GB2412705A

GB2412705A - A bolt tensioner

Info

Publication number: GB2412705A
Application number: GB0513012A
Authority: GB
Inventors: Anthony Scrivens
Original assignee: Tentec Ltd
Current assignee: Tentec Ltd
Priority date: 2004-08-13
Filing date: 2005-06-27
Publication date: 2005-10-05
Anticipated expiration: 2025-06-27
Also published as: GB0418053D0; GB2412705B; GB0513012D0

Abstract

A bolt tensioner (1) comprises a body (2) having a central threaded bore (5) for engagement with a bolt. A piston (4) is located in a cylinder (3) in the body (2). The piston (4) and a wall of the cylinder (3) define a first chamber (7) and a second chamber (8). Pressurisation of the first chamber (7) results in the piston being moved towards an end of the body (1). Pressurisation of the second chamber (8) results in the piston being moved towards another end of the body (2). The piston may be extended using hydraulic fluid but retracted using pneumatic fluid.

Description

24 1 2705

A BOLT TENSIONER

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. Such a bolt tensioner is hereinafter referred to as "of the kind described".

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-linearily 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 stoke. 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.

According to the present invention a bolt tensioner is provided of the kind described which comprises: 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 to 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 pressurising the first chamber acts to move the piston towards the upper end of the body as the volume of the first chamber expand and pressurizing the second chamber acts to move the piston towards the lower end of the body as the volume of the second chamber increases.

By providing a piston within the tensioner body which can be pushed in either direction under pressure from fluid, the need for a return spring is removed, which can lead to improvements in the accuracy of the tensioner.

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 second chamber is preferably adapted to receive fluid which is held in the second chamber under constant pressure.

Filling the second chamber with a fluid held under constant pressure is advantageous because it ensures that a constant force opposing movement of the piston is provided at all times.

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 chamber.

The constant pressure can be supplied from a supply line connected to a constant pressure source.

The fluid in the second space is preferably a compressible fluid. Because the change in volume of the second chamber and the supply line to the chamber will be relatively small as the tensioner is placed under load by introducing a fluid to the first chamber. Since the fluid is compressible, the small change in volume results in a substantially constant pressure opposing movement of the piston, improving accuracy of the tensioner.

It will be appreciated that by constant pressure we mean that the pressure of the fluid in the chamber is constant at least during loading of the tensioner.

The pressure of the fluid in the second chamber should be chosen such that it applies enough force to the piston to cause it to retract when the pressure in the first chamber is released.

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 nitrite seal, and may be a snap fit with the piston.

The seal may be a dual acting seal which prevents passage of hydraulic fluid and air past the seal.

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. When finished, air can be introduced into the second space to push the piston back to its retracted position, thereby expelling fluid from the first space.

A cap may be provided which retains the piston within the body. This may be removable to permit the piston to be exchanged for a different piston. This allows different pistons with different threaded bores to be inserted, allowing the tensioner to be used with studs having different threads. The cap may be fixed to the body by a plurality of bolts spaced around a circumference of the tensioner body. Alternatively, the cap may be threadedly secured to the body.

The first passage connecting the first space to the first inlet may extend through the piston The second passage may extend through the body of the tensioner to connect the second space to the second inlet valve.

According to a second aspect the invention provides a combination of a tensioner according to the first aspect of the invention, a source of hydraulic fluid in communication with the first chamber and a source of compressible fluid held at a constant pressure in communication with the second chamber.

The source of compressible fluid may comprise a pump connected to a supply line which is in communication with the second chamber.

The supply line may be replaced by a self-contained source of compressible fluid, such as a gas bottle which is in communication with the second chamber.

