US3583158A

US3583158A - Transducer for converting fluid pressure oscillations into mechanical oscillations

Info

Publication number: US3583158A
Application number: US790374A
Authority: US
Inventors: Keith Foster; John Fredericton Duff
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1968-01-12
Filing date: 1969-01-10
Publication date: 1971-06-08
Anticipated expiration: 1988-06-08
Also published as: GB1252941A

Abstract

A transducer for converting fluid pressure oscillations into mechanical oscillations comprising a piston member moveable in a cylinder, first and second opposing faces of the piston member being disposed in first and second chambers of the cylinder, both chambers being adapted to contain pressure fluid, means to connect at least the first chamber to a source of oscillating pressure fluid to cause the piston member to oscillate relatively to the cylinder, and cooling means to permit a restricted flow of pressure fluid sequentially through one said chamber and then through the other said chamber to a relatively low pressure region, whereby to cool the transducer.

Description

United States Patent Inventors Appl. No.

Filed Patented Assignee Priority Keith Foster Birmingham, England;

John Fredericton Duff, New Brunswick, Canada Jan. 10, 1969 June 8, 1971 National Research Development Corporation London, England Jan. 12, 1968 Great Britain TRANSDUCER FOR CONVERTING FLUID PRESSURE OSCILLATIONS INTO MECHANICAL I 44 I 4r Primary Examiner-Paul E. Maslousky Attorney-Cushman, Darby & Cushman ABSTRACT: A transducer for converting fluid pressure oscillations into mechanical oscillations comprising a piston member moveable in a cylinder, first and second opposing faces of the piston member being disposed in first and second chambers of the cylinder, both chambers being adapted to contain pressure fluid, means to connect at least the first chamber to a source of oscillating pressure fluid to cause the piston member to oscillate relatively to the cylinder, and cooling means to permit a restricted flow of pressure fluid sequentially through one said chamber and then through the other said chamber to a relatively low pressure region, whereby to cool the transducer.

PATENTEI] JUN 8 ISYI SHEET 1 OF 2 PATENTEDJUH 8197i 35 3158 SHEET 2 OF 2 /z/// a g L 6., w; 22

TRANSDUCER FOR CONVERTING FLUID PRESSURE OSCILLATIONS INTO MECHANICAL OSCILLATIONS piston member moveable in a cylinder, first and second opposing faces of the piston member being disposed in first and second chambers of the cylinder, both chambers being adapted to contain pressure fluid, means to connect at least the first chamber to a respective source ofoscillating pressure fluid to cause the piston member to oscillate relatively to the cylinder, and cooling means to permit a restricted flow of pressure fluid sequentially through one said chamber and then through the other said chamber to a relatively low pressure region, whereby to cool the transducer.

The average magnitude of the said restricted flow of fluid per cycles of oscillation of the piston member may be percent to 20 percent (preferably 5 percent to l0 percent) of the cyclic swept volume of the first chamber.

By average magnitude" we mean the average magnitude per cycle taken over several cycles of oscillation of the piston member, and by swept volume" we mean the area of the fist piston face multiplied by the piston stroke.

Preferably the one chamber is the first chamber and {the cooling means may comprise a first vent means operative at least when the piston member has moved towards the first chamber beyond a predetermined position to permit the restricted flow of fluid from the second chamber to the relatively low pressure region, and second vent means adapted to permit the restricted flow offluid from the first chamber to the second chamber.

The first vent means may comprise a port in a wall of the cylinder member which is connected to the said low pressure region, said port being connected to the second chamber via a longitudinally extending groove when the piston member has moved towards the first chamber beyond a predetermined position.

The second vent means may comprise a duct through which the first chamber is in restricted communication with the second chamber.

The duct may contain a valve which when in a first position, allows a relatively unrestricted flow of fluid to pass from the first chamber to the second chamber, and when in a second position allows relatively restricted flow from the first chamber to the second chamber.

