GB2204386A

GB2204386A - Hydraulic transmission

Info

Publication number: GB2204386A
Application number: GB08710850A
Authority: GB
Inventors: Viljo K Valavaara
Original assignee: INVENTORS COMPENDIUM
Current assignee: INVENTORS COMPENDIUM
Priority date: 1987-05-07
Filing date: 1987-05-07
Publication date: 1988-11-09
Also published as: GB8710850D0

Abstract

An hydraulic variable speed transmission of the type having an input shaft (1) driving a hydraulic pump/motor unit, which in turn drives a second hydraulic pump/motor unit driving an output shaft (13), has pump/motor units of the swash plate type, at least one of which is of variable capacity. The pump/motor units have pistons moving in a common rotating cylinder barrel (10), within which normal fluid circulation through the units is confined. Valves in the units are controlled by rotating the pistons (9, 21) which are configured to interact with ports (31, 43, 33, 50) in the cylinder barrel to control fluid flow within the units. <IMAGE>

Description

Transmission This invention relates to integrated hydraulic variable speed transmissions of the type in which an input shaft drives a hydraulic pump/motor unit, which in turn drives a second hydraulic pump/motor unit driving an output shaft, at least one of the units being of variable capacity so as to change the transmission ratio.

Various proposals have been made for transmissions of this type, because they provide a gear ratio which is infinitely variable within its working range and is relatively easily controlled, but to the best of applicant's knowledge such integrated transmissions have not met with wide acceptance, two pr-incipal reasons being low efficiency and high cost. The low efficiency arises from the fact that in most arrangements there are substantial pumping and motor losses under normal running conditions, whilst under starting conditions the motor is operating at low speeds and thus low efficiency. Proposals have been made to overcome this problem by connecting one of the rotor and stator of the first unit with the input shaft and the other to the output shaft so that rotational velocity of the output shaft may be algebraically summed with that of the input shaft.This permits a direct drive condition in which neither pump nor motor is operating, and a starting condition in which the motor is rotating at the same rate as the input shaft but in the opposite direction so that the net output rotational velocity is zero, although both pump and motor are operating at efficient speeds. Such an arrangement is described in U. S. Patent No. 3,131,539 issued May 5, 1974 to Creighton et al. That transmission employs two axial piston swash plate pump/motor units. The input shaft is connected to the rotor of a first unit whose swash plate is mounted in the transmission casing for tilting to adjust the displacement of the unit. The swash plate of the second unit is connected to the output shaft whilst its rotor is connected to the input shaft.Under direct drive conditions, the swash plate of the first unit is set to adjust the displacement of the unit to zero, which also effectively locks the second unit to provide straight through drive without pumping losses. Under starting conditions when the input shaft is rotating and the output shaft is stationary the swash plate is set so that the displacement of the first unit equals the displacement of the second unit at the same speed, the valving between the units being arranged so that the rotor of the second unit rotates in the opposite direction relative to its swash plate so jhat the net output shaft velocity is zero.

As described by the patentees provision may also be made for reverse and overdrive operation.

A disadvantage of such arrangements is the bulk of the resulting unit, in which the two swash plate pump/ motor units are arranged in line with each other, separated by relatively complex stationary and rotating valve plates necessary to port fluid between the two.

We have now found that a much more compact arrangement can be achieved by employing a common element to form the rotor for the two pump/motor units, with separate sets of pistons engaging the swash plates of the two units, and using movements of the pistons in the rotor to perform the porting of fluid between the units.

According to the invention, there is provided an infinitely variable power transmission comprising a driving shaft, a driven shaft, a pair of hydrostatic units disposed in a closed hydraulic circuit, each unit having a rotor, fluid displacement elements carried by said rotor, means for controlling the displacement of the elements of each unit, the rotor of each unit being connected to said power input shaft, the displacement controlling means of one unit being held from rotating about the axis of said one unit and the displacement controlling means of the other unit being connected to said power output shaft, wherein the rotors are formed as an integral rotating unit containing fluid displacement elements of both rotors, and said closed hydraulic circuit is contained within said integral rotating unit.

Further details of the invention will be apparent from the following description of a preferred embodiment thereof with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through a transmission; and Figure 2 is a transverse section on hte line A-A in Figure 1.

Referring to the drawings, the transmission unit shown is suitable for use in a truck or other vehicle powered by an internal combustion engine coupled to an input shaft 1 of the transmission through a driver plate 2. A front housing 7 of the transmission is secured to the engine through a bell housing. An output shaft 13 of the transmission is supported in a rear housing 11 by a bearing 14 and in a barrel 10 by a further bearing 37.

