GB2261029A - Fluid operated acuator overstroke prevention. - Google Patents

Abstract

A hydraulic bolt tensioner (10) (Fig. 2) includes an annular piston (15) and cylinder (20) defining chamber (19). To prevent inadvertent piston overstroke a piston skirt (152) is provided with a cam surface (27) opposite a relief passage (31) from the passage (17) that contains a relief valve (30). The valve body has a seating face (35), and a stem that extends through the valve seat and ends in a rounded cam follower (37), such that as the piston moves to overstroke, the cam surface displaces the valve body and relieves pressure in chamber (19) by permitting both supply fluid, and fluid ejected from chamber (19) by bolt tension reaction, to vent to the environment.

Description

Fluid operated actuator overstroke prevention This invention related to fluid operated actuators, and particularly, but not exclusively to such actuators used as, or in, bolt tensioning devices.

A bolt tensioning device is well known which attaches to one end of a threaded bolt or the like and, by interaction with a surrounding surface, tensions the bolt axially by a predetermined amount so that a cooperatively threaded nut can be applied to hold the bolt in said predetermined tension after the tensioning device is removed. Clearly, such tensioning has to be performed precisely to laid down standards both in respect of the functioning and safety of the bolted items and safety of the bolting operation itself.

In practice a bolt tensioning device comprises a piston, attachable to the bolt to be tensioned, contained within a cylinder arrangement that is seatable on the apparatus through which the bolt extends, hydraulic fluid being applied under pressure to a chamber defined between the piston and cylinder wall so that the piston, in tending to move along the cylinder, applies tension to the bolt and, insofar as the bolt elongates under tension up to its yield point, actually moves as the tensioned bolt stretches.

Hitherto, the degree of tension created within the bolt has been computed in relation to the bolt extension it produces and in accordance with this a level of tension predefined for any particular bolting operation is determined in accordance with the pressure of hydraulic fluid supplied.

It will be appreciated that exceeding the predefined bolt tension may have serious consequences both economic in terms of having to repeat the operation with a new bolt and in terms of safety should the bolt reach its yield point or the piston stroke exceed the capabilities of the apparatus.

In particular, if the stroke capacity of the piston and cylinder arrangement is exceeded then a sudden rupture of seals defining the chamber may occur. In respect of the problems consequent upon piston overstroke, the onset of such an undesirable, and incipiently dangerous, condition is wholly dependant upon the operator observing a limit marker or the like on the piston becoming exposed or otherwise made visible by a corresponding degree of movement by the piston relative to the cylinder and reacting to this by preventing more fluid from being supplied to the actuator. Unfortunately, because of the situations and orientations in which such bolt tensioning devices are used and the slow rate of piston stroke that makes the marker appear only slowly it is not unknown for an operator to not register, or at least not respond to, the appearance of the marker.

It has been proposed to prevent piston over stroke with a bolt tensioning device by providing a fluid pressure relief valve which is actuated in accordance with position relative to the cylinder. However, whereas it is simple in principle to provide such a relief valve, in practice this is less easily achieved without introducing constraints on tensioning design device that make it less satisfactory and commercially viable.

It is becoming increasingly necessary to provide tensioning devices with minimal overall dimensions to permit operation in confined situations and with easier handling and this is viable only through the use of higher fluid pressures, of the order of 1400 Bar (20,000 psi). This cause and effect place constraints on the provision of any additional facilities. The design of any additional relief valve is complicated by the operating pressures it has to withstand and the limited space available for additional features makes it difficult to provide such a relief valve of economical design.

One known form of bolt tensioning device includes a relief valve incorporating a spring loaded ball valve lifted off a seat by an actuator rod, extending through the piston wall in the direction of piston motion, encountering an abutment plate positioned to interact the piston, or rather the rod, path at the predefined stroke limit. At the onset of over stroke further rod motion is prevented and its relative movement through the piston opens the relief valve.

Such a relief valve arrangement may be considered unsuitable because of the requirement for a larger overall dimension of tensioning device by including the abutment plate and the cost penalty associated with the complexity of integrating the relief valve with the tensioning device.