There may be provided a conduit adapted to contain further pressure fluid and communicating with said one chamber via a regulator valve so that the pressure of the fluid in the conduit is greater than the algebraic mean pressure of the fluid in the said one chamber, and control valve means to supply the pressure fluid from the conduit to the other chamber when the piston member moves excessively towards that chamber, whereby to resist said excessive movement.

A pressure accumulator may be provided in the conduit.

The control valve means may comprise a port in the wall of the cylinder member which communicates with the conduit, and a drilling or groove in the piston member which communicates with the port when said excessive movement occurs.

The invention also provides a source of oscillating pressure fluid and a transducer as set forth above, the oscillating pressure fluid being connected to the said first chamber and the piston member being connected to a driven load.

A further source of pressure fluid may be connected to the second chamber, the further source of pressure fluid being in antiphase relationship with the first-mentioned source.

It will be appreciated that the present invention can also be applied to transducers in which the cylinder moves relative to the piston member, and it is therefore to be understood that references to movement of the piston member relative to the cylinder are to be interpreted (where the context so permits) so as to include movement of the cylinder relative to the piston member.

The invention will be described, merely by way of example, with reference to the accompanying drawings, wherein FIG. 1 shows a transducer according to the present invention,

FIG. 2 shows a modified construction of part of the transducer of FIG. 1.

The terms left" and right as used herein are to be understood as referring to the directions seen in the drawings.

Referring to FIG. I, a transducer according to the present invention has a cylinder 10 with a piston member I2 mounted therein for sliding movement. The piston member 12 has a central relatively large diameter portion 14, and two

outer portions

16 and 18 of smaller diameter. The diameter of the outer portion 18 is intermediate that ofthe portions l4, 16.

An end face 20 of the outer portion 16 constitutes a first operative face of the piston member, and a shoulder 22 formed by the change in diameter between the central portion 14 and the outer portion 18 of the piston constitutes a second operative face of the piston member. The

faces

20 and 22 are parallel and oppositely directed, so that respective fluid pressure forces acting on the respective faces are in opposition.

Face 20 is disposed in a first chamber 24 on the cylinder 10, and face 22 is disposed in a second chamber 26 of the cylinder. Each chamber is adapted to contain fluid under pressure.

The first chamber is connected to a source of oscillating pressure fluid 28 via line 30. The oscillating pressure fluid causes relative oscillating movement of the piston member 12 and the cylinder 10, the fluid in chamber 26 constituting a fluid spring. The volumes of the

chambers

24 and 26 are chosen, having regard to the mass of the oscillating parts of the transducer and the frequency of oscillation of the oscillating pressure fluid, so that the chamber 24 functions as a capacitor chamber, the pressure fluid oscillations therein being phase advanced relative to the oscillations in the chamber 26, resulting in the mechanical oscillations being in phase with the fluid pressure oscillations produced by the source 28. A minimum loss of energy is thus obtained.

In this exemplary embodiment of the invention, the transducer is embodied in a rock drill, the cylinder 10 forming the body of the drill and being provided with handles (not shown) for ease of operation. The oscillations of the piston 12 are communicated to a bit 32 of the rock drill, to produce useful work. The piston is not connected to the bit 32 but strikes it to produce an almost instantaneous force suitable for rock drilling.

The transducer may alternatively be connected to, for example a tine or group of tines of earth-moving equipment, or moving parts of an agricultural implement, such as the blades ofa grass-cutting machine.

It will be appreciated that the drawing is purely diagrammatic, and that in practice the cylinder 10 is made up of several parts to facilitate assembly. Consequently, in practice, there are several interfaces through which slight, but almost unavoidable, leakage from either of the

chambers

24, 26 may occur. In particulanleakage may occur via

bores

34, 36 in the walls of the

chambers

24, 26 through which the piston member 12 passes.

There is therefore provided in the

bores

34, 36 respective

annular leakage ports

35, 37 in to which leakage flow may pass. Another leakage port 39 is provided to receive leakage from the right-hand end of the cylinder 10. The leakage ports are respectively connected via

lines

41, 42, 43 to a tank 40 which is at relatively low pressure. The leakage fluid is pumped back into the system via a makeup pump 44, a nonreturn valve 46 and a connection 48 to line 30.

