GB1584158A

GB1584158A - Tensioning device for tensioning a rod

Info

Publication number: GB1584158A
Application number: GB2000777A
Authority: GB
Original assignee: Sulzer AG; Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1976-05-13
Filing date: 1977-05-12
Publication date: 1981-02-04
Also published as: DK146976B; DK146976C; IT1075339B; DE2625407B1; DK194677A; JPS52137556A; JPS5527968B2; DE2625407C2; CH610803A5; ATA407676A; AT357958B; SU969146A3

Abstract

The tightening device has a hydraulic arrangement consisting of cylinder (1) and piston (2) for producing a tensile force on the screw bolt and has a flexible spacer device (15), arranged between cylinder (1) and piston (2), and a restoring spring (18) exerting a restoring force on the piston (2). The spacer device (15) secures a minimum axial initial extent (H) of the pressure space (8) between cylinder (1) and piston (2). This initial extent (H) is necessary for releasing the nut (21) on the screw bolt without restraint. In its compressed state, the restoring spring (18) serves as a stop which prevents the piston (2) from moving out of the cylinder (1). The device is intended in particular for use in the case of tie rods of diesel engines. <IMAGE>

Description

(54) TENSIONING DEVICE FOR TENSIONING A ROD (71) We, SULZER BROTHERS LIMITED, a Company organised under the laws of Switzerland, of Winterthur, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a tensioning device for tensioning a screwthreaded rod during the tightening or loosening of a nut thereon, the device including: a piston and a cylinder of which one affords a screwthreaded bore for the reception of the rod, the cylinder and piston together defining a pressure chamber in which the fluid pressure, in use of the device, tends to move the piston in a direction away from a bottoming position and out of the cylinder to tension the rod.Such a device may be used where the rod, such as a tie bolt, is used to secure a number of mechanical parts together, for example to secure the parts of a large diesel engine together.

When screwed connections are provided between mechanical parts by means of relatively large screwthreaded fasteners and correspondingly large nuts, it is desirable to obviate the torsional stressing which tightening of the nut normally produces in the screwthreaded fastener. To this end, it has long been the practice to use tensioning devices which first apply a tension to the screwthreaded fastener to impart to it a corresponding elongation. The nut is then tightened with a low torque, whereafter the loading applied to the fastener by the tensioning device is removed.

It is necessary to ensure that when the device is used to loosen a nut, the piston is spaced away from the bottoming position before pressure is built up in the chamber because it is not otherwise possible to allow the tension in the rod completely to relax so that great care is required in setting up the device.

According to the present invention, a tensioning device for tensioning a screwthreaded rod during the tightening or loosening of a nut thereon includes: a piston and a cylinder of which one affords a screwthreaded bore for the reception of the rod, the cylinder and piston together defining a pressure chamber in which the fluid pressure, in use of the device, tends to move the piston in a direction away from a bottoming position and out of the cylinder to tension the rod; a resilient spacing means acting between the piston and cylinder respectively and tending, over at least the part of the piston stroke which is closest to the bottoming position, to hold the piston away from the bottoming position.The dimensions of the spacing means should, in practice, be so selected that complete relaxation of stressed rods with which the device is to be used can occur without the piston bottoming in the cylinder.

Preferably, the device also includes a resilient restoring means acting between the piston and the cylinder and tending, over at least the part of the piston stroke which is remote from the bottoming position, to move the piston towards the bottoming position. The restoring means and the spacing means thus tend to direct the piston to a middle zone away from the ends of its stroke and if the sum of the lengths of the parts of the piston stroke over which the spacing means and the restoring means are active is greater than the maximum piston stroke the piston will be directed towards a single starting position. The restoring means may have a limiting stressed condition which serves as a stop to prevent the piston moving out of the cylinder.This enables the maximum possible stroke of the piston in the cylinder to be utilised when necessary without risk of the piston coming out of the cylinder.

The invention may be carried into practice in various ways but one tensioning device embodying the invention will now be described by way of example with reference to the accompanying drawing which shows the device in vertical section and in use on a tie rod connecting together parts of a large diesel engine.

The device has a cylinder 1, a piston 2 movable therein and a number of guide rods in the form of screws 3 which are arranged in a circle and only one of which is shown.

The screws 3 pass through holes in a flange 11 on the piston 2 and are received in threaded bores in the end of the cylinder 1.

The screws 3 have shoulders which engage abutment surfaces 17 on the cylinder. The screws guide the piston for reciprocation relative to the cylinder. The piston affords two coaxial cylindrical surfaces 4, 5 which slide axially in corresponding bores in the cylinder 1. Between the cylinder 1 and the piston 2 there is an annular pressure chamber 8 which is sealed by ring seals 6 and 7.

