US3866418A

US3866418A - Hydraulic apparatus

Info

Publication number: US3866418A
Application number: US420862A
Authority: US
Inventors: John Henry Waters
Original assignee: Dowty Hydraulic Units Ltd
Current assignee: Dowty Hydraulic Units Ltd
Priority date: 1973-12-03
Filing date: 1973-12-03
Publication date: 1975-02-18
Anticipated expiration: 1992-02-18

Abstract

This invention provides a power supply assembly suitable for a vehicle such as a fork-lift truck in which one engine provides power for propulsion of the truck and for operation of the lift and tilt service.

Description

[ Feb. 18, 1975 United States Patent 1 1 Waters [56] References Cited UNITED STATES PATENTS i 1 HYDRAULIC APPARATUS Inventor: John Henry Waters, Cheltenham,

England 2,058,894 10/1936 2,867,091 1/1959 Orlot'fcl a1. 3,633,359 l/l972 [73] Assignee: Dowty Hydraulic Units Limited,

Cheltenham, England Dec. 3, 1973 App]. No.: 420,862

Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or Firm-Young & Thompson [57] ABSTRACT This invention provides a power supply assembly suitable for a vehicle such as a fork-lift truck in which one engine provides power for propulsion of the truck and for operation of the lift and till service.

6 6 00 2mm M 0 m 6 0 m 2 N 4 n n 0 6 u ,1. a s m U .l

[58] Field of Search 60/420, 423, 427, 431, 60/443, 444, 468 494 13 Claims, 3 Drawing Figures PATENTED FEB 1 8 I975 sum 3 BF HYDRAULIC APPARATUS This invention relates to a power supply assembly to provide a variable speed variable torque drive, and is an improvement of the invention in my'prior applicaton Ser. No: 146,105 filed May 24, 1971, which issued as US. Pat. No. 3,797,244 on Mar. 19, 1974.

In accordance with the present invention a power supply assembly comprises a positive-displacement pump, a variable speed power source adapted to drive the pump, a positive-displacement motor fed with hydraulic liquid delivered by the pump, a variable by-pass to cause some of the hydraulic liquid delivered by the pump to by-pass the motor in order to vary the motor speed and means for controlling the speed of the power-source said means being responsive to the bypass flow and so arranged that a reduction in the by-pass flow will cause an increase in the speed of the power source and vice versa.

The pump may be of fixed positive-displacement or of smoothly variable positive-displacement.

The motor may be of fixed positive-displacement or of smoothly variable positive-displacement. In the latter case the motor speed may be varied both by variation of motor displacement and by variation of liquid flow from the pump to the motor.

The variable by-pass may comprise a variable dividing valve and a variable throttle valve, the dividing valve acting to by-pass part of the pump delivery to a by-pass circuit in the sense to maintain a constant pressure drop across the throttle valve through which liquid flows to the motor.

The by-pass circuit may include a flow sensing means for generating a pressure drop as a result of flow of bypass liquid therethrough and the means for controlling the speed of the power source may comprise a springloaded variable volume device connected to respond to pressure drop across the flow sensing means for adjustment of power source speed.

The flow sensing means may comprise a restrictor or alternatively may comprise a restrictor and a springloaded check valve in parallel connection with one another.

The by-pass circuit may include one or more open centre control valves and a service connected to the or each open centre valve such that the open centre valve may either permit unrestricted flow in the by-pass circuit or may throttle the bypass circuit to direct liquid at pressure to the service. The service may be singleacting or double-acting.

The flow sensing means may be placed downstream of the open centre valve and means associated with the open centre valve may divert return flow from the service away from the flow sensing means. This will avoid response of the flow sensing means to possible high flow rates resulting from high return flow rates from the service which might otherwise cause excessive reduction of power source speed.

One embodiment of the invention for use in providing the propulsion and service on a fork-lift truck will now be particularly described with reference to the accompanying drawings, in which FIGS. 1 and 2 jointly form a circuit diagram similar to the diagram in my said prior application illustrating the pump and propulsion motor control circuit, and

FIG. 3 is a circuit diagram showing the by-pass circuit and service control associated with FIG. 1.

Reference is made initially to FIG. 1. The power source for the transmission comprises, an engine 1 which may be of any conventional type capable of speed adjustment, for example, a diesel engine or a petrol engine. The transmission comprises a fixed positivedisplacement pump 2 for example of the gear type, hydraulically connected to drive a variable positivedisplacement motor 3 which in turn is mechanically connected to ground-engaging wheels or the like for propelling the fork-lift truck. The motor 3 may be of the swashplate type whose displacement is adjustable by movement of a plunger 4 relative to the motor. As shown, outward movement of the plunger 4 increases motor displacement to a maximum. Pump 2 draws liquid from a low pressure reservoir 5 and delivers liquid at pressure to a pipe 6 which passes to a flow-dividing valve 7. The valve 7 comprises a spool valve 8 slidable within a cylinder 9 having three ports ll, 12 and 13 formed therein. The pipe 6 connects to port 12. The spool member 8 includes three lands l4, l5 and 16 of which the central land 15 is somewhat narrower than the port 12 and by variation in position is adapted to divide the flow from port 12 between the ports 11 and 13. The lands 14 and 16 define a pair of working spaces 17 and 18 at the two ends of the cylinder 9. The working space 18 is connected by an internal passage 19 within the spool member to receive the pressure within the port 11. Working space 17 contains a spring 21 and receives liquid at pressure from a pipe 22 through a restrictor 23. A pipe 24 carries liquid from port 11 to a variable throttle valve 25. The port 13 is connected to a by-pass passage 26 extending to the by-pass circuit shown in FIG. 3 which will be described later in the specification.

