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Publication numberUS3834494 A
Publication typeGrant
Publication dateSep 10, 1974
Filing dateFeb 23, 1973
Priority dateFeb 23, 1973
Also published asCA971861A1
Publication numberUS 3834494 A, US 3834494A, US-A-3834494, US3834494 A, US3834494A
InventorsBates L, Caldwell M
Original AssigneeRaymond Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Manually-controlled hydraulic actuator systems
US 3834494 A
Abstract
The power which a lift truck operator can demand by maximum deflection of a manual lifting control is made to vary automatically as a function of the load being lifted, thereby limiting power consumption to a safe amount but allowing small loads to be lifted at high speeds. In a main embodiment hydraulic fluid is bled from a master-slave system which liks the operator control to a controllable hydraulic pump or motor, with the amount of fluid bled varying in accordance with the load. Means for providing various desired operating characteristics are illustrated.
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United States Patent 1191 Bates et al. Sept. 10, 1974 MANUALLY-CONTROLLED HYDRAULIC 3,630,025 12/1971 Henry 187/9 ACTUATOR SYSTEMS [75] Inventors: Leo D. Bates; Melvin R. Caldwell, 'f R1chard schacher both of Greene N Y Ass1stantExammerJeffrey V. Nase Attorne A ent, or Firm-Richard G. Ste hens [73] Assignee: The Raymond Corporation, Greene, y g p [57] ABSTRACT 22 Fl (1: F b. 23 1973 1 1e 8 The power which a lift truck operator can demand by PP 335,030 maximum deflection of a manual lifting control is made to vary automatically as a function of the load 52 7 4 being lifted, thereby limiting power consumption to a m be at [58 Field of Search 187/9 38' 214/674- high a main embodiment hydraulic fluid is 60/445 bled from a master-slave system which liks the operator control to a controllable hydraulic pump or motor, [56] References Cited with the amount of fluid bled varying in accordance with the load. Means for providing various desired op- 3 543 508 gig? SK E PATENTS 60/445 erating characteristics are illustrated.

c wa 3,595,343 7/1971 Williamson 187/9 15 Claims, 7 Drawing Figures PATENTED SEPI 0 m4 sum 1 m MANUALLY-CONTROLLED HYDRAULIC ACTUATOR SYSTEMS Various material-handling devices such as lift trucks employ piston-cylinder hydraulic actuators to lift and lower heavy loads at varying speeds controlled by an operator. In one common form of lift truck a variabledisplacement hydraulic pump is arranged to run at a nominal speed when a lifting operation is to occur, and pump displacement is controlled by the operator to control fluid flow rate to the actuator, thereby controlling the speed at which the load is lifted. On batterypowered lift trucks the hydraulic pump is ordinarily driven at the nominal speed by an electric motor, which is switched on by the operator when a lifting operation is desired. On engine-driven trucks the hydraulic pump is ordinarily driven at a speed proportional to engine speed whether or not a lifting operation is being performed.

In the case of battery-powered lift trucks, it is especially desirable that the electric motor-hydraulic pump combination operate as efficiently as possible, since a large fraction of the power stored in the truck battery is frequently used for a series of lifting operations. While a given motor and a given pump easily may be selected so as to provide maximum power efficiency when lifting a given load at a given speed, material handling operations ordinarily require that a wide variety of different size loads be lifted at many different speeds. A main object of the present invention is to provide an improved hydraulic actuator system having improved efficiency over more varied speed and load conditions. A more specific object of the invention is to provide in a hydraulic system designed to move a variety of different loads, means for automatically limiting maximum power consumption as a function of the magnitude of the load.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts, which will be exemplified in the con struction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a mechanical-hydraulic schematic diagram illustrating one form of the present invention.

FIG. la is a mechanical schematic diagram illustrating portions of one possible modification of the system of FIG. 1.

FIG. 1b is a mechanical-hydraulic schematic diagram illustrating portions of a different possible modification of the system of FIG. 1.

FIG. 1c is a mechanical-hydraulic schematic diagram illustrating portions of yet another possible modification of the system of FIG. 1.

FIG. 1d is a graph illustrating the operation of the systems of FIGS. 1 and la 1c.

