Background Known hydraulic power steering rotary valves typically employ an input-shaft which extends upwardly from the steering gear assembly and is connected by a flexible coupling to the steering wheel shaft. For this purpose, the extended end of the input- shaft is usually externally splined.

The steering gear assembly is provided with a valve housing containing an input- shaft and a sleeve member supported thereon. For the purposes of reducing valve operating friction some manufacturers support the sleeve member on the input-shaft via needle roller bearings, these needle roller bearings serving to maintain a small radial clearance between the outside diameter of the input shaft and inside a diameter of the sleeve. However the vast majority of manufacturers journal the sleeve member directly on the outside diameter of the input-shaft. For the purposes of this specification both these types of support will be referred to as"journalling".

The input-shaft extends through the sleeve and is journalled with respect to the steering gear driven member, which for a rack and pinion steering gear, is the pinion.

For an"integral"steering gear box, the driven member is normally the worm portion of a recirculating ball nut assembly.

The sleeve is usually arranged to be driven in a slack-free manner by a drive pin extending radially from the pinion although, in other less common hydraulic valves, this slack-free drive is facilitated by the sleeve and pinion being manufactured as an integral arrangement. The downwardly-extending end of the input-shaft is splined in a loose-fitting manner to the pinion so allowing"limited relative rotation"between the input-shaft and the pinion, and hence also between the input-shaft and the sleeve.

The relative rotation must be limited to allow manual operation of the steering gear if the hydraulic power assistance fails.

Both the input-shaft and the sleeve member have respectively outwardly and inwardly facing longitudinal chambers formed on their interfacing surfaces which constitute an open centre four way valve operable on relative angular rotation of these components.

The sleeve operates within the valve housing and is provided with several circumferential grooves and seals which allow oil under pressure to be directed to and from an external hydraulic pump and to and from left and right assist cylinders in the manner well known in the art. The input-shaft and the sleeve are normally biased towards a neutral position by a torsion bar secured at its lower end to the pinion. The aforementioned slack-free drive of the sleeve therefore effectively means that the sleeve and torsion bar are rotationally connected via the pinion, the driven member in this case. The torsion bar is secured to the input-shaft at the upper end of the torsion bar, from the pinion, henceforth referred to as the"fixing end"of the torsion bar.

The general method of operation of such rotary valves is well known in the art of power steering design and so will not be described in any greater detail in this specification. A description of this operation is contained in U. S. Pat. No. 3,022,772 (Zeigler), commonly held as being the"original"patent disclosing the rotary valve concept.

It is a requirement of operation of most steering gears that the left and right turn hydraulic assist characteristics be as nearly as possible identical, and this symmetry of operation can only be established at the time of assembly of the valve due to the finite tolerances assigned to the various components involved. Great accuracy is required in determining the neutral position of the valve components to ensure this symmetry. Moreover, once this position is determined, it must be retained for the life of the steering gear.

For most power steering gears the theoretically ideal neutral position of the input- shaft and sleeve components can be defined as the position about which an equal

angular rotation or an equal input torque applied to the input-shaft in either direction, with respect to the sleeve, will result in equal magnitudes of differential pressure of hydraulic fluid being supplie to the left and right assist cylinders during valve operation. The neutral position is typically determined by adjustment of the angular disposition of the input-shaft with respect to the torsion bar whilst angular rotation of the input shaft with respect to the sleeve, or alternatively the input torque applied to the input-shaft (which is temporarily locked to the torsion bar), is measured against the valve inlet pressure. The operation of determining and fixing the neutral position is referred to as a"balancing"operation.

The balancing operation is performed in a"balancing machine". Such balancing machines take many different formats and may be either a conventional hydraulic balancing machine or a pneumatic balancing machine as described in US Pat. No.

5,727,443 (Baxter et. al.).

The neutral position is fixed by joining the fixing end of the torsion bar and the input- shaft, the resulting joint will henceforth be referred to as the"torsion bar to input-shaft joint". The fixing end of the torsion bar is typically cylindrical and extends through an axial extending bore in the input-shaft, the torsion bar outer surface and input-shaft bore having"clearance"to allow relative rotation during the balancing operation.

