 fixed upon a shaft 68 which is journaled in remains open until it is engaged by the next cam face.

For the purpose of illustration, the cam wheel 64 is shown provided with four cam faces which extend through 45 and are spaced 45 apart so that the switch is closed four times per revolution of the cam wheel 64 and is held closed during alternate eighths of a revolution.

The mechanism which actuates each switch 61 is identical to the mechanism which actuates the switch 62 except for the manner of driving the same, and like parts thereof have been indicated by like reference numerals with the exponent 0 added.

Each shaft 68* is driven through a conical pulley 70 which is secured thereon and connected by a belt 71 to a conical pulley 72 connected to the roll 2, and the belt '71 may be shifted along the pulleys '70 and 72 by a belt shifter '13 to vary the speed of the cam wheel 64 relatively to the speed of the roll 2.

As all of the rolls 2 are operated by drives which are identical to each other, the operation and control of but one of the drives will be described.

The stroke of the pump 10 is first adjusted to Y cause the roll 2 to rotate at a predetermined speed relatively tothe speed of the roll 1, then the mechanism which controls the switch 61 is so adjusted that the cam wheels 64 and 64 rotate at the same speed and a cam face on the wheel 64 engages and closes the switch 62 when the center of a cam face on the wheel 64 is in engagement with the switch 61, and then the rheostat 57 is adjusted to regulate the speed of the motor 52 and cause it to rotate at the same speed as the motor 48.

The cam faces of the cam wheels 64 and 64 hold each of the switches 61 and 62 closed during one-eighth of a revolution but, with the cam wheels out of phase as above described, either one or both switches are open during three-sixteenths of a revolution. Consequently, the grid circuit of the tube 58 is closed during only one-sixteenth of a revolution of the cam wheels and open during the following thr'ee-sixteenths of a revolution.

When either'the switch 61 or the switch 62 is open, the motor 52 receives its entire supply of current through h rheostat 57 and this current ment of the rheostat 57 to equal exactly the.

speed of the motor 48.

With the motors 48 and 52 rotating at the same speed, the pintle 18 of the pump 10 remains stationary and the speed of the roll 2 remains constant relatively to the speed of the roll 1.

The speed of the roll 2, however, tends to vary in response to several factors such as variations in the work performed by the roll or variations in the temperature and viscosity of the oil which drives the motor 8, either of which will cause a variation in the slip or internal leakage of the hydraulic transmission and thereby affect the speed of the motor 8, or variations in the speed of the pump 10 relatively to the speed of the pump 5 should the pumps not be driven in synchronism as by the pump 10 being driven by a separate electric motor or by a belt from the line shaft 14.

If, for any reason, the speed of the roll 2 should decrease relatively to the speed of the roll 1, the cam wheel 64 will rotate slower than the cam wheel 64 and each cam face on the wheel 64* will hold the switch 61 closed for a longer period of time during which the switch 62 is also closed, thereby increasing the time during which the larger current flows to the motor 52 and increasing the speed thereof above the speed of the motor 48.

Increasing the speed of the motor 52 above the speed of the motor 48 causes the stroke changing mechanism of the pump 10 to be operated, as previously described, and the stroke of the pump 10 increased to increase the speed of the roll 2 and the speed of the cam wheel 64.

The roll 2 will rotate slightly faster than its normal speed until the cam faces on the wheels 64 and 64 are in their initial relative positions and then the roll 2 will rotate at its normal initial speed relatively to the speed of the roll 1.

If the speed of the roll 2 should increase relatively to the normal speed of the roll 1, the switch 61 will be held closed for a shorter period of time during which the switch 62 is closed, thereby decreasing the time during which the larger current flows to the motor 52 and decreasing the speed thereof below the speed of the motor 48.

Decreasing the speed of the motor 52 causes the stroke changing mechanism to operate in the manner previously described but in the opposite direction and the roll 2 is thereby decelerated to its normal speed.

If it is desired to increase the speed of the roll 2 relatively to the speed of the roll 1, the belt shifter '73 is operated to move the belt- 71 outward along the pulleys 70 and '72 to thereby decrease the speed of the cam wheel 64.

The first efiect of decreasing the speed of the cam wheel 64 is to lengthen the time during which the switch 61 is held closed. This causes the motor 52 to increase its speed and operate the stroke changing mechanism to increase the stroke of the pump 10, thereby increasing the speed of the roll 2 until the cam wheel 64 is again rotating in synchronism with the cam wheel 64. The speed of the roll 2 then remains constant but higher than the previous speed.

