EP0088794A4

EP0088794A4 - Hydrostatically augmented mechanical drive unit.

Info

Publication number: EP0088794A4
Application number: EP19820902952
Authority: EP
Inventors: James A Stark
Original assignee: Individual
Current assignee: Stark James A(deceased) Legally Represented By S
Priority date: 1981-09-18
Filing date: 1982-09-07
Publication date: 1984-03-01
Also published as: EP0088794A1; WO1983001096A1

Abstract

The disclosed drive unit (10) includes an input shaft (90) and an output shaft (20). An inner axial face cam (92) is attached to the input shaft (90), and a cylinder block (40) which defines an outer ring of cylinders and an inner ring of cylinders, nested within the outer ring, is attached to the output shaft (20). An outer axial face cam (143) is mounted to a housing (130), such that its tilt angle can be adjusted, and pistons are disposed within the cylinders to slide around the inner and outer cams. The inner cam and pistons act as a pump which pressurizes a fluid that is then conducted to the outer pistons and cam, which act as a motor. A valve head (100) is mounted to a tension member (94) which extends through a central bore (102) in the cylinder block (40) and is secured to the input shaft (90) to hold the cylinder block (40) in place against the inner cam (92). A motor activation valve (176) for reducing hydraulic pressure on the motor while hydraulically locking the pump, and a soft-start valve (190) for providing a neutral mode, are also disclosed.

Description

HYDROSTATICALLY AUGMENTED MECHANICAL DRIVE UNIT

BACKGROUND OF THE INVENTION

This invention relates to an improved hydrostaticall augmented mechanical drive unit which has excellent operating efficiency and torque multiplication character¬ istics, and which can be embodied in compact, light weight designs.

It has been recognized for some time that hydrostatic transmissions provide the important advantage of a continuously variable^"ratio between the speed of the input shaft and that of the output shaft. One commonly used type of hydrostatic transmission provides a hydraulic pump which is driven by an input shaft and is hydraulicall coupled to a hydraulic motor which in turn drives an output shaft. In transmissions such as those shown in U.S. Patents 3,362,161 to Flint and 4,075,843 to Leker, the only means for transferring power from the pump to the motor is by means of pressurized hydraulic fluid. In general, this arrangement provides the disadvantage that high speeds for the output shaft are obtained through high volume operation of the pump. Therefore, pumping losses tend to be high when the output shaft is rotating at or near the speed of the input shaft. A second type of hydrostatic transmission of the prior art differs from the first in that both mechan¬ ical and hydraulic means are provided for transmitting torque and power from the pump to the motor. The trans- missions disclosed in U.S. Patents 2,679,139 to Posson, 2,633,710 to Jarmann and 3,698,189 to Reimer are examples of this second type. These transmissions can provide the advantage of near zero hydraulic pumping activity when operating at 1:1 ratio between the input and output shaft speeds. However, the designs mentioned above are not without their disadvantages. All three utilize an opposed piston design in which the valving apparatus is situated in a relatively inaccessable position between the motor and the pump, and the length of the transmission is therefore greater than the sum of the individual lengths of the pump and motor. Furthermore, all three include variable angle pump cam plates and. therefore variable displacement pumps which are capable of generat¬ ing extremely high hydraulic pressures when the pump cam plates are oriented at small angles with respect to the input shafts. In practical operation, such extremely high pressures are often undesirable, for they may require the use of safety valves which pose a potential reliability problem and which can only operate to reduce - the operating efficiency of the transmission. Additional examples of hydrostatic transmission of this second type include those disclosed in U.S. Patent 2,141,166 to Bischof, U.S. Patent 2,186,556 to Robbins,^" U.S. Patent 2,571,561 -to Genety, U.S. Patent 3,313,108 to Allgaier, and British Patent 10,108 to Renault.

SUMMARY OF THE INVENTION

The present invention is directed to an improved drive unit which to a large extent overcomes the aforemen¬ tioned disadvantages. According to this invention, a drive unit is provided which includes a first axial face cam mounted to a first shaft and first and second cylinder blocks mounted to a second shaft. A second axial face cam is mounted to provide a reaction point with respect to the cylinder blocks, and first and second pistons are slide- ably mounted in cylinder bores defined by the first and second cylinder blocks such that differential rotation between the first cam and the first cylinder block actuates the first pistons and differential rotation between the second cylinder block and the second cam actuates the second pistons. Valve means are provided for conducting hydraulic fluid between the first and second cylinder blocks to transmit power hydraulically between the first and second pistons.

The drive unit of this invention has been optimized for efficient operation at a mechanical ratio of 1:1, the ratio at which many drive units operate for the great majority of time. Additive torque is trans- mitted mechanically from the input shaft to the output shaft by means of reaction torques on the first pistons and the first cylinder block. When the first pistons are hydraulically locked in place in the first cylinder block, the input and output shafts rotate together, coaxially, and in unison, without pumping activity or pumping losses. In effect, the input and output shafts have been locked together, so they rotate as a unit, without gearing or pumping losses. When the first pistons are allowed to pump fluid to the second cylinder block, the second cylinder block, second pistons and second cam operate as a variable displacement hydrostatic motor which hydraulically augments the torque on the output shaft.

