US3703806A

US3703806A - Control system for lift trucks

Info

Publication number: US3703806A
Application number: US89444A
Authority: US
Inventors: William A Williamson
Original assignee: Clark Equipment Co
Current assignee: Doosan Bobcat North America Inc
Priority date: 1970-11-13
Filing date: 1970-11-13
Publication date: 1972-11-28
Anticipated expiration: 1989-11-28

Abstract

The system employs hydrostatic drive for a lift truck in combination with hydraulic system elements which control not only the hydrostatic drive and truck speed through the accelerator setting, but also afford elements and controls for actuation of the lift mechanism which make possible a speed up of the engine to provide a fast lift while the truck is driven at a controlled slow speed.

Description

United States Patent Williamson [451 Nov. 28, 1972 [62] Division of Ser. No. 79l,354, Jan. 15, 1969,

Pat. No. 3,595,343.

[52] US. Cl ..60/19, 60/52 HE, 60/53 R [5l] Int. Cl ..F02b 41/00 [58] Field of Search;.. ..60/19, 52 HE, 53 R 5 6] References Cited UNITED STATES PATENTS 2,942,42l m 6/1960 l-lann et al. 6 0/l9 3,036,435 5/ l 962 Samuely ..60/ l 9 X 3,187,497 6/ 1965 Granryd ..60/19 3,533,230 10/1970 Draper ..60/19 3 ,543,508 12/ l 970 Schwab ..60/19 Primary Examiner-Edgar W. Geoghegan Attorney-Witt, Wiessler, Johnson & Lamm 7] ABSTRACT The system employs hydrostatic drive for a lift truck in combination with hydraulic system elements which control not only the hydrostatic drive and truck speed through the accelerator setting, but also afford elements and controlsfor actuation of the lift mechanism which make possible a speed up of the engine to provide a fast lift while the truck is driven at a controlled slow speed.

5 Clains, 3 Drawing Figures SHEET 1 [1F 2 CONTROL SYSTEM FOR LIFT TRUCKS This is a division of application Ser. No. 791,354, filed Jan. 15, 1969 now US. Pat. No. 3,595,343, issued July 27,1971.

BACKGROUND OF THE INVENTION In maneuvering a lift truck prior to and during the operation of lifting a load as well as during the placing of the load on the lift fork members, it is quite generally the situation that the forward movement of the vehicle, or rearward as the case may be, is a very slow movement while the rate of the lift of the load-carrying member is desired to be relatively fast. It is necessary in order to accomplish the above outlined results, that special precautions'be taken by the operator to cause the truck to move very slowly. This is quite commonly accomplished either by special so called inching devices or by the well knownexpedient of slipping the clutch, both of which result in rapid clutch wear. Also,

this type of maneuver under the conditions outlined above is often difficult.

Furthermore, the conventional type of lift unit is operated by hydraulic means which operates by a pump driven by the vehicle engine. Therefore, increase in rate of lift requires an increase in engine speed which is difficult to obtain while accomplishing at the same time, the slow movement of the truck.

In view of the above conditions, it would be most advantageous if the vehicle control could be set to maintain a relatively low forward speed while the engine of the lift truck vehicle could be accelerated at will to a higher speed needed for most effective lift.

it is also a problem in a lift truck to maintain a relatively slow forward speed without undue manipulation of the accelerator pedal, leaving the operator more freedom to manipulate the lift controls; ltwould be advantageous to have a governor type of control for the forward speed of the vehicle, when desired, so that it could be set to be independent, within limits, of change in torque requirements of the vehicle.

SUMMARY Hydraulic system control elements in combination with hydrostatic drive for moving the truck forward as well as providing, by hydraulics, a load lift control and a governor type of vehicle control. The hydrostatic drive makes possible a control over vehicle speed separate from the engine speed, and the accelerator control is made to control the speed of the truck, within limits, independent of engine speed changes, thus making possible a variation in engine speed for the purpose of driving the lift pump fast enough for fast lift while the truck vehicle is operating at a slow speed.

It is therefore a primary object of the invention to provide a lift truck control system accomplishing controlled slow speed of the truck while allowing the en.-

gine to be speeded up to drive the lift pump for relative- It is another object to provide a control mechanism whereby the accelerator control member controls the speed of the truck independently, within limits, of changes in speed requirements of the lift system.

The above and other objects will appear more fully from the following more detailed description of an illustrative structure and by reference to the accompanying drawings.

DRAWINGS FIG. 1 is a side view of a type of vehicle known as a lift truck on which the present control system may be used.

