GB2457138A

GB2457138A - Bolt Tensioner with Sealable Chamber

Info

Publication number: GB2457138A
Application number: GB0802282A
Authority: GB
Inventors: David Michael Evans
Original assignee: Tentec Ltd
Current assignee: Tentec Ltd
Priority date: 2008-02-07
Filing date: 2008-02-07
Publication date: 2009-08-12
Anticipated expiration: 2028-02-07
Also published as: GB0802282D0; GB2457138B

Abstract

A bolt tensioner 1 has a body 2 with a central bore 3 receiving a cylindrical piston 4 having an annular protrusion 6 around the outer face, the protrusion abutting the sides of the bore 3 to define two chambers 7, 8, each having an inlet port 10, 12 for the introduction of fluid, the upper chamber 8 being sealable such that when the piston 4 moves upwards, reducing the volume of chamber 8, the pressure in the chamber is significantly increased, there further being a central threaded bore 5. Hydraulic fluid may be introduced and removed from chamber 7 to move the piston, and air or another compressible fluid may be used in chamber 8. A seal 9 may be provided between the chambers. Chamber 8 may have a channel or groove (20, figure 4a) such that the volume of the chamber never reaches zero. A one-way or selective release valve may be provided in fluid inlet 12 such that air can be introduced to chamber 8, but cannot escape. Thus a reliable return force may be provided without the use of a spring.

Description

BOLT TENSIONERS

This invention relates to a bolt tensioner of the kind comprising a body having a central screw-threaded bore for threaded engagement with a bolt projecting from a load bearing surface, and including at least one piston in the body which is located within a cylinder for displacement in a direction parallel to the longitudinal axis of the central bore and operable to exert a thrust force on the load bearing surface.

The term "bolt" is intended to include not only a bolt but also a comparable element such as a stud, screw-threaded bar, rod or shaft.

Bolt tensioners are the most consistently accurate method of applying accurate pre-load to bolts. However, a potential problem with thread tensioners of this kind stems from the need to provide a mechanism by which the piston is returned to a retracted position to reset the tensioner.

If this is not done then the piston will run Out of stroke after extended use.

The returning mechanism used can affect the accuracy with which the tensioner can work. For example, in one known arrangement a spring is provided that applies a return force to the piston. In use the fluid pressure overcomes the force of the piston to tension a stud. When pressure is released then the spring forces the piston back expelling any remaining fluid.

The applicant has appreciated that the use of a spring can, in some instances, introduce errors when tensioning if there is any non-linearity in the spring rate of the spring over its range of travel. This means that the force exerted on the piston by the spring will vary depending on where the piston is within its stroke. If this is so then it becomes important to measure the start position of the piston in order to provide a precise load to the piston. Making such a measurement is not always practical, is time consuming and can add to the cost of the tensioner. An alternative solution is to ensure that the piston is always fully reset before each use which again is time consuming.

We are also aware of UK Patent Application Publication no GB 2 412 705, which discloses a hydraulic bolt tensioner which uses a constant pressure air line to retract the piston.

According to the present invention there is provided a bolt tensioner comprising a body having a central screw-threaded bore for threaded engagement with a bolt projecting from a load bearing surface, and including at least one piston in the body which is located within a cylinder for displacement in a direction parallel to the longitudinal axis of the central bore and operable to exert a thrust force on the load bearing surface; further comprising: a tensioner body having an upper end and a lower end and a central bore defining an outer wall of a cylinder; a piston located within the bore defined by the body and having a central co-axial bore suitable for receiving a threaded stud and further in which the outer face of the piston is stepped such that the piston and the wall of the cylinder define a first chamber and a second chamber; a first passage extending from a first inlet port to the first chamber for introducing fluid to the first chamber; a second passage extending from a second inlet port to the second chamber for introducing fluid to the second chamber; and in which the first and second chambers are so arranged that moving the piston towards the upper end of the body increases the volume of the first chamber and moving the piston towards the lower end of the body increases the volume of the second chamber, and in which the second chamber is sealed such that fluid within the second chamber is prevented from escaping as the piston is moved towards the upper end of the body, causing a significant increase in pressure as the fluid is compressed.