Commonly a"sealing member"such as an o-ring is used to prevent hydraulic fluid from leaking through the clearance. Various methods can be used to create the torsion bar to input-shaft joint. The most common method is to join the fixing end of the torsion bar and input-shaft by a pin pressed through a diametrically disposed hole drilled and reamed through both components during the balancing operation.

An alternative method of creating the torsion bar to input-shaft joint is the torsion bar having a pre-formed axially extending bore into which one or more diameteral members, such as a ball, are press fitted thereby expanding the outer surface of the fixing end of the torsion bar and forming an"interface"with the input-shaft bore. The input-shaft bore surface or the torsion bar fixing end outer surface forming the interface are commonly smooth but may include a keying formation such as a spline on at least a portion of one or both surfaces. The sealing member may or may not be included. This joining method will henceforth be referred to as the"ball press fit"

method. Such a method is described in US Patent No. 5,431,379 (Takagi) and in International Patent Application No. PCT/AU98/00697 (Bishop Steering Pty Limited).

Another alternative method of creating the torsion bar to input-shaft joint is by bonding comprising adhesive bonding, soldering, brazing, welding or a mechanical keying agent, as described in US Pat. No. 5,727,443 (Baxter et. al.).

An alternative to hydraulic power steering is electric power steering wherein assistance is facilitated by an electric motor. In this case the input-shaft forms part of a"torque-sensing rotor"instead of a hydraulic valve. The torque-sensing rotor differs from a hydraulic valve in that the sleeve and the hydraulic chambers are replace by a device that measures the relative displacement between the input-shaft and the driven member.

The integrity of the torsion bar to input-shaft joint is critical to the safe operation of the motor vehicle. In service, the joint is subjected to repeated torsional cycling and random shock and vibration loading. The joint must have adequate torsional and axial strength to perform satisfactorily. If the joint is not strong enough then it could fail during operation, which would cause the driver to lose control of the vehicle. It is desirable to test the joint strength of every valve produced, in particular if the joint comprises the ball press fit or bonding method, because weak joints can be produced by these methods if manufacturing tolerances or parameters vary. However, it is not possible to apply a large enough torque to the joint to test its torsion strength because of the limited relative rotation possible between the input-shaft and the pinion.

The essence of the present invention is the recognition of the relationship between the torsion and axial strength of the torsion bar to input-shaft joint, since both relate to the shear strength of the interface between the input-shaft bore and the outer surface of the fixing end of the torsion bar. The torsion joint strength (measured as the breakaway torque) is approximately equal to the axial joint strength (measured as the breakaway force) multiplied by the radius of the input-shaft bore. This means that the torsion joint strength, which cannot be measured directly for reasons described above, can be indirectly measured by application of an axial force.

Summary of Invention In a first aspect, the present invention consists in a method of testing the integrity of a joint between an input-shaft and a torsion bar of a power steering input device, wherein the torsion bar has a fixing end extending through an axially disposed bore in the input-shaft, the outer surface of the fixing end of the torsion bar and the bore of the input-shaft having clearance to allow relative rotation therebetween before joining, characterised in that the method comprises the steps of: a) applying a predetermined axial force to the joint; and b) measuring the relative axial displacement between the input-shaft and the torsion bar as a result of the predetermined axial force being applied; and wherein, when the relative axial displacement is greater than a predetermined value, then the joint is deemed to have failed the test.

Preferably the joint between the input-shaft and the fixing end of the torsion bar comprises a pre-formed axially extending bore in the fixing end of the torsion bar into which one or more diametral members are press fitted thereby expanding the outer surface of the fixing end of the torsion bar and forming an interface with the input- shaft bore.

Preferably the surface of the input-shaft bore and the outer surface of the torsion bar fixing end forming the interface are both smooth.

Alternatively the surface of the input-shaft bore and the outer surface of the torsion bar fixing end forming the interface include a keying formation, on at least a portion of one or both surfaces.

Still alternatively the joint between the input-shaft and the fixing end of the torsion bar comprises adhesive bonding, soldering, brazing, welding or a mechanical keying agent.

In some applications of the present invention, the power steering may be hydraulically assisted and the input device comprises a balance hydraulic rotary valve having a sleeve journalled to the input-shaft.

In other applications of the present invention, the power steering assistance may be facilitated by an electric motor and the input-shaft forms part of an assemble torque- sensing rotor.

