GB2168791A

GB2168791A - Variable-ratio power transmission mechanism

Info

Publication number: GB2168791A
Application number: GB08432596A
Authority: GB
Inventors: Albert Bromhorst
Original assignee: General Motors France SA
Current assignee: General Motors France SA
Priority date: 1984-12-22
Filing date: 1984-12-22
Publication date: 1986-06-25
Also published as: GB2168791B; GB8432596D0

Abstract

A variable-ratio power transmission mechanism comprises an input shaft (10) rotatably drivable about its longitudinal axis by the input drive means, an output shaft (12) rotatably drivable about its longitudinal axis, and a variable-capacity axial-piston hydraulic pump and motor unit (24). The input shaft (10) rotatably drives the first (26) and second (28) drums of the unit (24). A hydraulic torque path is provided by a first fluid-displacement unit (76) comprising a rotationally fixed valve plate (34), the first drum (26) and a variable-angle swash plate (60); and a second fluid-displacement unit (78) comprising a rotatable valve plate (36), the second drum (28), and a fixed angle rotatable swash plate (68). The output shaft (12) is rotatably driven by the rotatable valve plate (36) and fixed angle swash plate (68). Hydraulic conduits 50, 54 and 52, 56 are formed in the respective valve plates 34, 36 and the angular position of the plate 60 is adjustable by a hydraulically actuable piston and cylinder unit 66 or by an electric device. The said conduits communicate with the cylinders of a plurality of positive displacement pistons (48) which connect each drum with its respective swash plate. <IMAGE>

Description

SPECIFICATION Variable-ratio power transmission mechanism This invention relates to a variable-ratio power transmission mechanism, for example for use as a vehicle drive.

Hydrostatic (positive-displacement) transmissions have been applied to various industrial uses, and as vehicle drives. For example the Janney transmission comprises a variable-ratio axial-piston hydraulic pump and motor unit in a coaxial arrangement with aligned input and output shafts. The pump unit includes a variable-angle swash plate, and the motor unit includes a fixed-angle swash plate, and the pump and motor units have respective cylinder-carrying rotary drums (cylinder blocks) separated by a fixed valve plate that is provided with fluid ports for establishing a hydraulic drive in the pump and motor unit.

By the present invention there is provided a variable-power transmission mechanism comprising an input drive means, an input shaft rotatably drivable about its longitudinal axis by the input drive means, an output shaft rotatably drivable about its longitudinal axis, and a variable-capacity axial-piston hydraulic pump and motor unit that is connected to be driven by the input shaft, the pump and motor unit comprising axially spaced first and second rotatable piston-carrying drums; first and second ported valve plates that are arranged for co-operation with the first and second drums respectively, the first valve plate being rotationally fixed and the second valve plate being rotatable relative to the second drum and the input shaft and being connected to the output shaft to rotatably drive the output shaft, the first drum and the second drum being connected to be rotatably driven by the input shaft; hydraulic conduits fluidly interconnecting ports in the first and second valve plates respectively; a rotationally fixed variable-angle swash plate that is engageable by a plurality of positive-displacement pistons carried by the first drum; and a fixed-angle swash plate that is connected to rotate with the second valve plate, the fixed-angle swash plate being engageable by a plurality of positive-displacement pistons carried by the second drum, whereby the first valve plate, the first drum and its positive-displacement pistons, and the variable-angle swash plate co-operate to form a first fluid-displacement unit, and the second valve plate, the second drum and its positive-displacement pistons, and the fixed-angle swash plate co-operate to form a second fluid-displacement unit that in operation is arranged to be driven hydraulically by the first fluid-displacement unit and mechanically by the input shaft, the rotational speed of the output shaft being a function of the rotational speed of the input shaft and the angle of the variable-angle swash plate.