The variable throttle valve 25 comprises a cylinder 27 having five

ports

28, 29, 31, 32 and 33 formed therein. A spool valve member 34 slides in the cylinder 27 and has three

lands

35, 36, and 37 connected by

tapered portions

38 and 39. The

lands

35 and 37 define working spaces 41 and 42 at the two ends of cylinder 27. The working space 42 in addition to receiving hydraulic liquid at pressure also contains a pair of

loading springs

43 and 44. The loading spring 43 is retained on the spool member 34 so as to permit a small degree of movement thereof before spring load in either direction is applied from the spring 43. The spool 34 is locatable in either of the limits of free movement by means of a pawl 45 engaging a flange 46 on the spool 34. The spring 44 is so arranged, that after the spool 34 has moved a predetermined distance in either direction when compressing spring 43, to add a further spring load resisting further movement of the spool 34. The force required to move the spool 34 is effected by hydraulic pressures fed to the working spaces 41 and 42. The hydraulic motor 3 is connected by

pipes

47 and 48 to the

ports

29 and 32 so that depending on the position of spool 34, flow from pipe 24 may pass to either of the two

pipes

47 or 48. The return flow from the motor which flows along either

pipe

47 or 48 will, depending on the position of spool 34, enter either of the

ports

28 or 33. These ports are connected together by a pipe 49 for connection to a braking valve 51. A pair of

auxiliary ports

52 and 53 open into the cylinder 27 on either side of the port 31 and are alternatively closable by the land 36. The

ports

52 and 53 are connected together externally of the throttle valve 25 to the pipe 22 which is connected to various valves within the circuit. In particular, the pipe 22 connects through restrictor 23 to the working space 17. I

The braking valve 51 comprises a cylinder 54 having a pair of

ports

55 and 56 formed therein and a spool 57 slidable in the cylinder. The spool 57 includes a pair of

lands

58 and 59 which control the flow between the

port

55 and 56 and which also define a pair of

working spaces

61 and 62 at the ends of the cylinder 54. The working space 62 includes a spring 63 acting on the spool. The working space 61 is fed with liquid by a restricted passage 64 within the spool. Working space 62 is fed with liquid by a restricted passage 65 within the spool.

Pressure control within working space 62 is effected both by a pilot valve 66 and a pilot valve 67. The pilot valve 66 is a spring loaded valve and responds to hydraulic pressure from the pipe 49 to connect working space 62 to drain through

pipes

68 and 69. The pilot valve 66 will connect working space 63 to drain when pressure in pipe 49 approaches 2,000 p.s.i. The pilot valve 67 will connect working space 62 to drain through pipes 71 and 72 in response to pressures in pipe 22 above about 250 p.s.i. The reduction of pressure is working space 62 by operation of either of the

pilot valves

66 or 67 opens a connection in valve 51 between

ports

55 and 56 permitting return flow liquid from the motor from port v28 or 33 to pass through to port 56.

A boost pressure valve 73 comprises a cylinder 74 having three

ports

75, 76 and 77 formed therein and a spool 78 slidable therein. The spool 78 includes two

lands

79 and 81 which control the connection between port 76 to either of the ports or 77. The spool 78 is acted upon by a spring 82 to urge it against the force in a small working space 83 formed by an auxiliary piston-and-cylinder and connected to the pipe 22. The port 75 receives liquid from pipe 6 at pump delivery pressure whilst the port 77 is connected to reservoir 5. Port 76 is connected to port 56 of braking valve 51. When the pressure in working space 83 is high the spool 78 will move to connect port 76 without restriction to port 77 thereby allowing return liquid pass through valve 51 to pass directly to reservoir. When the pressure in working space 83 is low, e.g., about 250 p.s.i. port 77 is closed and there is a restricted connection between

ports

75 and 76 allowing a small make up flow of liquid to pass through

ports

76 and 56 and through a pair of

non-return valves

84 and 85 into either of

ports

29 or 32 of the throttle valve 25.

A direction selecting valve 86 is controlled in position by a manual lever 87 having positions corresponding to forward, neutral and reverse. The valve 86 comprises a cylinder 87 having a spool 88 therein comprising three

lands

89, 91 and 92. The

lands

89 and 91 control ports 93 and 94 which are connected through

pipes

95 and 96 to the working spaces 41 and 42 of the throttle valve 25. A port 97 enters the cylinder 87 between the ports 93 and 94 and depending on the position of the spool 88, port 97 is connected to either of the ports 93 or 94. A detent flange 98 on the spool 88 is engageable by a pawl 99 controlled by a piston-and-cylinder unit 101. The piston-and-cylinder unit is spring-loaded so that as the spool disengages from the flange 98 it is connected through

pipes

102 and 103 to respond to the difference in pressures between the working

spaces

61 and 62 of the braking valve 51. A small pressure difference only, for example about 25 p.s.i., is sufficient to urge the pawl 99 inwardly to engage flange 98.

