HYDROSTATIC TRANSMISSION WITH MECHANICAL LIMIT STOP

Abstract: The hydrostatic transmission (6) has a mechanical limit stop (92) located entirely within hydraulic manifold (90). The limit stop (92) includes a shaft (104) which rotates in direct relation to a motor output shaft (44) of the hydrostatic transmission (6) and causes movement of a sleeve (112) having stops (114, 116) engageable with the wobbler (38) of a variable displacement pump (10) of the hydrostatic transmission to assure return of the wobbler to a neutral or zero position when the driven member (80) has reached a desired limit position and the electro-hydraulic servo valve (50) fails to operate to return the wobbler (38) to neutral position.

Description: HYDROSTATIC TRANSMISSION WITH MECHANICAL LIMIT STOP Description Technical Field This invention pertains to a hydrostatic transmission operable to drive a member to a desired position and which has a mechanical limit stop operable to stop the drive by the hydrostatic transmission in the event the control for the hydrostatic transmission fails. More particularly, the pump and motor com¬ ponents of the hydrostatic transmission, as well as the hydraulic components of the control therefor, are po¬ sitioned in a hydraulic manifold and the mechanical limit stop is also entirely within the hydraulic manifold to eliminate a leakage interface which exists when a mechanical limit stop has parts both inside and outside of the hydraulic manifold. Background Art Some systems that utilize a hydrostatic transmission to drive a driven member to a desired position require a limit stop at the end of travel of the driven member. The limit stop function can be accomplished by either electrical or mechanical means and, when the driven member has reached the desired position, the limit stop operates to adjust the dis¬ placement of the wobbler of the variable displace¬ ment pump of the hydrostatic transmission to a zero position and there is resultingly no further move¬ ment of the driven member. One method of achieving the control of the drive of the hydrostatic transmission is through an electro-hydraulic servo control which has an input provided by an error signal representing the dif¬ ference between a commanded position of the driven member and an actual position thereof and, as the error signal diminishes to zero, the wobbler of the variable displacement pump is brought to a zero position to stop the drive. Although such a con¬ trol is a normally acceptable limit stop means, it is ineffective to stop the drive if there should be a failure in the electro-hydraulic servo valve. The illet Patent No. 2,381,795 discloses a hydrostatic transmission and control having a variable displacement pump. The displacement of the pump is set by the position of a servo control piston. Feedback means including a rotatable shaft responsive to rotation of the motor of the hydro¬ static transmission causes one or the other of a. pair of members to act on linkage connected to the servo control piston to provide a mechanical feedback of actual position of a driven member whereby the pump will be brought to zero displace¬ ment when the desired position of the driven member is achieved. The Swartz et al Patent No. 2,601,760 discloses a hydrostatic transmission wherein the control of the displacement of the variable dis¬ placement pump includes feedback means for bringing the pump displacement to a zero position when the driven member reaches the desired position. The foregoing patents have the mechani¬ cal structures operable as a mechanical feedback signal which nulls out the input signal to the control for the hydrostatic transmission. These patents do not show a mechanical limit stop which is independent of the primary control of the hydrostatic transmis- sion as provided by an electro-hydraulic servo valve. The foregoing prior art has not had a me¬ chanical limit stop operable independently of the primary control for the hydrostatic transmis- sion and which is totally integrated inside the hydraulic ' manifold having the hydraulic components to eliminate a leakage interface where flexible drive shafting between the variable displacement pump control piston and the driven member enters the hydraulic manifold as well as minimizing instal¬ lation time and rigging. Disclosure of the Invention A primary feature of the invention is to provide a hydrostatic transmission with a control there¬ for for driving a driven member to a desired position and with a mechanical limit stop effective in the event of a failure in the primary control for the hydrostatic transmission to bring the hydrostatic transmission to a stop position when the driven member reaches the de¬ sired position. The mechanical limit stop is located within a hydraulic manifold housing the hydraulic components of the transmission and control whereby the structural association is simplified and there is no leakage interface at the hydraulic manifold because of the use of a mechanical