GB2535173A

GB2535173A - Piston rod-piston assembly for use in an actuator

Info

Publication number: GB2535173A
Application number: GB1502199.1A
Authority: GB
Inventors: G Holtgraver Edward; Howard James
Original assignee: QTRCO Inc
Current assignee: QTRCO Inc
Priority date: 2015-02-10
Filing date: 2015-02-10
Publication date: 2016-08-17
Also published as: GB201502199D0

Abstract

A piston/piston rod assembly comprises a piston 70 having a body 100 with a blind threaded bore 106 having a bottom surface 108. A piston rod 58 has a threaded portion 58A which is screwed into the blind bore 106 until an end surface 58C abuts the bottom surface 108 of the blind bore 106. To prevent removal of the piston rod 58 a set screw 116 is screwed into transverse threaded hole 114 until metal distorting formation 116A engages and deforms threaded portion 58A. In another embodiment set screw (234, fig 5) may register with a relieved portion (244). Reference is also made to an actuator, e.g. a scotch yoke actuator (10, fig 1), comprising a housing (12) having a first end (14) and a second end (16). The second end (16) is provided with a threaded travel stop (82) which is adjusted to alter an amount of travel of the piston 70.

Description

Title: Piston Rod-Piston Assembly for Use in an Actuator

Description of Invention

The present invention relates to piston and piston rod assemblies and actuators employing such assemblies.

BACKGROUND OF THE INVENTION

Actuators used for moving or controlling a mechanism are well known and widely used in many applications. Non-limiting examples of actuators include linear actuators, and rotary actuators which in turn can be hydraulic, pneumatic, electric, mechanical etc. Many actuators employ a piston/piston rod assembly as part of a prime mover, the movement of the piston/piston rod assembly resulting in linear or rotational movement of a driving member.

Two common types of actuators are spring return (SR) actuators and double acting (DA) actuators. In a typical SR actuator, in the power stroke, a piston/piston rod assembly, driven by pneumatic or hydraulic pressure, compresses a spring, the movement of the piston/piston rod assembly resulting in linear or rotary movement of a driving member. In the return stroke, the fluid pressure is released and the compressed spring moves the piston/piston rod assembly back to its original position or to some pre-set position. In prior art SR actuators, disconnecting the piston from the piston rod or bolt can result in serious injury because of the rapid release of the compressed spring force.

In DA actuators, and as is well known to those skilled in the art, fluid pressure reciprocates a piston/piston rod assembly in a housing. Typically, the housing is a cylindrical casing having a first end wall and a second end wall, the first end wall being interconnected to a housing for a driving assembly, the driving assembly having a driving member, e.g., a shaft. The second end wall typically has an adjustable positioner or travel stop screw which is threaded through the second end wall into the interior of the housing to a desired degree so as to pre-set a stop position for the return movement of the piston, the piston engaging the stop. In a typical prior art piston/piston rod assembly used in DA actuators, one end of the piston rod is threadedly received into a threaded bore in the piston body. However, in these prior art designs the end of the piston rod in the threaded bore does not contact the bottom surface of the threaded bore in the piston body. Accordingly, if the piston is traveling at high speed on the return stroke, the high speed impact between the piston body and the travel stop or positioner can fracture the piston resulting in entry of the travel stop screw into the piston bore with concomitant loss of stop position as well as pressure. Alternatively if the piston rod passes through the piston body, the piston rod contacts the stop screw again resulting in loss of stop position and pressure leakage.

SUMMARY OF THE INVENTION

In one aspect the present invention provides a piston/piston rod assembly which can be used in actuators, both linear and rotary.

In this first aspect, there is provided a piston and piston rod assembly comprising a piston comprising a piston body, said piston body having a peripheral edge, a front side and a back side, a blind bore with a bottom surface being formed in said front side, said blind bore having a threaded portion, and a piston rod, said piston rod having a first end forming a first end surface, a second end, a first threaded portion complimentary to said threaded portion in said blind bore and proximal said first end, and an elongate, first thread-free portion, whereby when said piston rod is threadedly received in said blind bore, said first end surface of said piston rod abuts said bottom surface of said blind bore.