Such a transmission mechanism is potentially relatively simple, light-weight and low-cost. The pump and motor unit may if desired be based on a conventional positive-displacement unit. There is no need for clutches, belts or fluid coupling, and the transmission mechanism is potentially energy-saving because maximum engine power can be utilized throughout the range of transmission ratios. Essentially, in such a transmission mechanism, continuous variation of the output speed is available by means of the hydrostatic drive.

A co-axial arrangement utilising aligned input and output shafts is readily possible.

The first and second valve plates are conveniently disposed adjacent each other, possibly with the interposition of a thrust bearing. Such an arrangement simplifies the external connections, for example for the supply of make-up fluid to maintain the requisite amount of hydraulic fluid (oil) in the positivedisplacement system. The valve plates may each include an opposed pair of circumferentially extending C-shaped grooves for co-operating with the cylinders of the respective piston-carrying drum, with the grooves of the respective valve plates communicating with ports that are hydraulically interconnected to provide, in operation, the hydraulic drive in the pump and motor unit.

The swash plates conveniently surround the input shaft with the interposition of respective bearings; the variable-angle swash plate may be mounted on a swivel pin that is carried by its respective bearing or by the housing within which the pump and motor unit is housed.

A variable-ratio power transmission mechanism in accordance with the present invention may have the input shaft thereof driven by, for example, an engine that normally operates at constant speed, with the angular position of the variable-angle swash plate controllable by a single regulating device responsive to transmission output speed, for example by means of a hydraulic piston and cylinder arrangement or by an electronic arrangement (eg. a servomotor), in response to the power developed by the engine. Thereby, it is readily possible to provide at the output shaft, progressively, a range of reverse ratios, a neutral condition (stall condition), and a range of forward ratios including a direct-drive ratio and a range of overdrive ratios.

Power transmission mechanisms in accordance with the present invention are applicable to various industrial uses, and in particular as motor vehicle drives, for example in passenger cars, public transport vehicles, heavy-duty goods vehicles, specialised vehicles and waterborne vehicles.

In the drawings: Figure 1 schematically illustrates in fragmentary longitudinal section, with parts in elevation, a preferred embodiment of a variable-ratio power transmission mechanism in accordance with the present invention; Figure 2 illustrates C-shaped ports in a pair of valve plates forming part of the transmission mechanism shown in Figure 1; Figure 3 illustrates the flow conditions prevailing at nine exemplary settings of a variable-angle swash plate forming part of the transmission mechanism shown in Figure 1; Figure 4 is a graph illustrating changes in various parameters in the transmission mechanism shown in Figure 1; Figure 5is a schematic diagram of the hydraulic circuit of the mechanism shown in Figure 1; and Figure 6 is a cross sectional view through the variable-angle swash plate of the mechanism shown in Figure 1.

As is shown in Figure 1 of the drawings, a variable-ratio power transmission mechanism in accordance with the present invention, applicable for use as a motor vehicle drive, includes a rotary input shaft 10 that is coaxially aligned with a rotary output shaft 12.

Coaxially surrounding the rotary input shaft 10 there is a variable-capacity axial-piston hydraulic pump and motor unit 24 of the swash-plate type, forming a hydrostatic (positive-displacement) transmission that is connected to provide a hydraulic torque path from the input shaft 10 to the output shaft 12. Specifically, the pump and motor unit 24 comprises axially spaced first and second annular drums 26 and 28 which coaxially surround the input shaft 10 and are connected to the input shaft by means of splined couplings 14 and 16, such that the annular drums are thereby connected to be driven by, and rotate with, the input shaft.

Axially between the first and second annular drums 26 and 28 there are first and second ported annular valve plates 34 and 36 which coaxially surround the input shaft 10 and are arranged for co-operation with the first and second drums respectively. The first valve plate 34 is held rotationally fixed by co-operation with a stationary housing 40 of the pump and motor unit 24. The second valve plate 36 is rotatable relative to the input shaft 10 and drum 28, and connected to the output shaft 12 by a generally annular connecting portion 70 to rotatably drive the output shaft. Athrust bearing 38 is positioned between the valve plates 34,36.