limit stop since the mechanical limit stop is entirely within the hydraulic manifold. The hydrostatic transmission and control has a variable displacement pump in closed circuit with a motor having an output shaft for driving a driven mem¬ ber to a desired position and an electro-hydraulic servo valve for controlling the position of a control piston connected to the wobbler of the variable dis¬ placement pump. These components are located within a hydraulic manifold. The electro-hydraulic servo valve responds to an error signal generated as a difference between a commanded position of the driven member and the actual position thereof for controlling the posi¬ tion of the wobbler whereby when the driven member reaches a desired position, the wobbler is brought to a neutral or zero position. The mechanical limit stop is positioned within the hydraulic manifold and has a member movable in a path of movement with positions directly related to the positions of the driven member. In the event the electro-hydraulic servo valve fails to stop the driven member within the proper limir position, the movable member will have engaged the wobbler of the variable displacement pump and moved the wobbler to a neutral position to stop the drive of the driven member. The movable member and associated structure are mounted entirely within the hydraulic manifold. More particularly, the movable member of the mechanical limit stop is mounted for linear movement on a rotatable shaft which is gearingly connected to the output shaft of the motor and has a loose connection to the wobbler at a side of the pivot axis of the wobbler which is opposite to the connection of the wobbler to a control piston which is under the control of the electro-hydraulic servo valve. In normal opera¬ tion, the electro-hydraulic servo valve brings the control piston to a position to achieve a neutral po¬ sition of the wobbler as the driven member reaches the desired position; however, if such does not occur be- cause of a failure in the electro-hydraulic servo valve, the movable member of the mechanical limit stop will have been moved to a position to effect the required movement of the wobbler to stop the hydrostatic trans¬ mission. An object of the invention is to provide a new and improved hydrostatic transmission and control with a mechanical limit stop to stop the drive of ^ the hydrostatic transmission when a failure in the control of the hydrostatic transmission renders the control ineffective to stop the hydrostatic transmission at the limit of movement of a member driven by the hydro¬ static transmission. Another object of the invention is to provide a hydrostatic transmission and control and a mechani- cal limit stop with all of the hydraulic components and the mechanical limit stop positioned within a hydraulic manifold to avoid a potential leakage interface, as would result from use of a previously known mechanical limit stop because of parts thereof being both inside and outside of the hydraulic manifold. Still another object of the invention is to provide a hydrostatic transmission having a variable displacement pump in closed circuit with a hydraulic motor, an electro-hydraulic servo valve associated with a control cylinder having a control piston con¬ nected to a wobbler of the variable displacement pump for positioning the wobbler to set the displacement of the pump, a control for generating an error signal dependent upon the difference between a commanded position and the actual position of a driven member driven by the hydrostatic transmission and which operates the electro-hydraulic servo valve, and a mechanical limit stop operable independently of the electro-hydraulic servo valve and the control piston for bringing the wobbler to a neutral position when the driven member reaches a limit position in the event the electro-hydraulic servo valve fails to do so. Another object of the invention is to pro- vide a mechanical limit stop for an electrically- controlled hydrostatic transmission which is mounted within a hydraulic manifold and drivingly connected to the output shaft of the hydrostatic transmission motor and selectively engageable with the wobbler of a variable displacement pump of the hydrostatic trans¬ mission to bring the wobbler to a neutral position after a predetermined amount of rotation of the motor output shaft which is indicative of movement of a driven member to a desired position in the event that the electrical control has failed to do so. Description of the Drawing The Figure is a schematic view of the hydro¬ static transmission and associated control and the mechanical limit stop associated therewith. Best Mode for Carrying Out the Invention The hydrostatic transmission 6 is of a conventional construction well known in the art and has a variable displacement pump 10 in closed circuit with a hydraulic motor 12 by means of a pair of connecting fluid lines 14 and 16. Also as well known in the art, a pair of over-pressure