Notwithstanding the above as a specific example of there it is desired to respond to the onset of piston overstroke, it will be appreciated that there are many other examples of where it is desired to prevent the onset of overstroke of a piston within a cylinder.

According to a first aspect of the present invention a fluid operated actuator comprises a piston, a cylinder containing at least part of the piston and a fluid passage by which fluid under pressure is admitted to a chamber, defined between portions of the cylinder and piston, to act on, and displace, the piston relative to the cylinder, the piston being provided with a skirt portion not forming part of the chamber extending alongside a wall of the cylinder and having a cam surface defined thereon, and the cylinder including a relief valve having a cam follower associated therewith adapted to be engaged by the cam surface over a range of piston positions and, at a predefined limit of piston travel representing the onset of overstroke, to operate the valve such that the pressure of fluid in said chamber is relieved.

According to a second aspect of the present invention a method of preventing overstroking of a skirted piston within a cylinder by fluid applied under pressure to a chamber, defined between a non-skirt portion of the piston and a portion of the cylinder, by way of a fluid passage, comprises defining a cam surface on the skirt portion such that the local separation between of the skirt surface and the cylinder wall varies as the piston approaches an overstroke condition, and providing within the cylinder a relief valve having a cam follower associated therewith and engaged by the cam surface and arranging that the cam follower is displaced by the cam surface at the onset of overstroke to relieve the pressure of fluid in the chamber.

In this specification a piston skirt is used to mean any extension of a piston not forming part of a pressure chamber wall with the cylinder.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a sectional elevation through one half of a generally annular fluid operated actuator according to the present invention in the form of a generally annular bolt tensioning device showing the piston fully retracted, and Figure 2 is a sectional elevation through a slightly modified form of the device of Figure 1 showing the piston at full stroke and operation of an overstroke prevention arrangement.

Referring to Figure 1 this shows a sectional elevation through half of an annular bolt tensioning device 10 according to the present invention that is symmetrical about an axis 11. As is well known in the art, a bolt tensioning device of this general type is used to create a predetermined tension in a bolt, prior to finally positioning a nut on the bolt, by securing a puller 12 to an extension of the bolt (not shown) which passes along a central passage 13 of the annular tensioning device and applying to the puller an axial force reacted by way of a bridge 14. against the body (not shown) through which the bolt protrudes.

The functional part of the tensioning device comprises a piston 15, a cylinder, shown generally at 16, containing at least a part of the piston and a fluid passage 17 by which hydraulic connection 18 to a chamber 19 defined between portions of the cylinder and piston.

The cylinder comprises a body 20, located on bridge 14, which is stepped in wall thickness at cylinder shoulder 21 to provide axially separated cylinder wall parts 201 and 202 of different radius. The piston 15 is likewise stepped to have a head portion 151 and a skirt portion 152 separated by piston shoulder 22 and the chamber 19 is defined between these shoulders and portions of the cylinder wall and piston skirt. To this end, the piston head 151 carries an annular 'outer' seal 23 which engages the inner wall of the cylinder part 201 and the inner wall of cylinder part 202 carries an annular 'inner' seal 24 which engages the skirt portion 152 of the piston.

The piston head 151 also has formed at its extremity a radially extending load bearing shoulder 25 on which the puller 12 sits and by way of which axial displacement of the piston relative to the cylinder or bridge is translated into a corresponding extension of, and generation of tension in, the bolt.

Referring also to Figure 2 this shows the formation of the chamber 19 by shoulders 21 and 22 and cylinder and piston skirt walls as the piston is displaced axially relative to the cylinder and shows the axial length of cylinder wall and piston skirt required to facilitate a predetermined maximum piston stroke. The piston head 151 also carried an overstroke indicator 26 in the form of a visually distinctive notch or mark on its outer cylindrical wall.

In normal operation the indicator is hidden within the cylinder as shown ip Figure 1 but at the maximum permitted piston stroke, that is, at the onset of overstroke, becomes visible as shown in Figure 2.