The lever 87 controls the spool 88 through the medium of a caged spring 104.

A master piston-and-cylinder unit 105 is formed by a piston 106 slidable in a cylinder 107 against a spring 108. The piston 106 is urged into cylinder 107 by means of a servo piston 108 slidable within a cylinder 109. Cylinder 109 receives liquid at reduced pressure through pipe 111. A rod 112 extends between the

pistons

108 and 106 through a reservoir chamber 113. A priming passage 114 extends from the reservoir chamber 113 into cylinder 107 at a position where it is just opened by piston 106 when in its fully retracted position. A bleed passage 115 extends through piston 108 and rod 112 to open into the reservoir chamber 113, the opening being controlled by a sleeve 116 slidable on rod 112. The position of sleeve 116 is adjusted by means of a foot pedal 117, the sleeve 116 being restored to its right hand position as shown by means of a spring 118 acting on the pedal 117.

A pressure reducing valve 119 receives liquid at pressure from the pump delivery connection 6 and by virtue of conventional spring action supplies reduced pressure through restrictor 121 to a port 122 opening into the cylinder 87 of selector valve 86. An internal passage 123 within the land 92 of the selector valve connects port 122 to reservoir when lever 87 is in the neutral position thus ensuring that when lever 87 is in the neutral position no servo liquid at pressure can be delivered to servo cylinder 109.

The servo motor for adjustment of displacement of the motor 3 is shown at 124. This servo motor is of the differential area piston type comprising a stepped cylinder 125 within which a stepped piston 126 is slidable providng a pair of working

spaces

127 and 128 of which the working area on the piston of space 127 is one half the working area on the piston of space 128. A control cylinder 129 within the piston carries the servo valve 131 provided with lands so as to connect working space 128 either through a restricted passage 132 to the working space 127 or to drain through a central passage 133 within the spool valve 131 and passage 134 extending from the end of the piston. A spring 135 within cylinder 129 urges the spool 131 in a downward direction. The piston 126 further defines with its stepped bore a working space 136 connected by pipe 137 with the master cylinder 107. The working space 127 is fed with high pressure liquid from the

motor connections

47 and 48 by virtue of two

non-return valves

138 and 139 which select the higher pressure from the motor connections to feed to the working space 127.

The motor servo 124 also includes an override valve 141 whose function is to cause the servo motor 124 to move over-ridingly to a larger motor displacement when the pressure exceeds 3,000 p.s.i. The valve 141 is fed with high pressure liquid from the working space 127 of the servo through a pipe 142, such pressure being fed to act on a spool 143 against the compression of a spring 144. If the pressure exceeds a predetermined value, say, 3,000 p.s.i., causing the spool 143 to move against the spring 144, the movement of the spool will connect a pipe 145 extending from the working space 128 to a pipe 146 extending to reservoir 5. The control pressure in the space 128 will cause movement of the servo piston to over-ridingly increase motor displacement until the pressure in working space 127 reduces below the level, i.e., 3,000 p.s.i., to which the valve 141 responds.

Reference is now made to FIG. 3 of the drawings, which illustrate the by-pass circuit. The flow from pipe 26 enters an open-centre control valve unit 151 and liquid flowing in the bypass circuit may be selected by the open-centre valve unit to operate the lift-jack 152 of the fork-lift truck and the tilt-jack 153. By-pass liquid flowing from the pipe 26 will for the most part leave the open-centre valve unit 151 through a pipe 154 to enter a flow-sensing unit 155, such flow leaving the flowsensing unit through a pipe 156 to flow through a cooler 150 to reservoir 5. A signal pressure developed at the flow-sensing unit 155 is fed through pipe 157 to an engine speed control unit 158 which reacts on the engine speed governor 159 forming part of the engine The open-centre control valve unit 151 comprises, a pair of open-

centre control valves

161 and 162 having separate manually

operable levers

163 and 164 respectively. The lift-jack 152 which is controlled by the valve 161 is a single-acting service, that is to say, hydraulic liquid is fed to only one working space 163 of the jack through a pipe 164. The weight of the fork-lifting apparatus on its own is normally sufficient under the action of gravity always to be capable of returning the lift-jack 152 when its working space is connected to reservoir. The tilt-jack 153 is a double-acting jack having a pair of opposed working

spaces

165 and 166 fed respectively through pipes 167 and 168. An any instant when liquid at pressure is supplied to one of these working spaces to increase its volume, then return flow liquid must flow from the other working space.

