GB2092088A

GB2092088A - Tension limiting device and method for drawing cable through ducting

Info

Publication number: GB2092088A
Application number: GB8102853A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-01-30
Filing date: 1981-01-30
Publication date: 1982-08-11
Also published as: NZ199554A; ES509205A0; AU551141B2; ES8301395A1; AU7978682A; GB2092088B

Abstract

A device to limit the tension applied to an optical cable being manually drawn through ducting with the aid of a draw rope (4) has a pulley system incorporating a capstan pulley (1), and jockey pulley (2) arranged such that when the tension in the part of the rope around the jockey pulley exceeds a predetermined threshold the movement of the jockey pulley applies a brake to the capstan. <IMAGE>

Description

SPECIFICATION Tension limiting device and method for drawing cable through ducting This invention relates to a device for limiting the tension applied to cable as it is drawn through ducting. This finds particular but not necessarily exclusive application in manually drawing optical cable through ducting.

When the cable is being drawn in with the aid of a motor driven winch, the maximum tension applied to the cable can be set by limiting the torque output of the motor, and this approach has been successfully used in drawing optical cable into Post Office ducting between Stevenage and Hitchin using a special winch comprising an electrically driven capstan powered by a petrol/electric generator controlled by an electronic controller.

An entirely different approach is required when the cable is to be drawn through the ducting by direct manual pulling on the draw rope. If, in the manual drawing in of a cable, some obstruction is met, there is a natural tendency for the installation team to try without thinking to clear the obstruction by applying just a short burst of extra effort. With susceptible cable this extra effort can cause permanent damage to the quality of the cable. One solution to this probiem has been to include a weak link in the draw rope that will break before excessive tension is applied to the cable.

The chief drawback to this approach is that replacement of the broken weak link is liable to be time consuming. The present invention concerns a device that will limit the tension by alternative means that does not require replacement of parts whenever the predetermined limiting tension is reached. A weak link set to break at a slightly higher tension may however additionally be used to provide a fail-safe back-up facility for the device.

According to the present invention there is provided a tension limiting device for drawing cable through ducting with the aid of a drawing rope, which device includes a pulley system for the rope including a capstan pulley, a jockey pulley, and at least one other pulley, arranged such that the rope can be threaded through the pulley system such that it passes around each of said three pulleys reversing its direction on passing round the jockey pulley, wherein the jockey pulley is biassed by compliant means against movement under the influence of tension in the rope passed around it and wherein the device includes a braking system acting on the capstan pulley and actuated by such movement of the jockey pulley.

The invention also provides a method of drawing cable through ducting wherein the cable is attached to one end of a draw rope and tension is applied to the other end and wherein intermediate its ends the draw rope threads a tension limiting device rigidly fixed in relation to the ducting, in which device the rope is wrapped around a capstan pulley, a jockey pulley and at least one other pulley arranged such that the rope reverses its direction on passing round the jockey pulley, wherein the tension limiting device is arranged such that when the tension in the portion of the rope passing round the jockey pulley increases beyond a predetermined threshold it moves the jockey pulley against the bias of compliant means so as to cause the operation of a braking system acting on the capstan pulley.

There follows a description of a tension limiting device embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure 1 is a graph depicting the ideal characteristic relating transmitted tension to applied tension, Figure 2 is a schematic representation of the device, Figure 3 depicts the layout of the pulleys of Figure 2, and Figure 4 depicts the mounting of the jockey wheel of Figures 2 and 3.

Ideally the tension limiting device should transmit all the applied tension up to a predetermined adjustable limit beyond which any further increase in applied tension results in no further increase in transmitted tension. Referring now to Figure 1 this ideal characteristic is graphically represented by the straight line from the origin to the point A corresponding to the transmitted tension value Tp. At A this straight line meets the straight line AB whose gradient is zero.

In a practical device there is liable to be some departure from this ideal characteristic. Thus in particular a small rate of increase in transmitted tension above the value Tp may be acceptable, giving rise to the characteristic from A to C. Over this region the gradient must be small so that large increases in applied tension produce only small increases in transmitted tension.

