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 B1 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 5 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 R1 which is greater than the mean radius'BM'of the bends B1, B2 and is also greater than the inner radius'Ri'and the outer 10 radius'Ro'of the inner wall 15.

The length of the inner cable 11 through the bends B1, B2 is therefore greater than if it were to follow the radius'Ri'because the distance round the bend of 90° is equal to IIR/2 where'R'is the radius of curvature.

In addition the shortest route between the inner radii of the bends B1 15 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 B1, B2.

During the transition from the relaxed state to the taut state any slack in 5 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 10 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 15 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 B1 at one end and a second bend B2 at 20 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 5 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 B1, so that it 10 follows the route of smallest radius of curvature round the bend B1. 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 B1, a guide section formed from two adjacent portions 15 B3, B5 of opposite curvature. As can be seen in Figure 4 the first portion B3, furthest from the bend B1, is of radius R2 and has a centre of curvature located a point A and the second portion B5, closest to the bend B1, is of radius BM and has a centre of curvature located at point B.

The first portion B3 extends berween 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 B1.

As shown in Figures 3 and 4 the second portion B5 is an extension of the bend B1 and in effect is an overbend of the bend B1 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 applied to the inner cable 111 so as to maintain contact with the inner radius Ri of the bend B1.

The inner cable 111 is biased against the inner radius Ri of the bend B1 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 5 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 10 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 line between the ends of the tube.

As shown in Figure 5 by introducing a three bends B10, B11, B12 to 15 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 20 within the tube.

The bends B10 and B12 are of opposite curvature to the bend B11 and produce a section of tube with an effective diameter that is similar to the diameter of the inner cable. The bend B10 is of radius R3, the bend B11 is of radius R4 and the bend B12 is of radius R5.