US3850323A

US3850323A - Overload protection device with differential pressure sensing

Info

Publication number: US3850323A
Application number: US00319068A
Authority: US
Inventors: G Ekstrom
Original assignee: Eaton Corp
Current assignee: Yale Materials Handling Corp
Priority date: 1972-12-27
Filing date: 1972-12-27
Publication date: 1974-11-26
Anticipated expiration: 1991-11-26
Also published as: JPS5911519B2; FR2212286A1; GB1461914A; DE2364198A1; FR2212286B1; IT1000607B; JPS49108758A

Abstract

An overload protection device for a vehicle, such as a lift truck, having a mast pivotally mounted on a frame and moved by a pair of double acting, fluid actuated piston-cylinders. The overload protection device includes a pressure actuated valve operable in response to a difference in pressure between two sources to prevent movement of the mast in one direction when the difference in pressure exceeds a given value. A shuttle check valve interconnects the front chambers within each cylinder and another shuttle check valve interconnects the rear chambers within each cylinder. Each shuttle check valve is operative to transmit only the highest pressure of the fluid existing within the respective front and rear chambers to the pressure actuated valve. The shuttle check valves constantly monitor the pressure of the fluid within the chambers at each end of the two cylinders.

Description

United States tt 1 Ekstrom Nov. 26, 197

[ OVERLOAD PROTECTION DEVICE WITH DIFFERENTIAL PRESSURE SENSKNG [75] Inventor: George A. Ekstrom, Churchville, Pa. [73] Assignee: Eaton Corporation, Cleveland, Ohio [22] Filed: Dec. 27, 1972 [21] Appl. No.: 319,068

Primary Examiner-Robert J. Spar Assistant Examiner-Lawrence J. Oresky Attorney, Agent, or Firm-Teagno & Toddy ABSTCT An overload protection device for a vehicle, such as a lift truck, having a mast pivotally mounted on a frame and moved by a pair of double acting, fluid actuated piston-cylinders. The overload protection device includes a pressure actuated valve operable in response to a difference in pressure between two sources to prevent movement of the mast in one direction when the difference in pressure exceeds a given value. A shuttle check valve interconnects the front chambers within each cylinder and another shuttle check valve interconnects the rear chambers within each cylinder. Each shuttle check valve is operative to transmit only the highest pressure of the fluid existing within the respective front and rear chambers to the pressure actuated valve. The shuttle check valves constantly monitor the pressure of the fluid within the chambers at each end of the two cylinders.

11 Claims, 2 Drawing Figures OVERLOAD PROTECTION DEVICE WITH DIFFERENTIAL PRESSURE SENSING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an overload protection device for vehicles having a load carrying member mounted for movement relative to a frame by means of two or more motors.

2. Description of the Prior Art Overturning of vehicles, such as a lift truck having a load carrying member or mast mounted for rotation about an axis on a frame to move fore and aft of a vertical plane by means of a pair of motors in the form of double acting, fluid actuated piston-cylinder assemblies connected to the frame and mast, is a problem. Attempts to solve the problem have included, among others, an overload protection device which prevents further movement of the mast relative to the frame when the moment created by a load on the mast tends to overturn the truck. One such device is described and illustrated in US Pat. No. 3,007,593, Hancock. The device of the patent includes a valve that moves when the difference of the pressures of. the fluids on the opposite sides of a piston of one of the piston-cylinder assemblies exceeds a predetermined value. Action of the valve prevents further increase in the forward overturn ing moment by preventing further fore movement of the mast. The value of the difference of the pressures of the fluids corresponds to the force exerted on the rod of the piston-cylinder assembly by the load. The force on the rod in turn corresponds to the forward moment tending to overturn the truck. Although such an overload protection device is of great benefit, problems may be encountered in its operation when: there are misaligned or improperly adjusted cylinders and rods, i.e., one piston will make contact with the end of the respective cylinder before the other piston; the pistoncylinder assembly providing the fluid pressure differential signal fails; or loads are unevenly applied to the mast.

SUMMARY OF THE INVENTION The present invention has application to any vehicle of the type including a load carrying member, such as a mast, boom, or arm, that is movable relative to the vehicle frame by at least two motor members interconnecting the load carrying member and the frame.

According to one feature of the invention the force exerted on two motors tending to move a load carrying member is sensed and a signal representative of the magnitude of force on each motor is generated. The signals are compared and the one representative of the greatest magnitude of force is transmitted to a device which operates to prevent further movement of the load carrying member when the transmitted signal is representative of a magnitude of force exceeding a predetermined value.