control valves 18 and 20 prevent over-pressure in the lines 16 and 14, respectively, by opening com¬ munication between the lines when there is an excessive pressure in one or the other of the lines. A charge pump 22 is connected to a fluid source by a line 24 and through a line 26 supplies make-up fluid to the fluid lines 14 and 16 through a pair of charge check valves 28 and 30 which assure that the fluid line functioning as the return line from the motor to the pump is full of fluid at a pressure at least equal to charge pressure. Addi¬ tionally, there is a hydraulic release valve 32 to permit fluid to circulate in an open circuit during manual backdriving of the hydraulic transmission, as more fully described hereinafter. The variable displacement pump 10 is of a well-known construction and has a rotatable cylinder block (not shown) driven by an input shaft 34 which is driven by an induction motor 36 which also drives a charge pump 22. The cylinder block of the pump carries a circularly-arranged array of axially-reciprocal pistons with their stroke being under the control of a specially- constructed wobbler 38 which can pivot about an axis identified at 40. A pair of the pistons are identified at 42 and the wobbler is shown in a displacement-establishing position wherein the pistons 42 are caused to reciprocate as the cylinder block is driven by the input shaft 34 for delivering fluid under pressure to the motor 12. The fixed displacement motor 12, also of a well-known construction, has a rotatable cylin¬ der block (not shown) connected to an output shaft 44 and the cylinder block carries a cir- cularly-arranged array of axially-reciprocal pistons, two of which are shown at 46, and which, coact with a fixed cam surface 48 which provides a reaction surface to fluid pressure acting on the pistons 46 whereby there is resultant rotation of the cylinder block of the motor and the output shaft 44. The induction motor 36 rotates in one direction only and the direction of rotation of the motor output shaft 44 is dependent upon the inclination of the wobbler 38 of the variable displacement pump. The wobbler 38 is shown to one side of a neutral position, with the neutral posi¬ tion having the wobbler 38 generally upright with resulting non-stroking of the pump pistons 42. In the position shown, the motor output shaft 44 will rotate in one direction and, with the wobbler 38 to the other side of neutral position, the motor output shaft 44 will rotate in the opposite direction. The servo control for setting the position of the wobbler 38 includes the electro-hydraulic servo valve, indicated generally at 50, which is supplied with control fluid by the charge pump 22 through a line 52 extending to an inlet 54 of the servo valve and with this line having- a filter and bypass valve 56 and a charge relief valve 58 connected to drain. The electro-hydraulic servo valve is of a construction well known in the art and may be of the type shown in Buraridt and Markunas Patent No. 4,487,109. An electrical signal is applied to coils which control the posi¬ tion of an armature for establishing a differential pressure in fluid lines 60 and 62 which extend to opposite ends of a control cylinder 64 having a, displacement-setting member in the form of a control piston 66 and a rod 68, with the latter being pivotally connected at 70 to an end of the wobbler 38. The electrical error signal applied to the coils of the electro-hydraulic servo 50 is derived from a signal-summing circuit 70 which receives an electrical command input through a line 72 and an actual-position electrical feedback signal through a line 74. The motor output shaft 44 has a gear 76 thereon which meshes with a gear 78 for driving a driven mem¬ ber, such as a ball screw 80, to a desired position. The desired position of the driven member 80 is re¬ lated to the amount of rotation of the motor output shaft 44 and this rotation is read by means of a position transducer 82 connected to the gear 76 through a gear 84 and which outputs the feedback signal to the signal-summing circuit 70. Any difference between the command input for the desired position of the driven member and the actual position thereof results in an error signal which is applied to the coils through a line 86. The fluid line 52, extending from the charge pump 22, also extends to a hydraulically-released brake 88 which automatically releases when pressure builds up at the electro-hydraulic servo valve 50 to permit ro¬ tation of the motor output shaft 44. This brake can also be manually released to permit the hydraulic motor to backdrive, with there being free fluid circulation in the closed loop of the hydrostatic transmission through opening of the previously-described hydraulic release valve 32. All of the foregoing hydraulic components are located within a hydraulic manifold, indicated generally by the broken line 90. The hydraulic manifold contains fluid, and the only potential for loss of hydraulic fluid from the