According to a first feature of this invention, the first shaft is coupled to a prime mover such as a heat engine or an electric motor, and the second shaft is mounted coaxially with the first shaft and is connected to a load such that all torque on the first shaft is transmitted directly to the second shaft via the first cam, pistons and cylinder block, without relative movemen between the cylinder blocks and the second shaft. When hydraulic fluid is allowed to pass from the first to the second cylinder blocks, the second cam, pistons and cylinder block then serve to provide additional torque in ^' the forward reduction mode which adds directly to the torque transmitted to the output shaft via the first cam, pistons and cylinder block. This feature of the inventio provides the important advantages of relatively low pumping speeds, low motor speeds, and efficient operation. In the preferred embodiment, the first cam is mounted at a fixed tilt angle. This arrangement provides the addi¬ tional advantage of low peak hydraulic pressures within the transmission.

According to a second feature of the invention, the first and second cylinder blocks are nested, one within the other, such that one of the two sets of pistons is situated within the perimeter defined by the other set of pistons. This geometry significantly reduces the axial length of the drive unit and it allows both cams to be placed on one side of the cylinder blocks and all necessary valving to be placed on. the o er side of the cylinder blocks, where the valves can readily be serviced if necessary and can readily be structurally reinforced in an adequate manner. According to another feature of this invention, the first cylinder block is held in place by a tension member, which passes through the first cylinder block, and a valve head, which is attached to the tension member and serves to control the axial position of the first cylinder block. Considerable axial forces can be develope by the action of the first pistons against the first cam, and the valve head and tension member serve to control these forces centrally, without the need for peripheral bearings or the like on the first cylinder block. Preferably, the valve head defines the ports and passage¬ ways needed to conduct fluid to and from the first pistons.

Another aspect of this invention relates to a motor activation valve which selectively interrupts the flow of high pressure fluid from the first to the second cylinder blocks in order simultaneously (1) to reduce hydraulic pressure on the second pistons, and (2) to hydraulically lock at least some of the first pistons in place in the first cylinder block. This valve is in- tended for use when the transmission is in the 1:1 mode in order to reduce unnecessary wear and friction associ¬ ated with unnecessary hydraulic pressure on the second pistons.

A further advantageous feature of this inven- tion relates to a soft-start valve which, when actuated, allows hydraulic fluid to recirculate within the cylin¬ ders of the first cylinder block, bypassing the cylinders of the second cylinder block. This soft-start valve provides a true neutral, hydraulically disengaging the first and second shafts. Furthermore, this valve allows the second shaft to be gradually engaged with the first shaft by gradually closing the soft-start valve.

The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF TEE DRAWINGS

FIG. 1 is a cross sectional view of the presently preferred embodiment of the drive unit of this invention. FIG. la is a cross sectional view taken along line la-la of FIG. 1.

FIG. 2 is a cross sectional view taken along line 2-2 of FIG. la. FIG. 3 is a cross sectional view taken along line 3-3 of FIG. la.

FIG. 4 is a perspective view of the valve head 100 of the embodiment of FIG. 1.

FIG. 5 is a cross sectional view taken along ^' line 5-5 of FIG. 4.

FIG. 6 is a cross sectional view taken along line 5-6 of FIG. 4.

FIG- 7 is a cross sectional view taken along line 7-7 of FIG. 4. FIG. 8 is a cross sectional view taken along line 8-8 of FIG. 4.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Turning now to the drawings, FIGS. 1 and la show two cross-sectional views of the presently preferred embodiment of the drive unit of this invention. In FIG. 1, the reference numeral 10 is used to refer gen¬ erally to this drive unit. The drive unit 10 includes an output shaft 20 which defines a circular array of aperture 22 and a central recess 24. In use, the output or driven shaft 20 would be coupled to a load, such as to the driving wheels of a land vehicle for example.

-The drive unit 10 also includes a cylinder block 40 which is provided with an annular skirt 42 extending- from one and. The cylinder block 40 defines a central bore 44 extending completely therethrough and a central recess 46 surrounded by the skirt 42. A plurality of threaded apertures 48 are formed in the extreme end portion of the skirt 42, and are positioned to mate with the circular array of apertures 22 in the output shaft 20, A plurality of threaded fasteners 50 are positioned in the apertures 48,22 to secure the output shaft 20 rigidly to the cylinder block 40 such that the two rotate as a unit.

The skirt 42 defines a first array of radial openings 52 which extend roughly in a circle around the periphery of the skirt 42. Similarly, the skirt 42 defines a second array of radial openings 54, which also extend around the periphery of the skirt 42. Preferably, adjacent openings in each array are staggered with respect to one another in order to maximize the strength of the skirt 42. The cylinder block 40 defines an inner array of cylinder bores 56, each of which extends in the axial direction, and each of which terminates in a respective fluid port 58. Similarly, the cylinder block 40 also defines an outer array of cylinder bores 60, each of which terminates in a narrowed respective fluid port 62. Thus, the inner portion of the cylinder block 40 which defines the inner bores 56 can be thought of as an inner cylinder body, and the outer portion of the cylinder block 40 which defines the outer bores 60 can be thought of as an outer cylinder body. As best shown in FIG. 2, each of the arrays of cylinder bores 56,60 comprises a plurality of individual cylinder bores, each of which is spaced at a predetermined radial distance from the center of the central bore 44. A respective inner piston 64 is slidingly and sealingly received in each_. of the cylinder bores included in the inner array of cylinder bores 56. Each of these inner pistons 64 terminates at one end with a spherical head 66. In each case an actuating shoe 68 is pivotably mounted on the respective spherical head 66 such that each actuating shoe 68 is free to articulate with respect to the respective inner piston 64. Similarly a respective outer piston 70 is slidingly and sealingly received in each of the outer array of cylinder bores 60. Each of the outer pistons 70 defines a ^'respective spherica head 72 which serves as a pivoting mount for a respective actuating shoe 74.