The control valve unit 151 is of conventional structure, each of the

control valves

161 and 162 sliding respectively in

cylinders

169 and 171. Each

control valve

161 and 162 includes a caged spring unit respectively 172 and 173 whose function is always to tend to restore its valve to a neutral position against deflection manually in either direction from the neutral position. Whilst the

valve cylinders

169 and 171 both include exactly the same port formation, the

valves

161 and 162 are themselves of slightly differentstructure to accommodate the single-acting and double-acting nature of the services. The valve 161 includes three

lands

174, 175 and 176 which co-operate with five

ports

177, 178, 179, 181 and 182. The pipe 26 connects to both

ports

179 and 181. In the neutral position of the valve 161, the port 181 connects to the port 182 from which the open-centre passage 183 extends to the valve 162. The port 178 is connected by pipe 164 to the working space 163 of liftjack 152. The port 177 is connected through pipe 184 and pipe 156 directly to reservoir through the cooler 150. The

lands

174, 175 and 176 are so located that during movement from the neutral or hold position to select upward movement of the lift-jack, the land 175 will open a connection between ports 178 and 179 a little before it closes 181 from the port 182 so that the operator by carefully operating the handle may throttle the flow from port 181 to 182 thus generating pressure in the by-pass circuit, such pressure being connected via

ports

179, 178 to the working space 163 to raise the lift-jack. Movement of the valve 161 to lower the liftjack makes no alteration in the connections in the neutral position other than to connect port 178 to 177 by movement of land 174, a variable throttling effect being obtained depending on the degree of opening.

The valve 162 comprises, four

lands

185, 186, 187 and 188 which co-operate with eight

ports

189, 191, 192, 193, 194, 195, 196 and 197 opening into the cylinder 171. Port 189 is directly connected to the return line 184. Port 191 is connected to the working space of jack 153. Port 192 is directly connected to the port 179, port 193 is connected to pipe 154 carrying the by-pass flow, port 194 is internally connected to passage 183 and to port 182, port 195 is internally connected to the by-pass pipe 26, port 196 is connected through pipe 168 to working space 166 of the tilt-jack and port 197 is connected directly to the return pipe 156.

Movement of valve 162 in either direction from its neutral position will initially connect the working

spaces

165 and 166 in one sense or the other to the pipe 26 and to the pipe 156, further movement then acting to throttle the flow between

passages

193 and 194. The lands and 186 control the connection of working space 165 to either of the

pipes

26 or 156 through either of the

ports

192 or 189. The

lands

187 and 188 control the connection of the working space to either of the

pipes

26 or 156 through the ports or 197. The

lands

186 and 187 irrespectively of the movement of the valve from neutral will throttle and close the open-centre passage between

ports

193 and 194. It will be seen that whatever direction of selection is applied to the valve 162 to move the tilt-jack, the return flow from the tilt-jack pass through the pipe 184 to reservoir without passing through the flow-sensing means 155. For selection of movement of either the lift-jack or the tilt-jack, the control exerted by the operator is normally such that the by-pass flow passage from pipe 26 through to pipe 154 will not be completely closed, the flow which is then permitted to pass through the flow-sensing device being such as to enable the operator to control the speed of the engine.

The flow-sensing means comprises a spring-loaded check valve 198 parallelly connected with a restrictor 199 whereby liquid flowing through pipe 154 may pass to the reservoir through valve 198 and restrictor 199 jointly into the pipe 156. In the present embodiment the spring-loading of the valve 198 is such that a pressure of 200 p.s.i. is necessary before the check valve will open. For flow rates from pipe 154 through restrictor 199 which do not demand a pressure drop at the restrictor greater than 200 p.s.i. the flow to reservoir will be completely through the restrictor providing a normal square law relation between flow rate and pressure drop. The pipe 157 carries the pressure signal upstream of the check valve 198 to the engine speed controller 158 which comprises a closed container 201 having a plunger 202 slidable through a sealed opening in a wall thereof, movement of the plunger being opposed by a spring 203. The plunger 202 externally of the casing 201 is connected to adjust the setting of the engine speed governor 159. Spring loading of the plunger 202 is so arranged that at a low pressure of about 50 p.s.i. in the casing the spring will move the speed setting to maximum, the engine speed being reduced to half speed as the pressure increases up to about 150 p.s.i. At 150 p.s.i. the shoulder 204 on plunger 202 will engage the interior of the casing to prevent further movement. Further increase in flow rate in pipe 154 will cause rise in pressure drop at the restrictor up to 200 p.s.i. at which point the check valve 198 will carry the further flow preventing the pressure drop from further increase.

Various functions for the transmission on the fork-lift truck will now be described.