FIG. 2 is a showing of the control system with the several elements, both hydraulic and mechanical in diagrammatic form and disclosing their relationship.

FIG. 3 is a diagram showing the hydrostatic drive from the-engine to the vehicle wheels with the elements thereof in diagrammatic form, together with a showing of the engine and the several pumps driven thereby forming a part of the control system. Certain relief valves and cross check valves being omitted as not pertinent to the description.

DESCRIPTION OF AN ILLUSTRATIVE STRUCTURE Referring to the drawings, FIG. 1 shows an industrial lift truck 102 in which the control system shown in FIG. 2 is used. Lift truck 102 is supported on forward drive wheels 106 and rear wheels 108. Mounted at the forward end of the lift truck is a lift upright 110 which includes a vertically movable carriage 112 which carries a pair of forwardly extending fork arms 114 which support the load to be carried. The carriage 112 is connected to a vertically directed fluid motor, so-called lift cylinder 116, location of which is designated adjacent the carriage112 in FIG. 1 and which is shown in the diagram of FIG. 2. Certain of the elements of the control system shown diagrammatically in FIG. 2 are located in FIG. 1 by a general designation of location by a lead line from the corresponding reference numeral. It is understood that all of the elements of the control system shown in FIGS. 2 and 3 are present on the lift truck of FIG. 1, with only representation elements being indicated as to their general location in FIG. 1.

Referring particularly to FIG. 2, the engine-driven fixed-displacement pump numeral 1 is connected by line 2 to deliver oil under pressure over a variable orifree 5 formed by a spool 5a, movable in a valve body 4. Spool 5a is movable longitudinally of the valve body 4 by accelerator pedal 15 connected as shown in FIG. 2 so that variation in the position of the spool 5a varies the orifice 5 to control restriction in flow from line 2 as later described.

The variable orifice spool 5a and a flow regulator spool 3 are both housed in housing 4 and are in alignment with the connection to the accelerator pedal 15. A spring 5b is provided between orifice spool 5a and the top of flow regulator spool 3 and another spring 50 is on the bottom side of regulator spool 3.

A spring-loaded hydraulic cylinder 9 is connected to V a hydrostatic transmission stem-type servo control valve '10 and hydraulically connected to the flow regulator 3 by a line 11, and regulator 3 in turn is connected to line 2 thru line 16. This is a connection to actuate the hydrostatic transmission shown in PK]. 3, with further functions to be'deseribed hereinafter.

Another hydraulic cylinder 12 is connected by suitable linkage to the carburetor butterfly valve 13. This hydraulic cylinder is hydraulically connected to be actuated from line 2 by line 14.

The drive of the vehicle wheels departs from the conventional geared transmission structure most commonly used, by providing a hydrostatic drive shown diagrammatically in FIG. 3. The hook up includes a variable displacement hydrostatic pump 27 driven by vehicle engine 30, which by the hydraulic connections and hydrostatic assembly 113 shown in FIG. 3 drives a hydrostatic motor 1181 adjacent the drive wheels to propel the axle which drives'the ground wheels 106 of the truck. Between line 17 and motor 118 is a manually operated directional control valve 119. The vehicleiengine 30 also drives another pump, a fixed displacement so-ealled lift pump 26 (FIG. 3). This pump is for the purpose of actuating the liftmechanism of the lift truck and is a partof the lift truck arrangement. However, like the hydrostatic drive, when combined in the system, it is a part of the combination herein disclosed. Also the engine 30 of the truck drives still another fixed displacement pump, the previously mentioned pump 1, which is shown in the diagram of FIG. 2, as well as FIG. g

[t is also noted that a hydrostatic valve servo actuates a so-called swash plate control, known in the art, for the hydrostatic pump 27 and that this servo mechanism operates the swash plate to change the displacement of the pump and thereby changes the speed of the vehicle wheels. It therefore follows that the total vehicle speed will result from the combination of engine speed and pump displacement.

in operation, when the engine is idling, a given amount of oil is pumped by the control system pump 1 through line 2 across orifice 5 and to the tank. This creates'a back pressure in line 2 which is reflected in line 14 and in cylinder 12. This pressure in cylinder 12 is sufficient to override the balance spring to cause the cylinder 12 to assume a retracted position. This, in turn, causes the carburetor butterfly valve 13 to be nearly closed. This pressure is also seen in line 7, but it is blocked by spool 8 whenthere is no actuation of the lift control.