Disposed between the

chambers

24, 26 of the cylinder 10 is a further chamber 38 which is open to the atmosphere. This chamber receives any excess leakage that may pass the

ports

35 and 37, and also ensures that the face 21 between the

portions

14 and 16 of the piston member 12 is not disposed in a closed volume. if this were so, that volume would act as a further fluid spring, and may reduce the efficiency of the transducer.

If during operation of the rock drill the drill bit suddenly breaks in to a region of relatively soft material, or if the drill body is withdrawn quickly, excessive rightward movement of the piston member 12 may occur. This is undesirable since in extreme cases the portion 14 of the piston member 12 may strike the end wall of chamber 26.

Furthermore, for efficient operation of the transducer, it is desirable that the piston member 12 is initially spaced from the drill bit 32 at the beginning of its power stroke and strikes it after a predetermined fraction of the power strike. It is therefore necessary to control the mean position of the piston member 12 in the cylinder 10.

Therefore, in order to limit the aforementioned excessive movement of the piston member and also to control the mean position thereof, a supply of high-pressure fluid is connected to the chamber 26 to force the piston leftward relative to the cylinder 10.

The piston member 12 thus comprises in its outer portion 18 a circumferential groove 52 which is normally closed by the walls of a bore 54 within which the portion 18 moves. The groove 52 is connected by means ofducting comprising radial drillings 56 a leftwardly extending substantially axial drilling 58, and further radial drillings 60 to a region of the outer portion 18 which is in contact with the chamber 26.

A circumferential groove 62 is provided in the wall of the bore 54 of the cylinder 10, the groove being connected to a source ofhigh-pressure fluid.

The source of high-pressure fluid comprises a conduit 64 provided with a pressure accumulator 66. The conduit 64 communicates with the first or capacitor chamber 24 via a flow regulatingvalve 68 of the nonreturn type. The valve 68 is arranged to permit fluid flow from the chamber 24 to the conduit 64 when the pressure in the chamber 24 exceeds the pressure in the pressure accumulator 66, but to substantially impede or even prevent flow in the opposite direction. This ensures that the algebraic mean pressure in the accumulator 66 is greater than the algebraic mean pressure in the chamber 24, so that the algebraic mean pressure in the conduit 64 is therefore greater than the algebraic mean pressure in the chamber 24.

Excessive rightward movement of the piston member 12 towards the second chamber results in the

grooves

52, 62 intercommunicating thus allowing pressure fluid from conduit 64 to pass to the second chamber 26. Since the mean pressure in conduit 64 exceeds the mean pressure in chamber 24 so that the force on face 22 exceeds that on face 20, the piston member 12 is pushed to the left.

The pressure accumulator 66 may be of a conventional type, comprising an elastic container for pressure fluid. Opening of the valve means 52, 62 allows the elastic container to contract, forcing into chamber 26 a volume of pressure fluid sufficient to move the piston 12 sufficiently leftward. Alternatively the pressure accumulator may merely comprise a volume of fluid with sufficient elasticity having regard to the frequency of the oscillating pressure fluid.

The

grooves

52, 62 thus constitute control valve means which, when opened, permit further pressure fluid to pass from the conduit 64 through ducting constituted by the

drillings

56, 58, 60 to the second chamber 26 to correct the excessive movement of the piston member 12 towards that chamber.

in order to cool the transducer it is necessary to arrange cooling means to provide a throughflow of fluid through one of the chambers and then through the other chamber and thence back to the relatively low pressure tank 40.

Therefore, the cooling means comprise in the second chamber 26 a first vent means constituted by the port 37 in the wall of the cylinder and at least one, preferably four, longitudinal grooves 70 in the portion 14 of the piston member.