In a first embodiment of the first aspect of the present invention, the predetermined force is applied in a direction such that, if axial motion occurred, it would result in relative axial displacement of the input-shaft towards a driven member connected to the torsion bar. The predetermined force may be applied to the input-shaft exterior whilst the driven member is supported. Alternatively the predetermined force is applied to the torsion bar via the driven member whilst the input shaft is supported. In a further alternative the predetermined force may be applied to the fixing end of the torsion bar whilst the input shaft is supported.

In a second embodiment of the first aspect of the present invention, the predetermined force is applied in a direction such that, if axial motion occurred, it would result in relative axial displacement of the input-shaft away from the driven member. The predetermined force may be applied to the input shaft exterior whilst the driven member is held. Alternatively, the predetermined force may be applied to the torsion bar via the driven member whilst the input-shaft is held. In a further alternative the predetermined force may be applied to the fixing end of the torsion bar whilst the input shaft is supported.

Preferably the relative axial displacement between the input-shaft and the torsion bar is determined by measuring the axial distance between a datum point on the driven member and a datum point on the input-shaft before and after applying the predetermined force.

In a second aspect, the present invention consists in a method of testing the integrity of a joint between an input-shaft and a torsion bar of a hydraulic valve for a power steering gear, the input-shaft having a first axial bore and the torsion bar having a

second axial bore in a fixing end thereof, the surface of the first axial bore and the outer surface of the fixing end of the torsion bar forming an interface therebetween, the joint comprising a diametral member press fitted within the second axial bore thereby expanding the outer surface of the fixing end of the torsion bar and engaging with the surface of the first axial bore, characterised in that the method comprises the steps of: a) applying a predetermined axial force to the input-shaft whilst the torsion bar is supported; and b) measuring the relative axial displacement between the input-shaft and the torsion bar as a result of the predetermined axial force being applied; and wherein, when the relative axial displacement is greater than a predetermined value, then the joint is deemed to have failed the test.

In a third aspect, the present invention consists in a machine for testing the integrity of a joint between an input-shaft and a torsion bar of a power steering input device, wherein the torsion bar has a fixing end extending through an axially disposed bore in the input-shaft, the outer surface of the fixing end of the torsion bar and the bore of the input-shaft having clearance to allow relative rotation therebetween before joining, characterised in that the machine comprises: a) means for applying a predetermined axial force to the joint; and b) means for measuring the relative axial displacement between the input-shaft and the torsion bar as a result of the predetermined axial force being applied.

Preferably the machine forms part of a balancing machine.

Preferably the machine comprises means to determine relative axial displacement between the input-shaft and the torsion bar by measuring the axial distance between a datum point on a driven member connected to the torsion bar, and a datum point on the input-shaft before and after applying the predetermined force.

In a first embodiment of the third aspect of the present invention, the machine comprises a means for holding the power steering input device by the driven member, and means for urging the driven member towards the input-shaft whilst the end of the input-shaft is supported by a means for supporting. The driven member is urged until the predetermined axial force is reached.

Preferably the means for holding the driven member is fixed to a carriage, which can slide on linear bearings.

Preferably the means for urging the driven member is a motor, which moves the carriage.

Preferably the means for supporting the input-shaft includes a compression spring.

Preferably the means for supporting the input-shaft inclues a force transducer.

Preferably the machine comprises a precision linear measuring device.

In a second embodiment of the third aspect of the present invention, the machine consists of a means of holding the power steering input device by the driven member, and comprises means for applying a predetermined force to the end of the input-shaft.

Preferably the means for applying the predetermined force comprises a ram in contact with the end of the input shaft, and the ram is also connected to a piston, which is urged by a pressurised fluid.

Preferably a precision linear measuring device indicates the position of the ram.

Brief Description of Drawings A prior art valve and the present invention will now be described with reference to the followingdrawings: Fig 1 a is a sectional view of a typical hydraulic power steering valve; Fig 1 b is a sectional view of the valve of Fig 1 a through line I-I; Figs 2a and 2b depict a first embodiment of the method of testing the joint between the input-shaft and torsion bar according to the present invention; Figs 3a, 3b and 3c depict a second embodiment of the method of testing the joint between the input-shaft and torsion bar according to the present invention; Fig 4 depicts a first embodiment of a machine to implement the method of testing according to the present invention; Fig 5 depicts a second embodiment of a machine to implement the method of testing according to the present invention.