Forward Propulsion In order to propel the vehicle forwardly the direction lever 87 is moved to the appropriate forward position, moving the spool 88 to the left as seen in the drawing thus connecting the master cylinder 107 through pipe 137, ports 97 and 93 of valve 86 and pipe 95 to the lefthand working space 41 of the throttle valve 25. Assume the engine is running at its normal speed. In order to propel the vehicle the pedal 117 is then depressed from the position A towards position B. The fact that the engine is running and the pump delivering means that liquid under pressure is supplied through pipe 6 to the pressure reducing valve 119 and the selection of forward on lever 87 will dis-connect the reservoir passage 123 from port 122 thus ensuring the delivery of liquid at reduced pressure to the servo motor 109. Depression of the pedal will move sleeve 116 to cover passage 155 so that pressure is developed against the servo piston 108 to move it to the-left, moving master piston 106 into cylinder 107 to cutoff port 114 and thus to pressurise the liquid in the master cylinder. This liquid is transferred through pipe 137 to working space 41 urging the spool-34 to the right, firstly displacing the locating pawl 45 and then slightly compressing the first spring 43. At this position, the land 36 will uncover the auxiliary port 53 and will open a restricted connection between

ports

31 and 29, so that high pressure liquid from the pump through pipe 24 may flow through port 29 into motor connection 47 thus causing the motor to rotate. The dividing valve 7 will receive in its working spaces 17 and 18 the pressures respectively from auxiliary port 53 and from port 31 such forces urging the spool 8 to the right as seen in the drawing to partially open the flow from port 12 into port 11 and also to partially close a throttle path from port 12 into port 13. This action will provide a flow from the pump delivery into the ports 11 and 13, the proportion between these two flows being dependent on the pressure drop from port 31 to port 29. The adjustment will be such that the pressure drop is maintained at a constant value depending on the load of the spring 21. In other words, the flow to the motor will be in direct proportion to the movement of the throttle valve spool 34 which in turn is in proportion to the pressure generated in the master cylinder 107. Up to position B for the pedal, a substantial proportion of the liquid delivered by the pump 2 and divided by the valve 7 will flow through the by-pass circuit to and through the valve unit 151 and through pipe 154 into the flow-sensing means. The substantial flow will ensure that the check valve 198 is open and a maximum pressure drop is developed which when fed to the engine speed controller 158 will urge the plunger 202 outwardly to the limit thus setting the engine governor at half speed.

Return flow from the motor passes along pipe 48 to port 32 of the throttle valve and then into port 33. From port 33 the flow will enter pipe 49 and port 55 of the braking valve 51. The pilot valve 66 will also receive the return pressure from pipe 49 which will be quite low, i.e., considerably less than the pressure at which the pilot 66 will start to vent working' space 62. The pressure from the auxiliary port 53 which is the pressure in the pipe 47 carrying flow to the motor, will react on the pilot valve 67 causing full opening thereof so that the working space 62 of braking valve 51 is vented directly to reservoir. Thus the pressure in the return flow of liquid in port 55 is communicated by restricted passage 64 to working space 61 and acts on the spool 57 urging it to the right against its spring 63 and connecting port 55 to port 56 without further restriction. From port 56 the return liquid enters the port 76 of the boost pressure valve 73. The working space 83 of valve 73 will receive liquid at high pressure from pipe 22 urging the spool 78 to the right as seen in the drawing providing a substantially unrestricted connection from port 76 into port 77 and so to reservoir.

Assume now that the driver wishes the vehicle to go faster. He will depress pedal 117 to the B position which increases pressure in the master cylinder to cause movement of the throttle valve spool 34 to the right to the extent that the first spring 43 is fully compressed and engagement in just made to start compressing the spring 44. However this would demand very substantial increase in pressure in working space 41 which will not be available at position B. The driver depresses slightly beyond position B and the pressure developed in the master cylinder 107 and acting in the space 136 of the servo motor 124 will now be sufficient to move the pilot valve 131 against the load of its spring 135, thus connecting high pressure to the working space 128 causing the servo piston 126 to move upwardly as seen in the drawing to reduce motor displacement. The flow rate of liquid fed to the motor is that determined by the throttle valve at the position where it is just about to start compressing spring 44 and increase in speed is obtained by virtue of the fact that the motor displacement is reduced whilst the liquid flow through it is maintained constant. As the driver pushes the pedal 117 from position B to position C the liquid displaced from the master cylinder 107 will cause displacement of the pilot spool 131 so that the servo piston 126 must follow. At any point, if the driver does not move the pedal further, the pilot valve 131 will stop moving and accordingly the servo piston 126 will stop moving. Within this operation the engine still rotates at half speed and the by-pass flow through the flowsensing means is sufficient to hold the engine speed at half speed.

Assume now that speed has increased to the extent that the motor is at minimum displacement. This will correspond to position C for the pedal. Again assume that the driver wishes to go faster, he will then depress the pedal from position C to position D. The servo piston 126 will be on its minimum displacement stop and the pilot-valve 131 will also be held against thestop in its cylinder so that no further increase in volume of working space 136 is possible. The further movement of the pedal 117 will therefore increase pressure in the master cylinder to the extent that the pressure increase in the working space 41 will cause the spool 34 to move against compression of both

springs

43 and 44. This movement will reduce the restricting effect between

ports

31 and 29 and the dividing valve 7 in its attempt to maintain a constant pressure drop between these ports will cause a greater proportion of pump delivery to pass through the throttle valve to the motor. In turn this means that the bypass flow through the by-pass circuit and through the flow-sensing means will become smaller and just at the position C for the pedal 117 the by-pass flow will reduce to the extent that the pressure at the flow-sensing means 155 will drop below 150 psi. whereby the spring 203 will act on plunger 202 to increase the speed setting of the engine governor. Thus movement of the pedal from position C to position D will adjust the restricting effect between