The drive unit 10 also includes an input or driving shaft 90 which in use would be coupled to a prime mover such as a heat engine or an electric motor. This input shaft 90 defines a fixed-angle, axial face, inner cam 92, which preferably is integrally formed with the input shaft 90 such that the cam 92 rotates in unison with the input shaft 90. The input shaft 90 also in- eludes an axially extending tension member 94 which terminates in a threaded end portion 96. As shown in FIG. 1, the tension member 94 extends through the central bore 44 in the cylinder block 40. Furthermore, the inner array of pistons 64 is positioned such that the actuating shoes 68 of the inner pistons 64 bear on the inner cam 92. Thus, differential rotation between the input shaft 90 and the output shaft 20 causes the inner cam 92 to cause the inner pistons 64 to reciprocate within the inner array of cylinder bores 56 in the cylinder block 40. A valve head 100 is mounted to the tension member 94 within the skirt 42 of the cylinder block 40. This valve head 100 defines a central bore 102 sized to receive the threaded end portion 96 of the^" tension member 94. A nut 98 threadedly engages the threaded end portion 96 of the tension member 94 in order to securely hold the valve head 100 in place on the .tension member 94. A key 103 and keyway 105 are provided to ensure that the angular orientation of the valve head 100 on the tension member 94 remains constant. A constant force spring washer 99 is positioned within the recess 24 to bear on the nut 98 in order to bias the valve head 100 toward the cylinder block 40.

The valve head 100 defines a number of cham- bers, ports and passageways. In the following descrip¬ tion, certain elements of the valve head 100 will be defined as either high pressure or low pressure elements. It is to be understood that the terms high pressure and low pressure have been selected merely to facilitate an understanding of this embodiment, and that they designate the relative pressures customarily found in these elements during normal operation of the drive unit 10 in the forward reduction mode when power is being transmitted from the input shaft to drive a load attached to the output shaft. It should be understood that in other modes of operation the relative pressures in the elements of the valve head 100 may be reversed.

The valve head 100 defines an annular high pressure chamber 104 and a spaced annular low pressure chamber 106, both of which extend completely around the periphery of the valve head 100. As shown in FIG. la, when the valve head 100 is assembled within the skirt 42 of the cylinder block 40, the high pressure chamber 104 is positioned adjacent the first array of radial open- ings 52, and the low pressure chamber 106 is positioned adj cent the second array of radial openings 54. The valve head 100 also defines an arcuate high pressure port 108 and an arcuate low pressure port 110. As best shown in FJG. 3, each of the arcuate ports 108,110 extends over somewhat less than a 180° at a fixed radius from the center of rotation of the valve head 100. Internal passageways 112,114 are provided within the valve head 100 to interconnect the high pressure port 108 with the high pressure chamber 104 and the low pressure port 110 with the low pressure chamber 106, respectively. A valve plate 116 is pinned to the valve head 100 so as to prevent relative movement therebetween. This valve plate 116 defines two arcuate cutouts 118,120 which correspond in shape and orientation to the high pressure port 108 and the low pressure port 110, respectively. The valve plate 116 is positioned between the cylinder block 40 and the valve head 100 to provide a low friction sliding joint therebetween. The drive unit 10 also includes a housing 130 which is provided with two tapered roller bearings 132,134 which serve to position the input shaft 90 with respect to the housing 130 while allowing rotary motion of the input shaft 90. A gear pump 136 is mounted to the housing 130. This gear pump 136 is provided with a pump inlet 138 and a pump outlet 140 and is driven by the input shaft 90. The gear pump 136 functions as a replen¬ ishing pump, as will be explained in detail below.

The housing 130 also defines a cylindrical support bearing 142 which serves to support a non- rotating, axial face, outer cam 143. This outer cam 143 is an annular cam positioned around the inner cam 92 such that the shoes 74 of the outer pistons 70 slide on the outer cam 143, and reciprocation of the outer pistons 70 in the outer cylinder bores 60 causes the cylinder block 40 to rotate with respect to the outer cam 143 and the housing 130. The outer cam 143 is supported by an outer cam support member 144 which defines^" a cylindrical support surface 146. This cylindrical support surface 146 is provided with a radius of curvature matching that of the cylindrical support bearing 142 defined by the housing 130. The outer cam support member 144 cooperates with the cylindrical support bearing 142 to prevent rotary motion of the outer cam 143 about the axis of the input shaft 90. The outer cam support member 144 is provided with a central opening 148 through which the input shaft 90 and the inner cam 92 pass.

An actuator 150 is mounted between the housing 130 and the outer cam support member 144. The axial length of the actuator 150 can be changed in order to adjust the relative position of the outer cam support member 144 with respect to the housing 130, and therefore the tilt angle of the outer cam 143 with respect to the input shaft 90. The housing 130 is also provided with a fill plug 152 and a drain plug 154. In normal use the interior of the housing 130 is substantially filled with an incompressible hydraulic fluid.

