Галерея 3106108

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Галерея 3106108
Oct. 8, 1963 o. THOMA ETAL HYDRAULIC APPARATUS 2 Sheets-Sheet 1 Filed Sept. 15, 1960 FORWARD \REVERSE LOW REVEPSE D m W R O F INVENTORS 0 0A 1. 7740M Ca wARn V- WARD ATTORNEYS Oct. 8, 1963 THOMA ETAL 3,106,108
HYDRAULIC APPARATUS Filed Sept. 15, 1960 2 Sheets-Sheet 2 FORWA P D REVERSE ATTORNEYS United States Patent Ofitice Patented Oct. 8, 1963 HYDRAUMQ APPARATUS Oswald Thoma and Edward V. Ward, Cheltenharn, lingland, assignors to Dowty Hydraulic Units Limited,
Tewkesbury, England, a British company Filed Sept. 15, 1960, Ser. No. 56,286 Claims priority, application Great Britain ept. 17, 1959 6 Claims. (Cl. 74-733) This invention relates to hydraulic apparatus, more particularly to hydrostatic power transmission, having a variable positive displacement pump adapted to be driven by a power source, such as a prime mover, and a positive displacement motor fed with hydraulic liquid from the pump, the motor being adapted for mechanical connection to drive the load. Such a hydrostatic transmission has a speed ratio range within which infinite speed variation is possible, extending from zero at zero pump displacement to a maximum at maximum pump displacement. Torque multiplication varies inversely with speed ratio over the majority of the speed ratio range where the transmission is efiicient, but at low speed ratios it is usually found that torque multiplication cannot increase above a certain value. This is due to the fact that high torque multiplication demands high hydraulic pressures in the transmission, which must necessarily be limited to a maximum value for safety, and that at such high pressures and low pump displacements leakage of hydraulic liquid considerably reduces transmission efliciency. These difiiculties occur in hydrostatic transmissions whether the motor is of fixed or variable positive displacement.
The object of the present invention is to extend, in a simple mechanical way, the effective torque multiplication range of a hydrostatic transmission.
In a hydrostatic power transmission, including a vari able positive displacement pump and a positive displacement motor hydraulically interconnected one with the other, the present invention is characterized by the provision of a mechanical change speed gear having at least two fixed gear ratios located between the motor and the load, and a synchronizing device operable during change in mechanical speed gear to adjust pump displacement to cause the motor to rotate at a speed appropriate to the rotational speed of the load through the gear to be engaged. In this way, smooth and effective changing of the mechanical gear is possible. The mechanical gear may be adjustable into any of its gear ratios by direct mechanical engagement between the gears or rotating members attached to the gears. Alternatively, the gear ratios may be selected by engaging one or other of a number of frictional clutches. This latter method is preferred, since it eliminates the problem of engaging relatively moving gears or dog clutches and, when combined with the synchronizing device of the invention, enables particularly smooth gear changing to be efiected.
The invention is of particular use where the load has a substantial inertia such that its speed is not likely to change substantially in the short period of time involved in changing the mechanical gear. A typical example of a load having substantially inertia is a vehicle propelled through the medium of the transmission, e.g. an earth mover. In order to make the changing of mechancial gear still smoother, it is further proposed, in accordance with this invention, to provide an unloading valve in the transmission which is so connected with the synchronizing device as to unload the transmission when the pressure difference exceeds a predetermined value at the period inbetween the engagement of alternative gears. Such partialunloading will allow the motor to rotate in response to hydraulic delivery from the pump without being able to exert any appreciable torque. Closing of the unloading valve occurs only when one or other mechanical gear has been engaged. The speed ratio of the transmission and the mechanical gear may be controlled by separate controls. It is also within the scope of the invention to combine the speed ratio adjustment and the mechanical gear for control from one lever, which determines the overall speed ratio and torque ratio of the transmission and mechanical gear box.
The synchronizing device may comprise a lever mechanism interposed between the speed ratio control of the transmission and the pump displacement control, arranged to vary the leverage during mechanical gear change in such manner as to alter pump displacement in accordance with the change in mechanical gear. Such lever mechanism may comprise a lever having an adjustable fulcrum or, alternatively, a lever of variable length. Where a single control of both speed ratio and mechanical gear is provided, one or more servomotors are arranged to operate simultaneously to change the mechanical gear and to adjust the lever mechanism.