Hitherto, the normal way of preventing piston overstroke in this type of tensioning device whilst avoiding undue complexity of design or increase in size has been for an operator to observe the piston and respond to this visual marker by interrupting the supply of further fluid to the chamber 19.

In accordance with the present invention the piston skirt portion 152 extends much further along the cylinder wall than is required to define chamber 19 and is provided with a cam surface 27 defined by a recess 28 in the wall of the skirt and a ramp portion 29 joining regions of reduced and normal thickness in the direction of piston motion relative to the cylinder when executing a stroke. The recess 28 and ramp portion 29 may extend circumferentially of the skirt, both for ease of manufacture and to obviate any dependency on piston orientation.

The cylinder 16 includes within its body portion 202 a relief valve shown generally at 30. The relief valve 30 comprises a relief passage 31, extending from the fluid passage 17 to an opening 32 in the inner cylinder wall.

The relief passage 31 extends substantially radially through the cylinder body and (at least as shown in Figure 1) substantially orthogonally to the device axis 11 such that opening 32 is opposite the piston skirt recess when the piston is fully retracted and opposite the cam surface 27 as the piston approaches onset of overstroke. The arrangement of Figure 2, described more fully hereinafter, differs only in that the relief passage axis is inclined to the radial direction orthogonal to the axial stroke of the piston in the direction of piston stroke.

The relief passage is restricted at one point, adjacent the opening 32, to define a conical annular valve seat 33, and an elongate valve body 34 is contained in the relief passage. The valve body 34 comprises a central body portion 35 intermediate its ends that forms a valve face 351, co-operable with seat 33 to make fluid tight contact therewith and define a valve closed position for the body as shown in Figure 1.

The valve formed by the body 34 and valve seat 33 essentially comprises a poppet valve and one end portion 36 of the valve body comprises a valve stem which extends by way of the valve seat aperture towards the piston skirt and protrudes through opening 32 into the cylinder space.

The end of this valve stem is formed with a rounded profile defining a cam follower 37 that is normally contained in skirt recess 28 but arranged to co-operate with the cam surface ramp 29 as the piston approaches overstroke.

Forces imposed by the cam surface on the follower displace it, and the valve body, from the position shown in Figure 1 along the relief passage towards a position depicted in Figure 2 The other end portion 38 of the valve body comprises a hollow cylindrical part 381 which is a slidable fit within the relief passage to locate the valve body and arranged, by means of axial and radial cross-drillings 382, 383 to permit hydraulic fluid from passage 17 and supplied by connection 18 to flow past the valve body, and , subject to the valve not being seated, by way of valve seat 33 and opening 32 into the space between the cylinder and piston skirt.The portion 38 also provides by its overall radially extending surface an effective valve closure biasing means which responds to fluid pressure in the absence of mechanical forces exerted by the cam surface to increase the valve seating force with increasing pressure.

Prior to the onset of overstroke (the Figure 1 situation) the pressure of the fluid in the relief passage 31 and chamber passage 17 forces the valve body onto the seat 33 and fully closes the valve.

To ensure such closure at start-up when fluid pressure is not fully established and at any time when supply fluid pressure is reduced and the cam surface is not acting on the valve the bias means may include resilient bias means in the form of spring 39 extending between the hydraulic connection 18 and the valve body to bias it to a fully closed position independently of fluid pressure.

When the valve is opened by the cam surface at the onset of overstroke, and in consequence thereof fluid escapes, it will be understood that the escape of fluid serves two purposes. In addition to relieving pressure in chamber 19 and preventing further stroking of the piston, it alerts the operator to the reaching of the predefined stroke limit, possibly in addition to the visual marker 26 which may be retained to advise the operator to remove the supply of hydraulic fluid.

Notwithstanding such removal of the fluid supply, the pressure of any fluid in relief passage 31 is lower than the pressure previously existing in chamber 19 and any reaction from the bolt being tensioned on the piston will ted to retract it, pressure generated in the fluid being relieved by fluid flow out of passage 17 and actual retraction of the piston. Such retraction is, of course, accompanied by reduction in cam surface force on the valve body and permits spring 39 and any resilient fluid pressure in the relief passage to bias the valve towards closure.

