WO2000019116A1

WO2000019116A1 - Cable actuators

Info

Publication number: WO2000019116A1
Application number: PCT/GB1999/002932
Authority: WO
Inventors: Michael Neil Basnett
Original assignee: Bayerische Motoren Werke Ag
Priority date: 1998-09-29
Filing date: 1999-09-03
Publication date: 2000-04-06
Also published as: GB9821013D0

Abstract

A mechanical control cable (110) is provided having a bent shape that ensures that an inner cable (111) supported by a rigid tube (113) follows the same route irrespective of whether the inner cable (111) is an relaxed or taut state.

Description

Cable Actuators

This invention relates to cable actuator systems and in particular to a

means for reducing unwanted slack in the cable of such systems.

It is well known to provide cable actuators, for example for vehicle

handbrakes, having a flexible inner cable made from several strands of

flexible steel wire covered with a low friction plastic sheath. The flexible

inner cable is slidingly supported by a flexible outer casing having a

wrapped wire construction covered in a plastic material.

It is also known to replace part of the outer casing with a rigid steel

tube or conduit where flexibility is not required. For example the rigid tube

is normally used to replace the flexible outer casing in the passenger

compartment of the motor vehicle and reverts to the flexible outer casing

before reaching the brake caliper or drum so as to provide the required

flexibility.

The use of such a rigid tube has the advantage that it is easier to secure

in place but has the disadvantage that the diameter of the tube must be

considerably greater than the diameter of the flexible inner cable due to the

presence of nipples secured to each end of the inner cable which have to be

fed through the tube. The result of this clearance is that the inner cable is not fully supported by the tube and so can distort introducing free play or

slack into the cable.

Such a prior art mechanical control cable is shown in Figures 1 and 2 of

the accompanying drawing and is described in greater detail hereafter.

It is an object of the invention to provide an cable actuator system.

According to the invention there is provided a cable actuator having a

flexible resilient inner cable, a conduit within which the inner cable is

supported with clearance, and actuator means for applying a tensile force to

the inner cable, wherein the conduit is formed for at least part of its length

by a rigid tube member shaped so as to cause the inner cable to follow the

same route therethrough irrespective of whether the inner cable is in a

relaxed or taut state.

Preferably said part includes a curved portion of the tube and the tube

is shaped such that the inner cable is held against the inner side of the tube

around substantially all of the curve.

Preferably at at least one end of the curved portion the tube includes a

guide portion curved in the opposite direction to said curved portion. Preferably the guide portion lies between the curved portion and a

straight portion of the tube and is arranged to guide the inner cable so that

it extends in a substantially straight line through the straight portion and

the guide portion until it contacts the inner side of the curved portion.

The invention will now be described by way of example with reference

to the accompanying drawing of which:-

Figure 1 is a cross-section through part of a prior art cable actuator

showing an inner cable thereof in a relaxed state;

Figure 2 is a cross-section as shown in Figure 1 but showing the inner

cable in a tensioned state;

Figure 3 is a cross-section of a cable actuator according to a first

embodiment of the invention showing an inner cable thereof in a relaxed

state;

Figure 4 is a cross-section of part of the cable actuator shown in Figure

3; and

Figure 5 is a cross-section of a cable actuator according to a second

embodiment of the invention showing an inner cable thereof in a relaxed

state. With reference to Figures 1 and 2 a prior art mechanical cable actuator

for the handbrake on a motor vehicle comprises an actuator cable 10 having

a flexible inner cable 11 made from a multi-strand flexible steel wire 16

slidingly supported within a low friction plastic sheath 12. The sheath 12 is

flexible enough not to affect the behaviour of the steel wire 16. The flexible

inner cable 11 is supported by a conduit in the form of a rigid steel tube 13

of inner diameter 'D'. Each end of the wire 16 has a nipple 14 fixed thereto

of greater external diameter than the external diameter of the sheath 12.