The cooling means also comprise a second vent means comprising a duct 74 containing a valve 76 to transfer fluid from the first chamber to replenish the second chamber. The valve 76 also is arranged to control the transducer. When in the first or open position, this valve 76 allows relatively unrestricted flow between the

chambers

24, 26 and thus the pressures therein are at this time in phase and equal. Thus no oscillation of the piston member 12 occurs. When the valve 76 is set to a second or "closed" position, it is such that flow through the duct 74 is prevented except for the restricted flow of makeup fluid, to compensate for the venting of fluid from chamber 26. Oscillating movement of the piston member 12 thus occurs. The valve 76 may be set to intermediate positions whereby control ofthe amplitude of the oscillations is effected.

The valve 76 is such that when in the "closed" position the average flow of makeup fluid therethrough (taken over several cycles of oscillation ofthe piston member) is 5 percent to 20 percent, preferably 5 percent to It) percent of the swept volume of the first chamber 24. The swept volume of the chamber 24 is of course equal to the gross oscillating pressure fluid flow into or out of the chamber 24 in each cycle. The makeup fluid, of course, returns to the tank 40 via the venting groove 70, and the port 37, or via the

leakage grooves

35, 39, and thus serves to cool the transducer. It has been found that the above-mentioned makeup fluid flow rate gives adequate cooling, surprisingly without seriously impairing the performance of the transducer. It will be appreciated however that other embodiments of the invention may require greater or lesser cooling flow. 1

As previously stated, it is desirable to maintain a predetermined gap between the drill bit 32 and the mean position of the piston member 12. This gap is controlled in the following way. When the transducer is stationary, the pressures in the

chambers

24, 26 are substantially equal, due to the duct 74, and the valve 76. The areas of the

face

20 and 22 of the piston member are chosen so that there is a net leftward force on the piston member which is thus predisposed to creep to the left in the cylinder 10. This is achieved by making the area of face 22 slightly greater than that of face 20. Alternatively, if use is made of the reaction force on the piston member 12 that occurs when it strikes the drill bit 32 during operation of the transducer, the area of face 22 can be very slightly less than that of face 20. The piston member will then be predisposed to creep leftwards during operation.

It can be shown that the velocity at which the piston member 12 strikes the drill bit 32, and therefore the force on the piston member 12, increases with increasing initial spacing of the piston member and drill bit, until an optimum spacing and impact velocity is obtained. This increases the predisposed leftward creep of the piston member 12, and this creep must be controlled so that the initial spacing of the piston member and drill bit is optimized.

The leftward creep of the piston member is controlled by means of the grooves and the port 37, since when the chamber 26 is vented to the tank 40, the pressure therein is reduced and the pressure in the chamber 24 pushes the piston member 12 to the right.

Most of the cooling flow via the duct 74 occurs whilst the pressure in the chamber 26 is reduced due to it being vented to the tank 40, since the pressure difference between the

chambers

24 and 26 is then greatest. lf the area of the piston face 20 is only slightly greater than that of the face 22 there may be at times during the cycle, some reversal of the flow through the duct 74, but this is acceptable provided the net cooling flow (e.g. 5 percentl0 percent) is maintained, If the face 22 is greater than the face 20, e.g. to provide a leftward drift of the piston then it is desirable to introduce a nonreturn characteristic into the valve 76 when it is in its closed position, (e.g. by employing a stop valve in parallel with a nonreturn valve as the valve" 76) so that it permits greater flow from the chamber 24 to the chamber 26 than in the opposite direction. The cooling flow in the duct 74 then occurs at times when the pressure in the chamber 24 is greater than that in the chamber 26.

It will be appreciated that the extent to which the grooves 70 overlap the port 37 is dependent upon the degree of leftward movement of the piston member 12. Thus the rate at which the chamber 26 is vented to the tank 40 is dependent on the fraction of the grooves 70 which is in communication with tank 40 by overlapping the port 37. Thus, the greater the leftward movement of the piston in the cylinder, the more rapidly is the chamber 26 vented. The position of the leakage port 37 and the length of the grooves 70 are of course chosen to ensure that the limit of leftward movement of the piston member I2 results in a gap between the means position of the piston member and the drill bit 32 consistent with optimum efficiency of the transducer.