With conditions as above, the variable speed engine portion of the circuit operates as follows: When it is desired to increase the engine 'speed, the operator presses down on the accelerator any desired amount. The variable orifice 5 is opened a corresponding amount which allows the pressure in line 2 to decrease. This pressure decrease is reflected in line 14 and in cylinder 12 such that the spring in the base of the cylinder causes the piston to move from right to left. This movement opens the carburetor butterfly valve 13 which causes the engine to increase in speed. As the en.- gine speed increases, the volume of oil delivered by fixed displacement pump 1 also increases. This increased volume is delivered across variable oriflce 5 and results in an increasing pressure in line 2, line 14 and cylinder 12. This in turn tends to cause the but-' terfly valve 13 to close. The more the engine speed increases, the farther the butterfly valve 13 will go toward its closed position. Eventually, a point of equilibrum is reached at which the butterfly opening matches the current engine speed and the increase in speed ceases. At this point, the pressure in line 2 is somewhat less than the initial idle pressure. If, during the operation of the truck there is any change in speed of the engine brought about by changing tractive conditions, the pressure in line 2 will automatically change accordingly. This pressure working through lines 14 and cylinder 12, will cause butterfly valve 13 to open or close to compensate for this change. thereby bringing the engine speed back to the original level called for by the accelerator pedal. It is important to note at this point that this is a governor-type control effective for each setting of the accelerator pedakln other words, it is the control of the speed of the truck, at one accelerator pedal position and the compensation is for varying tractive conditions, change in torque, or load.

Output speed of the hydrostatic transmission and consequently the truck speed is controlled by flow regulator spool 3 in the valve body 4. Normally this spool 3 is balanced by springs 5b and 50 on either end as shown, such that there is no oil delivered to cylinder 9 through line 11. Operation of the accelerator pedal 15 moves orifice member 5 downward and compresses the spring 5b on the top of the flow regulator spool 3, unbalancing the spool 3, and causing it to move downward. This opens a path from the pressurized source line 16 to line 11 in cylinder 9. This pressure causes cylinder 9 to stroke the pump servo 10 and start oil traveling through the hydrostatic transmission loop 113 causing the truck to move. Since the transmission is a closed-loop system and substantially all of the oil that is delivered by the pump ultimately reaches the oilreturn line 17 shown at the bottom of the valve body 4. This line 17 is also shown in FIG. 3 with orifice 18. The oil in line 17 passes acrossthe orifice 18 thereby creating a pressure drop. The magnitude of the pressure drop will be proportional to the amount of flow across the orifice. Thispressure drop is reflected at the bottom of the regulator spool valve 3 by line 19. This pressure drop will gradually build up as the flow of oil increases and eventually balances the forces on spool 3 causing it to close. Line 20 runs between the downstream side of the orifice l8 and the top of the flow regulator 3. This line cancels out any extraneous pressures in line 17 which might be brought about by dynamic braking and ensures that the flow regulator spool valve 3 only sees the pressure drop across the orifice. Any engine speed change that is not called for by the accelerator will be compensated for by pressure change in line 19. For example, if the engine speeds up causing the oil flow to the transmission to increase, a greater pressure will be developed in line 19 and cause the flow regulator spool to move upward opening the path from line 11 to line 21 and consequently the tank. This will cause the cylinder 9 and the serve 10 to destroke causing the flow through the transmission to decrease thereby bringing the pressure in line 19 back into balance.

Line 16 is connected to line 2 and, therefore, sees any pressure fluctuations that occur in line 2. The best way to explain the reason for this is to assign some values to these pressures. Assuming that cylinder 12 operates over a pressure range of to 50 PS1. That is, at 100 PSI the cylinder is Collapsed and the engine is idling at 500 rpm. At 50 PS1 the cylinder is extended and the engine is running at 2,200 rpm. Now let us assume that cylinder 9 operates over a pressure range of from 20 PSI to 40 PSI. Under normal operating conditions cylinder 9 will always see more pressure than is required to fully extend it. Therefore, its movement must be regulated by a volume change. Since spool 3 in body 4 constitutes a flow regulator, thiswill be no problem. The pressure drops in line 2, due to a slowdown of the engine. The carburetor butterfly opens, introducing more gasoline and air to the engine. This will attempt to bring the engine speed back up. If, however, the engine can not overcome the torqueload imposed upon it; the engine speed will continue to drop. As the speed continues to drop, the pressure drops below 40 PS1 in line 2 and line 16. While this has been happening, the orifice 18 has signaled flow regulator spool 3 calling for more displacement on the pump. if the pump has not fully stroked, it will go to full stroke which will, of course, add to the torque load on the engine. Since the flow regulator 3 is open, the pressure in line 16 is seen in cylinder 9. When the pressure drops below the 40 lbs. maximum for the cylinder 9, it will begin to destroke forcing the oil in the cylinder back through lines l1, l6 and 3, and into line 2. This destroking of the pump will have the same effect as shifting the'machine into higher gear, thereby relieving the torque load on the engine allowing it to speed up and reach a pressure equilibrium with cylinder 9.