A valve block 160 is secured to the housing 130 by means of a plurality of fasteners 162 such that the valve block 160 serves to seal off the interior of the housing 130. The valve block 160 cooperates with the valve head 100 in order to control the flow of hydraulic fluid between the inner and outer arrays of cylinder bores 56,60. For this purpose, the valve block 160 defines an annular high pressure reservoir 164 positioned adjacent the first array of radial openings 52 and the high pressure chamber 104, and an annular low pressure reservoir 166 positioned adjacent the second array of radial openings 54 and the low pressure chamber 106. The radial openings 52,54 ensure constant fluid communication between the high pressure chamber 104 and the high pressure reservoir 164, and the low pressure chamber 106 and the low pressure reservoir 166, respectively. A valve plate 168 is secured to the valve block 160 by pins such that the^"valve plate 168 is prevented from rotating with respect to the valve block

160. This valve plate 168, which is shown in FIG. 3, defines an arcuate high pressure port 170 and an arcuate

Q.! I low pressure port 172. As shown in FIG. 3, both the high pressure port 170 and the low pressure port 172 are somewhat less than 180° in angular extent, and each is situated at a constant radius from the center of rotation of the cylinder block 40.

Preferably, standard engineering principles are used to create hydraulic thrust bearings between the cylinder block 40 and the valve plates 116 and 168 and between the shoes 68,74 and the respective cams 92, 143. As is well known, this can be accomplished by providing a shallow recess on the sliding surface of each shoe 68,74 which is in fluid communication with the cylinder bore of the -respective piston 64,70, and by properly balancing the areas of the recesses and the areas of the cutouts 118,120, and the ports, 108,110, 170,172 such that hydraulic forces reduce frictional forces to the desired level.

A high pressure passageway 174a,174b extends between the high pressure reservoir 164 and the high pressure port 170. A motor activation valve 176 is mounted in the valve block 160 in order to selectively pass fluid from the high pressure passageway 174a to the high pressure passageway 174b. This valve 176 comprises a bore 178 defined by the valve block 160. A valve rod 180 is dimensioned to slide within the bore 178 in a sealing manner. -The position of the valve rod 180 within the bore 178 is controlled by an actuator 182. The valve rod 180 defines a region of reduced diameter 184 which is long enough to extend between the points at which the high pressure passageways 174a,174b communicate with the bore 178. When the actuator 182 is controlled to position the valve rod 180 as shown in FIG. la, the high pressure passageways 174a,174b are interconnected by means of the bore 178, and hydraulic fluid is thus free to flow from the high pressure reservoir 164 via the high pressure passageways 174a, 174b and the bore 178, to the high pressure port 170. Alternately, when the actuator 182 is positioned to cause the valve rod 180 to move to the left as shown in FIG. la, the valve rod 180 completely obstruct the high pressure passageway 174a, thereby completely interrupting the flow of fluid through the high pressure passageway 174a.

The valve block 160 also defines a low pressure passageway 186 which interconnects the low pressure reservoir 166 with the low pressure port 172. The low pressure passageway 186 also communicates via a soft- start valve 190 with the high pressure reservoir 164. This soft-start valve 190 includes a bore 192 defined by the valve block 160. A valve rod 194 having a reduced diameter portion 196 is dimensioned to slide within the bore 192 in a sealing manner. The position of the valve rod 194 in the bore 192 is controlled by an actuator 198. A passageway 200 extends between the bore 192 and the low pressure reservoir 166, and a passageway 202 extends between the bore 192 and the high pressure reservoir 164. When the actuator 198 is controlled to position the valve rod 194 as shown in FIG. la, the bore 192 serves to interconnect the passageways 200,202 such that fluid is freely passed directly between the high pressure reservoir 164 and the low pressure reservoir 166 without passing into the cylinder block 40. On the other hand, when the actuator 198 is used to move the valve rod 194 to the left as shown in FIG. la, the valve rod 194 obstructs the passageways 200,202, thereby preventing the free circula¬ tion of fluid from the high pressure reservoir 164 to the low pressure reservoir 166 via passageways 200,202. By gradually moving the valve rod from the position shown in FIG. la to ^'the left, the valve rod 194 can be used to

- O fr Λ-_* WI gradually cause more and more hydraulic fluid to be pumped from the high pressure reservoir 164 into the cylinder block 40, thereby providing a gradual transition from the neutral mode to the forward reduction mode. The valve block 160 also includes two check valves 210,212 each of which is connected to a respective one of the two passageways 200,202. The check valves 210,212 are in fluid communication with the gear pump outlet 140 by a conduit. The gear pump 136 serves to ^' pump hydraulic fluid at relatively low pressure from the interior of the housing 130 to the check valves 210,212. In the event that pressure in either the high pressure reservoir 164 or the low pressure reservoir 166 falls below that of the gear pump 136, the respective check valve 210,212 will open, allowing hydraulic fluid to pass from the gear pump 136 into the respective reservoir 164,166. The gear pump 136 serves to replenish hydraulic fluid lost by leakage from the reservoirs 164,166.

The drive unit 10 is designed to operate with maximum efficiency at a 1:1 mechanical ratio and in the forward reduction mode to rotate the output shaft 20 at a speed in the range between the speed- of the input shaf 90 and one-quarter the speed of the input shaft 90. In the following discussion the term "mechanical ratio" will be used to refer to the ratio between the speeds of the input and output shafts. Thus, for example, a mechanical ratio of 2:1 corresponds to the input shaft turning in the same direction as and with twice the speed of the output shaft. The. valve plate 116 rotates in unison with the inner cam 92 at an appropriate phase angle to ensure that the inner pistons 64 operate as a pump to pump fluid from the low pressure reservoir 166 to the high pressure reservoir 164. By way of example only, it will be assumed that, for every complete revolution of the inner cam 92 with respect to the cylinder block 40, the inner pistons 64 operate to pump 10 cubic inches of fluid from the low pressure reservoir 166 to the high pressure reservoir 164.