In the diagram of FIG. 1, lift pump control lever 10 comprises a manual control lever which is pivoted by the operator about pivot 11. Link 12 connects lever 10 to the piston within a conventional hydraulic master cylinder 13. Master cylinder 13 is connected via conduit 14 to a slave servo control cylinder 15. Clockwise movement of control lever 10 will be seen to expel fluid from master cylinder 13 through line 14 to displace piston 16' within servo control cylinder I5. Piston 16 is linked to vary the displacement of variabledisplacement hydraulic pump I7. The displacement of pump 17 may be varied from zero to a maximum amount to vary the rate of flow of fluid pumped and hence to vary the speed at which the load is lifted. In FIG. 1 pump displacement, and hence flow rate, is assumed to increase as piston 16 moves rightwardly. Electric motor M operates at a nominal, essentially constant speed, and drives pump 17. Output conduit 18 from pump 17 connects via check valve CV to lift cylinder 20, which is arranged to hoist a lift truck load carriage via a pulley and chain arrangement. The speed at which the load is lifted will be seen to be proportional to the flow rate out of pump 17. Neglecting system losses, the power required from battery B will be seen to be proportional to the weight of the load times the flow rate. Line 18 is pressurized only when control handle 10 is moved rightwardly to provide lifting. When handle 10 is returned to terminate lifting, fluid leaks from line 18 back through pump 17 so that pressure in line 18 decreases to a low value, but check valve CV prevents flow from lift cylinder 20, so that the load remains at the level to which it was lifted. Lowering of the load is ordinarily accomplished by bleeding fluid from the lift cylinder, by controllably opening lowering valve LV and allowing the weight of the load and load carriage to expel fluid from lift cylinder 20 back into the hydraulic reservoir 21 from which pump 17 draws hydraulic fluid. In one practical system incorporating the invention pump 17 comprised a Vickers Type PVBIS variable displacement piston pump with stem servo control, available from Vickers, Incorporated, of Troy, Michigan. Springs are shown within master and slave cylinders 13 and 15 to return the pistons therein when control handle is returned leftwardly. No spring is necessary, of course, in master cylinder 13 if link .12 is arranged to pull the master cylinder piston. No spring is necessary in either cylinder 13 or cylinder 15 if a spring (not shown) within the pump 17 adjustment servo provides a return force.

As thus far described the apparatus of FIG. 1 is completely conventional. In typical applications, the maximum load to be lifted might be 4,000 pounds, for example. The pulley arrangement frequently is arranged to provide double speed, so that such a load imposes a weight of more than 8,000 pounds on the lift cylinder. The lift piston typically might have an effective area of several (e.g. 2.4) square inches, so that a maximum load creates a pressure of approximately 2,500 psi in the lift cylinder. With a lift piston area of 2.4 square inches, hydraulic pump 17 must provide approximately 2,310 in. /min. or 10 gpm. to raise the lift piston at a speed of 39 feet per minute, so as to raise the load at twice that speed, or at 78 feet per minute. Assuming no system losses, it requires Wv/33,000 horsepower to raise a load of W pounds at v feet per minute, and with system losses, the motor in a typical prior application as that described would have a maximum horsepower rating exceeding I-I.P., so as to be able to raise 4,000 pounds at 78 feet per minute.

In an electric lift truck limitations on the battery size and the motor size limit the maximum horsepower available to drive the hydraulic pump. While lifting a maximum load at a given speed will require power input to the pump approaching the maximum available from the battery and motor, it will be seen that lifting a much lighter load at the same speed will require much less power, and hence that a light load may be lifted at a much greater speed without exceeding the battery and motor power output capabilities. Since it is highly desirable that the truck be capable of lifting light loads at much greater speeds than the speed at which a maximum load may be lifted, the variable displacement pump is ordinarily arranged so that the operator may obtain flow rates for high speed lifting greatly exceeding the low flow rate allowable with a maximum load. However, if the operator attempts to obtain too great a lifting speed for the particular load being lifted at a given time, an excessive amount of power will be drawn by the motor, sometimes with the possiblity of damaging the motor or the battery, and sometimes wasting power because the motor-pump combination is used outside an efficient operating region. The operator frequently does not know, of course, the weight of the particular load being lifted. While fuses may be employed to protect the motor, frequent replacement of fuses is wasteful and time-consuming. Use of a circuit breaker is undesirable because of its expense, because of the space it would consume aboard the truck, and because the need to frequently re-close it would delay lifting operations. Furthermore, the use of a circuit breaker would require that the operator initially determine the maximum lifting speed attainable with a given load by gradually increasing the pump flow rate until the circuit breaker tripped, and that he then not exceed that flow rate after he re-closed the circuit breaker. He would have to allow at least some margin for error in his positioning of the control, and hence would not be using the full capability of the lift system. The present invention automatically controls the lift systemso that the operator is allowed to provide the maximum allowable flow rate and lifting speed consonant with a given load, but prevented from demanding an excessive amount of power.