By allowing the fluid in the second chamber (typically a gas) to be compressed in this manner, the increase in pressure can act in a similar manner to a return spring. According to Boyle's law, at constant temperature for a sealed system: pV=k where p is pressure, V is volume and k is a constant. This means that the relationship between the pressure and hence the force on the piston due to the compressed fluid will follow a predetermined relationship. Where the second chamber has constant cross section, the pressure on the piston (and therefore the force, assuming the area of the top face of the piston does not change) will be inversely proportional to the displacement of the piston from the (possibly notional) zero second chamber volume point.

As such, in order to be significant, the change in volume of the second chamber between the ends of travel of the piston may preferably be at least 10%, and preferably 50% or 100%, of the minimum volume of the second chamber.

Whilst a nominal zero volume point is referred to above, preferably even at the ends of the travel of the piston, the volume of the second chamber never falls to zero. If this were not the case, then the pressure in the chamber would theoretically rise to infinity as the volume approached zero; in practice, either a seal would fail, or it would be impossible to provide sufficient force to the piston with the hydraulic fluid at some point before the end of travel of the piston. To this end, the second chamber may be provided with a groove in at least one of its walls.

The bore of the piston may be threaded so as to receive a stud having a complimentary thread. Alternatively, a threaded insert may be located within the bore of the piston, perhaps with a flange against which the piston can react.

The fluid in the second chamber is preferably a compressible gas or mixture of gases, and is most preferably air. It may be supplied, for example, from a pressurised air supply line connected to the second inlet.

In use, the second chamber may be pressurised before the bolt tensioner is used to tension a bolt, and possibly even before the tensioner is installed upon a bolt.

The tensioner may be provided with a valve at the second inlet, which may selectively allow the introduction of compressible fluid into the second chamber. The valve may comprise a simple open/shut valve, but preferably comprises a one-way valve that in normal use only allows the introduction of fluid into, and not escape of fluid out of, the second chamber. This allows the second chamber to be pre-charged as discussed above.

The second chamber may also be provided with a selective release valve, which in normal use is closed, but can be opened to allow escape of the fluid in the second chamber once use of the tensioner has finished. This may be incorporated into the one-way valve, which hence may have a manual override.

The first chamber may be adapted to receive a substantially incompressible fluid, such as a hydraulic fluid.

A seal may be provided between the piston and the wall of the cylinder to separate the first chamber from the second chamber. The piston may carry the seal such that the seal moves with the piston in the cylinder.

The seal may be a nitrile or polyurethane seal.

The seal may be a dual acting seal which prevents passage of hydraulic fluid and air past the seal. Alternatively, a two separate seals may be provided that separately seal the first and second chambers -this allows for the makeup of each seal to be adapted to the expected contents of each chamber. A seal for the first chamber may be formed of polyurethane, whereas a seal for the second chamber may be formed of nitrile or polyurethane.

The piston may have a stepped outer wall defining a raised step part way along its length, an outer face of the step cooperating with the wall of the cylinder through the seal.

Thus, hydraulic fluid can be introduced under pressure into the first space to stretch a bolt.

The second chamber may comprise a further chamber spaced from the main body of the second chamber but in fluid communication therewith.

This allows for variance in the volume of the second chamber and so control of the operating point in the pV=k given above. However, as discussed above, the combined volume of the second chamber, comprising its main body defined by the piston the further chamber, should be such that a significant change in pressure occurs over the travel of the piston.

According to a second aspect of the invention, there is provided a method of using a bolt tensioner according to the first aspect of the invention, the method comprising pressurising the second chamber with a compressible fluid before using the bolt tensioning to tension a bolt with the second chamber pressurised.

By keeping the second chamber pressurised -above local atmospheric pressure -a controllable, predictable force can be applied to retract the piston. Hydraulic fluid may be supplied to the first chamber in order to drive the piston to tension the bolt; when the pressure in the hydraulic fluid is released, the pressure of the compressible fluid in the second chamber may act to predictably retract the piston.

The second chamber may be pressurised to a pressure of at least 50psi (340 kPa), preferably at least or around 8Opsi (550 kPa).