The outer pistons 70 cooperate with the adjust¬ able, outer cam 143 to form a variable displacement motor, wherein the displacement of the motor is a func¬ tion of the tilt angle of the outer cam 143. Here, the tilt angle is defined as the angle between the plane of the outer cam 143 and a plane perpendicular to the axis of rotation of the input shaft 90. By way of example only, in the following discussion it will be assumed that, for each revolution of the cylinder block 40 with respect to the outer cam 143, the outer pistons 70 pass

3 approximately 30, 20, 10 and zero cubic inches (492cm ,

328cm 3, 164cm3, and 0cm3) of fluid from the high pressure reservoir 164 to the low pressure reservoir 166 at outer cam tilt angles of 15°, 10°, 5° and 0°, respectively. In order to follow the operation of the drive unit 10, it is important to understand that the inner pistons 64 cooperate with the inner cam 92 to form a fixed displacement pump; that the outer pistons 70 cooperate with the outer cam 143 to form a variable displacement motor, and that it is the ratio between the pump displacement and the motor displacement which determines the mechanical ratio and the torque multipli¬ cation of the drive unit 10.

For example, when the tilt angle is set equal to zero, the displacement of the motor goes to zero, thereby preventing the flow of fluid out of the high pressure reservoir -164 and hydraulically locking the inner pistons 64 in place in the cylinder block 40. In that the inner pistons 64 are locked in place, relative

OM rotation between the inner cam 92 and the cylinder block

40, and therefore relative rotation between the input shaft 90 and the output shaft 20, are prevented. A tilt angle of 0° therefore corresponds to a mechanical ratio of 1:1 and a torque on the output shaft 20 which is equal to that on the input shaft 90. The mechanical ratio of

1:1 is achieved without any pumping activity or pumping losses. In effect, the input and output shafts are mechanically locked together to rotate in unison as a single, coaxial unit.

When the motor displacement is 10 cubic inch 3 (164 cm ) per revolution (tilt angle of 5°), one complete revolution of the inner cam 92 with respect to the cylinder block 40 results in one revolution of the cylinder block 40 with respect to the outer cam 143, for a mechanical ratio of 2:1 and a torque on the output shaft 20 which is twice that on the input shaft 90.

Similarly, when the motor displacement is 30 cubic inch

3 (492 cm ) per revolution (tilt angle of 15°), one com- plete revolution of the inner cam 92 with respect to the cylinder block 40 results in one-third of a revolution of the cylinder block 40 with respect to the outer cam 143, resulting in a mechanical ratio of 4:1 and a torque on the output shaft 20 which is four times the torque on the input shaft 90.

From the foregoing examples, it should be apparent that the drive unit 10 is capable of operating at mechanical ratios between 1:1 and 4:1 in a stepless manner while providing efficient torque multiplication. Considerable axial forces are developed by the inner pistons 64 as a result of their pumping action. In the drive unit 10, the tension member 94 and the valve head 100 have been designed to cooperate with the inner cam S2 to bear these axial forces simply and efficiently, - -

without the need for additional thrust bearings. This is because the inner pistons 64 are captured between the valve head 100 and the inner cam 92 such that the tension member 94 holds the cylinder block 40 in place adjacent the inner cam 92. Axial pumping loads are transmitted centrally via the tension member 94 rather than being supported by the housing 130 or the valve block 160, and the need for external thrust bearings to carry the axial pumping loads is thereby eliminated. Furthermore, the closer the mechanical ratio of the drive unit is to 1:1, the lower the relative rotational speed of the valve head 100 with respect to the cylinder block 40.

The motor activation valve 176 operates further to reduce wear and frictional losses when the drive unit 10 is being used, at a mechanical ratio of 1:1. As explained above, a mechanical ratio of 1:1 corresponds to a tilt angle of 0° and therefore a zero stroke for the outer pistons 70. At a tilt angle of 0°,. the outer pistons 70 do not contribute torque to the output shaft 20, and they can therefore be isolated from the high pressure reservoir 168 without interfering with the operation of the drive unit 10. The motor activation valve 176 performs just this function by interrupting the flow of hydraulic fluid from the high pressure reservoir 164 to the outer pistons 70. Because the valve 176 simultaneously prevents the flow of fluid out of the high pressure reservoir 164 via the high pressure passageway 174a, the inner pistons 64 are still hydraulically locked in place and the drive unit operates at a mechanical ratio of 1:1, even though the outer pistons 70 are not subjected to high pressure fluid.

The valve 176 provides the significant advan¬ tage that it allows an important reduction in wear and friction by isolating the outer pistons 70 from high

-^JRE ^"OMP pressure fluid when the drive unit 10 is operating at a mechanical ratio of 1:1. In this way the operating efficiency of the drive unit at a mechanical ratio of 1:1 is maximized: because the outer pistons 70 are not pushed with great force against the outer cam 143 fric¬ tion in the motor is at a minimum, and because the inner pistons 64 are locked in position and there is no relative rotation between the cylinder block 40, the valve head 100 and the inner cam 92^ friction in the pump is also at a minimum. Thus, at a mechanical ratio of 1:1 the input and output shafts are locked together as a single, coaxial unit, without any pumping or gearing activity of any kind.

The soft-start valve 190 acts to provide a true neutral mode of operation which is effective for all angular positions of the output shaft 20. When the valve rod 194 is in the position shown in FIG. la, high pressure fluid is free to flow directly from the high pressure reservoir 164 to the low pressure reservoir 166 via the passageways 200,202, thereby bypassing the outer pistons 70 completely. In this way the inner pistons 64 are hydraulically disconnected from the outer pistons 70 and hydraulic power transmission is interrupted. By gradually moving the valve rod 194 from the position of FIG. la to the left as seen in FIG. la, the bypass passageways

200,202 can be gradually disconnected, thereby gradually applying more and more hydraulic power to the outer pistons 70.

