Background Art In motor vehicles the steering column contains a shaft one end of which is fixed to the steering wheel and the other end of which is connected to the steering mechanism. Because of the geometry of the vehicle, the shaft will contain at least one universal joint. There may be two separate single Hooke's joints which'will give approximately constant velocity or, if the geometry requires a greater articulation angle in the shaft, there may be a double Hooke's joint which is complicated and expensive.

Unlike constant velocity j oints used in drive shafts in the transmission of motor vehicles steering joints must have no circumferential backlash. Steering joints also rotate at slow speeds so that dynamic balance is not a problem but a large articulation angle, up to 45 degrees, is necessary.

FR-A-2 244 377 describes a constant velocity joint which includes two shafts which are connected by a first ball and socket joint, an arm carried by each shaft and, adjacent to one end of the arm, pivotally mounted on the shaft about an axis which is perpendicular to the longitudinal axis of the shaft, the arms being connected, adjacent to their other ends by a second ball and socket joint, the plane containing the centres of articulation of both of the ball and socket joints being the bisector plane between the longitudinal axes of the shafts as the constant velocity joints articulates.

In principle the design of this prior art joint is suitable for use as a steering joint, however the detailed design suffers from some disadvantages because the second ball. and socket joint is of smaller diameter than the first ball and socket joint and thus the torque which can be transmitted is limited by the size of the second ball joint.

However the ball and socket mechanisms will wear with use introducing clearance which will result in backlash. This is not acceptable in a steering joint.

The object of the invention is to provide a constant velocity joint which is an improvement on that shown in FR-A-2 244 377.

Disclosure of the Invention According to the invention we provide a constant velocity joint which includes two rotary members which are connected by a first ball and socket joint, an arm carried by each member and, adjacent to one end of the arm, pivotally mounted on the member about an axis which is perpendicular to the rotary axis of the member, the arms being connected, adjacent to their other ends, by a second ball and socket joint, the plane containing the centres of articulation of both of the ball and socket joints being the bisector plane between the rotary axes of the members as the constant velocity joint articulates: characterised in that the balls of both joints are of substantially the same diameter and are connected to their respective arms by necks of the same diameter and in that the wrap angle of the two sockets is the same and in that the socket of each ball and socket joint includes means to hold the ball in the socket and a spring-loaded seat which pushes the ball against said means so that there is no axial clearance between the ball and the socket.

When a torque is applied to the joint it can be resolved into a force acting through the centre of the second ball and socket joint. For the systems to be in equilibrium an equal and opposite force acts through the centre of the first ball and socket joint. The forces on the two ball joints are therefore equal and for efficient use of material the joints should be of the same size.

Moreover the bending angle of each joint is limited by the engagement of the socket of the joint with the neck connecting the ball to the rotary member. For optimum results therefore the ratio of ball diameter to neck diameter for each joint should be the same.

By wrap angle we mean the angle subtended at the centre of the ball by the free edge of the socket or ball retaining means in the socket.

The holding means may be a seating ring held in position by a spring ring received in an internal groove in the socket or the end of the socket may be crimped over to hold the ball in place. Preferably two retaining seats which are in angular contact with the balls are used to prevent backlash, the spring loading acting on one of the seats.

Preferably each of said arms is of U-shape with the member to which it is pivoted received between the legs of the U. Preferably the arms are pivoted to the rotary members at positions offset from the longitudinal axes ofthe members. Bythis means, the change in overall diameter of the joint when it is articulated is minimise.

In one construction, under normal light torque loads there is a clearance between the limbs of the U of an arm and the member on which it is mounted but under heavier torque loads one of the limbs engages the member. This allows a lightweight construction with low hysteresis of the joint for normal low operating torques but retained the ability to transmit occasional high torques.

The ball and socket joints may be enclosed by protective bellows.

Brief Description of the Drawings The invention will now be described in detail by way of example with reference to the accompanying drawings in which:- Figure 1 is a longitudinal section through a joint constituting a first embodiment of the invention; Figure 2 is an end elevation of the joint of Figure 1; Figures 3 and 4 show the limits of the joint articulation; Figures 5A and 5B are longitudinal sections through one of the ball and socket joints; Figure 6 is a longitudinal section through one of the joint members; Figure 7 is an end elevation of the joint member shown in Figure 6; Figure 8 is a diagram illustrating the dimensions of the ball and socket joint; Figures 9 and 10 show a modified form of joint ; Figure 11 shows a further modified form of joint ; and Figure 12 shows a still further modified form of joint.