Springs 18, 20 ensure the drums 26,28, respectively remain in contact with the valve plates 34,36 if there is no pressure in the hydraulic circuit 80 (described below).

As is shown in Figure 2 of the drawings, the ports in each of the valve plates 34 and 36 include first and second circumferentially extending C-shaped fluid-distribution grooves 42 and 44 (Figure 2) formed in an end face of the respective valve plate. Each of the annular drums 26 and 28 correspondingly has formed in an end face thereof, and extending axially through the drum to the opposite end face of the drum, a circumferentially extending series of cylinder bores 46 that slidably accommodate individual positivedisplacement pistons 48 and are at the same diameter as, and communicate with the C-shaped fluid-distribution grooves 42 or 44 in the respective valve plate 34 or 36.

As is also shown in Figure 1 of the drawings, the fixed valve plate 34 extends annularly around the rotatable valve plate 36. The first C-shaped grooves 42 of the respective valve plates 34 and 36 are hydraulically interconnected by means of a first hydraulic conduits 50 and 52 formed in the valve plates 34 and 36 respectively, and correspondingly the second C-shaped grooves 44 of the respective valve plates are hydraulically interconnected by means of a second hydraulic conduits 54 and 56 formed in the valve plates 34 and 36 respectively. Ring seals 72 are interposed between the valve plates 34 and 36 to minimise leakage of hydraulic fluid (oil) contained in the cylinder bores 46, and in the ports in the valve plates 34 and 36 and the conduits 50, 52, 54 and 56.The hydraulic circuit 80 for the mechanism is shown schematically in Figure 5, which illustrates the interconnection of the hydraulic conduits 50, 52, 54 and 56 between the fluid displacement units 76,78 (described below). The rotary seals between the fixed valve plate 34 and the rotatable valve plate 36 are shown as parts 82, 84. The hydraulic circuit 80 includes a feed pump 86 fed from a tank 88, a heat exchanger 90, and two one-way feed valves 92, 94. The circuit 80 also includes two load relief valves 96, 98 and a feed relief valve 100 connected to a tank 102 which ensure the fluid pressure does not exceed a predetermined level. The circuit 80 also includes a standstill (stall) safety valve 104 which is normally closed.The solenoid of this valve 104 is excited, and the valve opened, thereby stopping transmission, when a predetermined event occurs (for example, in the case of a motor vehicle, when the brake pedal is depressed).

The positive-displacement pistons 48 of the first annular drum 26 project from the opposite end face of the drum for co-operation with a rotationally fixed variable-angle swash plate 60 (first swash plate). The variable-angle swash plate 60 coaxially surrounds the rotary input shaft 10. The swash plate 60 is pivotally mounted on a swivel pin (pivot pin) 62 carried by a bearing 64 in the stationary housing 40 of the pump and motor unit 24 (see Figure 6). The angular position of the variable-angle swash plate 60 is adjustable by means of a hydraulically actuable piston and cylinder arrangement 66 in a stepless fashion throughout the working range of the variable-angle swash plate. A spring 64 acts on the variable-angle swash plate 60 to bias it against the piston and cylinder arrangement 66 for all angles of the swash plate 60.Alternatively, the angular position of the variable-angle swash plate 60 may be adjusted by an electric device. For the purpose of description, and for correlation with Figure 3 of the drawings, nine exemplary angular positions (settings) of the variable-angle swash plate 60 are identified in Figure 1 as, successively, I, II, III, IV, V, VI, VII, VIII, IX.

The positive-displacement pistons 48 of the second angular drum 28 project from the opposite end face of the drum for co-operation with a fixed-angle swash plate 68 (second swash plate) that is connected by means of the generally annular connecting portion 70 to rotate with the second valve plate 36.