_The drive unit 10 described above provides a number of important advantages. First, this transmission operates to provide efficient torque multiplication and speed reduction with a minimum of pumping losses. At a mechanical ratio of 1:1, substantially all pumping and frictional losses are eliminated. Because the input shaft 90 reacts directly against the cylinder block 40 (which is in a sense an extension of the output shaft 20) torque is efficiently and directly transmitted from the input shaft 90 to the output shaft 20 at all mechanical ratios from 1:1 to 4:1, or even lower, if so designed.

Furthermore, this drive unit operates the pump comprising the inner pistons 64 at a low speed, always less than the speed of the input shaft 90 when the drive unit is operating in the forward reduction mode. For example, the pump speed is only 75% of the input shaft speed at a 4:1 mechanical ratio and is actually zero at a 1:1 mechanical ratio. In fact, in the forward reduction mode, the sum of the pump speed and the motor speed is always equal to the input shaft speed. Because the tilt angle of the inner cam 92 is fixed, the drive unit 10 provides the further advantage that, for a given maximum torque on the input shaft, the maximum hydraulic pressure generated by the input shaft 90 in the drive unit is a known, fixed value. This is in marked contrast to conventional, variable tilt angle pumps which can generate extremely high hydraulic pressures■ at small tilt angles. By using a fixed angle inner cam 92, the drive unit 10 eliminates such high hydraulic pressures and provides a simple, reliable design which does not rely heavily on pressure relief valves when operating in the forward reduction mode.

The nested configuration of the cylinder block 40, in which the- inner pistons 64 are positioned within the circle_ defined by the outer pistons 70, results in a compact design-which is extremely short in the axial direction. Furthermore, this nested configuration allows all valving means to be placed conveniently on one side of the cylinder block 40, thereby facilitating the placement and structural reinforcement of the necessary valving means.

The enlarged valve head 100 and the tension member 94 further contribute to the efficient operation of the drive unit 10 by providing a direct means for properly positioning the cylinder block 40 against the cams 92,143, which entirely eliminates the need for peripheral thrust bearings around the cylinder block 40 and which minimizes unnecessary relative movement. As explained above, when the drive unit is operating at a mechanical ratio of 1:1, all differential movement between the cylinder block 40 and the thrust bearing valve plate 116 is eliminated. It should be understood that the use of the thrust member 94 and valve head 100 is not limited to drive units of the type described above. A similar construction can be used to hold either a motor cylinder block or a pump cylinder block against a shaft mounted cam and to accept axial forces developed during the operation of the motor or pump. From the foregoing, it should be apparent that an efficient hydrostatically augmented mechanical drive unit has been described which is efficient in operation and small, lightweight, and compact in size. This drive unit is well suited for use in land vehicles, in which the input shaft would be coupled to an engine, such as a diesel engine for example, and the output shaft would be coupled to a load, such as the driving wheels or tracks of the vehicle for example. This drive unit is also well suited for other applications, such as constant velocity drives, where its light weight and small.,size are importan

Of course, it should be understood that various changes and modifications to the presently preferred embodiment described above will be apparent to those skilled in the art. For example, the drive unit of this

- δ^

C . I invention can be embodied in forms which utilize the standard sequential position for the motor and pump rather than the nested configuration described above. Furthermore, the motor activation and soft-start valves can be used with other types of drive units, including some of those discussed above. Of course, the specific angles, displacements and dimensions of the drive unit can readily be altered to fit individual applications. It is therefore intended that the foregoing description be regarded as illustrative of the presently preferred embodiment rather than as limiting the scope of the invention. It is the following claims, including all equivalents which are intended to define the scope of this invention.

Claims

CLAIMS :

1. In combination with a prime mover coupled to rotate a driving shaft, a driven shaft, and a load drivingly connected to the driven shaft, an improved drive unit comprising: a first axial face cam coupled to rotate with the driving shaft; a first cylinder body coupled to rotate coaxi- ally and as a unit with the driven shaft, said first cylinder body defining a first array of bores; a first set of pistons, each slidingly received within a respective one of the first array of bores, said first array of bores oriented such that the first set of pistons are actuated by the first cam; a second cylinder body coupled to rotate coaxially and as a unit with the driven shaft, said second cylinder body defining a second array of bores; a second set of pistons, each slidingly re¬ ceived within a respective one of the second array of bores; a second axial face cam positioned to actuate the second set of pistons; means for adjusting the tilt angle of the second cam with respect to the axes of the second array of bores; and valve means for conducting a hydraulic luid between the first and second arrays of bores

-said driving shaft being mounted coaxially with the driven shaft such that when the firs_.t set of pistons are hydraulically locked, in place, torque is directly transmitted from the driving shaft, via the first cam, the first pistons, and the first cylinder body, directly to the driven shaft, without relative movement between the driven shaft and the first cylinder body.

2. The invention of Claim 1 further comprising: means, included in the valve means, for select- ively interrupting the flow of high pressure hydraulic fluid from the first array of bores in order simultane¬ ously to hydraulically lock at least a portion of the first array of pistons in place in the first cylinder - body and to reduce hydraulic pressure on at least a portion of the second array of pistons.

3. The invention of Claim 1 further comprising: means, included in the valve means, for select¬ ively recycling hydraulic fluid among the first array of bores in order to reduce the flow of pressurized hydraulic fluid from the first array of bores to the second array of bores.