According to another feature of the invention wherein the motor members comprise double acting fluid actuated piston-cylinder assemblies having first and second ends, a device is provided to sense the pressure of the fluid at the first end of each cylinder and transmit a signal representative of the highest pressure to a valve which also receives a signal representative of the pressure of the fluid at the second end of one cylinder. The valve precludes further movement of the load carrying member relative to the frame when the signals differ in a given manner.

According to a further feature of the invention the signal from the second end is representative of the highest pressure of the fluids at the second ends of the cylinders.

According to still another feature of the invention the pressure of the fluid at the first end of each cylinder is constantly monitored and the signal is only representative of the highest existing pressure at any given point in time.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention is disclosed in the accompanying drawings, wherein:

FIG. 1 is a perspective view of a lift truck; and

FIG. 2 is a schematic plan view of a portion of the hydraulic circuit used for tilting the mast of the lift truck illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS The industrial lift truck 10 of FIG. 1 comprises a frame 12 supported on a pair of rear steering wheels 14 and a pair of front drive wheels 16. An upright telescoping load carrying member in the form of a mast 18 mounted for pivotal movement about an axle of wheels 16 has a pair of fixed upright channel members 20 interconnected by support members 22. Connected to the side portion of each channel member 20 is a bracket 24 to which is pivotally connected one end of a piston rod 26 of a motor in the form of a double acting, fluid actuated piston-cylinder assembly 28 which is pivotally connected at its base end to a bracket portion 30 of frame 12. The two piston-cylinder assemblies 28 are identical in construction and are adapted to actuate mast 18 in a tilting movement fore and aft of a vertical position. Mounted rearwardly of mast 18 is an operators compartment having a seat 32 mounted on frame 12, a steering wheel 34, and a tilt selector control 36 for controlling the operation of piston-cylinder assemblies 28. Mast 18 includes a fork carriage 38that is adapted to be elevated and lowered in a known manner by a double acting lift piston-cylinder assembly 40 through the use of a lift selector control 42 of a known design.

Briefly, the operation of the circuit illustrated in FIG. 2 is based on the known concept that the moment tending to overturn the truck forwardly about the axle of wheels 16 is proportional to the tensional forces exerted on rods 26. The tensional force on each rod can in turn be shown to be related to the magnitude of the force exerted on the opposite sides of the piston by the pressure of the fluid acting against the sides of the pistons in each piston-cylinder assembly 28. The concept, as well as a portion of the construction and operation of the hydraulic circuit of FIG. 2, is described in US. Pat. No. 3,007,593, Hancock.

The circuit of FIG. 2 includes a hydraulic fluid feed line 44 from a pump (not illustrated) leading to tilt selector control 36. A line 46 interconnects the tilt selector control and the full area sides 48 of tilt pistons 50 which are connected to rods 26 and are located within piston-cylinder assemblies 28. The full area side of each piston 50 is further connected via

respective lines

54 and 56 to opposite sides of a shuttle check valve 58 of a known design. A line 60 interconnects the center tap of shuttle check valve 58 with a pressure chamber 62 in a tilt limiting valve 64. The annular sides 66 of pistons 50 are connected via

lines

68 and 70 with tilt limiting valve 64. The annular side of each piston 50 is further connected via

respective lines

72 and 74 to opposite sides of a shuttle check valve 76 of a known design. The center tap of shuttle check valve 76 is connected with a pressure chamber 78 in tilt limiting valve 64 through a line 80. One type of shuttle check valve which may be used is Model 2456C-l made by Kepner Products Co., Summit, Illinois.

Tilt limiting valve 64 includes a piston 82 biased on its full area side 84 by a compressed coil spring 86 located in pressure chamber 62. Full area side 84 of piston 82 communicates with chamber 62. A spool 88 formed with an annular peripheral chamber 90 is connected to piston 82 and is slideable within tilt limiting valve 64. Pressure chamber 78 communicates with side 92 of spool 88. The proportion of the area of side 92 to the area of side 84 is equal to the proportion of the area of side 48 to the area of side 66 of each piston 50. Peripheral chamber 90, when spool 88 is in a normal position as illustrated in FIG. 2, communicates with line 70, and to line 94. Another line 96 interconnects line 94 and tilt selector control 36. An outlet line 98 connects tilt selector control 36 to a reservoir (not illustrated). A bypass line 100 is connected to the annular sides 66 of pistons 50 via line 68 and to

lines

94 and 96. Line 100 incorporates a nonretum check valve 102 through which fluid may pass only from the

lines

94 and 96 to line 100.