in the operation of the lift truck, it is often desirable to lift the load at the maximum rate of speed possible while driving the truck at a relatively low rate of speed. The lift speed is dependent upon two things: (1) the amount that the control valve is opened and (2) the speed of the engine. The part of the system including valve spool 8 and control 22 are constructed to control both of these functions. The best way to describe this action is to take an example. In this example, it is assumed that the driver wants to move the truck at a very low rate of speed while elevating the load at a very high rate of speed. The driver sets his truck speed by movement of accelerator pedal a small amount. This causes a small pressure drop in line 2, moves flow regulator spool 3 and causes the operations hereinbefore described to take place, thus setting a low truck speed rate. To lift the load, the operator moves value handle 22 (FIG. 2) from right to left. This causes pressurized oil from line 23 to communicate with line 24 which is connected to the lift cylinder. This much of the spool 8 is of a conventional configuration and permits a certain amount of throttling of the oil. In moving spool 8, the movement of control handle 22 also uncovers the port which is connected to line 7. This allows oil to bleed off to tank and causes the pressure in line 2 to drop. The pressure drop in line 2 is, as described earlier, accompanied by a corresponding increase in carburetor butterfly 13 opening and consequently an increase in engine speed. This increase in engine speed is compensated for in the truck speed by orifice 18 in the return line of the hydrostatic drive (see above discussion). The engine speed increase has not been brought about by the operator changing the accelerator position. The connections through variable orifice 5 and regulator spool 3 have not changed, therefore, the increased delivery through return line 17 will be compensated for by destroking the pump. Valve spool 8 was designed to progressively uncover the opening from line 7 thus permitting throttling of the oil and consequently progressive decrease in pressure in line 2. This permits selective increase of engine speed to whatever the operator desires in order to accomplish a satisfactory rate of lift without materially increasing the speed of the truck.

It is understood in the above discussion that when the term destroking the pump" of the hydrostatic drive is used, it means that the effective drive ratio between the engine and the wheels of the truck has been in the direction corresponding to a higher gear" ratio in a conventional gear transmission, andconversely stroking the pump means a shift in drive corresponding to a lower gear ratio.

In the above connection, it is also to be noted that in the description above where the pressure build up in line 19 accomplished by a relatively large pressure drop across orifice 18, indicating relatively large flow of oil in line 17, that the resulting movement of spool 3 will open line 11 to line 21 thereby allowing cylinder 9 to move toward a retracted position and thus destroke the pump changing to a higher effective drive ratio and consequently lower truck speed. Compensating changes are thus made in effective gear ratio so any change in ground speed of the truck not called for by change in accelerator 15 setting will be thus compensated by a change in pressure in line 19 stroking or destroking" the pump as may be called for by existing conditions.

Actuation of the lift control lever 22 and spool 8 will cause the engine speed to increase in proportion to the amount of lever 22 movement, thereby providing maximum lift rate capabilities. For any specific operating position of accelerator 15 any change in engine speed willbe automatically compensated for by a change in pump displacement, and change in associated drive ratio of the vehicle hydrostatic transmission, thereby leaving the vehicle speed unchanged.

it is understood that the terms lift and lift truck as used herein are considered as applicable to lift truck mechanisms wherein a lift function is accomplished such as dump trucks, tractor scrapers, paving machines, all of which have engine-driven groundwheel drives, and auxiliary power devices which have independently variable control requirements to which the structure disclosed herein could be adapted.