Hydraulic liquid can enter the pressure chamber 8 via an inlet passage 9 and can be vented through a vent passage 10. The piston 2 is formed with a central tapped hole 28 into which a ring bolt (not shown) can be screwed to lift the tensioning device.

Between the cylinder end face 12 and an opposite cooperating piston surface 13 afforded by the flange 11 of the piston there is a space 14 which accommodates a resilient spacing means 15 which, in this case, is formed by a number of spring units 16 one of which extends around each of the screws 3. The spring units 16 are prestressed although this is not essential. A single spring unit can be provided instead of a number of spring units 16, the single spring unit extending around the cylindrical surface of the piston in the space 14. Each spring unit 16 is made up of a number of dished spring discs or washers but alternatively could consist of a number of discs of a resiliently compressible material or of one or more helical springs.The spring units 16 are such that in the piston bottom dead-centre position in which the spring units 16 are fully compressed, the pressure chamber 8 has at least a minimum necessary axial dimension H (as will be explained below). This dimension increases during the tensioning operation and the spring units 16 expand, in accordance with the elongation of the tie rod which is being tensioned.

Disposed between the top surface 26 of the flange 11 and the heads of the screws 3 is a resilient restoring means 18 which consists of spring units 19 extending around the screws 3. The spring units 19 can also be prestressed and, if desired, replaced by a single large spring unit. As with the spacing means, each spring unit 19 consists of a number of dished spring washers but could consist of a number of discs of resiliently compressible material or one or more helical springs.

When the restoring means 18 is fully compressed, it serves as an abutment which determines the top dead centre position of the piston, thus preventing the piston from moving out of the cylinder, and also limiting the maximum elongation of the tie rod produced by the tensioning device. Consequently, the spring travel of the restoring means 18 must correspond at least to the total elongation to be undergone by the tie rod. The restoring means also serves to restore the piston 2 to a predetermined position relative to the cylinder 1 - i.e., so that the piston 2 returns to a predetermined position when the pressure in the chamber 8 is reduced to zero on the termination of a tensioning operation.The spring units 16, 19 need not necessarily extend around the same screws 3; the spring units 16 may be on some of the screws 3 with the spring units 19 on others of the screws 3.

During the tensioning process, the nut 21 is screwed along the tie rod 20 so as to engage with the part 22 - the outermost of the parts which have been or are to be secured together. The piston 2 is screwed on the end of the tie rod 20 until the cylinder 1 also engages the part 22, the nut 21 being received in a bore 23 in the cylinder 1. The periphery of the nut 21 is formed with a number of radial bores 24 accessible by way of apertures 25 in the cylinder 1. A tommy bar (not shown) can be inserted through the apertures 25 into the bores 24 to turn the nut 21.

When it is required to use the device to tension the tie rod 20 and then tighten the nut 21, the pressure chamber which at this time has its minimum axial length of, for example, 4 mm - is filled with a hydraulic liquid, e.g. oil, through the inlet passage 9 and a hydraulic pressure builds up in the chamber 8 and presses the cylinder against the part 22 and acts through the piston 2 to produce in the tie rod a predetermined tension and an associated predetermined elongation. As the tie rod is stretched, the piston 2 moves in the cylinder 1 and the axial dimension H of the chamber 8 increases. When the predetermined elongation of the tie rod has been achieved, the nut 21 is screwed tight on the tie rod 20. The hydraulic liquid is then discharged from the cylinder 8 so that the pressure therein falls to zero. The piston is then unscrewed from the tie rod and the tensioning device is removed, the restoring means 18 then restoring the piston and the cylinder to their predetermined relative position.

When it is required to use the tensioning device to release the nut 21 from the tie rod 20, a similar procedure is followed inasmuch as the tensioning device is screwed on to the rod 20 as previously and a hydraulic pressure is built up in the chamber 8 to press the piston 2 upwards and to produce a tension in the tie rod 20 to produce an elongation thereof. In this case, that portion 27 of the tie rod 20 which is below the nut 21 is already in the stretched state which was produced when the nut was tightened in the first place.

To release the nut 21 the total tension must be restored to near its original value. When the nut is released and the liquid is discharged from the chamber 8 so that the pressure therein falls to zero, the tension is removed from the rod 20 so that the rod shortens abruptly by the whole of the original amount of elongation; the piston moves into the cylinder 1 by a distance equal to the elongation of the tie rod and the axial length of the chamber 8 decreases by the same amount.

Consequently, if at the beginning of the operation to release the nut 21 the initial length of the chamber 8 is less than the original elongation of the tie rod 20, the piston will bottom in the cylinder and the tie rod will not be completely relaxed so that a substantial stress is produced between the rod 20, the piston 2, the cylinder 1 and the part 22 and this stress makes it difficult or impossible to demount the tensioning device from the tie rod 20 and the part 22 and may make it necessary to apply a considerable torque to the nut 21 in order to unscrew it.