Fore and aft stops 104 and 106 are respectively located on annular side 66 and full area side 48 of each piston 50. In each assembly 28, stops 104 and 106

seal lines

68 and 46, respectively, when piston 50 approaches the respective end of the cylinder. The sealing of the respective line prevents the piston from bottoming, i.e., contacting the end of the cylinder, and maintains a quantity of fluid under pressure between the piston and the end of the cylinder.

Lines

54, 56, 72 and 74 are not sealed by stops 104 and 106.

To tilt mast l8 fore of the vertical position, tilt selector 36 is moved to a position connecting line 44 with line 46 and line 96 with line 98. Fluid contacting full area sides 48 of pistons 50 forces the pistons to the left in FIG. 2. The fluid at annular sides 66 of pistons 50 flows through line 68, line 70, peripheral chamber 90, assuming spool 88 is located in the normal position, line 94, line 96, tilt selector 36, line 98 and into the reservoir. In tilting mast l8 aft of the vertical position, tilt selector 36 is moved to connect line 44 with line 96 and line 98 with line 46. fluid flowing through line 96 is introduced through line 68 to annular sides 66 of pistons 50 via line 94, peripheral chamber 90, and line 70 assuming spool 88 is located in the normal position, or otherwise via nonretum check valve 102 and line 100. Fluid exits from full area sides 48 of pistons 50 via line 46, tilt selector 36 and line 98 to the reservoir.

The pressure of the fluid at annular sides 66 of pistons 50 acts through

lines

72 and 74 on shuttle valve 76 forcing ball 108 located therein to seal the line transmitting the lowest fluid pressure and permit the line transmitting the highest fluid pressure to communicate via the center tap, line 80, and chamber 78 of tilt limiting valve 64 with side 92 of spool 88. The pressure of the fluid at full area sides 48 is transmitted via

lines

54 and 56 to shuttle valve 58. The line with the highest fluid pressure will force ball 110 in shuttle valve 58 to seal the line having the lowest fluid pressure. The highest fluid pressure existing on the sides 48 will thereby be transmitted, via the center tap, line 60 and chamber 62 to the full area side 84 of piston 82. Movement of spool 88 from the normal position to a blocking position where peripheral chamber 90 is out of register with lines and 94 occurs when the force exerted on spool 88 by the pressure of the fluid in chamber 78 exceeds the combined force exerted on spool 88 by the pressure of the fluid in chamber 62 and the force of spring 86. The rate and preload of spring 86 is chosen so that peripheral chamber is out of register with

lines

70 and 94 when the moment tending to overturn truck I0 is slightly less than the moment needed to overturn the truck.

When peripheral chamber 90 moves out of register with

lines

70 and 94, the escape of fluid from annular sides 66 of pistons 50 is prevented thereby preventing piston-cylinder assemblies 28 from further extension and fore movement of mast 18 and accordingly preventing an increase in the moment. When spool 88 is in the blocking position, piston-cylinder assemblies 28 can be released for contraction and aft movement of mast 18 by connecting lines 44 with line 96 and line 46 with line 98. Such connection permits fluid to be forced through line 96, past one-way check valve 102, and then through lines and 68 to annular sides 66 of pistons 50. If communication between

lines

70 and 94 has not been closed by spool 88 then fluid may be pumped to annular sides 66 of pistons 50 through peripheral chamber 90 and line 68.

The use of shuttle check valves 58 and 76 in sensing the fluid pressure at the annular and/or full area sides of tandem operating piston-cylinder assemblies 28 results in the transmittal of the higher of the two fluid pressures from each of the respective sides of pistons 50 to tilt limiting valve 64. This operation allows spool 88 to operate properly in the event of: improper loading of the mast; failure of one of the stops 104 and 106 in each assembly 28, or one 'of the

lines

54 and 56 or one of the

lines

72 and 74.

Although shuttle valves 58 and 76 are illustrated at each end of piston-cylinder assemblies 28, shuttle valve 58 can be eliminated if forward overturning of truck 10 is the only consideration. When a forward overturning moment exists on truck 10, relatively high fluid pressures exist at annular sides 66 of pistons 50 and relatively low pressures exist at full area sides 48 of the pistons. The highest fluid pressure existing at annular sides 66 of the pistons must be fed to side 92 of spool 88 along with a low pressure from full area sides 48 of the pistons to side 84 of piston 82 in order to prevent forward overturning. If the pressure of the fluid at side 84 is nonrepresentative of the tensional forces on rods 26 premature movement of spool 88 will occur. Although such premature movement will affect vehicle productivity it will not result in forward overturning of lift truck 10.