The input shaft 1 is supported in a sleeve secured to the front housing and carrying an external sun gear 27, and is coupled to the rotating unit or barrel 10, which is supported by roller bearings 22 and restrained against axial motion by a thrust bearing 23. A first bearing plate 5 of a first hydrostatic unit, referred to for convenience as the pump, is carried by a thrust bearing 4 in a swash plate in the form of a yoke 24 supported in the front housing by a saddle bearing 25 so that the angle of inclination of the swash plate to the longitudinal axis of the transmissions may be controlled by links connecting the yoke to control pistons 8 housed in cylinders 38 formed in the front housing 7.A second bearing ring 17 of a second hydrostatic unit, referred to for convenience as the motor, is supported by a thrust bearing in a swash plate 12 keyed to the output shaft 13 and provided with a forward extension formed with an external sun gear 20.

The barrel 10 is formed with a ring of cylinders 39 and a ring of cylinders 40, both rings extending parallel to the axis of the input and output shafts. The cylinders 39 and 40 alternate, five cylinders 39 being associated with the pump, and five cylinders 40 with the motor. The cylinders 39 contain fluid displacement elements in the form of pistons 9 having ball and socket couplings to the first bearing ring 5, and the cylinders 40 contain fluid displacement elements in the form of pistons 21 having ball and socket couplings to the second bearing ring, so that the swash plates act as means to control the displacement of the pistons.Each piston 9 carries a planet gear 26 which remains in engagement with the sun gear 27 regardless of the axial position of the piston, and each piston 21 carries a planet gear 19 which remains in engagement with the sun gear 20 regardless of the axial position of the piston. The gears 26 and 27 and 19 and 20 are of the same diameter, so that for each revolution of the barrel 10, each piston 9 will complete a revolution in its cylinder 39, and for each revolution of the output shaft relative to the barrel, each piston 21 will complete a revolution in its cylinder 40.

Each cylinder 39 has an inlet port 31 commun1- cating with a chamber or manifold 41 within the barrel 10, and an outlet port 43 communicating through passages 48, 49 with a chamber or manifold 42 within the barrel 12. Each cylinder 40 has an inlet port 33 communicating with the chamber 2 and an outlet port 50 c':)mrThlnicating through passages 51, 36 with the chamber 41. The hydraulic circuit comprising the cylinders 39 and 40, the ports 3i, 43, 33 and 50 and the chambers 41 and 42 is cnarced with hydraulic fluid through a port 3 which communicates through a rotary union 44 and a passage 45 in the input shaft 1 with non-return valves 28 and 30 communicating respectively with the chambers 1 and 42.Relief valves 32 normally held closed by springs 34 are provided in a valve block separating the charters 41 and 42. These do not function during normal operation or the transmission but avoid development of excessive pressure differences between the chambers and excessive stress o the trarsmiss ion in circumstances such as sudden halting of the vehicle which might otherwise cause damage.

Each piston 9 and 21 is cut away on one side to form a recess 45 communicating with a port 46 in the head of the piston, and each cylinder is provided with a cylinder head 47 which also defines part of a passage 36 or 49 associated with an outlet port 50 or 43. The engagement of the gears 26 and 19 with the gears 27 and 20 is such that the angular position of each piston in in its cylinder is dependent upon the angular oisplac- ment of the barrel 10 relative te a reference point on the gears 27 and 20 and thus the swash plates 24 and 12. The angular position of the recess 45 in a piston is thus also dependent upon its angular position relative to the associated swash plate so that communications between the inlet and outlet ports and the cylinders through the ports 46 alternate according to which half of the diameter of the swash plate a particular cylinder is adjacent, such that the inlet port is open when the piston is moving outwardly and the outlet port is open when it is moving inwardly.

In operation, the unit is charged through the port 3 by an external hydraulic pump (not shown) from an external reservoir (not shown). The ratio of the transmission is controlled by differential admission or exhaust of fluid from the cylinders 38, thus tilting the swash plate 24, operation being generally similar to that described in U.S. Patent No. 3,151,539. If the swash plate 5 is tilted to a vertical position, no longitudinal motion will be imparted to the pistons 9 connected to it as the input shaft 1 rotates the barrel 10. As a result, there is no displacement of fluid in the system, there is no fluid delivered to the motor cylinders 40, and there is therefore no relative rotation of the output shaft 13 and swash plate 12 relative to the input shaft 1 and barrel 10.The transmission thus provides a straight through connection between the input and output shafts, and operates at high efficiency since there is no displacement of hydraulic fluid.

As the swash plate 24 is tilted towards the position shown in Figure 1, the stroke of the pistons 9 will increase, the timing of the gears 26 and 27 being arranged so that as the pistons move outwards, they draw fluid into the cylinders 39 from the chamber 41 through the ports 31, recesses 45 and ports 46, whilst during the inward movement of the piston and the fluid is forced into the chamber 42 tyhrough the ports 46 and 43 and the passages 48 and 49. Certain of the ports 33 associated with the pistons 21 will be open (those whose relation to the swash plate 12 is such that their movement is outwards), so that the pressurzied fluid from the chamber 42 urges these pistons 21 outwardly, thus rotating the swash plate 12 and producing a differential movement of the output shaft 13 relative to the input shaft 1, hence reducing the relative velocity of the former.The remaining inwardly moving pistons 21 will be expelling fluid through the ports 50 into the chamber 41, thus completing the circulatory path.