The nipples 14 are used to attach the inner cable 11 to the mechanical

devices it is used to connect. In this case one end is connected to a

handbrake lever and the other end to a brake.

The rigid tube 13 has a first bend Bl at one end and a second bend B2

at the other end both of which are of a mean radius 'BM', with a straight

portion therebetween.

The inner diameter "D" of the tube 13 has to be considerably greater

than the outer diameter "d" of the sheath 12 so as to permit one of the of

nipples 14 to be fed through the tube 13. The result of this clearance

between the inner cable 11 and the internal wall of the tube 13 is that the

inner cable 11 is not fully supported by the tube 13 and so can follow a non-

uniform route that is not directly related to the route followed by the inner wall 15 of the tube 13 thereby introducing free play or slack into the inner

cable 11.

This can best be seen by comparing the route taken by the inner cable

11 in Figure 1 with the route shown in Figure 2. In Figure 1 the cable is in

a relaxed state with no longitudinal, or axial, loads applied to the inner

cable 11 through the nipples 14. The resilient nature of the inner cable 11

is such that when following a curve in the rigid tube 13 it will adopt the

largest possible radius Rl which is greater than the mean radius 'BM' of the

bends Bl, B2 and is also greater than the inner radius 'Ri' and the outer

radius 'Ro' of the inner wall 15.

The length of the inner cable 11 through the bends Bl, B2 is therefore

greater than if it were to follow the radius 'Ri' because the distance round

the bend of 90° is equal to πR/2 where 'R' is the radius of curvature.

In addition the shortest route between the inner radii of the bends Bl

and B2 is that shown by the line S-S on Figure 1 which is the route followed

when the inner cable 11 is put in tension as shown in Figure 2. The

deviation of the cable 11 from the line S-S indicates that a further amount

of slack is present in the relaxed cable 11. As shown in Figure 2, when the inner cable 11 is put into tension by

applying end loads 'L' to it the inner cable 11 follows the shortest route and

lies adjacent to the inner radii Ri of the tube 13 through the bends Bl, B2.

During the transition from the relaxed state to the taut state any slack in

the inner cable 11 has to be taken up and this will result in free play in the

handbrake lever.

Referring now to Figures 3 and 4 a cable actuator according to the

invention will now be described which in many respects is the same as that

previously described and is intended to be used in place of such a prior art

actuator.

The handbrake actuator cable 110 has a flexible inner cable 111 made

from a multi-strand flexible steel wire 116 slidingly supported by a low

friction plastic sheath 112. The flexible inner cable 111 is supported by a

conduit in the form of a rigid steel tube 113. To allow the inner cable 111 to

be connected to the mechanical devices it is used to connect, in this case a

handbrake lever 120 and a brake 122, each end of the wire 116 has a nipple

114 fixed thereto of greater external diameter than the external diameter of

the inner cable 111.

The tube 113 has a first bend Bl at one end and a second bend B2 at

the other end both of which are of a mean radius 'BM' each of which is arranged to bend the inner cable through an angle of 90°. The portion of

tube 113 between the two bends is straight.

The diameter "D" of the tube 113 is considerably larger than the

external diameter "d" of the sheath 112 covering the wire 116 so as to

permit one of the of nipples 114 to be fed through the tube 113.

To prevent the occurrence of slack or free play in the inner cable 111

when it is in a relaxed state as shown in Figure 3 the tube 113 is shaped so

as to maintain contact between the inner cable 111 and the part of the

inside 115 of the tube 113 that is on the inside of the bend Bl, so that it

follows the route of smallest radius of curvature round the bend Bl. This

ensures that the inner cable 111 follows the same route irrespective of

whether the inner cable 11 is in a relaxed or taut state.