Both piston-cylinder assemblies 28 are monitored through shuttle check valves 58 and 76. The shuttle check valves always communicate the higher of the fluid pressures being sensed from the given side of the piston to the tilt-limiting. Thus, improperly adjusted piston rods 26 are of no consequence to the proper functioning of the valve 64. This is of major importance after the piston-cylinder assemblies 28 have been repaired, removed or disconnected from the mast for any reason. Further, in the absence of one of the seals 104 and 106 making contact with the cylinders to seal one of the

lines

68 and 46, the pressures of the fluids'at the corresponding annular sides and at the corresponding full area sides of the pistons are equalized via the

respective lines

68 and 46. Once one of the

lines

68 or 46 is sealed by one of the seals 104 or 106, respectively, the pressures of the fluids in the corresponding sides having the sealed line are no longer equalized. It is at this point in the operation of lift truck 10, i.e., when one or more seals cover either

line

68 or 46, that shuttle check valves 76 and 58 see their greatest usage and benefit in the illustrated embodiment.

I claim: 1. An overload protection device for a vehicle having a frame, load carriage mounted for substantially vertical movement on a mast which is pivotally mounted on said frame for tilting motion relative thereto and'two motors operatively connected between said frame and said mast to tilt said mast, the overload protection device comprising:

means for generating a signal at each motor representative of the magnitude of the force on the motor needed to maintain the mast at its position relative to the frame; means for comparing the signals from each motor and for transmitting the signal representative of the greatest magnitude of force on the motors, means operativefor receiving the transmitted signal and for preventing tilting of the mast in at least one direction relative to the frame by preventing operation of the two motors in at least their corresponding direction when the magnatude of the tramsitted signal is above a given preselected magnitude. 2. An overload protection device for a vehicle having a frame, a load carriage mounted for movement on a mast pivotally mounted on the frame, first and second piston-cylinder assemblies each having a respective first and second piston, each piston-cylinder assembly having first and second ends connected to the frame and the mast, the assemblies being operative in response to a flow of fluid into the chamber at one end of each cylinder and out of the chamber at the other end of each cylinder to pivotally move the mast relative to the frame, and a fluid circuit for providing a flow of fluid to the chamber at one end of each cylinder and for collecting fluid from the chamber at the other end of each cylinder, the overload protection device comprismg:

first means for transmitting a signal representative of the highest pressure of the fluids in the chambers at the first ends of the cylinders to the operative means; and second means for transmitting a signal representative of the pressure of the fluid in the chamber at the second end of at least one of the cylinders to the operative means and means operative in response to'the magnitude of the difference between the two signals for preventing pivotal movement of the mast'in at least one direction by preventing movement of the first and second pistons in the corresponding direction. 3. An overload protection device according to claim 2 wherein the first means continuously monitors the pressure of the fluids in the chambers at the first ends of the cylinders.

4. An overload protection device according to claim 2 wherein the second means transmits a signal representative of the highest pressure of the fluids in the chambers at the second ends of the cylinders to the operative means.

5. An overload protection device according to claim 4 wherein the first and second means continuously monitor the pressures of the fluids in the chambers respectively at the first and second ends of the cylinders.

6. An overload protection device for a vehicle having a frame, a load carriage mounted on a mast pivotally mounted on the frame, a fluid transmitting circuit having a fluid outlet and a fluid inlet, the overload protection device comprising:

a first cylinder having first and second ends and a chamber;

a first piston located within the chamber in the first cylinder and movable between the first and second ends;

a second cylinder having first and second ends and a chamber;

a second piston located within the chamber in the second cylinder and movable between the first and second ends;

means adapted for connecting the cylinders and the pistons to the frame and the mast to pivotally move the mast relative to the frame in response to movement of the first and second pistons relative to the first and second cylinders;

means adapted for connecting the fluid inlet of the circuit to the first end of each of the cylinders;

means adapted for connecting the fluid outlet of the circuit to the second end of each of the cylinders; first means to produce a signal representative of highest pressure of the fluids at the first ends of the cylinders and second means to produce a signal from the second ends of the cylinders means operative in response to the magnitude of the difference between the two signals for preventing pivotal movement of the mast in at least one direction by preventing movement of the first and second pistons in the corresponding direction.

7. An overload protection device according to claim 6 wherein the second means transmits the highest pressure of the fluids at the second ends of the cylinders to the operative means.