The invention has been described by reference to an illustrative structure, but the invention is intended to be limited only by the scope of the following claims.

lclaim:

1. A control system for a lift truck, said truck being of a type having a hydraulic actuated lift and load support member, with an engine for propelling the truck;

said control system comprising:

a fluid pump driven by said engine;

an acceleration control member for said engine;

a variable outlet orifice controlled by said acceleration control member;

a main delivery line connecting said fluid pump with said variable outlet orifice;

a carburetor valve controlling fuel flow to said engine;

a hydraulically actuated cylinder and piston resiliently biased to open said carburetor valve, said valve being actuated toward closure by hydraulic pressure; I

an auxiliary delivery line connecting said main delivery line with said resiliently biased hydraulic cylinder and piston at a position between said engine driven pump and said variable orifice thereby supplying fluid under pressure to said piston in a direction to close said'carburetor valve against the resilient bias; I

whereby a movement of said acceleration control member from one setting to ,a new position to change said variable orifice will result in change of engine output followed by compensating change in said valve and gradually reach a balance corresponding to the setting of said acceleration control member and thereby further to provide governing control of said engine for varying torque resulting from variation in tractive and other conditions of the truck vehicle for the various positions of said acceleration control member.

2. A control system as in claim 1 wherein there is provided:

a hydrostatic drive actuated by the truck engine for propelling the truck;

a second resiliently biased hydraulic cylinder and piston for controlling said hydrostatic drive;

a second auxiliary delivery line from said main delivery line serving said second hydraulic cylinder;

a flow regulator in the form of a spool member in said second auxiliary delivery line actuated by said acceleration control member to control flow to said second hydraulic cylinder;

means mounting said flow regulator for actuation by said acceleration control member;

said means including a spool member balanced by resilient means at each end thereof normally balanced to provide no flow to said second hydraulic cylinder for said hydrostatic drive;

means actuated by movement of said acceleration control member toward a speed increase to unbalance s'aid spool toward position to direct flow to said second hydraulic cylinder controlling said hydrostatic drive.

3. A control system as in claim 2 wherein there is provided:

a hydraulic fluid flow line in said hydrostatic drive, the flow rate thru which is indicative of the speed generated by the drive for propelling said truck;

an orifice in said hydraulic fluid flow line;

a first line flow regulator connection from the upstream side of the orifice in said fluid flow line to the end of said flow regulator in which pressure will cause said flow regulator to reduce flow to said hydrostatic control cylinder;

second line flow regulator connection from the downstream side of said orifice to the end of said flow regulator in which pressures will cause said flow regulator to increase flow to said hydrostatic control cylinder to change the effective drive ratio; whereby any engine speed change not called for by the acceleration control member will be compensated by agressure change and build up of a pressure in sal first line connection rebalancmg said flow regulator'and during such rebalancing effecting a change in effective drive ratio in said hydrostatic drive.

4. A control system for a lift truck, said truck having an engine and an acceleration control member, a lift control, a fuel flow control, and a hydrostatic drive 0 control of varying ratio to propel the truck from said engine and a valve body;

a fluid pump driven by said engine; a hydraulic fluid delivery line from said pump to said valve body; an orifice forming spool movable in said valve body with said acceleration control member and providing restriction by said orifice to generate a back pressure in said hydraulic fluid delivery line for actuating said controls;

spaced from said orifice forming spool;

a resilient connection in an end chamber between an end of said flow control spool and said orifice forming spool with said space therebetween forming a pressure chamber;

a further chamber between the opposite end of said flow control spool and the end of said valve body;

a resilient member in said further chamber which with said resilient connection at the opposite end of said spool balances said spool in said valve body;

formation on and connections from said flow control spool and valve body so formed and positioned that with said spool in balance restricts flow from said delivery line to minimize variation in said hydrostatic drive ratio, but when unbalanced by movement of said orifice forming spool by movement of said acceleration control member, will unbalance said flow control spool and cause flow to vary said hydrostatic drive ratio.

5. A control system as in claim 4 wherein there is provided: I

a hydraulic fluid flow line in said hydrostatic drive, the flow rate thru which is a function of the speed generated by the drive for propelling the truck;

an orifice in said hydraulic fluid flow line;

a first line flow regulator connection from the upstream side of said orifice in said fluid flow line to the end of said flow regulator spool in which pressure will cause said spool to restrict flow to vary said hydrostatic control; second line flow regulator connection from the downstream side of said orifice to the end of said flow regulator spool in which pressures will cause said spool to increase flow to said hydrostatic control to change the effective drive ratio; whereby an engine speed change not called for by the said acceleration control member will be compensated by a pressure change and build up of pressure in said first line connection rebalancing said flow regulator and during such rebalancing effecting a change in effective drive ratio in said hydrostatic drive.