4. The invention of Claim 1 wherein the first cylinder body is disposed within the second cylinder body such that the first array of bores is nested within the second array of bores.

5. The invention of Claim 4 wherein the second cam is positioned on the same side of the first and second cylinder bodies as is the first cam.

6. In combination with a prime mover coupled to rotate a driving shaft, a driven shaft, and a load drivingly connected to the driven shaft, an improved drive unit comprising: pump means, actuated by the first cam, for pumping a hydraulic fluid and for mechanically trans- ferring torque from the driving shaft to the driven shaft, said pump means comprising: a first axial face cam coupled to rotate with the driving shaft; a pump cylinder block coupled to rotate coaxi¬ ally and as a unit with the driven shaft and defining a plurality of pump cylinders; and a set of pump pistons, each slidingly received in a respective one of the pump cylinders and positioned to be activated by the first cam; motor means, actuated by hydraulic fluid pumped by the pump means, for supplying torque to the driven shaft, said motor means comprising: a second axial- face cam mounted to provide a reaction point with respect to the driving and driven shafts; a motor cylinder block coupled to rotate coaxially and as a unit with the driven shaft and defining a plurality of motor cylinders; and a set of motor pistons, each slidingly received in a respective one of the motor cylinders and positioned to be activated by the second cam; means for adjusting the tilt angle of the second cam with respect to the motor cylinder block; and valve means for regulating the flow of hydraulic fluid between the pump means and the motor means; said driving shaft being mounted coaxially with the driven shaft such that when the pump pistons are hydraulically locked in place, torque is directly trans- mitted from the driving shaft, via the first cam, the pump pistons, and the pump cylinder block, directly to the driven shaft, without relative movement between the driven shaft and the pump cylinder block.

O A... VΓI 7. The invention of Claim 6 further comprising: means, included in the valve means, for select¬ ively interrupting the flow of high pressure fluid from the pump means to the motor means in order simultaneously to hydraulically lock at least a portion of the pump pistons in place in the pump cylinder block and to reduce hydraulic pressure in the motor means.

8. The invention of Claim 6 further comprising: means, included in the valve means, for select- ively causing fluid to bypass the motor means in order to interrupt the hydraulic transmission of power from the driving shaft to the driven shaft.

9. The invention of Claim 6 wherein the pump cylinder block is disposed within the motor cylinder block such that the pump cylinders are nested within the motor cylinders.

10. The invention of- Claim 9 wherein the second cam is positioned on the same side of the motor and pump cylinder bodies as is the first cam.

11. An improved drive unit comprising: a first shaft; a first axial face cam mounted to rotate with the first shaft; a second shaft; a second axial face cam mounted to provide a reaction point with respect to the first and second shafts; a cylinder block mounted to rotate coaxially and as a unit with the second shaft, said cylinder block defining a central bore, an inner array of cylinders, and an outer array of cylinders, wherein the inner array of cylinders is nested within the outer array of cylinders; a plurality of inner pistons, each slidingly received within a respective cylinder in the inner array of cylinders and oriented such that the inner pistons are actuated by one of the first and second cams; a plurality of outer pistons, each slidingly received within a respective cylinder in the outer array of cylinders and oriented such that the outer pistons are actuated by the other of the first and second cams; means for adjusting the tilt angle of a se¬ lected one of the first and second cams with respect to the cylinder block; and valve means for controlling the flow of a hydraulic fluid between the inner array of cylinders and the outer array of cylinders, said valve means com¬ prising: a valve head defining an axial port surface; and a tension member coupled between the valve head and the first shaft and extending through the central bore in the cylinder block such that the cylinder block is captured between the first cam and the axial port surface of the valve head, and axial forces generated by the inner pistons are transmitted by the tension member between the first cam and the valve head; said first shaft being mounted coaxially with the second shaft such that when the pistons actuated by the first -cam are hydraulically locked in place, torque is directly transmitted between the first and second shafts via the first cam, the pistons actuated by the first cam and the cylinder block, without relative movement between the second shaft and the cylinder block. 12. The invention of Claim 11 wherein both the first and second cams are positioned on the same side of the cylinder block such that the inner and the outer pistons contact the first and second cams, respectively, on the same side of the cylinder block.

13. The invention of Claim 11 or 12 wherein the valve means comprises means for conducting high pressure fluid between the inner array of cylinders and the outer array of cylinders and the invention further comprises means for selectively interrupting the flow of hydraulic fluid through the conducting means in order to hydraulic¬ ally lock at least a portion of the pistons actuated by the first cam in place in the cylinder block and to reduce hydraulic pressure on the pistons actuated by the second cam.

14. An improved drive unit comprising: a first shaft; a first axial face cam mounted to rotate with the first shaft; a second shaft; a housing; a second axial face cam mounted to the housing; a cylinder block mounted to rotate with the second shaft, said cylinder block defining a central bore, an inner array of cylinders, and an outer array of cylinders, wherein the inner array of cylinders is nested within the outer array of cylinders; a plurality of inner pistons, each slidingly received within a respective cylinder in the inner array of cylinders and oriented such that the inner pistons are actuated by one of the first and second cams; a plurality of outer pistons, each slidingly received within a respective cylinder in the outer array of cylinders and oriented such that the outer pistons are actuated by the other of the first and second cams; means for adjusting the tilt angle of a selected one of the first and second cams with respect to the cylinder -block; valve means for controlling the flow of a hydraulic fluid between the inner array of cylinders and the outer array of cylinders; and means for holding the cylinder block in posi¬ tion against the first and second cams.