8. An overload protection device according to claim 6 wherein the first means for transmitting continuously monitors the pressure of the fluids at the first ends of the cylinders.

9. An overload protection device according to claim 8 wherein the first means for transmitting includes:

means defining first and second passages each having one end communicating with the first end respectively of the first and second cylinders and another end;

a valve having first and second ends respectively communicating with the other end of the first and second passages;

means in the valve operative in response to a difference in the pressure of the fluids in the first and second passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure with an exit passage; and

means defining a passage communicatingwith the exit passage and the operative means.

10. An overload protection device according to claim 9 further including a second means for continuously monitoring the highest pressure of the fluids at the second ends of the cylinders and transmitting it to the operative means comprising:

means defining third and fourth passages each having one end communicating with the second end respectively of the first and second cylinders and another end;

a valve having first and second ends respectively communicating with the other end of the third and fourth passages;

means in the valve operative in response to a difference in pressure of the fluids in the third and fourth passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure to an exit passage; and

means defining a passage communicating with the exit passage and the operative means.

11. An overload protection device for a vehicle having a frame, a load carriage mounted on the frame for substantially vertical movement on a mast which is pivotally mounted on the frame a fluid transmitting circuit having a fluid outlet and a fluid inlet, the overload protection device comprising:

a first cylinder having first and second ends and a chamber;

a second cylinder having first and second ends and a chamber;

means adapted for connecting the cylinders and the pistons to the frame and the mast to move the mast relative to the frame in response to movement of the pistons relative to the cylinders;

means adapted for connecting the fluid outlet of the circuit to the second end of each of the cylinders;

means operative in response to a given difference in pressure between two sources representative of the pressure differential across the-first and second cylinders for preventing at least certain tilting movement of the mast relative to the frame;

first means for continuously monitoring the highest pressure of the fluids at the first ends of the cylinders and transmitting it to the operative means including:

a. means defining first and second passages each having one end communicating with the first end respectively of the first and second cylinders and another end;

b. a first valve having first and second ends respectively communicating with the other end of the first and second passages;

c. means defining a first exit passage communicating the first valve with the operative means;

d. means in the first valve operative in response to a difference in the pressure of the fluids in the first and second passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure with the first exit passage; and

e. means disposed on the first and second pistons preventing movement of the pistons to axial positions effecting the sealing of the first and second passages at the first ends of the cylinders; and

second means for continuously monitoring the highest pressure of the fluids at the second ends of the cylinders and transmitting it to the operative means including:

a. means defining third and fourth passages each having one end communicating with the second end respectively of the first and second cylinders and another end;

b. a second valve having first and second ends respectively communicating with the other end of the third and fourth passages;

c. means defining a second exit passage communicating with the second valve and the operative means;

d. means in the second valve operative in response to a difference in pressure of the fluids in the third and fourth passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure to the second exit passage; and

e. means disposed on the first and second pistons preventing movement of the pistons to axial positions effecting the sealing of the third and fourth passages at the second ends of the cylinders.

Claims

1. An overload protection device for a vehicle having a frame, load carriage mounted for substantially vertical movement on a mast which is pivotally mounted on said frame for tilting motion reLative thereto and two motors operatively connected between said frame and said mast to tilt said mast, the overload protection device comprising: means for generating a signal at each motor representative of the magnitude of the force on the motor needed to maintain the mast at its position relative to the frame; means for comparing the signals from each motor and for transmitting the signal representative of the greatest magnitude of force on the motors, means operative for receiving the transmitted signal and for preventing tilting of the mast in at least one direction relative to the frame by preventing operation of the two motors in at least their corresponding direction when the magnatude of the tramsitted signal is above a given preselected magnitude.

2. An overload protection device for a vehicle having a frame, a load carriage mounted for movement on a mast pivotally mounted on the frame, first and second piston-cylinder assemblies each having a respective first and second piston, each piston-cylinder assembly having first and second ends connected to the frame and the mast, the assemblies being operative in response to a flow of fluid into the chamber at one end of each cylinder and out of the chamber at the other end of each cylinder to pivotally move the mast relative to the frame, and a fluid circuit for providing a flow of fluid to the chamber at one end of each cylinder and for collecting fluid from the chamber at the other end of each cylinder, the overload protection device comprising: first means for transmitting a signal representative of the highest pressure of the fluids in the chambers at the first ends of the cylinders to the operative means; and second means for transmitting a signal representative of the pressure of the fluid in the chamber at the second end of at least one of the cylinders to the operative means and means operative in response to the magnitude of the difference between the two signals for preventing pivotal movement of the mast in at least one direction by preventing movement of the first and second pistons in the corresponding direction.