Such closure prevents further loss of fluid, thereby stabilising the stroke position of the piston.

To improve the manner of valve closure the central body portion 35, at the junction of the valve seat 351 and end portion 36, may include a resilient annular seal ring 352 of suitable elastomeric material so that as the valve approaches the seat 33 the seal ring 352 closes the relief passage at the seat to further fluid and permits fluid pressure to build up quickly in the relief passage, thereby acting on the valve body to derive a closure bias force proportionally as large as the pressure to effect a good sealing engagement between the valve body face 351, and with seat 33.

It will be seen that the relief valve is provided in a manner that involves little additional structural variation to the basic tensioner design. That is, the relief passage is formed as a continuation of a radial drilling required in the cylinder body for attaching the hydraulic supply connection 18. Similarly the valve body, including the cam follower, is a unitary structure (with only spring 39 and seal ring 352) totally contained within the cylinder, either in the passage 31 or extending into the cylinder bore, and the actuator arrangement is a simple formation of a piston skirt, also within the cylinder bore.

The fluid ejected by pressure relief is also directed safely into the space between the cylinder wall and piston skirt rather than outwardly of the device where danger may ensue.

It will be appreciated that the overall dimensions of the tensioning device need not be increased by the provision of the relief valve of the invention.

It will also be appreciated that a number of constructional variations are possible, depending upon the precise construction of the basic bolt tensioning device without departing from the principle of the invention.

For instance, as shown in Figure 1 the relief passage may extend orthogonal to the direction of motion of the piston.

However, if it is desired to avoid any sideways forces on the valve body that may impede its axial displacement, the relief passage may be inclined to the direction of piston motion as shown in Figure 2.

The cam surface 27, in particular the ramp 29 may be formed other than by reference to an annular recess 28 in the piston skirt wall. For example, the recess and ramp may extend circumferentially for less than the whole periphery of the skirt, provided of course that piston orientation is observed. Also the cam surface may be formed not as a wall of a recess of lesser skirt thickness but as a wall portion of greater thickness adjacent a cylinder portion of lesser wall thickness. Furthermore, the ramp portion, instead of being an integral part of the piston skirt may be an additional component attached to the skirt.

It will be understood that although the above description has been confined to a hydraulic bolt tensioning device the principle of preventing the flow of additional fluid to a chamber, and which flow is the cause of piston motion relative to a cylinder by means of a valve actuated by cam forces operated by a cam surface on a skirt part of the piston is applicable to different fluid operated actuator configurations and uses.

Also, it will be understood that the fluid operated actuator of the invention is not limited to rectilinear motion of a piston within a cylinder but may be employed in respect of angular motions instead with suitable detail variations. In accordance with such variations it will be appreciated that any such piston and cylinder need not be of circular section nor indeed symmetrical. Furthermore, the actuating fluid need not be a liquid and in appropriate actuator configuration, and relief valve seating construction, may be a gas.

Claims (1)

1A A fluid operated actuator comprising a piston, a cylinder containing at least part of the piston and a fluid passage by which fluid under pressure is admitted to a chamber, defined between portions of the cylinder and piston, to act on, and displace, the piston relative to the cylinder, the piston being provided with a skirt portion not forming part of the chamber extending alongside a wall of the cylinder and having a cam surface defined thereon, and the cylinder including a relief valve having a cam follower associated therewith adapted to be engaged by the cam surface over a range of piston positions and, at a predefined limit of piston travel representing the onset of overstroke, to operate the valve such that the pressure of fluid in said chamber is relieved.

2 A fluid operated actuator as claimed in claim 1 in which the relief valve is operable to relieve pressure in the chamber by permitting the fluid from the chamber and fluid supplied under pressure to the actuator to flow through the relief valve to vent to the atmosphere.