manifold is across two dynamic carbon face seals (not shown) with one being for the pump input shaft 34 and the other being for the motor out¬ put shaft 44. In normal operation, the hydrostatic trans¬ mission operates as a result of a comman input through line 72, and the position transducer 82 feeds back to the signal-summing circuit 70 the actual position of the driven member 80. As the driven member 80 ap¬ proaches the final desired position, the error signal approaches zero and the electro-hydraulic servo valve 50 operates to reduce the differential pressure in the lines 60 and 62 to move the wobbler to neutral position and a zero error signal maintains the control piston 66 in a centered position and the wobbler 38 in a neutral position. There can be a hard over failure in the electro-hydraulic servo valve whereby normal operation to bring the wobbler to a neutral position does not occur. This can result in continued move¬ ment of the driven member 80 and resulting damage to the driven member and associated structure. In order to assure that driven member 80 does not move beyond a desired position, the hydrostatic transmission has a mechanical limit stop 92 asso- ~ ciated therewith and located entirely within the hydraulic manifold 90. The mechanical limit stop in¬ cludes a spur gear 94 on the motor output shaft 44 which drives a spur gear jack shaft 96 having a gear 98 in mesh with the gear 94 and a gear 100 meshing with a gear 102 on a rotatable shaft 904 rotatably supported by bearings 106 and 108. The shaft 104 is threaded along a section 110 adjacent one end thereof. A sleeve 112 is threaded onto the threaded section 110 of the rotatable shaft 104 and is associated with a yoke 115 at an end of the wobbler 38 opposite the pi-vot connection 70 which permits free movement of the sleeve 112 relative to the wobbler, but with the sleeve being prevented from, rotating by engagement with the yoke 115 of the wobbler. During normal opera¬ tion, the yoke at the end of the wobbler is free to move relative to the sleeve and a pin and slot con¬ nection between the yoke and the sleeve 112 permits this operation with a slot 113 in the wobbler. The sleeve has a pair of stops 114 and 116 at opposite ends thereof which may come into engagement with the wobbler yoke upon failure of the electro-hydraulic servo valve 50. With the parts as positioned as shown in the Figure, the motor output shaft 44 will be rotating in a direction to cause rotation of the shaft 104 of the mechanical limit stop in a direction to move the stop 114 on the sleeve toward the wobbler. If the control for the hydrostatic transmission should fail to move the wobbler to a neutral position as the driven member 80 approaches its desired position, the stop 114 will engage the wobbler and move the wobbler to neutral position as the driven member moves to its desired limit position. Limit switches, one of which is shown at 118, can signal that the limit position has been reached by being contacted by one of the sleeve stops. With the wobbler 38 brought to the neutral position, which would be an upright position as seen in the Figure, " the sleeve stop 114 would be positioned to the right from the position shown and would be in en- gagement with the wobbler yoke. Thus, the only subse¬ quent error signal applied to the electro-hydraulic servo valve which will cause positioning of the wobbler 38 is a signal to move the wobbler in a counterclock¬ wise direction about the axis 40. Any attempt to move the wobbler in a clockwise direction would be prevented by engagement with the sleeve stop 114. This assures that the driven member 80 will next be operated in the opposite direction of movement. The sleeve 112 is caused to move along a linear path of travel and have the stops thereof engage the wobbler at a side of the pivot axis 40 of the wobbler opposite from the pivot connection of the wobbler to the displacement-setting member 68. In an operation with the wobbler being moved in a counterclockwise direction, there will be oppo¬ site rotation of the motor output shaft 44 and movement of the sleeve 112 toward the left to bring the sleeve stop 116 into engagement with the wobbler to return the wobbler to neutral position in the event there is any failure in the electro-hydraulic servo valve 50. The movement imparted to the wobbler by the mechanical limit stop will bring the control piston 66 back to a neutral position as the wobbler is moved to neutral position. The mechanical limit stop provides a mechani¬ cal means to physically pull the pump wobbler to a zero or neutral position, regardless of the command of the electro-hydraulic servo valve when a driven member has reached a desired limit position. The mechanical limit stop is located entirely within the hydraulic manifold housing the hydraulic components of the system to avoid any passage of mechanical limit • stop structure through the wall of the hydraulic manifold and, thus, avoid a potential point of leakage.