Referring now to Figures 2 and 3, a preferred form of tension limiting device consists essentially of a pulley system, a brake system, and compliant means mounted on a rigid frame (not shown). To use the device this frame is held fixed in relation to the ducting through which the cable is to be drawn. The pulley system consists of a capstan pulley 1, a jockey pulley 2 and a third pulley 3. A draw rope 4 is passed at least once, and preferably twice, round the capstan pulley, and then is taken round the other two pulleys. The pulleys are positioned relative to each other so that the draw rope reverses direction as it passes over the jockey pulley. Secured to the axle of the capstan pulley is a brake disc 5. Its associated calliper assembly 6 is controlled by a master cylinder 7 operated by movement of the jockey pulley.

Referring now particularly to Figure 4, the jockey pulley rotates on an axle 20 in a block 21 that slides on p.t.f.e. runners 22, and is guided by two upper ball race bearings 23 and a single lower ball race bearing 24 engaged respectively in upper and lower fixed channel supports 25 and 26.

Tension in the draw rope passing over the jockey pulley imparts a twisting movement to the block 21 about a verticai axis. This movement is resisted by the two ball race bearings 23 which are mounted one behind the other respectively in front of and behind the line of the pulley axle 20.

This means that one of the bearings 23 is pressed against the side wall of the upper channel support while the other is pressed against the opposite side wall. Rocking of the block about the axis normal to the plane of the page of Figure 4 is prevented by the bearing 24.

Sliding motion of the block 21 is transmitted to the brake master cylinder 7 by means of push rods 8 and 9 positioned against opposite faces of a lever 10 pivoted at a fulcrum 11. On the other side of the fulcrum the lever 10 is biassed against an end stop 12 by compliant means 13.

When the lever is against the end stop no braking action is applied to the capstan pulley, and hence, neglecting the effects of friction, a tension applied to the draw rope where it leaves pulley 3 is transmitted in its entirety through the device to the draw rope where in joins the capstan pulley 1, T1 = T2. Since the draw rope reverses direction round the jockey pulley the force tending to impart sliding motion to the block 21 is, neglecting the effects of friction, equal to twice T2. As the tension T2 is progressively increased, the resulting moment applied to the lever via push rod 8 will eventually exceed the opposite moment applied by compliant means 13, with the result that a braking force is applied to the capstan pulley.This braking action will restrict the further increase of transmitted tension Tp provided that the draw rope is wrapped sufficiently far round the capstan to prevent slippage on the capstan surface. Generally such slippage can be prevented by a single turn round the capstan, but a second turn is preferred to give a large safety margin.

If the predetermined tension in T2 at which braking action commences is at a tension Tithe effect of increasing T2 beyond Tp can be analysed to see at what rate, if any, the transmitted tension T1 is allowed to rise beyond Tp with increasing values of applied tension T2. Initialiy this analysis will assume a high compliance for the compliant means 13, so that the force it applies to the lever is not significantly altered over the range of the full travel of the master cylinder push rod 9.Under these conditions the force applied to the master cylinder for an applied tension T2 in excess of Tp is 2(T2Tp). This force is transmitted via the braking system to provide a braking moment on the capstan equivalent to a force 2m(T2 -- Tp) acting tangentially at the capstan surface, where m is the mechanical advantage of the braking system.

Therefore the transmitted tension is given by the expression T1 = T22m(T2Tp) =T+(1 (12m)(T2Tp) This equation applied only when T2 is greater than Tp. When T2 is less than Tp, T1 is equal to T2.

Therefore it is evident that T1 never exceeds Tp provided that m, the mechanical advantage of the braking system, is greater than one half.

Inspection of the above analysis shows that the effect of a finite value of compliance for the compliant means 13 will mean that the mechanical advantage of the braking system will have to be somewhat greater than one half in order to prevent T1 from exceeding Tp.