Fig 6 is a sectional view of a torque sensing rotor of an electric power steering embodiment; Mode for Carrying out Invention Fig 1 is a sectional view of a typical hydraulic power steering valve. Valve 1 comprises input-shaft 2 having sleeve 3 journalled thereon. The manner in which input-shaft 2 and sleeve 3 operate to direct oil to and from a hydraulic pump and assist cylinders (not shown) is well known in the art and will not be further described here. The lower end of input-shaft 2 is journalled on the lower end of torsion bar 4.

Torsion bar 4 axially protrudes from and is rigidly connected to pinion 5. Sleeve drive pin 6, radially protruding from pinion 5, engages a hole in sleeve 3 to provide a slack- free connection between sleeve 3 and pinion 5.

Torsion bar 4 and input-shaft 2 are joined by pressing ball 7 into pre-formed bore 8, in torsion bar 4, thereby expanding the outer surface of the fixing end of torsion bar 4 and forming an interface 9 with input-shaft bore 11. Sealing member 10 may or may not be included. There are alternative methods of creating the torsion bar to input- shaft joint, such as a pin pressed through a diametrically disposed hole drilled and reamed through both components, or by bonding comprising adhesive bonding, soldering, brazing, welding or a mechanical keying agent.

Prior to the joining of input-shaft 2 and torsion bar 4, input-shaft 2 must be rotation oriented with respect to sleeve 3 such that the valve is in the neutral position. Joining torsion bar 4 to input-shaft 2 permanently fixes the neutral position. The operation of determining and fixing the neutral position is referred to as a balancing operation and is performed in a balancing machine.

Flats 12 on the lower end of input-shaft 2 and pocket 13 in pinion 5 limit the relative rotation between input-shaft 2 and pinion 5. The relative rotation must be limited to allow manual operation of the steering gear if the hydraulic power assistance fails.

Figs 2a and 2b depict a first embodiment of the method of testing the joint between input-shaft 2 and torsion bar 4 according to the present invention. Predetermined force 14 is applied to valve 1 such that the force is transmitted through the torsion bar 4 to input-shaft 2 joint. Predetermined force 14 is applied in an axial direction such that if axial motion occurred, would result in the relative axial displacement of input- shaft 2 towards pinion 5. Axial gap 16 between the lower end of input-shaft 2 and pinion 5 ensures that the load is transmitted through the torsion bar 4 to input-shaft 2 joint. Predetermined force 14 can be applied to the upper end 15 of input-shaft 2 whilst supporting pinion 5 as depicted in fig 2a. Alternatively, predetermined force 14 can be applied to pinion 5 whilst supporting input-shaft 2 as depicted in fig 2b.

Relative axial displacement between input-shaft 2 and torsion bar 4, as result of predetermined force 14 being applied is measured and if this displacement is greater than a predetermined value then the joint between input-shaft 2 and torsion bar 4 is deemed to have inadequate strength and therefore fails the test. The torsion bar 4 to

input-shaft 2 joint must have a minimum torsion and axial strength to perform satisfactorily. It is desirable to test the joint strength of every valve produced, in particular if the joint comprises the ball press fit or bonding method, because weak joints can be produced by these methods if manufacturing tolerances or parameters vary. However, it is not possible to apply a large enough torque to the joint to test its torsional strength because of the limited relative rotation possible between input-shaft and the pinion. The present invention overcomes this problem because the axial strength and torsion strength of the joint are relate. Therefore the predetermined force 14 is chosen such that if the joint passes the test it will have both adequate torsion and axial strength.

The relative axial displacement between input-shaft 2 and torsion bar 4 is typically determined by measuring the distance between a datum point on the input-shaft 2 and a datum point on the pinion 5, as indicated by example dimension 17, before and after applying predetermined force 14.

Figs 3a, 3b and 3c depict a second embodiment of the method of testing the joint between the input-shaft and torsion bar according to the present invention.

