Bidirectional Power Transmission System

Technical Field

This invention relates to a bidirectional power transmission system, and more specifically, to such a system wherein there is included a hydraulically en¬ gaged clutch for controlling the directional output of the system.

Background Art Many power transmission systems requiring a bi¬ directional rotary output utilize hydraulically en¬ gaged clutches for controlling the rotational direction of an output of the system. Such systems including hydraulically engaged direction control clutches may be found, for example, in construction vehicles or the like. In the usual case, an engine will provide a rotary, unidirectional input to a clutch assembly having two clutches, both of which may be hydraulically engaged and spring disengaged. When one of the clutches is en- gaged, the rotational output of the clutch assembly will have the same direction of rotation as that of the engine. When the other clutch is engaged, the rotational output of the clutch assembly will be in a direction opposite that of the engine. In a typical case, each of the clutches in the assembly will be formed of a clutch pack which may be compressed by a hydraulic piston when hydraulic fluid under pressure is applied against the piston. Typically, the clutch may brake a ring gear in a planetary gear set and, of course, the system will include means where¬ by both clutches cannot be simultaneously engaged.

The systems will usually include some sort of pressure modulating device interconnecting a source of

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hydraulic fluid under pressure such as a pump' and a valve which is utilized to select the direction of the assembly output, that is, control the engagement of one or the other of the clutches. The modulating means is typically a modulating relief valve and frequently, the same do not modulate pressure build-up to the desired extent, particularly at relatively low pressures. Thus, during initial engagement of such a clutch, "grabbing" in the clutch pack will occur rather than smooth en- gagement as desired. This, in turn, is communicated to components connected to the output of the clutch assem¬ bly as well as components therein and produces consider¬ able shock. Shock, in turn, reduces the useful life of such components causing premature wear and failure thereof.

Disclosure of the Invention

In one aspect of the present invention, there is provided in a bidirectional power transmission system including a bidirectional, hydraulically engaged clutch adapted to be coupled to a source of rotary power and to a rotary drive and having two hydraulic inlets, each for receiving hydraulic fluid under pressure, one to engage the clutch to cause one direction of rotation of the drive and the other to engage the clutch to cause the opposite rotation of the drive, a source of hy¬ draulic fluid under pressure, a directional control valve interconnecting the source and one or the other of the inlets, and a hydraulic pressure modulating de¬ vice connected between the source and the valve, the improvement including accumulator means connected to each of the inlets between the clutch and the valve. Each receives hydraulic fluid under pressure when the valve is directing fluid to an associated inlet for modulating pressure build-up therein to thereby provide smooth clutch engagement to minimize shock in the drive

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during initial engagement of the clutch.

Other objects and advantages of the invention will become apparent from the following specification taken in connection with the accompanying drawings .

Brief Description of Drawings

Fig. 1 is a schematic of a bidirectional power transmission system made according to one embodiment of the invention;

Fig. 2 is a graph illustrating the pressure rise in a prior art system;

Fig. 3 is a graph similar to that of Fig. 2 but illustrating the pressure rise in a system made accord¬ ing to the embodiment of the present invention.

Best Mode For Carrying Out The Invention An exemplary embodiment of a bidirectional, power transmission system made according to the invention is illustrated in Fig. 1 and is seen to be used in con¬ nection with an engine 10 having a rotary output shown schematically at 12. Ultimately, the engine 10 is to provide rotary power to a driven element 14. The driven element 14 includes a rotary input 16, shown schematicall .

Interconnecting the engine output 12 and the driven element input 16 is a clutch assembly, generally desig- nated 18. The clutch assembly 18 is for providing direction control in a conventional fashion. The same includes a forward clutch, generally designated 20, and a reverse clutch, generally designated 22. When the forward clutch 20 άs engaged, both the output 12 and the input 16 will be rotating in the same direction while when the reverse clutch 22 is engaged, the input 16 will be driven in a direction oppositely that of the output 12.

Each of the clutches 20 and 22 is comprised of a clutch pack including interleaved clutch disks.

_' Clutch disks 24 are, for example, splined to the housing 25 of ^" the assembly 18 while alternate ones of the clutch disks 26 may be splined, for example, to the radially outer surface of a ring gear in a conventional fashion.

Each clutch 20 and 22 includes a hydraulic piston 28 by which the pack comprising the disks 24 and 26 may be compressed. When such occurs, the disks 26 are no longer free to rotate thereby braking the associated ring gear. Disengagement of each clutch 20 and 22 is typically provided for by the use of a spring (not shown) biasing the associated piston 28 away from the pack. Hydraulic fluid under pressure may be directed to each of the pistons 28 through associated inlets 30 and 32 in the assembly 18 from a source 34 in the form of a hydraulic pump. Typically, but not always, the pump 34 will be driven by the engine 10.

The output of the pump 34 is connected to a modu¬ lating means in the form of a conventional modulating relief valve 36. The modulating relief valve 36 is operative to control the rate of pressure increase from the pump 34 but inherent in its construction is an inability to provide such control at relatively low pressures. For example, for a maximum pressure of about 300 psi, smooth pressure build-up provided by the modulating relief valve 36 can generally only be obtained from about 45-50 psi on upward to the maximum system pressure.