In principle the device can be operated in the reverse sense with the applied tension being the tension T1 applied to the capstan pulley and the transmitted tension being the tension T2. This is not the preferred way of operation because the equivalent equation relating applied and transmitted tensions when the applied tension exceeds the predetermined tension Tp is T2=T1 -2m(T2-T) = (Tr + 2m Tp)/( 1 + 2m) =Tp + (T1Tp)/(1 + 2m) Remembering that there is no braking effect until T1 is equal to or greater than Tp, it is seen that this shows that the transmitted tension continues to rise with increasing value of applied tension, but the gradient of the characteristic is reduced from unity to 11(1 + 2m) when the value of Tp is exceeded. This continued increase may be acceptable provided that the mechanical advantage of the braking system is large. On the other hand a large mechanical advantage implies a long travel in the operation of the master cylinder, and this makes the effects of a finite compliance more pronounced.

The mounting and brake assembly for the capstan pulley may conveniently be provided by a wheel bearing and brake assembly unit designed for a car. Slmilarly the compliant member can be a simple spring such as one designed for a car suspension unit. Unless such a spring is particularly long, its compliance is liable to be relatively low. A higher compliance can with advantage, and with relative compactness, be provided by making use of a pneumatic suspension unit designed for a car.

In principle the compliant member can be directly coupled to the push rod of the jockey wheel sliding block 21 so as to avoid the need for the lever 10. The advantage of the lever is that, by moving the position of its fulcrum relative to the push rod 8 and the compliant means 13, the mechanical advantage provided by the lever can be varied so as to vary the value Tp at which the onset of braking action occurs, and hence the limiting value of transmitted tension. With a direct coupling the adjustment of Tp is possible by moving the compliant member in the direction of movement of the sliding block 21. However it will be apparent that the adjustment thereby provided wouid be very small in the case of a compliant member with a particularly high value of compliance.

Claims

1. A tension limiting device for drawing cable through ducting with the aid of a drawing rope, which device includes a pulley system for the rope including a capstan pulley, a jockey pulley, and at least one other pulley, arranged such that the rope can be threaded through the pulley system such that it passes around each of said three pulleys reversing its direction on passing round the jockey pulley, wherein the jockey pulley is biassed by compliant means against movement under the influence of tension in the rope passed around it and wherein the device includes a braking system acting on the capstan pulley and actuated by such movement of the jockey pulley.

2. A device as claimed in claim 1 wherein the compliant means consists of or includes a unit possessing pneumatic compliance.

3. A tension limiting device substantially as hereinbefore described with reference to the accompanying drawings.

4. A method of drawing cable through ducting wherein the cable is attached to one end of a draw rope and tension is applied to the other end and wherein intermediate its ends the draw rope threads a tension limiting device rigidly fixed in relation to the ducting, in which device the rope is wrapped around a capstan pulley a jockey pulley and at least one other pulley arranged such that the rope reverses its direction on passing round the jockey pulley, wherein the tension limiting device is arranged such that when the tension in the portion of the rope passing round the jockey pulley increases beyond a predetermined threshold it moves the jockey pulley against the bias of compliant means so as to cause the operation of a braking system acting on the capstan pulley.

5. A method as claimed in claim 4 wherein the draw rope passes, from its point of attachment to the cable, first around the capstan pulley, and then around the jockey pulley.

6. A method as claimed in claim 4 or 5 wherein the draw rope is attached to the cable via a 'weaklink' device that will part at a predetermined tension.

7. A method as claimed in claim 4, 5 or 6 wherein the cable is an optical cable incorporating one or more optical fibres.

8. A method of drawing cable through ducting substantially as hereinbefore described with reference to the accompanying drawings.

New claims or amendments to claims filed on 17/7/81.

Superseded claim 1.

New claim:

1. A tension limiting device for drawing cable through ducting with the aid of a drawing rope, which device includes a pulley system for the rope including a capstan pulley, a jockey pulley, and at least one other pulley, arranged such that the rope can be threaded through the pulley system such that it passes around each of said three pulleys reversing its direction on passing round the jockey pulley, wherein the jockey pulley is biassed by compliant means against movement under the influence of tension in the rope passed around it and wherein the device includes a braking system acting on the capstan pulley and actuated by such movement of the jockey pulley, such that increasing tension in the part of the rope around the jockey pulley produces increasing braking effect upon the capstan pulley.