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a tensioner in accordance with the present invention; Figure 2 is a partially cut-away view of the tensioner of Figure 1; Figure 3 is an alternative plan view of the tensioner of Figure 1; Figure 4 is an enlarged, cut away, partial view of the tensioner fitted to a stud: (a) prior to tensioning the stud; and (b) whilst the stud is tensioned by applying hydraulic fluid to the tensioner; Figure 5 is the same view as Figure 4 showing the tensioner: (a) as hydraulic pressure is released from the tensioner; and (b) as pneumatic pressure is used to push the piston back to a fully retracted position.

In the embodiment shown in Figures 1, 2 and 3 the bolt tensioner 1 comprises a cylindrical body 2 having a central bore 3 which receives a cylindrical piston 4 having, in this example, a co-axial screw-threaded bore 5. In an alternative, a threaded insert could be proved which sits in the bore. An annular protrusion 6 extends around an outer face of the piston 4. The piston 4 and the bore 3 are so shaped as to define a first, lower chamber 7 and a second, upper, chamber 8 either side of the annular protrusion. A seal 9 on the outer face of the protrusion connects the piston 4 to the wall 3 of the central bore of the body 2, thus separating the first and second spaces. The seal 9 is double acting and prevents the passage of fluid from the first chamber to the second chamber and vice versa.

The piston 4 includes a passage 10 which extends from a connector 11 provided on an upper face of the piston 4 to an opening into the first chamber 7. A supply line (not shown) from a source of pressurised hydraulic fluid (not shown) may be connected to the connector 11. A valve in the supply line (also not shown) allows the pressure of the hydraulic fluid at the connector, and hence in the first chamber, to be adjusted.

A second passage 12 is also provided which provides communication between another connector 13 and the second chamber 8. This passage 12 can be seen in Figure 2 and runs through the body 2 of the tensioner 1.

The connector comprises a non return valve, normally allowing only the introduction of compressible fluid into the second chamber 8. The valve has a manual override 13a, which allows pressure in the second chamber to be released.

This connector 13 may be connected to a supply line (not shown) from a source of compressible fluid. A suitable source would be a pump which compresses air in a supply line to the second chamber 8.

As can best be seen in Figure 1, a lower end of the body 2 is extended downwards and has a cut-away 14 around at least a part of one side to permit access to the stud, and a nut on the stud, whilst the body is in place.

To use the tensioner 1 the air supply line is first connected and the hydraulic supply line is removed or the pressure is removed from the hydraulic line. The second chamber 8 is then charged with pressurised compressible fluid -in this embodiment, air -so that it pushes the piston 4 down to reduce the volume of the first chamber 7. The non-return valve in connector 13 prevents the pressurised compressible fluid from escaping. Once the chamber is pressurised to suitable pressure, say 8Opsi (55OkPa), the supply is removed; the air introduced into the second chamber will be held captive by the non-return valve.

The tensioner is then screwed down onto the stud to be tensioned. When the tensioner is fully screwed into position the lower end of the body will abut the load surface. In this position the tensioner is ready for a load to be applied to the stud.

Pressurised hydraulic fluid is then pumped along the supply line in to the first chamber 7 by passing through the passage 10 in the piston 4.

Initially the piston 4 will be at its lowest position as shown in Figure 4(a). As the pressure of the fluid in the space increase the piston is forced upwards, in turn applying tension to the stud. This can be seen in Figure 4(b). The movement of the piston 4 will be opposed by the pressure of the captive fluid in the second chamber 8.

Given that the amount of air within the second chamber 8 is held constant, then it is therefore relatively simple to allow for the opposing force of the air to be taken into account when calculating the load applied to the stud caused by the pressure of the hydraulic fluid in the first chamber. As discussed above, the relation pV = k will hold (where p is the pressure in the second chamber, V is the volume of the chamber, k is a constant and assuming the temperature remains constant) and so the force on the piston (the area of the face of the piston exposed to the second chamber multiplied by the pressure in the second chamber) can be predicted reliably.

As shown in Figures 4a and 4b of the accompanying drawings, the upper part of the body 2 of the tensioner is provided with a groove 20, in that part of the body 2 which forms a top wall 21 of the second chamber 8.

This groove in fact extends around the circumference of the top surface of the second chamber, but is not shown in the other Figures.