In another aspect the present invention provides a piston/piston rod 15 assembly which can be used in a DA actuator.

In still another aspect the present invention provides a piston/piston rod assembly which can be used in an SR actuator.

Further, optional features of the first aspect of the invention are set out in the dependent claims.

In yet a further aspect, the present invention provides an actuator of either the DA or SR type employing a piston/piston rod assembly according to the present invention.

In this second aspect, there is provided an actuator comprising a housing; a driving assembly mounted in said housing, said driving assembly including a moveable, driving member; a first force module connected to said housing, said force module comprising a generally cylindrical casing, said casing having a first end wall and a second end wall; a piston and piston rod assembly mounted in said casing for reciprocal motion therein, said piston and piston rod assembly comprising a piston comprising a piston body, said piston body having a peripheral edge, a front side and a back side, a blind bore with a bottom surface being formed in said front side, said blind bore having a threaded portion, a piston rod, said piston rod having a first end having a first end surface, a second end, a first threaded portion complimentary to said threaded portion in said blind bore, and an elongate, first thread-free portion, whereby when said piston rod is threadedly received in said blind bore, said first end surface of said piston rod abuts said bottom surface of said blind bore; a connector attached to second first end of said piston rod, said connector being operatively connected to said driving assembly whereby reciprocal motion of said piston and piston rod assembly causes movement of said moveable member; a stop mounted in said second end wall, and forming a stop surface interiorly of said casing, said stop surface being engaged by said back side of said piston body when said piston and piston rod assembly are moved sufficiently away from said first end wall toward said second end wall; a first force applicator in said casing to move said piston and piston rod assembly in a first direction away from said first end wall towards said second end wall.

Further, optional features of the second aspect of the invention are set out in the dependent claims.

These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings, and which description is presented by way of example only, showing non-limiting embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a top plan view, showing one form of actuator according to the present invention.

Fig. 2 is a view similar to Fig. 1 showing another type of actuator according to the present invention.

Fig. 3 is an enlarged view, partly in section, of the circled portion designated as A of Fig. 1.

Fig. 4 is an enlarged, elevational view showing the damaged threads of the piston rod caused by the set screw in the embodiment shown in Figs. 1 20 and 3.

Fig. 5 is an enlarged view, partly in section, of the circled area designated as C in Fig. 2.

DETAILED DESCRIPTION OF NON-LIMITING PREFERRED EMBODIMENTS

As used herein, the term "actuator" means any device, motor, prime mover etc., which supplies and transmits a measured amount of energy for the operation of another mechanism or system. Thus, non-limiting examples of actuators include hydraulic actuators, pneumatic actuators, mechanical actuators, linear actuators, rotary actuators etc. More specifically, the term actuator as used herein refers to any servo mechanism that supplies and transmits a measured amount of energy for the operation of another mechanism or system and which employs a piston/piston rod assembly. While this description makes particular reference to a scotch yoke actuator, it is to be understood that, in view of the above, it is not so limited.

Referring then to Fig. 1 there is shown a scotch yoke actuator, shown generally as 10, actuator 10 comprising a housing 12 having a first end 14 and an opposite, second end 16. Extending through a top wall 18 of actuator body 12 is a rotatable shaft 20, shaft 20 being interconnected in a well known manner to a yoke 22 having a first yoke slot, 24 and second yoke slot 26, slots 24 and 26 being disposed on diametrically opposite sides of shaft 20. A first yoke pin 28 which is connected to a connector linkage 30 is received in slot 24 while a second yoke pin 32 received in second yoke slot 26 is attached to a connector linkage 34.

Disposed on either side of and operatively attached to actuator housing 12 are first and second force modules 40 and 42, respectively. Since force modules 40 and 42 are identical in construction, only force module 40 will be described in detail.