As the inclination of the swash plate increases, a point will be reached at which the displacement of the pistons 21 will equal that of the pistons 9. At this point, at which the inclination of the swash plates is equal assuming pistons of equal diameter, the relative motion between the output shaft and the barrel cancels the angular velocity of the input shaft so that the angular velocity of the output shaft is zero. Further inclination of the swash plate will produce reverse motion of the output shaft. By tilting the swash plate 24 beyond the vertical in the opposite direction, an overdrive effect can be obtained.

Under overrun or overdrive conditions, the pump and motor functions of the two sets of pistons are in fact reversed, thus providing engine braking.

The external control of the transmission through operation of the cylinders 38 may be effected manually or automatically. Reference may be made to our Canadian Patent No. 1,220,405 for further discussion of methods for control of this general type of hydrostatic transmission.

Claims

WE CLAIM:

1. An infinitely variable power transmission mechanism comprising a driving shaft, a driven shaft, a pair of hydrostatic units disposed in a closed hydraulic circuit, each unit having a rotor, fluid displacement elements carried by said rotor, means for controlling the displacement of the elements of each unit, the rotor of each unit being connected to said power input shaft, the displacement controlling means of one unit being held from rotating about the axis of said one unit and the displacement controlling means of the other unit being connected to said power output shaft, wherein the rotors are formed as an integral rotating unit containing the fluid displacement elements of both rotors, and said closed hydraulic circuit is contained within said integral rotating unit.

2. A transmission according to Claim 1, wherein the fluid displacement elements are pistons movable on axes parallel to the rotational axis of the rotating unit in cylinder defined in the latter, wherein the pistons forming the fluid displacement elements of each rotor are arranged in a ring around said rotational axis, and wherein the displacement controlling means are swash plates arranged at opposite ends of said rotating unit.

3. A transmission according to Claim 2, wherein that swash plate held against rotation has a variable inclination to the rotational axis of the rotating unit, whereby to vary the transmission ratio of said unit.

4. An infinitely variable power transmission mechanism constructed and arranged substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

4. A transmission according to Claim 2 or 3, wherein the rotating unit defines two manifolds, and further including fluid porting means between said cylinders and said manifolds, whereby cylinders of the one unit are in communication with one of said manifolds only when their associated pistons are moving axially outwards under control of one swash plate, and cylinders of the other unit are in communication with the other of said manifolds only when their associated pistons are moving axially outwards under control of the other swash plate.

5. A mechanism accdrding to Claim 4, wherein the fluid porting means include cooperating ports formed in the pistons and the cylinders, and means to rotate the pistons in the cylinders in accordance with their angular relationship to the associated swash plate.

6. A mechanism according to Claim 5, wherein the means to rotate the pistons includes planet gears mounted on the pistons and cooperating with sun gears which have an agularly fixed relationship to the swash plate associated with the pistons whose planet gears they engage.

Amendments to the claims have been filed as follows CLAIMS

1. An infinitely variable power transmission mechanism comprising an integral rotating unit having a rotational axis, a driving shaft connected to the rotating unit, a driven shaft, and a pair of'hydraulic units associated with the rotating unit and each having aswash plate and pistons movable on axes parallel to the said rotational axis in cylinders defined in the rotating unit, the two swash plates being arranged at opposite ends of the rotating unit for controlling the displacement of the pistons of each hydraulic unit, the swash plate of one hydraulic unit being held from rotating about the axis of said one hydraulic unit and the swash plate of the other hydraulic unit being connected to said driven shaft, the rotating unit defining two manifolds and fluid porting means between the cylinders and the manifolds to provide a closed hydraulic circuit within the rotating unit, the cylinders of one hydraulic unit being in communication with one of said manifolds only when their associated pistons are moving axially outwards under control of one swash plate, and cylinders of the other hydraulic unit being in communication with the other of said manifolds only when their associated pistons are moving axially outwards under control of the other swash plate, wherein .the fluid porting means include cooperating ports formed in the pistons and the cylinders and wherein means are provided to rotate the pistons in the cylinders in accordance with their angular relationship to the associated swash plate.

2. A mechanism according to claim 1, wherein the means to rotate the pistons includes planet gears mounted on the pistons and cooperating with sun gears which have an angularly fixed relationship to the swash plate associated with the pistons whose planet gears they engage.

3. A mechanism according to claim 1 or 2, in which the swash plate held against rotation has a variable inclination to said rotational axis whereby to vary the transmission ratio of the mechanism.