* III t ii a flow control spool positioned in said valve body

Claims

1. A control system for a lift truck, said truck being of a type having a hydraulic actuated lift and load support member, with an engine for propelling the truck; said control system comprising: a fluid pump driven by said engine; an acceleration control member for said engine; a variable outlet orifice controlled by said acceleration control member; a main delivery line connecting said fluid pump with said variable outlet orifice; a carburetor valve controlling fuel flow to said engine; a hydraulically actuated cylinder and piston resiliently biased to open said carburetor valve, said valve being actuated toward closure by hydraulic pressure; an auxiliary delivery line connecting said main delivery line with said resiliently biased hydraulic cylinder and piston at a position between said engine driven pump and said variable orifice thereby supplying fluid under pressure to said piston in a direction to close said carburetor valve against the resilient bias; whereby a movement of said acceleration control member from one setting to a new position to change said variable orifice will result in change of engine output followed by compensating change in said valve and gradually reach a balance corresponding to the setting of said acceleration control member and thereby further to provide governing control of said engine for varying torque resulting from variation in tractive and other conditions of the truck vehicle for the various positions of said acceleration control member.

2. A control system as in claim 1 wherein there is provided: a hydrostatic drive actuated by the truck engine for propelling the truck; a second resiliently biased hydraulic cylinder and piston for controlling said hydrostatic drive; a second auxiliary delivery line from said main delivery line serving said second hydraulic cylinder; a flow regulator in the form of a spool member in said second auxiliary delivery line actuated by said acceleration control member to control flow to said second hydraulic cylinder; means mounting said flow regulator for actuation by said acceleration control member; said means including a spool member balanced by resilient means at each end thereof normally balanced to provide no flow to said second hydraulic cylinder for said hydrostatic drive; means actuated by movement of said acceleration control member toward a speed increase to unbalance said spool toward position to direct flow to said second hydraulic cylinder controlling said hydrostatic drive.

3. A control system as in claim 2 wherein there is provided: a hydraulic fluid flow line in said hydrostatic drive, the flow rate thru which is indicative of the speed generated by the drive for propelling said truck; an orifice in said hydraulic fluid flow line; a first line flow regulator connection from the upstream side of the orifice in said fluid flow line to the end of said flow regulator in which pressure will cause said flow regulator to reduce flow to said hydrostatic control cylinder; a second line flow regulator connection from the downstream side of said orifice to the end of said flow regulator in which pressures will cause said flow regulator to increase flow to said hydrostatic control cylinder to chAnge the effective drive ratio; whereby any engine speed change not called for by the acceleration control member will be compensated by a pressure change and build up of a pressure in said first line connection rebalancing said flow regulator and during such rebalancing effecting a change in effective drive ratio in said hydrostatic drive.

4. A control system for a lift truck, said truck having an engine and an acceleration control member, a lift control, a fuel flow control, and a hydrostatic drive control of varying ratio to propel the truck from said engine and a valve body; a fluid pump driven by said engine; a hydraulic fluid delivery line from said pump to said valve body; an orifice forming spool movable in said valve body with said acceleration control member and providing restriction by said orifice to generate a back pressure in said hydraulic fluid delivery line for actuating said controls; a flow control spool positioned in said valve body spaced from said orifice forming spool; a resilient connection in an end chamber between an end of said flow control spool and said orifice forming spool with said space therebetween forming a pressure chamber; a further chamber between the opposite end of said flow control spool and the end of said valve body; a resilient member in said further chamber which with said resilient connection at the opposite end of said spool balances said spool in said valve body; formation on and connections from said flow control spool and valve body so formed and positioned that with said spool in balance restricts flow from said delivery line to minimize variation in said hydrostatic drive ratio, but when unbalanced by movement of said orifice forming spool by movement of said acceleration control member, will unbalance said flow control spool and cause flow to vary said hydrostatic drive ratio.

5. A control system as in claim 4 wherein there is provided: a hydraulic fluid flow line in said hydrostatic drive, the flow rate thru which is a function of the speed generated by the drive for propelling the truck; an orifice in said hydraulic fluid flow line; a first line flow regulator connection from the upstream side of said orifice in said fluid flow line to the end of said flow regulator spool in which pressure will cause said spool to restrict flow to vary said hydrostatic control; a second line flow regulator connection from the downstream side of said orifice to the end of said flow regulator spool in which pressures will cause said spool to increase flow to said hydrostatic control to change the effective drive ratio; whereby an engine speed change not called for by the said acceleration control member will be compensated by a pressure change and build up of pressure in said first line connection rebalancing said flow regulator and during such rebalancing effecting a change in effective drive ratio in said hydrostatic drive.