Force module 40 comprises a generally cylindrical casing 44 having a first end wall 46 and a second end wall 48. A plurality of circumferentially displaced tie rods 50 extend from and through first and second end walls 46 and 48, tie rods 50 being threaded at their respective ends, nuts 52 being received on the threaded ends. Thus when nuts 52 are tightened, casing 44 will be securely clamped between end walls 46 and 48.

First end wall 46 has a boss 54 with an opening 56 through which extends one end of a piston rod 58, piston rod 58 being attached to connector 30. A flanged adapter 60 has a flange 62 which is bolted to first end wall 46, adaptor 60 also being connected to housing 12 in a manner not shown but well known to those skilled in the art.

Casing 44 in conjunction with end walls 46 and 48 forms a generally cylindrical chamber 66. Disposed in chamber 66 is a compression spring 68, one end of which engages end wall 46 the other end of which engages a piston 70. As seen, compression spring 68 is concentrically disposed in chamber 66 relative to piston rod 58. Piston rod 58 is threadedly connected to piston 70 in a manner described more fully hereafter.

Piston 70 has a piston body 100 with a generally circular periphery 76, an annular, radially inwardly extending groove being formed in periphery 76.

As will be understood by those skilled in the art, groove 78 is adapted to receive a seal ring.

Second end wall 48 has a threaded through bore 80 in which is threadedly received an adjustable, threaded stop 82, stop 82 having an end surface as seen more clearly in Fig. 3 which forms a stop surface interiorly of chamber 66. A lock nut 84 is threadedly received on adjustable stop 82 such that when adjustable stop 82 is positioned as desired to stop the travel of piston 70 towards end wall 48, threaded travel stop 82 can be locked in position. In effect, travel stop 82 is an adjustable positioner which determines the amount of travel of piston 70 toward end wall 48.

There is a port 88 formed in end wall 48, pod 88 being connected in a well known manner to a source of pressurized fluid.

In operation, when pressurized fluid is introduced via port 88 into the space between end wall 48 and piston 70, piston 70 is forced towards first end wall 46 compressing spring 58. This movement of piston 70 also moves piston rod 68 in a direction towards end wall 46 which in turn drives connector 30 toward actuator housing 12 with the net result that yoke pin 28 moves yoke 22 in the direction of arrow B. This in turn causes rotation of driving shaft 20, which in turn can be connected to a valve having a rotatable valve element as is well known to those skilled in the art. It will be appreciated that in operation both force modules 40 and 42 are supplied with pressurized fluid and operate as described above with respect to force module 40.

Reference is now made to Figs. 3 and 4 for a detailed description of the structure shown in the circled area A of Fig. 1. Referring then to Fig. 3, circled area A is shown in enlarged fashion. Piston body 100 has a circular periphery as described above with reference to Fig. 1, a generally central hub 102 being formed on a front side 104 of piston body 100. a threaded, blind bore 106 being formed in hub 102. Blind bore 106 has a bottom generally flat surface 108. As seen in Fig. 3, positioner 82 has a stop surface 110 which when piston 70 is in the position shown in Fig. 3 is engaged by the back side 112 of piston body 72.

There is a threaded hole 114 formed in hub 102, threaded hole 114 being transverse to and in open communication with threaded blind bore 106, a set screw 116 being threadedly received in hole 114.

Piston rod 58 has a threaded portion 58A which has threads complimentary to the threads of threaded bore 106. Piston rod 58 also has a first end 58B forming a first end surface 58C, surface 58C engaging to bottom surface 108 of threaded bore 106 wherein the piston rod 58 is fully received in bore 106. As can also be seen, first end 58B is unthreaded for a short portion extending from end surface 58C and has a diameter which is less than the crest of the threads on the threaded portion 58A of rod 58 and is generally approximately the diameter of the roots of the threads on threaded portion 58A. As can also be seen, bore 106 likewise has a thread-free portion proximal bottom surface 108 of bore 106. This is occasioned by the fact that it is not possible to thread bore 106 all the way to bottom surface 108.