In the system shown in FIG. 1, conduit 18a senses the output pressure of pump 17, which corresponds to the pressure in lift cylinder 20 while a load is being lifted, and which pressure depends directly upon the weight of the load being lifted. Line 18a is connected to cylinder 24 so that piston 25 therein is displaced against the force of compression spring 26 by an amount proportional to the load. Piston 25 is linked by rod 53 to piston 27 in cylinder 28, which is connected to line 14 by conduit 14a. An increase in the load will be seen to move pistons 25 and 27 rightwardly as viewed in FIG. 1, allowing fluid to flow into cylinder 28 from line 14. Removal of fluid from line 14 will be seen to decrease the amount of displacement of piston 15 obtainable by maximum deflection of control lever 10. Thus the greater the weight of the load carried, the less will be the flow rate which can be demanded from the pump by maximum deflection of control lever 10. Cylinders 24 and 28 are shown enlarged in FIG. 1 with a diameter approximating that of the lift cylinder solely for sake of clarity. and it is important to recognize that both of these cylinders may be very small and compact. The piston-cylinder 27-28 size merely need be large enough that piston 27 displacement volume equal the volume of fluid expelled from master cylinder 13 by full deflection of control lever 10, and in prior art systems that volume is only about one-half cubic inch. Since substantial hydraulic pressure exists in line 18a even when an unloaded load carriage is lifted, piston 25 has sufficient actuating force to reliably overcome friction even if the piston has a small diameter. The diameter of piston 25 need not correspond to that of piston 27 and is ordinarily made no larger than is necessary to avoid clogging. It will be seen that rod 53 interconnecting pistons 25 and 27 reduces the effective area of piston 25, for reasons which will be discussed below in connection with FIG. 2. Piston 25 may be connected to the lift pump through small tubing, which acts as a restriction, so that the piston will respond to pressure changes in the lift cylinder with a slight delay in some applications of the invention.

The amount by which maximum attainable flow rate is decreased per amount of load increase can be determined very simply by selection of the spring constant of spring 26, which, of course, can be made adjustable. With a simple linear spring provided, the maximum attainable lifting speed will decrease quite linearly with increase in load. In FIG. 1d the variation of maximum obtainable speed with load if a simple linear spring is used is shown at a. The deflection of the spring under such circumstances, and hence the amount of fluid bled from between the master and slave cylinders, is shown at b. If desired, a non-linear spring means may be used, to provide a desired non-linear relationship between load and maximum attainable lifting speed. For example, two different springs may be arranged as in FIG. la, so that one spring opposes movement of pistons 25 and 27 over a lower range of loads but both springs act when the load increases beyond a predetermined value. Such a system would provide a maximum speed vs. load characteristic of the nature shown at c in FIG. 1d, and would bleed fluid in the manner shown at d. Substantially any desired maximum speed vs. load characteristic may be obtained simply by interconnection of piston-cylinder assembly 24-25 and assembly 27-28 with an appropriate non-linear motion transmitting mechanism. In FIG. lb the pressure transmitted to cylinder 24 from the lift pump translates piston 25, rotating plate 29 about shaft 30 against the force of tension spring 31 attached to plate 29, thereby moving piston 27 with approximately cosinusoidal motion. A desired maximum speed vs. load characteristic may be provided by use of a selected non-linear relationship between piston 25 and the spring instead of, or in addition to, provision of a non-linear motion converter between the two piston-cylinder assemblies. In FIG. lb it will be seen that the amount which plate 29 rotates for a given load (pressure in cylinder 24') would differ if the stationary end of spring 31 were fixed at 31b or 31c, for example, instead of where shown. In FIG. 1c the pressure in line 18a translates pistons 25 and 27 in the same manner as in FIG. 1 during one range of load conditions. However, when the pressure increases above a predetermined value so that pistons 25 and 27 travel rightwardly beyond a predetermined amount, plate 53d attached to rod 53 strikes rod 53', and upon further increase in pressure, piston 25 is urged rightwardly in cylinder 24 against the force of compression spring 26', so that fluid is bled from the master-slave control assembly not only into cylinder 24 but also into cylinder 24', thereby causing the maximum speed obtainable to drop sharply, in the manner illustrated by curve 2 in FIG. 1d. It will now be apparent that a variety of techniques are readily available to provide a desired maximum speed vs. load characteristic very simply and inexpensively. It will be apparent that the system may be arranged, if desired, so that loads in excess of a predetermined weight translate pistons 25 and 27 to bleed sufficient fluid from line 14 that maximum deflection of control lever leaves pump 17 in its zero flow rate condition, thereby preventing the operation from doing any lifting whatever of a load which is deemed too heavy.