It will be appreciated that the combined volume of the second chamber and the supply line/gas bottle should be relatively large such that movement in the position of the piston does not alter the pressure of the fluid in the chamber. Alternatively, the second chamber may be sized such that the there is no significant change in pressure as the piston moves; in such a case the second chamber acts as the self-contained source of compressible fluid. The second chamber may be five, ten, twenty, fifty or a hundred times bigger than the maximum volume displaced by the piston.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure l is an isometric view of a tensioner in accordance with the present invention; Figure 2 is a partially cur-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; and Figure 6 shows an alternative embodiment of a tensioner according to the present invention.

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 HI 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 1 0 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.

Connected to this passage is a supply line (not shown) from a source of compressible fluid which is held under a constant pressure. A suitable source would be a pump which compresses air in a supply line to the second chamber 8. By arranging for the volume of the supply line to be far greater than the volume of the second chamber the pressure in the line should not vary much if at all during operation of the tensioner.

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 constant pressure of air in the second chamber 8 then pushes the piston 4 down to reduce the volume of the first chamber 7.

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 constant air pressure in the second chamber 8. However, as this is at a constant pressure, or substantially constant, the opposing force will be constant at all points in the stroke of the piston 4. 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.

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 constant 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.

In an alternative embodiment shown in Figure 6 of the accompanying drawings, corresponding features have been shown with the same reference numerals raised by 100. The tensioner 101 functions in the same manner as that of the first embodiment except that, rather than requiring continual connection in use to a constant pressure air supply, the second chamber 108 is enlarged with respect to the first chamber 8 of the first embodiment. In this ease, the air supply is connected to port 113 before use, to charge the second chamber 108 with sufficient air to reach a predetermined pressure. The air supply can then be disconnected, leaving the second chamber 108 at the predetermined pressure. Given that the stroke of the piston 104 is similar to that of the piston 4 of the first embodiment, the volume of the second chamber 108 will not change significantly as hydraulic fluid is introduced into first chamber 107. The pressure in the second chamber 108 will therefore stay approximately constant, leading to an approximately constant force being applied to the piston as it moves.

Claims

1. 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 pressurising the first chamber acts to move the piston towards the upper end of the body as the volume of the first chamber expand and pressurising the second chamber acts to move the piston towards the lower end of the body as the volume of the second chamber increases.

2. The bolt tensioner of claim 1, in which the second chamber is adapted to receive fluid which is held in the second chamber under constant pressure.

3. The bolt tensioner of claim 1 or claim 2 in which the second chamber is adapted to hold a compressible fluid.

4. The bolt tensioner of any preceding claim, in which the first chamber is adapted to receive a substantially incompressible fluid.

5. 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.

6. The bolt tensioner of all of claim 3, 4 and 5 in which the seal is a dual acting seal which prevents passage of hydraulic fluid and air past the I O seal.

7. The bolt tensioner of claim 5 or claim 6, in which the piston may 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.

8. The bolt tensioner of any preceding claim, in which a cap provided which retains the piston within the body.

9. The bolt tensioner of claim 8, in which the cap is removable to permit the piston to be exchanged for a different piston.

10. The bolt tensioner of any preceding claim in which the first passage extends through the piston.

11. The bolt tensioner of any preceding claim in which the second passage extends through the body of the tensioner.

12. A bolt tensioner of the kind set forth substantially as described herein with reference to and as illustrated in the accompanying drawings.

13. A combination of a tensioner according to any preceding claim, a source of hydraulic fluid in communication with the first chamber and a source of compressible fluid held at a constant pressure in communication with the second chamber.

14. The combination of claim 13, in which the source of compressible fluid comprises a pump connected to a supply line which is in communication with the second chamber.

15. The combination of claim 13 or claim 14, in which the source of compressible fluid is self contained.

16. The combination of claim 15 in which the source of compressible fluid is a gas bottle.

17. The combination of claim 15 or claim 16 in which the combined volume of the second chamber and the source of compressible fluid is such that movement in the position of the piston does not significantly alter the pressure of the fluid in the chamber.

18. The combination of claim 17 in which the second chamber acts as the self-contained source of compressible fluid.