It is further within the scope of this invention for the synchronizing device to comprise a friction clutch associated with each fixed mechanical gear ratio, a servomotor arranged for overriding control of pump displacement and valve means arranged to connect said servomotor to respond to transmission hydraulic pressure when a mechanical gear is being engaged, the friction clutch corresponding to the gear to be engaged exerting a torque on the motor to generate pressure in the transmission, the servo motor being responsive to said pressure to act on pump displacement in an overriding sense to tend to reduce the pressure to a minimum, thus to cause synchronization of the motor speed with the speed of the load through the gear ratio to be engaged.
Four examples of the invention will now be described with reference to the accompanying drawings, in which:
FIGURE 1 is a diagrammatic representation of the first example, 7
FIGURE 2 is a diagrammatic representation of an alternative mechanical gear box for inclusion in FIG- URE 1, and:
FIGURES 3 and 4 are diagrammatic representations of the third and fourth examples.
Reference is made initially to FIGURE 1 of the drawings. In this example, the transmission comprises a reversible variable positive displacement pump 1, a fixed positive displacement hydraulic motor 2 and a pair of pipelines 3 and 4 carrying liquid in opposite directions between pump and motor. Conventional means, not shown, are also provided for maintaining the transmission pump motor and'the pipelines primed with liquid. Ad-
justment of displacement of the pump 1 is effected by the gear box 9. An output shaft 11 extending from gear box 9 extends to drive the load. Within the gear box 9 a pinion 12 is fixedly secured to the shaft 8 and is con- .stantly in mesh with a larger gear 13 mounted for rotation on a secondary shaft 10 carried by suitable bearings within the gear box. The gear 13 through shaft 10 drives a further gear 14 of smaller size, which is in mesh with a larger gear 15 freely mounted on the output shaft 11, which extends from the gear 12 through gear 15. In between the two gears 12 and 15 the shaft 11 is splined as at 16 for reception of a dog clutch element 17 which is slidably mounted on the spline 16 and is engageable with dog teeth 18 and 19 formed respectively on the gears 12 and 15. The gear box 3, as shown, is a conventional gear box which is arranged when the dog clutch element 17 engages dog teeth 18 to cause rotational power from the shaft 8 to be transmitted directly to the shaft 11 whilst, when the dog clutch 17 engages dog teeth 19 the power is transmitted through gears 12, 13, 14 and to the shaft 11 rotating this latter at a lower speed than the shaft 8. If the hydrostatic transmission has a useful speed ratio range of 3:1 over which torque ratio varies efficiently inversely to the speed ratio, it is then desirable that the ratio between the two gear ratios in the gear box should also be 3: 1. For controlling movement of the dog clutch 17, a lever 21 is provided pivoted by a fulcrum 22 within the box 9. The lever 21 is extended for convenient operation by the operator of the transmission.
In order to adjust speed ratio of the transmission, a further control lever 22 is provided pivoted at fulcrum 23. Extension 24 of this lever carries a link 25 by means of pivot 26. Link 25 is pivotally connected at 27 to lever 28 which at its opposite end is pivotally connected to the servomotor control 6. Centrally of the length of the lever 23 a slot 29 is provided within which a fulcrum pin 31 is slidable. The fulcrum pin 31 is carried by a carrier 32, which is, in turn, slidably carried by engagement of pin 33 within a fixed slot 34. The carrier 32 is moved by means of a link 35 which is pivotally connected by pivot 36 to the gear change lever 21. An unloading valve 37 is connected across the transmission pipelines 3 and 4 and an operating lever 38 extending from valve 37 is pivotally connected to the rod 35. The valve 37 is so arranged that it opens to permit limited flow of liquid only at the centre of the range of movement of lever 38 but, at either limit of movement of lever 38, the valve 37 is closed. A clamp operated by lever 39 acts on rod 25 to clamp it in position when lever 39 is raised. A push rod 40 and cam 40a on link 35 raise lever 39 during movement of gear lever '21.