ports

31 and 29 demanding increase in flow to the motor 3 which in turn will cause a reduction in by-pass flow. This reduction in by-pass flow as detected by the flow-sensing means will cause an increase in engine speed, the increase in engine speed acting in effect to maintain flow in the by-pass circuit which at least will maintain the pressure at the flow-sensing means at 50 psi Pressure Over-ride Control In during propulsion at any instant, the pressure fed to the motor attains a value in excess of a pre-set maximum, e.g., 3,000 p.s.i., the pressure limit valve 141 will respond by connecting the working space 128 of the servo to reservoir. The high pressure in the working space 127 of the servo wil then move the servo piston in a motor displacement increasing direction, although the pressure in the working space 136 holds the pilot valve 131 fully depressed. This over-riding movement of the piston 126 will cause liquid to be displaced from the working space 136 to increase pressure in both the master cylinder 107 and in the working space 41 (assuming forward propulsion is selected). The increased pressure in the master cylinder 107 will not be felt on the pedal 117 because the master piston 106 is servo operated by virtue of sleeve 116. However, the increased pressure in the working space 41 will cause further movement of the spool valve member 34 to the right against the joint loading of both

springs

43 and 44 to increase the selected flow from pump 2 to the motor 3. Such increase in selected flow may involve reduction in by-pass flow to the extent that the pressure at the flow-sensing means 155 will drop below 150 p.s.i. and the engine speed will be increased to help to supply the increased flow rate. In this way substantially the same speed of the vehicle may be retained up to the maximum engine speed but the operating pressure of the liquid is maintained constant at say 3,000 p.s.i.

Braking When the driver wishes to reduce speed of the vehicle, he will merely raise the pedal 117 an amount in accordance with the speed reduction desired. The basic action is independent of the actual position of the pedal and is as follows. The raising of the pedal will either reduce the flow rate selected by the throttle valve 25 or increase motor displacement, such operation immediately serving to make the motor rotate at a speed greater than that dictated by the flow rate of liquid to the motor. There will be an instantaneous pressure reversal in the motor connections, the connection 47 carrying liquid to the motor suddenly dropping to a low pressure, and the connection 48 carrying liquid from the motor suddenly increasing to a high pressure. The flow path for the return liquid from pipe 48 is through

ports

32 and 33 of the throttle valve, pipe 49,

ports

55 and 56 of the braking valve and through non-return valve 84 into the port 29 leading to the flow pipe 47. Thereby a closed circuit is formed for the liquid flowing through the motor and substantial pressure is lost in this circuit in flow between the

ports

55 and 56. The pilot valve 67 by virtue of low pressure from pipe 22 is not operative to vent liquid from working space 62 of the braking valve, but the pilot valve 66 which now receives high pressure from the pipe 49 will permit a restricted flow of liquid from the working space 62 in the sense to ensure that a pressure drop ofsome 2,000 psi. will occur when liquid passes from port 55 to the port 56. This closed circuit within which liquid flows to and from the motor, will tend to lose liquid due to leakage and therefore liquid must be fed into the circuit to prevent cavitation. This function is performed by the boost valve 73 which on receipt of the comparatively low pressure from pipe 22 during braking, will slightly move the spool 78 to the extent to provide a restricted connection between the pump delivery pipe 6 through port 75 and into port 76 for feeding to the port 56, which is the position in the closed motor circuit having the lowest pressure during braking. This function will ensure that liquid at the pump delivery pressure during braking is fed into the closed motor braking circuit.

Prevention of Selecting Change in Direction Whilst Vehicle In Motion When the vehicle is in motion in a particular direction, the lever 87 will be in the forward or the reverse position in which the reservoir passage 123 is cut off from port 122 and in which the locking piston-andcylinder unit 101 is energised from the pressure drop at the braking valve to urge the pawl 99 to engage one or other side of the flange 98. Whilst the vehicle is in motion, the return flow of liquid from the motor through the braking valve, whether during braking or propulsion will produce a small pressure drop between the two working

spaces

61 and 62, this pressure drop acting on the piston-and-cylinder unit 101 against its light spring loading to hold pawl 99 in engagement with flange 98. Only when the vehicle has actually stopped movement and the motor has stopped rotation, does the pressure difference between the working spaces of the braking valve vanish, and at this instant the spring loading will withdraw the pawl 99 allowing selection of the alternative direction. If before the vehicle has stopped motion, the driver attempts to select the alternative direction of motion, he could just succeed by moving lever 87 to cause the reservoir passage 123 to coincide with port 122, thereby removing the pressure from the pipe 111. in this case the master piston 106 will immediately retract completely, and the action will be to cause vehicle braking to bring the vehicle to a complete halt. The pawl 99 will then remove and the alternative direction of propulsion can be selected.

Thus the operations of propulsion and braking have been described for one direction of propulsion only it will be appreciated that similar operations take place when the alternative direction of propulsion is selected.