The position of the piston member 12 is thus accurately controlled enabling the mean position of the piston member to be accurately spaced from the drill bit 32.

Should a sudden drop in pressure occur in the chamber 24, eg due to a fracture of the supply line 30, the piston may be urged rapidly leftward beyond its usual limit of travel. The grooves 70 then communicate with the chamber 38, venting the chamber 26 directly to atmosphere. This serves to reduce the leftward velocity of the piston member, and may prevent the piston striking the end wall 51 ofthe chamber 24.

A portion ofa modified form of transducer is shown in FIG. 2. This transducer is similar to that of FIG. 1 and thus will not be described in detail except where it differs therefrom. Parts common to FIGS. I and 2 have common reference numerals.

In FIG. 2, the

drillings

56,58, 60 are replaced by axially extending grooves 61 cut on the circumferential surface of the portion 18 of the piston member I2. Excessive rightward movement of the piston member 12 causes the ends of the grooves 61 to overlap the circumferential groove. Pressure fluid is thus fed to the chamber 26 via the grooves 61. The

grooves

61, 62 thus constitute control valve means to supply pressure fluid from the conduit.

In FIG. 2, no axially extending grooves 70 are provided; instead, the first vent means is constituted by a drilling 45 in the cylinder bore 36. When leftward movement of the piston member 12 beyond a predetermined position occurs, the right-hand end of portion 14 uncovers the drilling 45, permitting a restricted flow of fluid from the chamber 26 to the relatively low pressure region 40. The single drilling 45 is employed in this embodiment in preference to the annular port 37 since the latter would produce too sudden a venting flow when uncovered by the portion 14 and the degree of cooling and the leftward movement of the piston member could not be accurately controlled. It will be appreciated that the alternative features shown in FIG. 2 may be applied singly to the transducer of FIG. 1 if so desired.

It will be appreciated that, in either of the embodiments of FIG. I or FIG. 2 the piston member could consist of two relatively moveable parts. In operation, such a two-part piston would act as a one-piece piston member but the parts thereof would be capable of relative movement to accommodate malalignments in the cylinder bores. The term "piston member" as used throughout this specification is to be taken to include such a two-part piston.

It will be appreciated that the transducer of FIGS. 1 or 2 may be made double acting by connecting to the chamber 26 a source of oscillating pressure fluid which is 180 out of phase with the source 28 connected to chamber 24. Such a doubleacting transducer may produce up to twice as much power as the single-acting types hereinbefore described, but requires a further source of oscillating pressure fluid (which may be integral with the first-mentioned source in a double-acting generator) and two supply lines 30.

In order to provide the cooling flow, the transducer is arranged to be such that a flow occurs from the first chamber 24, and through the second chamber 26, either by employing a nonreturn valve between the first and second chambers or by arranging the areas of the piston faces such that the mean pressure in the second chamber is less than in the first chamber.

In either the single or double-acting embodiments of the transducers the supply line 30 leading to the first chamber is replenished with cool pressure fluid from a point near the generator (not shown). In the double-acting embodiments, the cooling flow can, instead of being vented to a low pressure region via a port in the cylinder wall be vented from a point in the supply line 30, extending from the generators to the second chamber 26. This point is preferably near to the generator, so that the line is itsclfcooled.

We have found that the provision of the cooling flow in a transducer according to this invention brings the advantage that the transducer is maintained at an acceptable working temperature. If it were not for the cooling flow, the fluid in the transducer, particularly that trapped in the second chamber of the single-acting embodiments would become undesirably hot. The cooling flow maintains the transducer at a satisfactory temperature less by actually cooling the fabric of the transducer than by replacing the hot pressure fluid with fluid at a lower temperature, thus removing the source of heat.

We have found that this is particularly useful when the transducer is embodied in a lightweight machine such as a hand-held rock drill or road breaker. In such a construction, the energy output of the transducer is the kinetic energy of a fast-moving relatively light piston. Consequently, there are appreciable viscous losses in the fluid, manifested as waste heat. Since the transducer is relatively small, little heat is lost to the surrounding air, and if it were not for the cooling flow the transducer would become too hot for the operator to handle.