If it is desired to decrease the speed of the roll 2 relatively to the speed of the roll 1, the belt shifter 73 is operated in the opposite direction to increase the speed of the cam wheel 64 with the resultant decrease in the speed of the roll 2.

The apparatus is very sensitive and it maintains the relative speeds of the several rolls absolutely constant and allows the speed of any roll 2 to be micrometrically regulated relatively to the speed of the roll 1.

In the application of the invention shown in Fig. 2, each diiferential gear 38 is driven by two self- synchronous motors 48 and 52 which perform the same functions as the motors 48 and 52.. That is, the motor 48 operates at a constant speed in accordance with the speed of the roll 1 and the speed of the motor 52-" may be varied to vary the speed of the roll 2.

The positions of the motors 48 and 52 are reversed from the positions of the corresponding motors 48 and 52 shown in Fig. 1, that is, the variable speed motor is connected to the shaft of the differential gear 38 and the constant speed motor is connected to the shaft 49.v Consequently, decreasing the speed of the variable motor 52 will increase the displacement of the pump 10 and decreasing its speed will increase the displacement of the pump 10.

nected to the stator winding of a self-synchronous generator '15 one of which is driven from each roll 2, and the rotors of all of the self-synchronous motors and generators are connected to a single phase alternating current power line 76.

The generators 7-1 and 75 are driven from the rolls 1 and 2, respectively, by mechanisms similar t6 those which drive the cam wheels 64 and 64 shown in Fig. 1, and like parts thereof have been indicated by like reference numerals.

The characteristics of self-synchronous motors and generators are described in bulletin GEA722A of the General Electric Company. Briefly, a self-synchronous motor-generator set connected in circuit has the characteristic that the rotor of the motor rotates in the same direction, at exactly the same time, at the same speed and through the same angular distance as the rotor of the generator.

The apparatus shown in Fig. 2 functions substantially the same as the apparatus shown in The pump 10 is adjusted to drive the roll 2 at the speed desired relatively to the speed of the roll '1 and the mechanisms which drive the motors '75 are adjusted to cause the motors 52 to rotate at the same speed as the motor 48, thereby holding the displacement of each pump con-' stant.

If the speed of a roll 2 decreases below the speed of the roll 1, the generator and the motor 52* are driven at a slower speed. Decreasing the speed of the motor 52 increases pump displace-- ment, as previously described, and the roll 2 is accelerated imtil it is operating at its initial speed and the motor 52* is being driven in synchronism with the motor 48'.

If the speed of the roll 2 increases above the speed of the roll 1, the speed of the motor 52 increases and causes a decrease in pump displacement to decelerate the roll 2 to its initial speed.

If it is desired to increase the speed of the roll 2' relatively to the speed of the roll 1, the belt shifter 73 is operated to move the belt 71 outward along the pulleys 70 and 72, thereby decreasing the speed of the motor 52 and increasing the speed of the roll 2, as previously described. The roll 2 will accelerate until the motor 52 is being driven in syn'chronism with the motor 48 at which time the roll 2 rotates at a constant speed which is higher than its former speed.

If it is desired to decrease the speed of the roll 2 relatively to the speed of the roll 1, the belt 71 is moved inward along the pulleys 70 and 72 to increase the speed of the motor 52 with the resultant decrease in the speed of the roll 2.

The invention herein set forth is susceptible of various modifications and adaptations without departing from the scope thereof as hereafter claimed.

The invention is hereby claimed as follows:

1. A hydraulic drive, for a mill having a master roll and a secondary roll, comprising a hydraulic transmission including a motor for driving said master roll at a predetermined speed and a pump for supplying liquid to said motor, a variable displacement hydraulic transmission including a motor for driving said secondary roll and a pump for supplying liquid to said moto means for driving said pumps, stroke changing mechanism for varying the relative displacements of the pump and motor of said variable displacement transmission to enable said transmission to drive said secondary roll at a speed proportional to the speed of said master roll, and means controlling said stroke changing mechanism and responsive to a variation in the relative speeds of said rolls to operate said stroke changing mechanism and vary the relative displacements of the pump and motor of said variable displacement transmission to thereby correct said variation in relative speeds.