As pump 17 supplies fluid to the lift cylinder in'FIG. 1, the pressure in line 18a and the deflection of spring 26 will remain approximately the same. If the truck is traveling on an uneven floor while lifting occurs, the load may tend to bounce. Check valve CV tends to prevent pressure oscillations which would result fr'om a bouncing load from causing pistons 25 and 27 to oscillate. The invention does not require the use of such a check valve, however. With control lever 10 in a given position, an oscillation of pistons 25 and 27 conceivably would cause some variation in the displacement adjustment of pump 17, and from a theoretical standpoint, one might suppose that an undamped oscillation might result. However, since a load does not ordinarily change in weight while it is being lifted, there is no requirement that piston 25 respond very rapidly to pressure changes in the lift cylinder, and thus the potentially oscillating closed loop which includes cylinders 24 and 28, slave cylinder 15, and pump 17 may be provided with a long time-constant and sufficient damping that such oscillations will not diverge and will readily decay. The friction of pistons 25 and 27 provides significant damping. Additional damping may be provided if desired by connecting cylinder 24 to lift cylinder through small tubing, and/or by connecting cylinder 28 to line 14 via small tubing. If desired, the ends of cylinders 24 and 28 shown open may be closed and provided with small bleed air holes, so tthat each cylinder also acts as a dashpot.

While the embodiments of the invention thus far discussed have included a hydraulic master-slave linkage connected between the operator control and the pump, it is within the scope of the invention to provide mechanical means which provide similar operation. In FIG. 3 control lever 40 is mounted in housing 41 to pivot about the axis of shaft 42. The lower end of lever 40 is formed as a bar of rectangular cross-section having an elongated through-slot 43. Cylinder 44 mounted in the base of housing 41 carries piston 45, and a pair of parallel spaced-apart plates 46, 47 are carried atop piston 45 on yoke 48. A pair of compression springs 49, 50 interposed between plate 47 and shelf means 51 in the housing urge piston 45 and plates 46,-47 downwardly. A circular rod 54 extends between plates 46, 47 and through slot 43 in lever 40. Link 53, which is only partially shown, has a hole in its left end into which rod 54 extends. Link 53 is connected to one end of lever 61 which is pivotally mounted at fixed pivot 62, and the opposite end of lever 61 is connected by link 63 to the displacement control of variabledisplacement pump 60. Pressure from lift pump 60 is connected to cylinder 44 via line 57.

Control lever 40 is pivotally adjustable between the position shown and a position determined by stop 58. Clockwise movement of control lever 40 will be seen to translate rod 54 leftwardly, pulling link 53 leftwardly, moving link 63 rightwardly and thereby increasing the displacement of variable-displacement pump which feeds lift cylinder 56. The amount by which links 53 and 63 are translated for a given movement of control lever 40 will be seen to depend upon the distance between rod 54 and pivot shaft 42. When a light load is present on lift cylinder 56 so that relatively low pressure is applied to cylinder 44, springs 49, 50 urge plates 46, 47, and piston 45 downwardly so that rod 54 lies near the bottom of slot 43, at a relatively large radius distance from shaft 42, and so that a given angular displacement of lever 40 causes relatively large adjustment of the displacement of pump 60. When a heavier load increases the pressure in cylinders 56 and 44, piston 45 and plates 46, 47 move rod 54 upwardly in slot 43, decreasing the operating radius and causing lesser adjustment of the pump displacement. FIG. 3 assumes that a rightward or inward thrust to the pump displacement adjustment means increases pump displacement. If pump displacement is arranged to decrease upon receipt of an inward adjustment, reversing lever 61 may be omitted, of course. In many applications it may be inconvenient to interconnect an operator-accessible control and the lift pump by means of rigid links, and in those applications it will be apparent that a flexible Bowden wire or push-pull cable may be used.