The whole transmission as described is suitable for power transmission between the engine and the ground engaging wheels of a vehicle. The engine is automatically governed to a constant speed. Assuming the transmission to be so installed, the vehicle driver when starting his vehicle will have lever 21 engaged in the low gear position and lever 22 at the central position corresponding to zero pump displacement. The rod 35 and the fulcrum carrier 32 will be at the right-hand extremity of their movement as shown in FIGURE 1. To start the vehicle, the driver will move the displacement control lever 22 to rock the lever 28 about its fulcrum pin 31 and so to give some displacement to pump 1, which will cause motor 2 to rotate output shaft 8. Such rotation is transferred through the reduced gear of gear box 9 to drive the output shaft 11 at a lower speed with a considerable torque. The vehicle will thus accelerate and at a certain speed, assuming the vehicle is running on the level, the driver will move the gear lever 21 counterclockwise to the high gear posit-ion. Such movement will cause rod 35 to move to the left, to displace the fulcrum pin 31 to the left, and so to reduce the displacement movement applied to servo control rod 6, thus reducing displacement of the pump 1. During movement of the gear lever 21 in between engagement of the two gears, lever 38 will be in its midposition and will allow flow of liquid between pipelines 3 and 4 if there is sufiicient pressure difierence. Because of such hydraulic unloading at this stage the motor exerts substantially no torque and the pressure differential between pipelines 3 and 4 is quite low. As displacement of the pump 1 is reduced, the pressure between pipelines 3 and 4 need only be suflicient to cause motor 2 to slow down to a speed corresponding to the displacement of pump 1. As the dog clutch 17 engages the dog teeth 18, the motor speed will have dropped to the extent that the speed of shaft 8 approximates the speed of shaft 11, and at the engagement of this dog clutch with teeth 18, lever 38 will move valve 37 to the closed position, thus enabling the motor again to exert torque. Also during movement of gear lever 21, red 25 and control lever 22 are locked by operation of clamp lever 39 by cam 4%. At either gear position of lever 21, earn 40a allows clamp lever 39 to move to the unlocked position. Power is now transmitted through the hydrostatic transmission employing the higher gear ratio of box 9. The vehicle speed may now be raised to any desired value as selected by the movement of lever 22. If the vehicle climbs a gradient the driver will necessarily move lever 22 towards a lower pump displacement position, to ensure that sufiicient torque is generated at the output shaft 11, so that the vehicle may climb the gradient. If the driver needs to pull lever 22 'close to the central position corresponding to a very small pump displacement, he will appreciate that the transmission is operating at its maximum torque capability and he will then move lever 21 from high to low gear position. Such movement will cause movement of fulcrum 31 to its [right-hand position increasing a selected pump displacement so that motor 2 rotates at a high speed at the actual instant of engagement of the lower gear by means of the dog clutch 17. The transmission can now operate within its range of efiicient torque conversion and the vehicle is able to climb the gradient. Gear changing and speed ratio changing operate similarly for reverse movement as selected by reverse movement of control lever 22.
If, when the vehicle is travelling at a higher speed in the higher gear, the driver attempts to engage the lower gear, the rod 6 will be moved to the maximum displacement position before the fulcrum pin 31 can reach the end of slot 29. Since under these conditions lever 28 is inclined to slot 34, it Will be seen that pins 31 and 33 will wedge in their slots at the full displacement position of rod 6 and the carrier 32 will be prevented from moving fully to the right to permit engagement of low gear. The driver will thus realizethat engagement of low gear is not possible at that vehicle speed and that vehicle speed will need to be reduced for low gear engagement.
An alternative to the mechanical engagement of moving members in the gear box to cause engagement of the gears is to use frictional engagement members, as shown by way of example in FIGURE 2. in this arrangement, the motor shaft '8 is connectable by means of a friction clutch 41a with the output shaft 11, in order to engage the higher speed drive. For the lower speed drive an epicyclic reduction gear is provided comprising a pair of epicyclic gears 42 and 43, rotatably mounted on a carrier 44- driven by the shaft 8. The gear 42 engages with a gear 4-5 fixed by means of sleeve 46 to the output shaft 11, whilst the gear 43 engages a gear 47 formed on a sleeve 43- secured to a rotary drum &9. The drum 43' may be prevented from rotation by engagement with a friction band 51. The control lever for this transmission is arranged to cause alternative engagement of the clutch 41a, or of the friction band 51, corresponding to high and low speed ratios. Since there is no provision for engagement of moving gear or dog clutch teeth in this arrangement, it is always ensured that gear engagement is silent. However, the provision of the synchronizing device of the invention comprised by rod 35 and carrier 32, together with adjustable fulcrum 3'1, ensures that during the changing of mechanical gear the actual engagement of friction clutch 41a, or the friction band 51, can be effected without causing a jerk or a shock in the transmission. Other than this, operation is substantially as described with reference to FIGURE 1.