To achieve this continuous contact the tube 113 is bent to provide at

each end of the bend Bl, a guide section formed from two adjacent portions

B3, B5 of opposite curvature. As can be seen in Figure 4 the first portion

B3, furthest from the bend Bl, is of radius R2 and has a centre of curvature

located a point A and the second portion B5, closest to the bend Bl, is of

radius BM and has a centre of curvature located at point B. The first portion B3 extends between the radially extending lines A-C

and A-B and is therefore adjacent the straight part of the tube 113 and the

second portion B5 extends between the radial lines A-B and B-D and is

therefore adjacent the bend Bl.

As shown in Figures 3 and 4 the second portion B5 is an extension of

the bend Bl and in effect is an overbend of the bend Bl causing the tube

113 to be bent through an angle greater than 90 degrees. The effect of the

bend B3 is to compensate for this overbend so that the cable 111 is guided in

the correct direction.

The bend B2 at the other end the tube has similar guide portions

formed at each of its ends.

The effect of the adjoining portions B3, B5 is to produce an 'S' shaped

bend structure into the tube 113 that maintains a small additional bending

moment on the inner cable 111 irrespective of the longitudinal loading

apphed to the inner cable 111 so as to maintain contact with the inner

radius Ri of the bend Bl.

The inner cable 111 is biased against the inner radius Ri of the bend Bl

at all times through the desired bend angle of 90° by the contact between

the inner cable 111 and the inner radius Rs of the first portion B3 of the tube 113. Then it departs from the inner radius Ri and extends in a

straight line which extends through the portions B3, B5 and on through the

adjacent straight part of the tube 113. In this way when a longitudinal load

is applied to the inner cable 111 to use it to transmit load there is no slack

present as the inner cable 111 already follows the shortest route.

With reference to Figure 5 there is shown a second embodiment of the

invention in which it is used to reduce the amount of slack that is present in

a straight length of rigid tube by reducing the amount of sag.

Normally when an inner cable extends along a long length of straight

tube the effect of gravity will cause the inner cable to sag and lie along the

lower internal surface of the tube. However when the inner cable is put into

tension the inner cable becomes taut and then extends in a straight fine

between the ends of the tube.

As shown in Figure 5 by introducing a three bends BIO, Bll, B12 to

form two 'S' shaped bends contact is maintained at all times between the

inner cable 211 and an inner wall of the tube 213. By including several of

such bend combinations along a long length of straight conduit it is possible

to maintain the inner cable in an unsagged state and if these are of different

radial orientation it is also possible to maintain the inner cable centrally

within the tube. The bends BIO and B12 are of opposite curvature to the bend Bll and

produce a section of tube with an effective diameter that is similar to the

diameter of the inner cable. The bend BIO is of radius R3, the bend Bll is

of radius R4 and the bend B 12 is of radius R5.

Claims

1. A cable actuator system having a flexible resilient inner cable, a

conduit within which the inner cable is supported with clearance, and

actuator means for applying a tensile force to the inner cable, wherein

the conduit is formed for at least part of its length by a rigid tube

member characterized in that the rigid tube member is shaped so as to

cause the inner cable to follow the same route irrespective of whether

the inner cable is in a relaxed or taut state.

2. A system according to claim 1 wherein the said part includes a curved

portion of the tube and the tube is shaped such that the inner cable is

held against the inner side of the tube around substantially all of the

curve.

3. A system according to claim 2 wherein at at least one end of the curved

portion the tube includes a guide portion curved in the opposite

direction to said curved portion.

4. A system according to claim 3 wherein the guide portion lies between

the curved portion and a straight portion of the tube and is arranged to

guide the inner cable so that it extends in a substantially straight fine

through the straight portion and the guide portion until it contacts the

inner side of the curved portion.

5. A system as claimed in any foregoing claim in which the inner cable is

covered by a low friction sheath to reduce friction between the inner

cable and an inner wall of the conduit.

6. A system as claimed in any foregoing claim in which each end of the

cable inner has a nipple secured thereto used to connect the wire to a

device to be operated.

7. A system as claimed in claim 6 in which the external diameter of each

nipple is greater than the diameter of the inner cable.

8. A mechanical control cable substantially as described herein with

reference to the accompanying drawing.