15. The invention of Claim 14 wherein both the first and second cams are positioned on the same side of the cylinder block.

16. A variable speed drive unit comprising: a driving shaft; means for connecting the driving shaft to a prime mover; an inner axial face cam fixedly mounted to rotate coaxially and as a unit with the driving shaft; a housing; an outer axial face cam mounted to the housing; means for adjusting the tilt angle of the outer cam; a driven shaft mounted coaxially with the driving shaft; means for connecting the driven shaft to a load; a cylinder block mounted to rotate coaxially and as a unit with the driven shaft, said cylinder block defining a central bore, an inner array of cylinders, and an outer array of cylinders, wherein the inner array of cylinders is nested within the outer array of cylinders; a plurality of inner pistons, each slidingly received within a respective cylinder in the inner array of cylinders and oriented such that the inner piston^'s are actuated by the inner cam; a plurality of outer pistons, each slidingly received within a respective cylinder in the outer array of cylinders and oriented such that the outer pistons are actuated by the outer cam; a tension member defined by the driving shaft and disposed within the central bore of the cylinder block; a valve head rigidly mounted to the tension member, said valve head defining at least first and second ports positioned adjacent the cylinder block such that the cylinder block is rotatably mounted on the tension member between the inner cam and the valve head and selected ones of the inner cylinders are in fluid communication with the first and second ports; conduits for conducting fluid between the first and second ports and first and second fluid reservoirs, respectively; and conduits for conducting fluid between the first and second fluid reservoirs and the outer pistons; said valve head acting to hold the- cylinder block in place against the inner and outer cams and to transfer axial forces associated with the reciprocation of the inner pistons to the tension member, such that when the inner pistons are hydraulically locked in place, torque is directly transmitted from the -driving shaft to the driven shaft via the inner cam, the inner pistons, and the cylinder block, without relative movement between the driven shaft and the cylinder block.

17. The invention of Claim 16 further comprising: valve means for selectively interrupting the flow of fluid from the inner array of cylinders to the outer array of cylinders in order to hydraulically lock at least some of the inner pistons in place in the cylinder block and to reduce fluid pressure on at least some of the outer pistons.

18. The invention of Claim 16 further comprising: means for selectively recirculating fluid between the first and second fluid reservoirs to bypass the outer array of cylinders, thereby selectively inter¬ rupting the hydraulic transmission of power to the outer array of cylinders.

19. A hydraulic apparatus comprising: a shaft extending in an axial direction; an axial face cam mounted to rotate with the first shaft such that forces exerted on the cam are transferred to the shaft; a tension member defined by the shaft; a valve head rigidly mounted to the tension member, said valve head defining at least first and second ports; a cylinder block defining a plurality of cylinder bores and an axial central bore sized to receive the tension member, said cylinder block rotatably mounted on the tension member between the valve head and the cam with the tension member passing through the central bore such that selected ones of the cylinder bores are in fluid communication with the first and second ports; a plurality of pistons, each slidingly and sealingly disposed within a respective one of the cylinde bores to interact with the cam such that rotary motion between the cam and the cylinder causes the pistons to 5 reciprocate in the respective cylinder bores; and conduits for conducting fluid between the first and second ports and first and second fluid reservoirs, respectively; said valve head acting to hold the cylinder 10 block in place against the cam and to transfer axial forces associated with the reciprocation of the pistons to the tension member.

20. A variable ratio drive unit comprising: a housing; 15 a first drive shaft rotatably mounted to the housing;

¹ a second drive shaft rotatably mounted to the housing, co-linear with the first drive shaft; a hydrostatic pump mounted between the first 20 and second drive shafts, said pump comprising: an axial face pump cam mounted to rotate as a unit with one of the first and second drive shafts; a cylinder block mounted to rotate as a unit with the other of the first and second drive shafts and 25 defining, a plurality of pump bores; and a plurality of pump pistons slidingly received i respective ones of the pump bores and actuated by the pump cam such that differential rotation between the ; first and second drive shafts causes the pump pistons to

_! 30 reciprocate in the pump bores;

I a variable displacement hydrostatic motor acting between the second drive shaft and the housing; and valve means for controlling the flow of hydrauli fluid between the pump and motor, said valve means having a first state in which at least a portion of the pump pistons are hydraulically locked in place and hydraulic pressure on the motor is reduced such that the first and second drive shafts rotate in unison and torque is mechanically transmitted between the first and second drive shafts via the cam, the pump pistons and the pump cylinder block; said valve means having a second state in which pressurized hydraulic fluid is conducted from the pump to the motor to reduce the rotational speed of the first drive shaft with respect to the second drive shaft and to increase the torque on the second drive shaft proportionally.

21. The invention of Claim 20 wherein the pump is a fixed displacement pump.

22. The invention of Claim 20 wherein the motor includes an axial face motor cam mounted to the housing adjacent to the pump cam, a motor cylinder block mounted to rotate in unison with the second drive shaft and defining a plurality of motor bores, and a plurality of motor pistons slidingly received in respective ones of the motor bores and actuated by the motor cam, and further, wherein the pump bores are nested within the motor bores.

23. -The invention of Claim 20 wherein the pump further comprises a high pressure port and a low pressure port and the valve means further comprises a conduit interconnecting the high and low pressure ports and a valve positioned in the conduit to selectively open the conduit, thereby interrupting the transmission of torque from the first to the second drive shafts.

24. The invention of Claim 23 wherein the valve comprises a throttle valve to effect gradual closing of the conduit and therefore gradual application of torque to the second drive shaft.