Reference is now made to FIGURE 3 of the accompanying drawings. Where possible, similar reference numerals will be used to those used in l. The hydrostatic transmission is again formed by the pump 1, the motor 2 and pipelines 3 and 4. The pump includes an input shaft 7 and the motor drives an output shaft 3 forming the input to a two-speed gear box 9 having an output shaft 11. An unloading valve 37 is also provided operated by lever 38. For control of the combined transmission, a single control lever 52 is provided movable within a gate from the central zero position giving forward speed in one direction and reverse in the opposite direction. The lever 52 is pivotally carried on a shaft 53. The lever 52 has mounted thereon a cylinder 54, within which a piston 55 and piston rod 56 are slidably mounted. The piston rod 56 extends in a direction away from the fulcrum 53 and is pivotally connected at its outer end 57 to a rod 58 which, in turn, is pivotally connected to the servo control rod 6 at position 59. The rod 58 is long in comparison with the stroke of the piston rod 56 and is arranged to extend substantially at right angles to the direction of movement of piston rod 56. The upper and lower ends of the cylinder 54 are fed with hydraulic liquid through flexible pipes 61 and 62.
The shaft 53 is rotatable with the lever 52 and it carries a cam 63 which co-operates with a follower 64. Follower 64- is spring loaded against the cam and controls a servo vent valve 65 enclosed in a casing 66. A pipe 67 extending from the casing 66 extends to the hydraulic reservoir at low pressure. A servo vent pipe 68 is controlled by the valve 65 and this pipe carries liquid from a pressure source 69 which may conveniently comprise the priming pump of the transmission. The supply of liquid from the source 69 to the vent pipe 68 takes place through a restrictor 71, the arrangement being that when the vent valve 65 is closed the pressure in pipe 68 is at the pressure within the source 69 whilst, when the valve 65 is open, flow takes place from the source through restrictor 71 and valve 65, causing a substantial pressure drop within pipe 68. The pipe 68 is connected by flexible pipe 61 to the upper end of the cylinder 54. The pipe 62 extending from the lower end of cylinder 64 is connected to the full pressure from the source 69 which flows through pipe 72. For the purpose of changing the mechanical gear, a further servo piston 73 is provided, slidably mounted in servo cylinder 74, piston rod 75 extending from the piston through the cylinder. Piston rod 75 is pivotally connected at 39a to the unloading valve lever 38 and at pivot 76 to the gear box lever 21. Pipe 70 extends from pipe 68 to the upper end of cylinder 74 whilst pipe 72 carrying full pressure from the source 69 extends to the lower end of cylinder 74. The two servo piston and cylinder units, 54, 55 and 73, 74 are both of the differential area type and the full upper areas of the pistons are acted upon by pressure within pipe 68. In both cases a piston rod extends downwardly from the piston through the cylinder, leaving a smaller lower area of the piston over which all pressure from the pipe 72 may act. Thus, when the valve 65 is open and pressure reduced in the pipe 68, both piston 73 and 55 will move to the upper ends of their cylinders whilst, when the valve 65 is closed and the pressure in pipe 68 is that of the source 69, both pistons will move to the lower end of their cylinders. The cam 63 over the central part of its surface is of circular form as indicated at '78, struck about the centre of shaft 53. The two outer portions 79 of the cam surface are also circular and are struck at a slightly smaller radius about the centre of shaft 53. Thus, over the central part of the movement 52, the follower 64 is depressed to close valve 65 and at the two extremes of movement of the lever 52 the valve 65 is lifted to vent pipe 68 to low pressure.
The transmission as described is intended for the propulsion of a vehicle by transmitting power from an engine to the ground engaging wheels. The engine is governor controlled to a constant speed and the driver merely needs to operate the lever 52 to control the speed of vehicle movement, and whether forward or reverse. At the central position of the lever, valve 65 is closed and pistons 55 and 73 are at their lowermost position, which means that low gear is engaged in gear box 9' by lever 21 and the pump 1 is at zero displacement. Slight counterclockwise movement of the lever 52 to move the vehicle, for example, forwardly, results in movement to the right of the lower end 57 of piston rod 56 moving the servo control rod
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