Selection of Lift.

When it is desired to operate the forks of the truck to raise a load, the driver will operate the handle 163 of valve 161 towards the lift position, such movement initially throttling the by-pass flow from pipe 26 and through port 181 to 182. The same movement will also connect the port 179 to the port 178 so that liquid may flow from the by-pass pipe 26 to the working space 163 of the lift-jack. The greater the throttling of the by-pass passage by land 175, the greater will be the pressure generated and the smaller the flow through the by-pass circuit into the flow-sensing means 155. If the remaining flow through the flow-sensing means becomes sufficiently small, the engine speed will be increased by the speed control unit 158. The operation of valve 161 to select lift may be either whilst the truck is being propelled or whilst it is stationary. If the lift is selected whilst the truck is stationary, there is no flow to the propulsion motor 3 and the dividing valve will be in position to close the flow path between ports 11 and 12 leaving the flow path between ports 12 and 13 fully open. Thus without the selection of lift the pump is substantially unloaded, the delivery passing completely through the by-pass circuit at low pressure. Selection of lift on the valve 161 will throttle the by-pass circuit and at the same time connect the jack working space 163 to'the pipe 26 through

ports

178 and 179. The liquid in the pipe 26 is thus pressurised and the jack 152 will lift. The speed of the engine is controlled by the driver by virtue of the throttling effect exerted on the by-pass circuit by the valve 161. If the liquid permitted to flow in the by-pass circuit is reduced to a very small amount, the flow detecting means will cause the engine speed to increase. If the selection oflift is made whilst the truck is moving, there will be pressure in the pump delivery circuit for feeding to the propulsion motor. The throttling of the by-pass circuit by valve 161 to produce pressurisation in pipe 26 will cause the dividing valve 7 to move slightly'to reduce the throttling effect between ports 12 and 13 thus providing pressure to operate the lift-jack.

When it is desired to lower the lift jack, the handle 163 is moved to the appropriate position causing upward movement of the valve 161 to initially open a connection between ports 177 and 178 so that the working space is connected to reservoir through ports 178 and 177, and

pipes

184 and 156. This flow does not generate a signal at the flow-sensing means. The land spacing on valve 161 is such that the return flow between ports 177 and 178 is opened well before the land 176 can start to throttle the by-pass circuit by closing the flow between the

ports

181 and 182.

Lowering of the lift-jack may be accomplished when the vehicle is stationary or in motion. Control of the rate of fall being dependent solely on the adjustment by the driver of lever 163.

Adjustment of Tilt-Jack For adjustment of the tilt-jack the driver will move the handle 164 to the appropriate tilt forward or tilt back position. The initial movement of the valve 162 is to connect one or other of the working

spaces

165 and 166 to receive liquid from the by-pass circuit. Working space 165 would receive liquid from the port 191 and the working space 166 would receive liquid from port 196. Further movement of the valve 162 after connecting the appropriate working space to the by-pass circuit is to throttle the by-pass circuit so that pressure may be generated for movement of a tilt-jack. Return flow liquid from the working

spaces

165 and 166 will flow either through port 189 or port 197 into the pipe 184 to return directly to reservoir without passing through the flow-sensing means 155. Liquid at pressure required from the tilt-jack 153 is very small and normally movement of the valve 162 to adjust the tilt-jack will not cause any substantial reduction in flow rate through the by-pass circuit into the flow-sensing means 155. The selection of any movement for the tilt-jack 153 will of course demand pressurisation of the flow in the pipe 26 from dividing valve 7 and as previously described as with respect to the lift-jack 152 such pressurisation of pipe 26 will cause a slight adjustment of the dividing valve 7 in order to provide such increased pressure if the vehicle is in motion.

Whilst the described embodiment of the invention has provided a pair of open-

centre selector valves

161 and 162 capable of using the by-pass flow from the dividing valve 7 to operate the services it will be appreciated that within the scope of the present invention there need not be any provision for operating the services in the by-pass circuit, the flow detecting means being solely in the by-pass circuit and acting to control engine speed. i

I claim:

1. A power comprising assembly comprsing a positive-displacement pump, a variable speed power source adapted to drive the pump, a positive-displacement motor fed with hydraulic liquid delivered by the pump, a variable by-pass to cause some of the hydraulic liquid delivered by the pump to by-pass the motor in order to vary the motor speed, and means for controlling the speed of the power source, said means being responsive to the by-pass flow and so arranged that a reduction in the by-pass flow will cause an increase in the speed of the power source and vice versa.

2. A power supply assembly as claimed in claim 1 wherein the pump is of fixed positive-displacement.

3. A power supply assembly as claimed in claim 1 wherein the motor is of variable positive-displacement.

4. A power supply assembly as claimed in claim 1 wherein the variable by-pass comprises a variable dividing valve and a variable throttle valve, the dividing valve acting to by-pass part of the pump delivery to a by-pass circuit in the sense to maintain a constant pressure drop across the throttle valve through which liquid flows to the motor.