2. A hydraulic drive, for a mill having a master roll and a secondary roll, comprising a hydraulic transmission including a motor for driving said master roll at a predetermined speed and a pump for supplying liquid to said motor, a variable displacement hydraulic transmission including a motor for driving said secondary roll and a pump for supplying liquid to said motor, means for said pumps, stroke changing mechanism for varying the relative displacements of the pump and motor of said variable displacement transmission to enablesaid transmission to drive said secondary roll at a speed proportional to the speed of said master roll, means controlling said stroke changing mechanism and responsive to a variation in the relative speeds of said rolls to operate said stroke changing mechanism and vary the relative displacements of are" and including a'pump and a motor having the a motor having the relative displacements the'rc- I of normally held at a given ratio, means for driving said pumps in unison to enable each variable 1 displacement transmission to drive the roll corchanging mechanismand vary the relative displacements of said pump and motor to thereby correct said variation in relative speeds.

4. A hydraulic drive, for a mill having a master roll and one or more other rolls, comprising a constant displacement hydraulic transmission connected to said master roll for driving the same and including a pump and a motor having the relative displacements thereof preset or fixed at a constant ratio, a variable displacement hydraulic transmission connected to each of the other rolls for driving the same and including a pump and a motor having the relative displacements thereof normally held at a given ratio, means for driving said pumps in unison to enable each variable displacement transmission to drive the roll corresponding thereto at a speed proportional to the speed of said master roll, stroke changing mechanism for varying the relative displacements of the pump and motor of each variable displacement transmission and normally maintaining the displacements of said pump and motor at said given ratio, means controlling each stroke changing mechanism and responsive to a variation in the relative speeds'of the corresponding roll and said master roll to operate said stroke changing mechanism and vary the relative displacements of said pump and motor to thereby correct said variation in relative speeds, andmeans for adjusting said controlling means to regulate the normal speed of said corresponding roll relatively to the speed of said master roll.

5. A hydraulic drive, comprising a hydraulic motor connected to a driven member for driving the same at a predetermined speed, a variable displacement pump hydraulically connected to said motor and driven at a predetermined speed to drive said motor at a predetermined speed and having means for changing its stroke, a spring urging said stroke changing means toward zero displacement position, a screw and nut connection for moving said stroke changing means toward maximum stroke position and normally holding it in a predetermined position to cause said pump to normally drive said motor at said predetermined speed, a differential gear case geared to said screw and nut connection for operating the same, two bevel pinions arranged within said case upon shafts carried by said case, two bevel gears arranged within said case in mesh with both of said bevel pinions and fixed upon shafts journaled in said case, a con- Lia Ill speed electric motor connected to the shaft of one of said bevel gears for rotating it in one direction at a given speed, a variable speed electric motor connected to the shaft of the other bevel gear for rotating it inthe opposite direction and normally rotating it at said given speed whereby said gear case remains stationary and holds said stroke changing means stationary, and means for varying the speed of said variable speed motor in response to a variation in the speed of said driven member to cause said differential case to rotate and operate said screw and nut connection and said stroke changing means to vary the displacement of said pump and thereby correct said variation in the speed of said driven member.

6. A hydraulic drive, comprising a hydraulic motor connected to a driven member for driving the same at a predetermined speed, a variable displacement pump hydraulically connected to said motor and driven at a predetermined speed to drive said motor at a predetermined speed and having means for changing its stroke, a spring urging said stroke changing means to zero displacement position, a screw and nut connection for moving said stroke changing means toward maximum stroke position and normally holding it in a predetermined position to cause said pump to normally drive said motor at said predetermined speed, a differential gear case geared to said screw and nut connection for operating the same, two bevel pinions arranged withinsaid case upon shafts carried by said case, two bevel gears arranged within said case in mesh with both of said bevel pinions and fixed upon shafts journaled in said case, a constant speed electric motor connected to the shaft of one of said bevel gears for rotating it in one direction at a given speed, a variable speed electric motor connected to the shaft of the other bevel gear for rotating it in the opposite direction and normally rotating it at said given speed whereby said gear case remains stationary and holds said stroke changing means stationary, means for varying the speed of said variable speed motor in response to a variation in the speed of said driven member to cause said differential case to rotate and operate said screw and nut connection and said stroke changing means to vary the displacement of said pump and thereby correct said variation in the speed of said driven member, and means for adjusting said speed varying means to regulate the normal speed of said driven member.