3. A fluid operated actuator as claimed in claim 2 in which the relief valve includes bias means operable to bias it towards closure and responsive to retractive motion of the piston upon relief of pressure in the chamber to effect such closure.

4. A fluid operation actuator as claimed in any one of the preceding claims in which the relief valve comprises a relief passage within the cylinder body extending from the fluid passage to an opening in the cylinder wall adjacent the piston skirt and a valve body moveable within the relief passage and displaceable with respect to a valve seat by the cam follower to permit such fluid to vent by way of the clearance between the piston skirt and the cylinder wall.

5. A fluid operated actuator as claimed in claim 4 in which the relief passage is restricted at one point to define an annular valve seat, and a valve body has i) a central body portion intermediate its ends forming an annular valve face co-operable with the seat to make fluid tight contact therewith and define a valve fully closed position for the body, ii) one end body portion comprising a valve stem, extending by way of the valve seat towards the piston skirt and having an end profile co operable with the cam surface, defining said cam follower and responsive to forces imposed on the cam follower by the cam surface to lift the valve body from its seat and displace it along the relief passage towards a valve fully open position, and iii) the other end body portion being adapted to locate the body relief passage to permit the passage of fluid along the relief passage to the valve seat and present a surface against which the pressure of fluid in the passage can act to provide valve closure bias means.

6. A fluid operated actuator as claimed in claim 5 in which the valve seat and the central body portions are tapered in the direction of fluid relief flow to provide conical engagement surfaces.

7. A fluid operated actuator as claimed in claim 5 or claim 6 in which the central body portion carries an annular resilient seal member disposed to interrupt the flow of fluid past the valve seat at low fluid pressure when the valve is permitted to close by the cam surface, a resultant increase in pressure in the inescapable fluid acting on the valve to provide an increased closure force operable to effect a high pressure fluid tight seal between the valve body and the seat.

8. A fluid operated actuator as claimed in any one of claims 3 to 7 in which the bias means includes resilient bias means operable to bias the relief valve body towards the seat independently of any fluid pressure acting thereon.

9. A fluid operated actuator as claimed in any one of claims 4 to 8 in which the relief passage extends through the cylinder in a direction substantially radial in relation to an axial stroke of the piston.

10. A fluid operated actuator as claimed in claim 9 in which the relief passage is inclined, in relation to a radial direction orthogonal to the axial stroke of the piston, in the direction of the piston stroke such that the cam follower is engaged by the cam surface with reduced lateral loading of the relief valve body.

11. A fluid operated actuator as claimed in any one of the preceding claims in which the cam surface is defined on the piston skirt by means of a recess in the wall thereof defining a region of reduced thickness and a ramp portion joining regions of reduced and normal thickness in the direction of motion relative to the cylinder when executing a stroke.

12. A fluid operated actuator as claimed in any one of the preceding claims in which a substantially cylindrical piston having a substantially cylindrical skirt is arranged to stroke axially along within the cylinder and the cam surface defined on the piston skirt extends circumferentially of the piston skirt.

13. A fluid operated actuator as claimed in any one of the preceding claims in which the cam surface is integral with the piston skirt.

14. A fluid operated actuator substantially as herein described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.

16. A bolt tensioning arrangement including a fluid operated actuator as claimed in any one of the preceding claims.

17. A method of preventing overstroking of a skirted piston within a cylinder by fluid applied under pressure to a chamber, defined between a non-skirt portion of the piston and a portion of the cylinder, by way of a fluid passage, the method comprising defining a cam surface on the skirt portion such that the local separation between the skirt surface and the cylinder wall varies as the piston approaches an overstroke condition, and providing within the cylinder a relief valve having a cam follower associated therewith and engaged by the cam surface and arranging that the cam follower is displaced by the cam surface at the onset of overstroke to relieve the pressure of fluid in the chamber.

18. A method of preventing overstroking of a piston within a cylinder, substantially as herein described with reference to, and as shown in, the accompanying drawings.

19. A method of preventing overstroking of a piston within a cylinder of a bolt tensioning arrangement substantially as herein described with reference to, and as shown in, the accompanying drawings.