We have found that further sources of heat for which the cooling flow compensates when long flexible supply lines 30, are used, are the hysteresis of the flexible material and the viscous losses through the pipe connections in the lines 30, which pipe connections are conventionally of smaller bore than the lines themselves.

The reduction of the working temperature of the transducer also extends the life of the seals employed to prevent leakage.

Surprisingly we have found that the cooling flow, when of the magnitude suggested above, does not seriously reduce the efficiency of the transducer, although it has hitherto been though that appreciable leakage in an oscillating pressure fluid transducer is undesirable, and should be prevented ifpossible.

We claim:

I. A transducer for converting hydraulic fluid pressure oscillations into mechanical oscillations comprising:

a cylinder member;

a piston member having first and second opposing faces and moveable within said cylinder member;

said cylinder member having first and second chambers adapted to contain hydraulic fluid under pressure;

said first and second opposing faces of said piston member bounding in part said first and second chambers, respectively, of said cylinder member;

first means adapted to connect said first chamber to a source of oscillating pressure hydraulic fluid for causing relative movement between said piston member and said cylinder member in a first relative direction; second means associated with the second chamber to cause relative movement between said piston member and said cylinder member in a second relative direction opposite to the first relative direction whereby said first and second means cause relative oscillating movement between said piston member and said cylinder member;

means predisposing the piston member to creep in the second direction relative to the cylinder member towards a predetermined position;

said last-mentioned means including means adapted to permit limited flow of hydraulic fluid from said first chamber to said second chamber;

valve means operable upon relative movement of said piston member and cylinder to said predetermined position for permitting flow of fluid to a low pressure region from said second chamber.

2. A transducer as in claim 1 wherein the valve means comprises:

a port in a wall of said cylinder member,

means connecting said port to said low pressure region, and

a longitudinally extending groove connecting said port to the second chamber when the piston member has moved relative to said cylinder member to said predetermined position.

3. A transducer as in claim 1 wherein the means adapted to permit limited flow of hydraulic fluid from said first chamber to said second chamber includes a flow passage containing a valve which is an open position allows a relatively unrestricted flow of fluid to pass from the first chamber to the second chamber to stop relative oscillating movement between the piston member and the cylinder member, and when in the second position allows only said limited flow of hydraulic fluid from the first chamber to the second chamber.

4. A transducer as in claim 1 further comprising:

a conduit adapted to contain hydraulic pressure fluid and communicating with said first chamber,

a regulator valve disposed in the conduit and adapted to maintain the pressure of the fluid in the said first chamber, and

control valve means to supply pressure fluid from the conduit to the second chamber when the piston member moves excessively towards that chamber thus resisting such excessive movement.

5. A transducer as claimed in claim 4 wherein a pressure accumulator is provided in the conduit.

6. A transducer as claimed in claim 4 wherein the control valve means comprises a port in the wall of the cylinder member which communicates with the conduit, and a drilling or groove in the piston member which communicates with the port when said excessive movement occurs.

7. Apparatus comprising a source of oscillating pressure hydraulic fluid and a transducer as in claim 1, the source of oscillating pressure fluid being connected to the said first chamber and the piston member driving a load.

8. A transducer as in claim 1 wherein the means adapted to permit limited flow of hydraulic fluid from said first chamber to said second chamber comprise a nonreturn valve permitting flow only from the first to the second chamber.

9. A transducer for converting hydraulic fluid pressure oscillations into mechanical oscillations comprising:

a cylinder member,

a piston member having first and second opposing faces and moveablc within said cylinder,

said cylinder member having first and second chambers with said first and second opposing faces of the piston member bounding said first and second chambers,

means for connecting the first chamber to a source of oscillating pressure fluid for urging said piston member towards the second chamber in a first stroke directed in a first direction said second camber being effective to contain pressure fluid for urging the piston member towards the first chamber in a second stroke directed in a second direction opposite to said first direction vent means for venting pressure fluid from said second chamber to a low pressure region upon excessive movement ofthe piston during said second stroke, and

a restricted flow passage between said first and second chambers for permitting a restricted flow therebetween to substantially replenish fluid vented by said vent means.