Hydraulic fluid under pressure from the valve 36 is conveyed via a conduit 38 to a direction selector spool valve 40 of conventional construction. The valve 40 can route hydraulic fluid under pressure to the in¬ lets 30 via a line 42, to the inlet 32 via a line 44, or to neither. When fluid under pressure is applied to the line 42, the forward clutch 20 will be engaged while when fluid under pressure is applied to the line

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44, the reverse clutch 22 will be engaged. When fluid under pressure is applied to neither line, neither clutch will be engaged.

An accumulator is connected to each of the lines 42 and 44 and serves to modulate pressure build-up in each line over the lower part of the range of system pressures. Thus, the accumulator acts over that part of the range wherein the valve 36 is ineffective to provide for smooth engagement of the clutches at low pressures, that is, during initial engagement.

According to the present invention, the accumula¬ tors are defined by a single, opposed spring accumulator, generally designated ^' 46. The same includes a body 48 having a bore 50 therein. A piston 52 is slidable with- in the bore 50 and divides the same into two chambers 54 and 56. The piston 52 carries a seal 58 so as to isolate the chambers 54 and 56 from each other. Addi¬ tionally, oppositely directed stops 60 project axially from both ends of the piston 52. Compression coil springs 62 in each of the chambers

54 and 56 abut opposite sides of the piston 52 and serve to bias the same to a predetermined position within the bore 50, normally, a central position as illustrated. To provide substantially identical pressure modulating characteristics irregardless of whether the forward clutch 20 or the reverse clutch 22 is being engaged, both of the springs 62 have substantially identical spring rates .

The system is completed by a line 64 connecting the chamber 54 to the line 42, and thus to the inlet

30 and by a line 66 connecting the chamber 56 to the line 44, and thus to the inlet 32.

Industrial Applicability

In operation, when it is desired to engage the forward clutch 20, the valve 40 will be shifted appro¬ priately to route fluid under pressure from the line 38

to the line 42. Typically, the valve 40 will include means whereby, for such a configuration, the line 44 will be connected to the hydraulic reservoir of the system. Fluid will flow through the line 42 through the inlet 30 to encounter the piston 28 for the forward clutch 20. The piston 28 will begin to move to engage the forward clutch pack. As the clutch pack begins to compress, resistence toward further movement of the piston 28 will increase resulting in pressure in the line 42 elevating. Such elevating pressure will result in the piston 52 in the accumulator 46 moving to the left against the bias of the leftmost spring 62. And, as the spring 62 compresses, it will offer increasing resistence to such movement of the piston 52.

In any event, the chamber 54 which varies in volume as the piston 52 moves will receive some of the ^' fluid from the pump 34 thereby modulating the pressure build-up in the line 42 so that the pressure build-up is gradual, in a large part, controlled by the value of the spring 62 resisting leftward movement of the piston 52.

Operation of the system when the reverse clutch 22 is engaged is essentially the same. The advantages of the system can be readily as¬ certained by a comparison of Figs. 2 and 3, the former illustrating pressure characteristics of a prior art system without the accumulator and the latter illustra¬ ting the pressure characteristics of a system with the accumulator.

Referring to Fig. 2 , typically, there will be a fairly abrupt increase in pressure from zero to about 15 psi as shown at A. This increase is tolerable with¬ in the system since the pressure is relatively low and is taken up by friction as well as resistence to move¬ ment of the pistons imposed by the springs (not shown)

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which bias the pistons to a non-engaged position!

Shortly thereafter, an abrupt increase, shown at B, occurs from 15 psi to 45 psi. This will occur upon initial engagement of the disks 24 and 26 and because the same is so abrupt, smooth engagement will not occur, but rather, there will be "jumping" which will impose shock loads upon components of the clutch 18 as well as the drive 14. As alluded previously, such shock loads increase wear and shorten the life of the system.

After about 45 psi has been attained in the system, the modulating relief valve 36 takes over and there will be a relatively smooth increase, shown at C, up to the maximum system pressure, for example, 300 psi.

In a system made according to the present inven¬ tion/ the initial increase to 15 psi, shown at A in Fig. 3 is also present. However, as mentioned, this is tolerable. The rate of pressure increase is shown in Fig. 3 at B ¹ over the range from 15 psi to 45 psi and it will be observed that it is relatively smooth, virtually a downward extension of the modulation provided at C ¹ by the modulating relief valve 36. Consequently, there will be no jumping within the clutch pack being engaged and no shock generated by such jumping. During the time period covering the pressure increase from 15 psi to 45 psi, the piston 52 of the accumulator will be shifting and at about 45 psi, it will be at the end of its stroke, one or the other of the stops 60 abutting the end of the bore 50 to which it is adjacent.

At this time, the modulating effect provided by the valve 36 will, of course, take over and cause smooth pressure increase up to maximum system pressure. In addition to the advantage of eliminating shock during initial engagement of the clutch, the exemplary

embodiment of the invention provides a further ad¬ vantage in terms of minimizing spacial requirements for the components thereof. The unique construction of the accumulator 46 provides for accumulators for both inlet ports in a single structure thereby con¬ siderably reducing the amount of space that would be required if two separate accumulators, one for each inlet, were employed.