An actual load-compensating device which performs the functions of parts 25-28 of FIG. 1 is illustrated in FIG. 2. Body member 70, which is generally rectangular in cross-section and mounted on bracket 76 contains a longitudinal bore 71. Rod 73 has a reduced diameter left-end portion 73a and an enlarged diameter right-ned portion 73b, the step or change in diameter being shown at 73c. Pilot line 18a extending from the lift pump is connected to connector 74 to admit fluid to bore 71 on the left side of step 730. Rod 73 is shown at its leftward limit of travel in FIG. 2. Rod 73 will be seen to comprise a piston having an effective piston area equal to the difference between the crosssectional areas of rod portions 730 and 73b. In the specific device illustrated in FIG. 2, portion 73a had a diameter of 0.25 inch and portion 73b a diameter of 0.3125 inch, so that the effective area of the piston acted upon by the lift pump pressure was 0.0276 sq. in. Use of a differential piston of the type shown allows one to greatly reduce the required opposing spring force, allowing one to use a more simple and moreeasily calibrated spring. A differential piston with an effective area of 0.0276 sq. in. requires only 41.4 lbs. of opposing spring force when subjected to a hydraulic pressure of 1,500 psi. While a simple piston and cylinder having a diameter of the order of O. l 88 inch and an area of 0.0276 sq. in. theoretically could be substituted, the use of such a small bore is disadvantageous because it would be much more subject to clogging, as well as being more difficult to construct with comparable accuracy.

The right-hand end 73b of rod 73 is rounded and fits into a recess or socket in guide rod 75, which is slidably supported in a hole through flange 76' of bracket 76 by flanged ring 77. Compression spring 78 acts between flanged ring 77 and a further flanged ring or collar 79,

with spring 78 urging collar 79 against a pair of nuts 80, 80, threaded on guide rod 75. The spring will be seen to urge guide rod 75 and piston rod 73 leftwardly with a spring force which may be adjusted by threading nuts 80, 80 along the guide rod. As pressure is applied from the lift pump via connector 74, rod 73 will be translated rightwardly against the force of the compression spring. Bolt 82 fixed in place by jam nut 83 serves as an adjust-- able stop which acts to limit the maximum travel of guide rod 75, piston rod 73, and hence to limit the maximum travel of piston 86.

Sleeve 85 threaded into body member 70 includes a bore 85a in which rod portion 73a extends to engage piston 86, which is equipped with seal 86a. Seal and seal retainer assembly 87 prevents leakage of fluid from bore 71 into bore 85a, and a seal and seal retainer as sembly 84 prevents fluid leakage from the right end of bore 71. Connector 88 threaded into the end of sleeve 85 connects a pilot line from the master-slave cylinder assembly to the portion of bore 85a leftward of piston 86. As pressure applied from the lift pump translates rod 73 rightwardly, the rightward movement of piston 86 admits fluid into sleeve 85, thereby decreasing the amount of slave cylinder travel which maximum movement of the control handle and master cylinder can provide. In the specific device shown in FIG. 2 piston 86 comprises a free piston, the right end of which abuts the left end of rod 73. It will be apparent that piston 86 could be fixedly attached to rod 73, if desired.

While FIG. 1 illustrates use of the invention in connection with a variable-displacement pump, it will become apparent that slave piston-cylinder assembly l6 could instead be mechanically connected to control a variable speed motor which drives a constantdisplacement pump, in a variety of applications where the maximum obtainable power consumption which an operator can demand is desired to be limited as a function of the load being moved. Inasmuch as a variety of known motor speed control devices can be adjusted by the motion of piston 16, none need be illustrated. Also, while the invention is illustrated in FIG. 1 showing use of a variable displacement pump feeding a hydraulic ram, it will be apparent that the invention is as well readily applicable to hydraulic drive systems wherein a variable displacement pump feeds various other forms of hydraulic motive means, either fixed or variable displacement hydraulic motors.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a material-handling vehicle having hydraulic cylinder-ram lifting means, a motor-pump means for supplying hydraulic fluid to said cylinder-ram lifting means to lift a load supported by said cylinder-ram lifting means, an operator-variable control variable between minimum and maximum positions for controlling the speed at which said load is lifted by said cylinderram lifting means, and connection means interconnecting said operator-variable control and said motor-pump means, the combination of automatic means responsive to the magnitude of said load for varying said connection means to lift the adjustment of said motor-pump means caused by movement of said operator-variable control to said maximum position to a value which varies as a function of the weight of said load.