Best Mode of Carrying out the Invention Referring now to Figures 1 and 2, the joint comprises two identical members 20 and 21 each rotatable about a longitudinal axis 22 and 23 respectively. As shown in Figures 6 and 7, for the member 20, each member has a bore 24 to receive a shaft and two ears 25 which can be pulled together by pinch bolt, not shown, to secure a shaft in the bore 24. The end of the member 20 has an attachment portion 26 which has an attachment lug 27 having a through bore 28. The portion 26 also has a longitudinal threaded bore 29.

Referring to Figure 1, a first ball and socket joint 3 0 is interposed between the members 20 and 21. The construction of the ball and socket joint is shown in Figure 5. The ball is indicated at 31 and has a neck32. The neck 32 carries a flange 33 which is adjacent to a threaded portion 3 5. The threaded portion 3 5 is received in the bore 29 of the member 20 and

the flange 33 engages the end surface of the attachment portion 26.

The socket is indicated at 36 and has a threaded stem 37. Internally it has a seating ring 38 having a conical seat 39 which is engaged by the ball. A spring 40 is arranged behind the seating ring and may be a resilient plastic washer or a wavy spring for example. The ball is held in position in the socket by a seating ring 41 having a conical seat to engage the ball. The ring 41 is held in position by a circlip 42 which engages in grooves 43 and 44 in the socket and ring respectively. In Figure 1 the stem 3 7 of the socket is received in the bore 29 of the member 21.

It will be seen therefore that the members 20 and 21 are connected by the first ball and socket joint 30.

The members 20,21 are also connected by a second ball and socket joint 45 which is identical to the first ball and socket joint 30 thus the diameter of the balls of the joints are the same and the diameter of the necks are the same. The second ball and socket joint 45 is connected to the members 20 and 21 by U-shaped arms 46 and 47 which are identical.

Referring to Figure 2 the arm 46 is of U-shape having limbs 48 and a cross-member 49 which has a threaded bore 50. The threaded stem of the socket 51 of the second ball joint is received in the bore 50 ofthe arm 46 and the threaded stem ofthe second ball 52 is received in the bore 50 of the arm 47. The arms 46 and 47 are connected to the members 20 and 21 respectively by pivots 53 and 54 received in the bores 28 and which may include needle roller bearings. It will be seen that the arms are connected to the members about axes which are perpendicular to the rotary axes 22 and 23 but are offset therefrom.

Figures 3 and 4 show the limits of articulation of the joint. In each case the angle of articulation is limited by the engagement of the free edge of the socket 36 with the neck 32 of the ball 31. It will be noted that, in all positions of the joint, the centres 55 and 56 of the balls 31 and 52 lie in the bisector plane 57 of the joint, i. e. the plane which bisects the angle between the rotary axes 22 and 23 of the members 20 and 21.

As described above, both ball and socket joints are identical and are of identical dimensions with the balls being of the same diameter and the necks being ofthe same diameter.

Each ball and socket joint experiences the same force during torque transmission and therefore requires, for maximum efficiency, to be of the same size.

Figure 8 is a diagram showing the relationship between the ball diameter R and the neck diameter r.

angle efd = angle abc = angle cde = y R cosx = ac + cd ac = abtany cd = r/cosy Rcosx-r/cosy = Rsinxtany R (cosx cosy-sinx siny) = r R= r cosx. cosy-sinx. siny The angle"x"is the safety angle or wrap angle which prevents the ball from pulling out of the socket. Angle"y"is the articulation angle. Thus for a particular articulation angle and a safety or wrap angle one can choose the appropriate relationship between the ball diameter and the diameter the neck. For a bending angle ouf 45', for example, and a safety angle of 15° the ratio of the ball diameter R to the neck diameter r is 2: 1.

Figures 9 and 10 show a modification of the joint shown in Figures 1 and 2. There are two modifications. In the first, the ball joints are surrounded by bellows 57 and 58 which retain lubrication and keep out dirt.

Secondly, the limbs 60 and 61 of one of the U-shaped arms has projections 62 and 63 respectively. These projections are normally clear of the joint member 64 so for light loads there is little friction between the arm and the joint member as the joint articulates. However when there is a heavy load, for example if the power steering fails, the projection 62 or 63 would come into contact with the member 64. This would provide a high friction situation but nevertheless would prevent there being undue, loads on the pivot 65.

Figure 11 shows a simplified form of joint in outline. The essential construction of the joint, however, is as described above.

Figure 12 shows a joint in which the socket 70 has its end crimped over at 71 to retain the ball 72 in position. There are two seating rings 73 and 74, the ring 73 being held by the end 71 and the ring 74 being acted upon by a resilient washer 75. The ball 72 is located without backlash by the rings 73 and 74.

It will be seen that the invention provides a constant velocity joint which is an improvement over the prior art and is suitable for use as a steering joint.