A valve engine (not shown) (input drive means) is connected to drive the rotary input shaft 10 of the transmission mechanism, and the rotary output shaft 12 of the transmission mechanism is connected to drive a pair of drive wheels (not shown) of the vehicle. The vehicle engine is controlled so as normally to operate at a variable or at a constant speed, and the piston and cylinder arrangement 66 for adjusting the angular position of the variable-angle swash plate 60 is controllable in response to the power developed by the vehicle engine.

In the variable-ratio power transmission mechanism which has been described, the first valve plate 34, the first annular drum 26 with its positive-displacement pistons 48, and the variable-angle swash plate 60 co-operate to form a first fluid-displacement unit 76, and correspondingly the second valve plate 36, the second annular drum 28 with its positive-displacement pistons 48, and the fixed-angle swash plate 68 co-operate to form a second fluid-displacement unit 78 which in operation, over the greater part of the working range of the variable-angle swash plate 60, hydraulically drives the first fluid-displacement unit 76.

Thereby, a hydraulic torque path is available from the rotary input shaft 10 by way of the first and second fluid-displacement units 76 and 78 to the rotary output shaft 12.

Overall, therefore, the variable-ratio power transmission mechanism which has been described utilises a steplessly variable-capacity positive-displacement hydraulic transmission (constituted by the fluiddisplacement units 76 and 78) in a split-torque configuration with a fixed-ratio mechanical torque path (constituted essentially by the rotary input shaft 10), with recombination of torque from the hydraulic and mechanical torque paths by the use of the rotation of the fixed-angle swash plate 68, being controlled by means of the second fluid-displacement unit 78.

In operation of the variable-ratio power transmission mechanism, rotation of the annular drums 26 and 28 with the rotary input shaft 10 produces positive displacement of the hydraulic fluid (oil) as the swash plates 60 and 68 co-operate with the positive-displacement pistons 48 to produce a pumping action of the positive-displacement pistons in the cylinder bores 46.In conjunction with this pumping action, the ported valve plates 34 and 36 provide appropriate selective fluid connections, inasmuch as each of the cylinder bores 46 draws hydraulic fluid from one of the fluid-distribution grooves 42 or 44 of the respective valve plate 34 or 36 during essentially one-half of a revolution of the input shaft 10, and then delivers hydraulic fluid to the other fluid-distribution groove of the valve plate during the succeeding one-half of a revolution of the input shaft, to provide a hydraulic drive in the pump and motor unit formed by the first and second fluid-displacement units 76 and 78.By varying the angular position of the variable-angle swash plate 60, which is of the over-centre type, and without the need for further control members, the transmission mechanism provides at the output shaft 12, progressively, a range of reverse ratios, a neutral (stall) condition, and a range offorward ratios including a direct-drive (1:1) ratio and a range of overdrive ratios.

Figure 3 of the drawings illustrates the hydraulic flow conditions prevailing in the pump and motor unit at nine specific angular positions of the variable-angle swash plate 60, these nine positions having been chosen from the full working range of stepless variation of swash plate angle to illustrate the types of transmission ratio available, and corresponding to the successive settings @@@ III, IV, V, VI, VII, VIII, IX shown for the variable-angle swash plate 60 in Figure 1.

In the following, rotational speeds, pumping volume and oil flow are summarised for each of the five settings I to IX of the variable-angle swash plate 60.

The calculations which follow are based on the following:- Ratio (R) = Output Speed (in rpm)/Input Speed (in rpm) Input Torque (in Nm) = Input Power (in Watts) x 60/2 x # X input Spped (in rpm) Output Torque (OT) (in Nm) = Input Torque (In Nm)/Ratio Output Speed (OS) (in rpm) = Input Speed (in rpm) x Ratio Relative Speed (RS) (in rpm) = Input Speed (in rpm) - Output Speed (in rpm) Oil Flow (Q) (in l/min) = Capacity of Unit 78 (in 1) x Relative Speed (in rpm) Oil Pressure (P) (in bars) = Output Torque x (in Nm) 15.9 x Capacity of Unit 78 (in I) Hydraulic Power Circulation (HPC) (in kw) = Oil Pressure (in bars) x Oil Flow (in l/min) 600 Change in Capacity of unit 76 (in @) = Oilflow (in l/min)/Input Speed (in rpm) Sine of the Variable Swash Plate Angle (α) = Oil flow (in l/min) x Sinαmax/Change in Capacity of unit 76 at αmax (in l) x Input Speed (in rpm) where a max is the maximum angle of tilt of the variable angle swash plate (60).