Claims: Claims 1. A hydrostatic transmission and control having a hydraulic motor with an output shaft rotatable to drive a driven member to a desired position and a hydraulically-connected variable displacement pump having an adjustable displacement-setting wobbler, an electro-hydraulic servo valve, a control cylinder connected to said servo valve and having a control piston connected to said wobbler, a hydraulic manifold housing the aforementioned components, and a control ° which causes the electro-hydraulic servo valve to move the wobbler toward a neutral position as said driven member approaches a desired position; the improvement comprising: a mechanical limit stop within the hydraulic manifold including means moved in response to rotation of the motor output shaft and engageable with the wobbler to move the wobbler toward a neutral position when said control fails to do so. 2. A hydrostatic transmission and control having a hydraulic motor with a rotatable output shaft for driving a driven member to a desired position and a hydraulically-connected variable displacement pump having an adjustable displacement-setting wobbler, electrically operable control means including a control cylinder having a control piston connected to said wobbler, and a position control which causes the electrically-operable control means to move the wobbler toward a neutral position as said member approaches a desired position; the improvement comprising: a mechanical limit stop including means moved in response to rotation of the motor output shaft and engageable with the wobbler to move, the wobbler toward a neutral position when said position control fails to do so. 3. A hydrostatic transmission and control as defined in claim 2 wherein said electrically-operable control means further includes an electro-hydraulic servo valve connected to said control cylinder. 4. The hydrostatic transmission and control of claim 2 further including a hydraulic manifold having said pump, motor and electrically-operable con- trol means therein and said mechanical limit stop con¬ trol is also positioned in said hydraulic manifold. 5. The hydrostatic transmission and control of claim 2 wherein said means engageable with the wob¬ bler includes a movable member movable along a linear path, said wobbler is pivoted about an axis generally intermediate its ends, and said connection of the control piston thereto is at a side of said axis opposite the linear path of the movable member. 6. In combination, a hydrostatic transmission having an electro-hydraulic control and a mechanical limit stop control operable upon failure of the electro- hydraulic control, said hydrostatic transmission and electro-hydraulic control comprising: a hydraulic motor having an output shaft for driving a driven member to a desired position; a variable displacement pump in cir- σuit with the hydraulic motor and having an adjustable wobbler for setting pump displacement to control opera- tion of the hydraulic motor; a servo control for the variable displacement pump including a displacement- setting member connected to the wobbler and a servo valve for setting of wobbler position, and means for generating an error signal for the servo valve repre¬ senting an error between a commanded and an actual position of the driven member; and said mechanical limit stop control comprising: a movable member; means for moving said movable member through a stroke proportional to the desired amount of rotation of the motor output shaft to achieve the desired position of the driven member; and said wobbler having a part in the path of the movable member to be engaged there¬ by and be moved to a neutral position in the event the hydraulic motor continues to operate after a failure in said servo control. 7. The combination of claim 6 wherein said wobbler is pivoted about an axis generally intermediate its ends and said connection of the displacement-setting member thereto is at a side of said axis opposite the path of the movable member. 8. The combination of claim 6 wherein said means for moving the movable member includes a rotatable shaft and gearing operatively connecting said rotatable shaft to the motor output shaft, and said movable mem¬ ber being mounted onto said rotatable shaft for linear movement in response to rotation of the rotatable shaft. 9. The combination of claim 8 wherein said movable member has a sleeve threaded on said rotatable shaft and a raised surface selectively engageable with said part of the wobbler, and said part of the wobbler is a yoke in contact with the sleeve. 10. In combination, a hydrostatic transmis- sion having an electro-hydraulic control and a mechani¬ cal limit stop control operable upon failure of the electro-hydraulic control, said hydrostatic transmis¬ sion and electro-hydraulic control comprising: a hydraulic motor having an output shaft; a variable displacement pump in circuit with the hydraulic motor and having an adjustable wobbler for setting pump displacement to control operation of the hydraulic motor; a servo control for the variable displacement pump including a control cylinder having a control piston connected to the wobbler, an electro-hydraulic servo valve hydraulically communicating with the control cylinder to position the control piston for setting of wobbler position, and means for generating an error signal for the electro-hydraulic servo valve represent¬ ing an error between a command input and the actual ro- tational output from the motor; and said mechanical limit stop control comprising: a movable member; means for moving said movable member through a stroke proportional to the desired amount of rotation of the motor output shaft; and said wobbler having a part 'in the path of the movable member to be engaged thereby and be moved to a neutral position in the event the hydraulic motor continues to operate after a failure in said servo control. 11. The combination of claim 10 wherein said wobbler is pivoted about an axis generally intermediate its ends and said- connection of the control piston thereto is at a side of said axis opposite the path of the movable member. 12. The combination of claim 10 wherein said means of moving the movable member includes a rotatable shaft and gearing operatively connecting said rotatable shaft to the motor output shaft, and said movable mem¬ ber being mounted onto said rotatable shaft for linear movement in response to rotation of the rotatable shaft. 13. The combination of claim 12 wherein said movable member has a sleeve threaded on said rotatable shaft and a raised surface selectively en- gageable with with part of the wobbler, and said part of the wobbler is a yoke in contact with the sleeve. 14. The combination of claim 10 further ' including a hydraulic manifold having said pump, motor and servo control therein and said mechanical limit stop control is also positioned in said hydraulic manifold.

Inventors: BURANDT WESLEY, BURANDT, WESLEY
Applicants: SUNDSTRAND CORP, SUNDSTRAND CORPORATION
Application year: 1986
Main classification: F16D31
IPC codes: F16H61