2. The combination according to claim 1 wherein said connection means comprises a hydraulic masterslave cylinder-pistion linkage interconnecting said operator-variable control and said motor-pump means and wherein said automatic means comprises a first spring-biased piston-cylinder means connected to receive the pressure which occurs in said lifting means during lifting and to adjust a second piston-cylinder means, said second piston-cylinder being connected to bleed hydraulic fluid from said master-slave linkage.

3. The combination according to claim 1 wherein said motor-pump means comprises an electric motor and a variable-displacement pump, said connection means being connected to adjust the displacement of said pump.

4. The combination according to claim 1 wherein said motor-pump means comprises a variable speed motor and a constant displacement pump, said connection means being connected to adjust the speed of said motor.

5. The combination according to claim 1 wherein said automatic means includes spring-biased pistoncylinder means hydraulically connected to receive the pressure which occurs in said lifting means during lifting, and means responsive to movement of said springbiased piston-cylinder means for varying said connection means.

6. The combination according to claim 1 wherein said connection means comprises a hydraulic master cylinder-piston assembly connected to be operated by said operator-variable control, a slave cylinder-piston assembly hydraulically connected to be operated by said master assembly and connected to adjust said motor-pump means, and wherein said automatic means comprises a first piston-cylinder assembly having its cylinder hydraulically connected to receive the pressure which occurs in the cylinder of said lifting means during lifting, spring means for biasing movement of the piston of said first piston-cylinder assembly relative to its cylinder, a second piston-cylinder assembly having its cylinder hydraulically connected between said master and slave cylinder-piston assemblies, and means connecting said first piston-cylinder assembly so that said relative movement of said first piston-cylinder assembly causes relative movement between the piston and cylinder of said second piston-cylinder assembly.

7. The combination according to claim 2 wherein said first spring-biased piston-cylinder means includes a cylinder and a piston extending through said cylinder, said piston having first and second portions of differing crosssectional area situated with said cylinder, whereby the force applied to said piston is commensurate with the difference incross-sectional area of said first and second portions.

8. The combination according to claim 5 having nonlinear spring means for biasing said piston-cylinder means.

9. The combination according to claim 5 having nonlinear motion-converting means connected between said piston-cylinder means and said means responsive to movement of said piston-cylinder means.

10.The combination according to claim having check valve means connected between the pump of said motor-pump means and said lifting means.

11. The combination according to claim 5 wherein said means responsive to movement of said pistoncylinder means comprises means for varying the connection point of said connection means to said operator-variable control.

12. The combination according to claim 1 wherein said connection means comprises a hydraulic masterslave linkage interconnecting said operator-variable control and said motor-pump means, and wherein said automatic means comprises means responsive to the pressure which occurs in said lifting means for varying the quantity of hydraulic fluid in said hydraulic masterslave linkage to vary said adjustment of said motor- 15. The combination according to claim 11 wherein said operator-variable control is rotatably variable about a pivot axis between said minimum and maximum positions, and said means for varying said connection point is operative to vary the distance of said connection point from said pivot axis.

U N ITED STATES PATENT OFFICE CERTIFICATE ()F CORRECTION Patent No.3, 83 1, 4 94 Dated September 10, 1974 'Inventor(s)Leo D. Bates and Melvin R. Caldwell I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 38, change "right-ned" to--- right-end-- Column 8,- line 3, change "lift" to --limit-' and in line 9 change "cylinder-pistion" to "cylinder-piston" 1 Column 10, line 6', change "liffing",to --liiriiting- Signed and sealed this 17th day oflDecember' .1974. l I

' (SEAL) Attest:

' McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer 1 Comissioner of Patents FORM PO-1050 (10-69) USCOMM'DC 60376-P69 u.s. eovnuuinr "mums omcs: 8 93

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3543508 *Oct 16, 1968Dec 1, 1970Hyster CoHydrostatic transmission with pressure control
US3595343 *Jan 15, 1969Jul 27, 1971Clark Equipment CoControl system for lift trucks
US3630025 *Jun 1, 1970Dec 28, 1971Allis Chalmers Mfg CoControl system for hydraulic devices
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US6135694 *Sep 29, 1998Oct 24, 2000Crown Equipment CorporationTravel and fork lowering speed control based on fork load weight/tilt cylinder operation
WO1999016698A1 *Sep 29, 1998Apr 8, 1999Crown Equip CorpProductivity package
Classifications
U.S. Classification187/224, 60/445
International ClassificationB66F9/20, B66F9/22
Cooperative ClassificationB66F9/22
European ClassificationB66F9/22