Assuming an engine rotating in the clockwise direction (corresponding to the arrow in Figure 1) at a constant speed of 5600 rpm (input speed), the capacity of unit 78 to be 0.1 and an input power of 100 kw, the calculations for each setting are set out in the following table: Setting Ratio Input Output Output Relative Oll flow Oll Hydraulic Changelin Sin α α Torque Torque Speed Speed (l/min) Pressure Power Capacity of (Nm) (Nm) (Nm) (Nm) (bars) Circulation Unit 76 (kw) (1) I -0.2 170 -850 -1120 6720 672 -535 -600 0.12 0.4226 25 II -0.1 170 -1700 -560 6160 616 -1069 -1098 0.11 0.3873 22.8 III 0 170 0 0 5600 560 0 0 0.10 0.3521 20.6 IV 0.1 170 1700 560 5040 504 1069 898 0.09 0.3169 18.4 V 0.5 170 340 2800 280 0 214 100 0.05 0.1761 10.1 VI 0.8 170 212 4480 1120 112 133 24.8 0.02 0.0704 4.0 VII 1.0 170 170 5600 0 0 107 0 0 0 0 VII 1.1 170 154.5 6160 -560 -56 97 -9.05 -0.01 -0.0352 -2.1 IX 1.2 170 141.6 6720 -1120 -112 89 -16.6 -0.02 -0.0704 -4.0 Although the calculated value of oil pressure for settings II and IV is very high in practice the load relief salves 96, 98 connected to the hydraulic conduits 50, 52, 54, 56 prevent excessive pressure and torque being developed.The negative oil flow for setting VII and IX means the oil flow is in the opposite direction to that or settings I to VI.

In Figure 1 of the drawings, the swash plates 60 and 68 are shown schematically. To reduce friction, either Dr both of the swash plates could in practice comprise a drive plate portion and a main swash plate portion, vith a bearing interposed to permit relative rotation of the said portions.

Figure 4 of the drawings is a graph illustrating the above-described calculations. The graph is thus plotted For an input shaft 10 speed of 5600 r.p.m. (1): variation of the input shaft speed (engine speed) would vary the angle (ss) of the straight-line characteristic of the output speed. For example, if the input speed was changed to 2800 r.p.m. (2); the straight-line characteristic of the output speed would be at angle 2.

Claims

1. Avariable-ratio power transmission mechanism comprising an input drive means, and input shaft rotatably drivable about its longitudinal axis by the input drive means, an output shaft rotatably drivable about its longitudinal axis, and a variable-capacity axial-piston hydraulic pump and motor unit that is connected to be driven by the input shaft, the pump and motor unit comprising axially spaced first and second rotatable piston-carrying drums; first and second ported valve plates that are arranged for co-operation with the first and second drums respectively, the first valve plate being rotationally fixed and the second valve plate being rotatable relative to the second drum and the input shaft and being connected to the output shaft to rotatably drive the output shaft, the first drum and the second drum being connected to be rotatably driven by the input shaft; hydraulic conduits fluidly interconnecting ports in the first and second valve plates respectively; a rotationally fixed variable-angle swash plate that is engageable by a plurality of positive-displacement pistons carried by the first drum; and a fixed-angle swash plate that is connected to rotate with the second valve plate, the fixed-angle swash plate being engageable by a plurality of positive-displacement pistons carried by the second drum, whereby the first valve plate, the first drum and its positive-displacement pistons, and the variable-angle swash plate co-operate to form a first fluiddisplacement unit, and the second valve plate, the second drum and its positive-displacement pistons, and the fixed-angle swash plate co-operate to form a second fluid-displacement unit that in operation is arranged to be driven hydraulically by the first fluid-displacement unit and mechanically by the input shaft, the rotational speed of the output shaft being a function of the rotational speed of the input shaft and the angle of the variable-angle swash plate.