Accordingly the thread-free portion on first end 58B proximal first end surface 58C, since it does not engage the thread-free portion of bore 106 adjacent bottom surface 108 ensures that piston rod 58 will bottom out in bore 106.

With continued reference to Fig. 3 and additional reference to Fig. 4, it can be seen that set screw 116 has an annular formation 116A on its outer most end forming somewhat of an annular knife edge. In effect formation 116A can be considered a metal distorting formation. In this regard and with reference to Fig. 4 when set screw 116 is tightly threaded into threaded hole 114 such that metal distorting formation 116A engages the threads of threaded section 58A, the threads are distorted or deformed as shown in Fig. 4. This distortion of the threads of threaded section 58A prevents removal of piston rod 58 from piston body 72 thereby retraining spring 68 compressed in a safe assembly.

It will be understood that metal distorting formation 116A can take many forms including being only a flat surface so long that the formation has the ability to distort or deform the threads enough to prevent unthreading of piston rod 58 from piston body 72.

Referring now to Figs. 2 and 5 there is shown another embodiment of the actuator and piston/piston rod assembly. Actuator 200 shown in Fig. 2 is substantially the same as actuator 10 shown in Fig. 1 with the exception that whereas actuator 10 is of the SR type, actuator 200 is of the DA type and as will be seen with respect to Fig. 5 the piston rod piston assembly is slightly different. Referring then to Fig. 2, it can be seen that there are two DA force modules 202 and 204 which are identical in construction. Accordingly, only force module 202 will be described in detail. Force module 202 has a generally cylindrical casing 206 and first and second end walls 208 and 210 which together form a generally cylindrical chamber 212. First end wall 208 has a first port 214 while second end wall 210 has a second port 216, ports 214 and 216 allowing for the ingress and egress of pressurized fluid. Disposed in chamber 212 and described in greater detail hereafter is a piston 218 connected to a piston rod 220.

Referring now to Fig. 5 there is shown the portion of Fig. 2 in circle C enlarged. Piston 218 comprises piston body 222 which, like piston 70 shown in Fig. 1 has a generally circular periphery with an annular, radially inwardly extending groove for receipt of a seal ring. Piston body 222 includes a central hub 224, and on at least partially threaded blind bore 226 formed in hub 224 of piston body 222, bore 226 having a substantially planar or flat bottom surface 228 and a threaded portion 230. There is a threaded hole 232 formed in hub 224, hole 232 being transverse to and in open communication with blind bore 226. Received in threaded hole 232 is a set screw 234.

As seen, piston rod 220 has a first end 240 forming an end surface 242 which when piston rod 220 is threadedly received in piston body 222 engages 20 bottom surface 228 of bore 226.

Piston rod 220 also has a thread free relief portion 244 which is adjacent a second threaded portion 246, relieved portion 244 separating threaded portion 246 from threaded portion 230.

As seen in Fig. 5, set screw 234 is in register with relieved portion 244. Accordingly, when piston rod 220 is fully received in threaded bore 226 such that end surface 242 contacts bottom surface 228 and set screw 234 is tightened, no threaded portion of piston rod 220 is engaged by set screw 234.

Accordingly, for disassembly purposes once set screw 234 is unthreaded sufficiently, piston rod 220 can be unthreaded from piston body 222.

Piston body 222 has a front side 260 and a back side 262. Received in a threaded through bore 264 extending through end wall 210 is a threaded positioner 270 similar if not identical to positioner 82 as shown in Fig. 1. As can be seen, positioner 270 extends into chamber 212 a desired amount to provide a stop surface 272 which when piston 218 is in the position shown in Figs. 2 and 5, is engaged by the back side 262 of piston body 222. It can be seen in Fig. 5 that positioner 270 is generally coaxial with bore 226 and piston body 222 and accordingly with piston rod 220.