The spacing means makes it possible to avoid this difficulty for when the rod 20 shortens by the amount of the orginal elongation and the piston moves by the same amount, the spring units 16 are compressed, but the axial length of the chamber 8 with the spring units 16 fully compressed is still large enough to allow the tie rod to become fully relaxed so that the tensioning device will be demountable from the rod 20 and the part 22 and the nut can easily be unscrewed without applying a large torque.

WHAT WE CLAIM IS: 1. A tensioning device for tensioning a screwthreaded rod during the tightening or loosening of a nut thereon, the device including: a piston and a cylinder of which one affords a screwthreaded bore for the reception of the rod, the cylinder and piston together defining a pressure chamber in which the Ifluid pressure, in use of the device, tends to move the piston in a direction away from a bottoming position and out of the cylinder to tension the rod; a resilient spacing means acting between the piston and cylinder respectively and tending, over at least the part of the piston stroke which is closest to the bottoming position, to hold the piston away from the bottoming position.

2. A device as claimed in Claim 1 which includes a resilient restoring means acting between the piston and the cylinder and tending, over at least the part of the piston stroke which is remote from the bottoming position, to move the piston towards the bottoming position.

3. A device as claimed in Claim 2 in which the sum of the lengths of the parts of the piston stroke over which the spacing means and the restoring means are active is greater than the maximum piston stroke.

4. A device as claimed in Claim 2 or Claim 3 in which the restoring means has a limiting stressed condition which serves as a stop to prevent the piston moving out of the cylinder.

5. A device as claimed in Claim 2 or Claim 3 or Claim 4 in which the restoring means comprises one or more spring units bearing on a piston surface in a direction to move the piston into the cylinder.

6. A device as claimed in any of the preceding claims in which the spacing means comprises one or more spring units disposed between a surface carried by the cylinder and a facing cooperating surface carried by the piston.

7. A device as claimed in Claim 5 or Claim 6 in which the or each spring unit comprises a number of discs of resiliently compressible material.

8. A device as claimed in Claim 5 or Claim 6 in which the or each spring unit comprises one or more helical springs.

9. A device as claimed in Claim 5 or Claim 6 in which the or each spring unit comprises a group of dished spring discs.

10. A device as claimed in any of the preceding claims which includes guide rods extending from the cylinder through bores in the piston to guide the movement of the piston relative to the cylinder.

11. A device as claimed in Claim 10 when appendant to Claim 5 in which the or each spring unit of the restoring means is disposed between the piston and an abutment on one of the guide rods.

12. A device as claimed in Claim 10 or Claim 11 when appendant to Claim 6 in which the or each spring unit of the spacing means is disposed around one of the guide rods.

13. A tensioning device substantially as described herein with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. be restored to near its original value. When the nut is released and the liquid is discharged from the chamber 8 so that the pressure therein falls to zero, the tension is removed from the rod 20 so that the rod shortens abruptly by the whole of the original amount of elongation; the piston moves into the cylinder 1 by a distance equal to the elongation of the tie rod and the axial length of the chamber 8 decreases by the same amount. Consequently, if at the beginning of the operation to release the nut 21 the initial length of the chamber 8 is less than the original elongation of the tie rod 20, the piston will bottom in the cylinder and the tie rod will not be completely relaxed so that a substantial stress is produced between the rod 20, the piston 2, the cylinder 1 and the part 22 and this stress makes it difficult or impossible to demount the tensioning device from the tie rod 20 and the part 22 and may make it necessary to apply a considerable torque to the nut 21 in order to unscrew it. The spacing means makes it possible to avoid this difficulty for when the rod 20 shortens by the amount of the orginal elongation and the piston moves by the same amount, the spring units 16 are compressed, but the axial length of the chamber 8 with the spring units 16 fully compressed is still large enough to allow the tie rod to become fully relaxed so that the tensioning device will be demountable from the rod 20 and the part 22 and the nut can easily be unscrewed without applying a large torque. WHAT WE CLAIM IS:

1. A tensioning device for tensioning a screwthreaded rod during the tightening or loosening of a nut thereon, the device including: a piston and a cylinder of which one affords a screwthreaded bore for the reception of the rod, the cylinder and piston together defining a pressure chamber in which the Ifluid pressure, in use of the device, tends to move the piston in a direction away from a bottoming position and out of the cylinder to tension the rod; a resilient spacing means acting between the piston and cylinder respectively and tending, over at least the part of the piston stroke which is closest to the bottoming position, to hold the piston away from the bottoming position.