2. A variable-ratio power transmission mechanism according to claim 1, in which the variable-capacity axial-position hydraulic pump and motor unit coaxially surrounds the input shaft, with the first drum and the second drum connected to rotate with the input shaft.

3. A variable-ratio power transmission mechanism according to claim 1 or claim 2, in which the first and second valve plates are disposed adjacent each other.

4. A variable-ratio powertransmission mechanism according to any one of the preceding claims, in which the ports in each valve plate include a pair of circumferentially extending C-shaped fluid-distribution grooves formed in an end face of the respective valve plate, each drum correspondingly has formed in an end face thereof a circumferentially extending series of cylinders that accommodate the individual positive-displacement pistons and communicate with the C-shaped grooves of the respective valve plate, and the hydraulic conduits each fluidly connect one of the C-shaped fluid-distribution grooves in one valve plate to the corresponding fluid-distribution groove in the other valve plate, to provide an arrangement in which in operation each cylinder draws hydraulic fluid from one of the fluid-distribution grooves of the respective valve plate during one-half of a revolution of the input shaft and then delivers hydraulic fluid to the other fluid-distribution groove of the valve plate during the succeeding one-half of a revolution of the input shaft, to provide the hydraulic drive in the pump and motor unit.

5. A variable-ratio power transmission mechanism according to claim 4, including a source of pressure fluid connected by means of two one-way valves to the fluid-distribution grooves, for the supply of make-up fluid for the hydraulic conduits.

6. A variable-ratio power transmission mechanism according to any one of claims 2 to 5, in which a thrust bearing is interposed between the first valve plate and the second valve plate.

7. A variable-ratio power transmission mechanism according to any one of claims 2 to 6, in which the variable-angle swash plate is pivotally mounted on a swivel pin that is carried by a bearing.

8. A variable-ratio power transmission mechanism according to claim 7, in which the swivel pin is carried by the housing within which the variable-angle swash plate is housed.

9. A variable-ratio power transmission mechanism according to any one of the preceding claims, in which the angular position of the variable-angle swash plate is adjustable by means of a hydraulically actuable piston and cylinder arrangement or by an electronic arrangement to provide at the output shaft, progressively, a range of reverse ratios, a neutral condition, and a range of forward ratios including a direct-drive ratio and a range of overdrive ratios.

10. A variable-ratio power transmission mechanism according to claim 9, in which the piston and cylinder arrangement for adjusting the angular position of the variable-angle swash plate is controllable in response to the power developed by the input drive means that normally operates at constant speed.

11. A variable-ratio power transmission mechanism according to any one- of claims 1 to 10, in which the output shaft is connected to drive a pair of drive wheels of a vehicle.

12. A variable-ratio power transmission mechanism according to any one of claims 1 to 11, in which the input drive means comprises a vehicle engine.

13. A variable-ratio power transmission mechanism as claimed in any one of the preceding claims, in which the input shaft rotatably drives the first drum and the second drum by splined couplings.

14. A variable-ratio power transmission mechanism as claimed in any one of the preceding claims, in which the fixed valve plate is attached to the housing within which the pump and motor unit is housed.

15. A variable-ratio power transmission mechanism as claimed in any one of the preceding claims, in which the fixed valve plate extends annularly around the rotatable valve plate to provide direct interconnection of the hydraulic conduits.

16. A variable-ratio power transmission mechanism substantially as hereinbefore described with reference to, and as shown in Figures 1 to 4 of the accompanying drawings.