10. A transducer for converting hydraulic fluid pressure oscillations comprising:

a piston member moveable in a cylinder,

the cylinder having first and second chambers, first and second opposing faces of the piston member bounding said first and second chambers of the cylinder,

means connecting the first chamber to a source of oscillating pressure fluid to urge the piston member towards the second chamber in a first stroke,

the second chamber containing pressure fluid which urges the piston member towards the first chamber in a second stroke, the second chamber being substantially full of pressure fluid throughout the said first and second strokes,

vent means for venting pressure fluid to a low pressure region from the second chamber upon excessive movement of the piston during said second stroke, and

a restricted but permanently open flow passage between the first and second chambers, permitting a restricted flow therebetween to substantially replenish fluid vented by said vent means.

Claims

1. A transducer for converting hydraulic fluid pressure oscillations into mechanical oscillations comprising: a cylinder member; a piston member having first and second opposing faces and moveable within said cylinder member; said cylinder member having first and second chambers adapted to contain hydraulic fluid under pressure; said first and second opposing faces of said piston member bounding in part said first and second chambers, respectively, of said cylinder member; first means adapted to connect said first chamber to a source of oscillating pressure hydraulic fluid for causing relative movement between said piston member and said cylinder member in a first relative direction; second means associated with the second chamber to cause relative movement between said piston member and said cylinder member in a second relative direction opposite to the first relative direction whereby said first and second means cause relative oscillating movement between said piston member and said cylinder member; means predisposing the piston member to creep in the second direction relative to the cylinder member towards a predetermined position; said last-mentioned means including means adapted to permit limited flow of hydraulic fluid from said first chamber to said second chamber; valve means operable upon relative movement of said piston member and cylinder to said predetermined position for permitting flow of fluid to a low pressure region from said second chamber.

2. A transducer as in claim 1 wherein the valve means comprises: a port in a wall of said cylinder member, means connecting said port to said low pressure region, and a longitudinally extending groove connecting said port to the second chamber when the piston member has moved relative to said cylinder member to said predetermined position.

4. A transducer as in claim 1 further comprising: a conduit adapted to contain hydraulic pressure fluid and communicating with said first chamber, a regulator valve disposed in the conduit and adapted to maintain the pressure of the fluid in the said first chamber, and control valve means to supply pressure fluid from the conduit to the second chamber when the piston member moves excessively towards that chamber thus resisting such excessive movement.

9. A transducer for converting hydraulic fluid pressure oscillations into mechanical oscillations comprising: a cylinder member, a piston member having first and second opposing faces and moveable within said cylinder, said cylinder member having first and second chambers with said first and second opposing faces of the piston member bounding said first and second chambers, means for connecting the first chamber to a source of oscillating pressure fluid for urging said piston member towards the second chamber in a first stroke directed in a first direction said second camber being effective to contain pressure fluid for urging the piston member towards the first chamber in a second stroke directed in a second direction opposite to said first direction vent means for venting pressure fluid from said second chamber to a low pressure region upon excessive movement of the piston during said second stroke, and a restricted flow passage between said first and second chambers for permitting a restricted flow therebetween to substantially replenish fluid vented by said vent means.

10. A transducer for converting hydraulic fluid pressure oscillations comprising: a piston member moveable in a cylinder, the cylinder having first and second chambers, first and second opposing faces of the piston member bounding said first and second chambers of the cylinder, means connecting the first chamber to a source of oscillating pressure fluid to urge the piston member towards the second chamber in a first stroke, the second chamber containing pressure fluid which urges the piston member towards the first chamber in a second stroke, the second chamber being substantially full of pressure fluid throughout the said first and second strokes, vent means for venting pressure fluid to a low pressure region from the second chamber upon excessive movement of the piston during said second stroke, and a restricted but permanently open flow passage between the first and second chambers, permitting a restricted flow therebetween to substantially replenish fluid vented by said vent means.