This groove means that, even if the piston 4 were driven to its uppermost extent against the top wall 21 of the second chamber 8, minimising the volume of the second chamber 8, the second chamber would still retain a non-zero volume. Compressible fluid in the second chamber can flow into the groove 20 in such case.

Figures 4a and 4b also show an alternative sealing situation to that depicted in the other Figures. In place of a double acting seal 9a, separate seals are provided to seal the first and second chambers from one another. A polyurethane hydraulic seal 9a is provided which prevents egress of hydraulic fluid from the first chamber into the second chamber, whereas a nitrile or polyurethane 0-ring seal 23 prevents egress of air from the second chamber into the first chamber. Furthermore, a set of 0-ring seals 22 are provided adjacent to the top wall 21 of the second chamber 8, so as to prevent egress of the captive air to atmosphere; such egress would effect the amount of air held captive in the second chamber and hence render the pressure less predictable.

Once the stud has been tensioned to the required level, the nut on the stud may be tightened down by hand and the pressure in the first space released to take the load from the stud. This can be seen in Figure 5(a).

The air pressure in the second chamber returns the piston to its position of rest as shown in Figure 5(b) with the hydraulic fluid expelled from the first chamber.

Claims

CLAIMS1. A bolt tensioner comprising a body having a central screw-threaded bore for threaded engagement with a bolt projecting from a load bearing surface, and including at least one piston in the body which is located within a cylinder for displacement in a direction parallel to the longitudinal axis of the central bore and operable to exert a thrust force on the load bearing surface; further comprising: a tensioner body having an upper end and a lower end and a central bore defining an outer wall of a cylinder; a piston located within the bore defined by the body and having a central co-axial bore suitable for receiving a threaded stud and further in which the outer face of the piston is stepped such that the piston and the wall of the cylinder define a first chamber and a second chamber; a first passage extending from a first inlet port to the first chamber for introducing fluid to the first chamber; a second passage extending from a second inlet port to the second chamber for introducing fluid to the second chamber; and in which the first and second chambers are so arranged that moving the piston towards the upper end of the body increases the volume of the first chamber and moving the piston towards the lower end of the body increases the volume of the second chamber, and in which the second chamber is sealed such that fluid within the second chamber is prevented from escaping as the piston is moved towards the upper end of the body, causing a significant increase in pressure as the fluid is compressed.
2. The bolt tensioner of claim 1, in which at the ends of the travel of the piston, the volume of the second chamber never falls to zero.
3. The bolt tensioner of any preceding claim, in which the second chamber is provided with a groove in at least one of its walls.
4. The bolt tensioner of any preceding claim, in which the fluid in the second chamber is a compressible gas, such as air.
5. The bolt tensioner of any preceding claim, in which the tensioner is provided with a valve at the second inlet, which selectively allows the introduction of compressible fluid into the second chamber; the valve comprises a one-way valve that in normal use only allows the introduction of fluid into, and not escape of fluid out of, the second chamber.
6. The bolt tensioner of any preceding claim, in which the second chamber is provided with a selective release valve, which in normal use is closed, but can be opened to allow the escape of the fluid in the second chamber once use of the tensioner has finished.
7. The bolt tensioner of any preceding claim, in which the first chamber is adapted to receive a substantially incompressible fluid, such as hydraulic fluid.
8. The bolt tensioner of any preceding claim, in which a seal is provided between the piston and the wall of the cylinder to separate the first chamber from the second chamber, the piston has a stepped outer wall defining a raised step part way along its length, an outer face of the step cooperating with the wall of the cylinder through the seal.: 25
9. The bolt tensioner of any preceding claim, in which the second S...chamber comprises a further chamber spaced from the main body of the *:*. second chamber but in fluid connection therewith. * .

*... . . .
10. A method of using a bolt tensioner according to any preceding :: 30 claim, the method comprising pressurising the second chamber with a *SS.

S S...compressible fluid before using the bolt tensioning to tension a bolt with the second chamber pressurised.
11. A bolt tensioner of the kind set forth substantially as described herein with reference to and as illustrated in all of the accompanying drawings. I... * * * ** * * S S.. S. * * SS * *. * . S... * I. 55. S... *..S