3. An overload protection device according to claim 2 wherein the first means continuously monitors the pressure of the fluids in the chambers at the first ends of the cylinders.

6. An overload protection device for a vehicle having a frame, a load carriage mounted on a mast pivotally mounted on the frame, a fluid transmitting circuit having a fluid outlet and a fluid inlet, the overload protection device comprising: a first cylinder having first and second ends and a chamber; a first piston located within the chamber in the first cylinder and movable between the first and second ends; a second cylinder having first and second ends and a chamber; a second piston located within the chamber in the second cylinder and movable between the first and second ends; means adapted for connecting the cylinders and the pistons to the frame and the mast to pivotally move the mast relative to the frame in response to movement of the first and second pistons relative to the first and second cylinders; means adapted for connecting the fluid inlet of the circuit to the first end of each of the cylinders; means adapted for connecting the fluid outlet of the circuit to the second end of each of the cylinders; first means to produce a signal representative of highest pressure of the fluids at the first ends of the cylinders and second means to produce a signal from the second ends of the cylinders means opeRative in response to the magnitude of the difference between the two signals for preventing pivotal movement of the mast in at least one direction by preventing movement of the first and second pistons in the corresponding direction.

9. An overload protection device according to claim 8 wherein the first means for transmitting includes: means defining first and second passages each having one end communicating with the first end respectively of the first and second cylinders and another end; a valve having first and second ends respectively communicating with the other end of the first and second passages; means in the valve operative in response to a difference in the pressure of the fluids in the first and second passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure with an exit passage; and means defining a passage communicating with the exit passage and the operative means.

10. An overload protection device according to claim 9 further including a second means for continuously monitoring the highest pressure of the fluids at the second ends of the cylinders and transmitting it to the operative means comprising: means defining third and fourth passages each having one end communicating with the second end respectively of the first and second cylinders and another end; a valve having first and second ends respectively communicating with the other end of the third and fourth passages; means in the valve operative in response to a difference in pressure of the fluids in the third and fourth passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure to an exit passage; and means defining a passage communicating with the exit passage and the operative means.

11. An overload protection device for a vehicle having a frame, a load carriage mounted on the frame for substantially vertical movement on a mast which is pivotally mounted on the -frame, a fluid transmitting circuit having a fluid outlet and a fluid inlet, the overload protection device comprising: a first cylinder having first and second ends and a chamber; a first piston located within the chamber in the first cylinder and movable between the first and second ends; a second cylinder having first and second ends and a chamber; a second piston located within the chamber in the second cylinder and movable between the first and second ends; means adapted for connecting the cylinders and the pistons to the frame and the mast to move the mast relative to the frame in response to movement of the pistons relative to the cylinders; means adapted for connecting the fluid inlet of the circuit to the first end of each of the cylinders; means adapted for connecting the fluid outlet of the circuit to the second end of each of the cylinders; means operative in response to a given difference in pressure between two sources representative of the pressure differential across the first and second cylinders for preventing at least certain tilting movement of the mast relative to the frame; first means for continuously monitoring the highest pressure of the fluids at the first ends of the cylinders and transmitting it to the operative means including: a. means defining first and second passages each having one end communicating with the first end respectively of the first and second cylinders and another end; b. a first valve having first and second ends respectively communicating with the other end of the first and second passages; c. means defining a first exit passage communicating the first valve with the operative means; d. means in the first valve operative in response to a difference in the pressure of the fluids in the first and second passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure with the first exit passage; and e. means disposed on the first and second pistons preventing movement of the pistons to axial positions effecting the sealing of the first and second passages at the first ends of the cylinders; and second means for continuously monitoring the highest pressure of the fluids at the second ends of the cylinders and transmitting it to the operative means including: a. means defining third and fourth passages each having one end communicating with the second end respectively of the first and second cylinders and another end; b. a second valve having first and second ends respectively communicating with the other end of the third and fourth passages; c. means defining a second exit passage communicating with the second valve and the operative means; d. means in the second valve operative in response to a difference in pressure of the fluids in the third and fourth passages for sealing the passage having the lowest pressure and communicating the passage having the highest pressure to the second exit passage; and e. means disposed on the first and second pistons preventing movement of the pistons to axial positions effecting the sealing of the third and fourth passages at the second ends of the cylinders.