5. A power supply assembly as claimed in claim 4 wherein the by-pass circuit includes a flow sensing means for generating a pressure drop as a result of flow of by-pass liquid therethrough, and the means for controlling the speed of the power source comprises a spring-loaded variable volume device connected to respond to pressure drop across the flow sensing means for adjustment of power source speed.

6. A power supply assembly as claimed in claim 5 wherein the flow sensing means comprises a restrictor.

7. A power supply assembly as claimed in claim 5 wherein the flow sensing means comprises a restrictor and a spring-loaded check valve in parallel connection.

8. A power supply assembly as claimed in claim 4 wherein the by-pass circuit includes one or more open centre control valves and a service connected to the or each open centre valve such that the open centre valve may either permit unrestricted by-pass flow in the bypass circuit or may throttle the by-pass circuit to direct liquid at pressure to the service.

9. A power supply assembly as claimed in claim 8 wherein the flow sensing means is placed downstream of the open centre valve and means associated with the open centre valve to divert return flow from the service away from the flow sensing means.

10. A hydraulic power transmission comprising a fixed positive-displacement-pump, a variable-positivedisplacement-motor, a first slave piston-and-cylinder unit arranged to adjust motor displacement, a main control, a variable by-pass for selectively by-passing a portion of pump delivery liquid, the remainder being fed to the motor, a second slave piston-and-cylinder unit to control the said variable by-pass, a master piston-and-cylinder unit adjustable by said main control to displace liquid to the slave piston-and-cylinder units, mechanical means operative on liquid displaced from the master piston-andcylinder unit to cause movement of the slave units in a desired sequence, an adjustable speed power source arranged to drive the pump, and flow responsive means to receive liquid bypassed from the pump delivery and to control power source speed in the sense that reduction ofby-pass flow through said flow responsive means will cause increase in engine speed and vice versa.

11. A hydraulic power transmission as claimed in claim wherein the sequence of slave piston-andcylinder operation provided by the mechanical means comprises firstly, adjustment of the variable by-pass by the second slave piston-and-cylinder unit to increase flow to the motor, secondly reduction of motor displacement by the iirst slave piston-and-cylinder unit to increase motor speed and thirdly, further adjustment of the variable by-pass by the first piston-and-cylinder unit to increase flow rate to the motor by reducing bypass flow to the flow responsive means to increase power source speed.

12. A hydraulic power transmission as claimed in claim 11 including a pressure responsive means responding to receive hydraulic pressure within the motor to overridingly increase motor displacement and to react on the first slave piston-and-cylinder unit to increase pressure therein and thereby to reduce the bypass flow to cause adjustment of the flow adjusting means to increase power source speed.

13. A hydraulic power transmission as claimed in claim 12 wherein the mechanical means comprises spring loading, one stage of spring-loading acting on the first slave piston-and-cylinder unit to provide said second adjustment and two stages of spring loading acting on the second slave piston-and-cylinder unit-t0 provide said first and third adjustments.

Claims

8. A power supply assembly as claimed in claim 4 wherein the by-pass circuit includes one or more open centre control valves and a service connected to the or each open centre valve such that the open centre valve may either permit unrestricted by-pass flow in the by-pass circuit or may throttle the by-pass circuit to direct liquid at pressure to the service.

10. A hydraulic power transmission comprising a fixed positive-displacement-pump, a variable-positive-displacement-motor, a first slave piston-and-cylinder unit arranged to adjust motor displacement, a main control, a variable by-pass for selectively by-passing a portion of pump delivery liquid, the remainder being fed to the motor, a second slave piston-and-cylinder unit to control the said variable by-pass, a master piston-and-cylinder unit adjustable by said main control to displace liquid to the slave piston-and-cylinder units, mechanical means operative on liquid displaced from the master piston-and-cylinder unit to cause movement of the slave units in a desired sequence, an adjustable speed power source arranged to drive the pump, and flow responsive means to receive liquid by-passed from the pump delivery and to control power source speed in the sense that reduction of by-pass flow through said flow responsive means will cause increase in engine speed and vice versa.

11. A hydraulic power transmission as claimed in claim 10 wherein the sequence of slave piston-and-cylinder operation provided by the mechanical means comprises firstly, adjustment of the variable by-pass by the second slave piston-and-cylinder unit to increase flow to the motor, secondly reduction of motor displacement by the first slave piston-and-cylinder unit to increase motor speed and thirdly, further adjustment of the variable by-pass by the first piston-and-cylinder unit to increase flow rate to the motor by reducing by-pass flow to the flow responsive means to increase power source speed.

12. A hydraulic power transmission as claimed in claim 11 includIng a pressure responsive means responding to receive hydraulic pressure within the motor to overridingly increase motor displacement and to react on the first slave piston-and-cylinder unit to increase pressure therein and thereby to reduce the by-pass flow to cause adjustment of the flow adjusting means to increase power source speed.

13. A hydraulic power transmission as claimed in claim 12 wherein the mechanical means comprises spring loading, one stage of spring-loading acting on the first slave piston-and-cylinder unit to provide said second adjustment and two stages of spring loading acting on the second slave piston-and-cylinder unit to provide said first and third adjustments.