'7. A hydraulic drive, for a mill having a master roll and one or more other rolls, comprising a constant displacement hydraulic transmission connected to said master roll for driving the same and including a pump and a motor having the relative displacements thereof preset or fixed at a constant ratio, a variable displacement hydraulic transmission connected to each of the other rolls for driving the same andincluding a pump and a motor having the relative displacements thereof normally held at a given ratio, means for driving said pumps in unison to enable each variable displacement transmission to drive the roll corresponding thereto at a speed proportional to the speed of said master roll, stroke changing mechanism for varying the relative displacements of the pump and motor of each variable displacement transmission and normally maintaining the displacements of said pump and motor at said given ratio, a differential gear connected to said stroke changing mechanism for operating the same, a constant speed electric motor connected to said differential gear and tending to operate said stroke changing mechanism in one direction, a variable speed electric motor connected to said differential gear and tending to operate said stroke changing mechanism in the opposite direction and normally rotating at the same speed as'said constant speed motor to thereby hold said stroke changing means against operating, and means responsive to the speed of the corresponding roll for controlling said variable speed motor and causing the same to operate at a speed proportional to the speed of said corresponding roll whereby a variation in the speed of said roll will cause said electric motors to operate said stroke changing mechanism and vary the relative displacements of said pump and motor to correct said variation in the speed of said roll.

- 8. A hydraulic drive, for a mill having a master roll and one or more other rolls, comprising a constant displacement hydraulic transmission connected to said master roll for driving the same .and including a pump and a motor having the relative displacements thereof preset or fixed at a constant ratio, a variable displacement hydraulic transmission connected to each of the other rolls for driving the same and including a pump and a motor having the relative displacements thereof normally held at a given ratio, means for driving said pumps in unison to enable each variable displacement transmission to drivethe roll corresponding thereto at a speed proportional to the speed of said master roll, stroke changing mechanism for varying the relative displacements of the pump and motor of each variable displacement transmission and normally maintaining the displacements of said pump and motor at said given ratio, a differential gear connected to said stroke changing mechanism for operating the same, a constant speed electric motor connected to said difierential gear and tending toioperate said stroke changing mechanism in one direction, a variable speed electric motor connected to said difierential gear and tending to operate said stroke changing mechanism in the opposite direction and normally rotating at the same speed as said constant speed motor to'thereby hold said stroke changing means against operating, means responsive to the speed of the corresponding roll for controlling said variable speed motor and causing the same to operate at a speed proportional to the speed'of said corresponding roll whereby a variation in the speed of said roll will cause said electric motors to operate said stroke changing mechanism and vary the relative displacements of said pump and motor to correct said variation in the speed of said roll, and means for adjusting each of said controlling means to vary the normal speed of the roll corresponding thereto.

9. A hydraulic drive, for a mill having a master roll and a secondary roll, comprising a hydraulic motor connected to each of said rolls for driving the same, a pump hydraulically connected to the master roll motor and having a preset or constant displacement, a variable displacement pump connected to the other motor and having mechanism for changing its stroke, means for driving said pumps, a diflferential gear connected to said stroke changing mechanism for operating the same, two self-synchronizing motors connected to opposite sides of said difieren tial gear and normally rotating at the same speed in opposite directions to hold said stroke changing mechanism against operating, a self-synchronizing master generator driven from said master roll and electrically connected to one of said self-synchronizing motors to drive the same at a speed proportional to the speed of said master roll, and a secondary self-synchronizing generator driven from said secondary roll and electrically connected to the other self-synchronizing motor to drive the same at a speed proportional to the speed of said secondary roll.

10. A hydraulic drive, for a mill having a master roll and a secondary roll, comprising a hydraulic motor connected to each of said rolls for driving the same, a pump hydraulically connected to the master roll motor and having a preset or constant displacement, a variable displacement pump connected to the other motor and having mechanism for changing its stroke, means for driving said pumps, a difl'erential gear connected to said stroke changing mechanism for operating the same, two self-synchronizing motors connected to opposite sides of vsaid difl'erential gear and normally rotating at the same speed in opposite directions to hold said stroke changing mechanism against operating, a self-synchronizing master generator driven from said master roll and electrically connected to one of said self-synchronizing motors to drive the same at a speed proportional to the speed of said master-roll, a secondary self-synchronizing generator driven from said secondary roll and electrically connected to the other self-synchronizing motor to drive the same at a speed proportional to the speed of said secondary roll, and means for varying the speed of said secondary self-synchronizing motor relatively to the speed of said secondary roll.

JOHN P. FERRIS.

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