As noted above, there are two ports, 214 and 216 in open communication with chamber 212. Piston 218, when in the position shown in Figs. 2 and 5 essentially divides chamber 212 to form a second, sub-chamber 212A between the back side 262 of piston body 222 and second end wall 210. To move piston and piston rod reciprocally through chamber 212, pressurized fluid is first introduced into chamber 212A via port 216 to force piston 218 and piston rod 220 in the direction of arrow D (Fig. 2) i.e., towards first end wall 208. During this power stroke, any fluid in chamber 212 is exhausted through port 214. In the return stroke, and to move piston 218 and piston rod 220 in the direction of arrow E (Fig. 2) pressurized fluid is introduced through port 214 into chamber 12, any fluid in chamber 212A being exhausted through port 216.

It is on the return stroke, at the end of the outward travel of piston 218 where one advantage of the present invention can be fully appreciated. In the normal case, and as discussed above back side 262 of piston body 222 would contact the stop surface 272 of positioner 270. If piston rod 220 was not fully bottomed in bore 226 such that when surface 242 of piston 220 contacted bottom surface 228 of bore 226, and if the piston 218 was traveling at high speed towards second end wall 210, the ensuing high speed impact between piston body 222 and positioner 270 could cause fracture of piston 218, entry of positioner 270 into bore 226 resulting in loss of stop position and the passage of pressurized fluid through piston 218. If on the other hand piston rod 220 were to pass through piston body 222, the end 242 of piston rod 220 would contact the surface 273 of positioner 270. I.e., piston body 222 would not contact the stop positioner 270. Again there would be loss of stop position of the piston and pressure.

Another advantage of the piston/piston rod assembly described herein and with reference to the embodiments shown in Figs. 2 and 5, is that in high speed applications and if the piston rod 220 were not bottomed out in the body 222 of piston 220 as described above, the momentum of piston 220 could result in undue stress on the threads which connect the piston rod 220 to the threads of the threaded bore 226. This undue stress could initiate failure after some number of cycles. In other words, rather than the bottom surface 228 of bore 226 absorbing the force, the threads of bore 226 would be absorbing the force.

Further, if the piston/piston rod assembly were designed such that there was a throughbore in the piston body and the piston rod extended through the throughbore to contact the stop, once the piston rod contacted the stop, it would stop the valve, but that would mean that thread engagement between the piston rod and the piston would be relied upon to stop further motion of the piston. In this regard such a pass through piston rod would stop the valve, but the piston would want to continue moving being pushed by inertia and the spring forces, again stressing the threads. Additionally, a piston rod which is threaded only partially into the bore such that the end of the piston rod does not contact the bottom of the bore in the piston body also stresses the threads as the piston rod wants to continue forward movement towards the stop pushed by inertia and the forces applied by the valve.

The disadvantages discussed above of not having the piston rod bottom out in a blind bore in the body of the piston could, be alleviated to some degree by making the piston body more robust e.g., thicker, and hence heavier. This points to another advantage of the present invention in that the piston can be made lighter thereby requiring less material and resulting in an actuator construction having greater efficiency.

Although specific, non-limiting embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiments shown and described are strictly exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof

Claims

WHAT IS CLAIMED IS: 1. A piston and piston rod assembly comprising: a piston comprising a piston body, said piston body having a peripheral edge, a front side and a back side, a blind bore with a bottom surface being formed in said front side, said blind bore having a threaded portion, and a piston rod, said piston rod having a first end forming a first end surface, a second end, a first threaded portion complimentary to said threaded portion in said blind bore and proximal said first end, and an elongate, first thread-free portion, whereby when said piston rod is threadedly received in said blind bore, said first end surface of said piston rod abuts said bottom surface of said blind bore.
2. The assembly of Claim 1 wherein there is a hub formed on said front side of said piston body and said blind bore is formed in said hub.
3. The assembly of Claim 1 or Claim 2 wherein there is a threaded hole transverse to and in open communication with said blind bore.
4. The assembly of Claim 3 wherein there is a set screw threadedly received in said hole.
5. The assembly of any preceding Claim wherein said piston rod has a second thread-free portion and a second threaded portion, said second thread-free portion being between said first threaded portion and said second threaded portion.
6. The assembly of Claim 5 wherein said hole is in register with said second thread-free portion when said piston rod is received in said bore and said first end surface abuts said bottom surface, said set screw being in engagement with said second thread-free portion.
7. The assembly of any preceding Claim wherein said second end of said piston rod has a connector for connecting to a driving assembly.
8. The assembly of any of Claims 3 to 7 wherein said first threaded portion spans said threaded hole wherein said piston rod is received in said blind bore.
9. The assembly of Claim 8 wherein when said piston rod is received in said bore, and said set screw is received in said hole, said set screw has an end surface which engages said first threaded portion.
10. The assembly of Claim 9 wherein said end surface of said set screw has a metal distorting formation formed therein.
11. The assembly of Claim 8 wherein said second end of said first piston rod has a connector for connecting to a driving assembly.
12. The assembly of any preceding Claim wherein the peripheral edge of said piston body has an annular, radially inwardly extending groove.
13. An actuator comprising: a housing; a driving assembly mounted in said housing, said driving assembly including a moveable, driving member; a first force module connected to said housing, said force module comprising a generally cylindrical casing, said casing having a first end wall and a second end wall; a piston and piston rod assembly mounted in said casing for reciprocal motion therein, said piston and piston rod assembly comprising: a piston comprising a piston body, said piston body having a peripheral edge, a front side and a back side, a blind bore with a bottom surface being formed in said front side, said blind bore having a threaded portion, a piston rod, said piston rod having a first end having a first end surface, a second end, a first threaded portion complimentary to said threaded portion in said blind bore, and an elongate, first thread-free portion, whereby when said piston rod is threadedly received in said blind bore, said first end surface of said piston rod abuts said bottom surface of said blind bore; a connector attached to second first end of said piston rod, said connector being operatively connected to said driving assembly whereby reciprocal motion of said piston and piston rod assembly causes movement of said moveable member; a stop mounted in said second end wall, and forming a stop surface interiorly of said casing, said stop surface being engaged by said back side of said piston body when said piston and piston rod assembly are moved sufficiently away from said first end wall toward said second end wall; a first force applicator in said casing to move said piston and piston rod assembly in a first direction away from said first end wall towards said second end wall.
14. The actuator of Claim 13 wherein said stop comprises an adjustable positioning member.
15. The actuator of Claim 13 or Claim 14 wherein there is a second force applicator in said casing to move said piston and piston rod assembly in a direction away from said second end towards said first end wall.
16. The actuator of Claim 13, 14 or 15 wherein said first force applicator comprises a compression spring disposed between with said first end wall and said front side of said piston body.
17. The actuator of Claim 15 or Claim 16 wherein said second force applicator comprises pressurized fluid between said second end wall and said back side of said piston body.
18. The actuator of any of Claims 13 to 17 wherein said first force applicator comprises a first pressurized fluid between said first end wall and said front side of said piston body.
19. The actuator of Claim 18 wherein said second force applicator comprises a second pressurized fluid between said back side of said piston 20 body and said second end wall.
20. The actuator of any of Claims 13 to 19 wherein said moveable member comprises a rotatable shaft.
21. The actuator of any of Claims 13 to 20 wherein said first force module is connected to a first side of said housing and there is a second force module connected to a second side of said housing.
22. The actuator of Claim 21 wherein said first and second force modules are spring return modules.
23. The actuator of Claim 21 or Claim 22 wherein said first and second modules are double acting force modules.
24. A piston and piston rod assembly substantially as hereinbefore described and/or as shown in the accompanying drawings.
25, An actuator substantially as hereinbefore described and/or shown in the accompanying drawings.
26. Any novel feature or novel combination of features substantially as described herein and/or as shown in the accompanying drawings.