Predetermined force 14 is applied to valve 1 such that the force is transmitted through the torsion bar 4 to input-shaft 2 joint. The only difference between this embodiment and the first embodiment is that predetermined force 14 applied in an axial direction such that if axial motion occurred, would result in the relative axial displacement of input-shaft 2 away from pinion 5. Relative axial displacement between input-shaft 2 and torsion bar 4, as result of predetermined force 14 being applied, is measured and if this displacement is greater than a predetermined value then the joint between input-shaft 2 and torsion bar 4 is deemed to have inadequate strength and therefore fails the test.

Typically, input-shaft 2 includes a shoulder 18 that can be used to apply predetermined force 14 to input-shaft 2. Fig 3a depicts predetermined force 14 applied by pulling pinion 5 whilst supporting input-shaft 2 at shoulder 18. Fig 3b depicts predetermined force 14 applied by pushing the upper end 19 of torsion bar 4 whilst supporting input-shaft 2 at shoulder 18. Fig 3c depicts predetermined force 14

applied by pulling input-shaft 2 whilst supporting the top face 20 of sleeve 3, which transmits the load to pinion 5 via sleeve drive pin 6.

Fig 4 depicts a first embodiment of a machine to implement the method of testing according to the present invention. Valve 1 is supported by block 21, which is attached to carriage 22. Carriage 22 contains linear bearings 23 that slide on rails 24.

Motor 27 rotates screw 26, which moves carriage 22 up and down via nut 25 fixed to carriage 22. Precision linear measuring device 28 measures the vertical position of carriage 22. Cap 29 can slide up and down on spigot 30, which is attached to force transducer 31. Cap 29 contacts the upper end 15 of input-shaft 2.

The length 17 of valve 1, before applying predetermined force 14, is measured by moving carriage 22 up and reading precision linear measuring device 28 at the exact instant that flag 33, attached to cap 29, is pushed upwards by valve 1 and triggers proximity switch 32. Predetermined force 14 is then applied to upper end 15 of input- shaft 2 by slowly moving carriage 22 further upwards, which compresses spring 34, until force transducer 31 indicates that predetermined force 14 has been reached.

Carriage 22 then moves downward to release predetermined force 14 and until flag 33 resets proximity switch 32. The length 17 of valve 1 after applying predetermined force 14 is measured by moving carriage 22 up again up and reading precision linear measuring device 28 at the exact instant that flag 33 triggers proximity switch 32. The relative axial displacement between input-shaft 2 and torsion bar 4 is the change in length 17 from before and after applying predetermined force 14. If the change in length is greater than a predetermined value then the joint between input-shaft 2 and torsion bar 4 is deemed to have inadequate strength and therefore fails the test. The apparatus depicted in Fig 4 can be included as part of a balancing machine.

Fig 5 depicts a second embodiment of a machine to implement the method of testing according to the present invention. Valve 1 is supported by block 35. Ram 36 moves up and down guided by bush 45 and contacts upper end 15 of input-shaft 2.

Precision linear measuring device 37 measures the vertical position of ram 36. Piston 38 is contained in cylinder 40 and is connected to ram 36. Ported valve 39 directs pressurised fluid, such as oil or air, to and from chambers 41 and 42 above and

below piston 38 respectively, via supply lines 43 and 44. Pressure gauge 46 indicates the pressure in chamber 41.

The length 17 of valve 1, before applying predetermined force 14, is measured by ported valve 39 admitting low-pressure fluid to chamber 41, which moves ram 36 downward until it contacts upper end 15 of input-shaft 2. This low-pressure should only apply enough force for the ram 36 to positively contact upper end 15 of input- shaft 2, and the length 17 is then determined by reading precision linear measuring device 37 at this instant. Predetermined force 14 is then applied to upper end 15 of input-shaft 2 by ported valve 39 admitting fluid to chamber 41 at a predetermined pressure. The predetermined pressure is calculated to give predetermined force 14 when acting over the area of piston 38. Predetermined force 14 is then released by ported valve 39 releasing fluid pressure from chamber 41. The length 17 of valve 1 after applying predetermined force 14 is measured by ported valve 39 again admitting low-pressure fluid to chamber 41, which moves ram 36 downward until it contacts upper end 15 of input-shaft 2, and the length 17 is then determined by reading precision linear measuring device 37 at this instant. The relative axial displacement between input-shaft 2 and torsion bar 4 is the change in length 17 from before and after applying predetermined force 14. If the change in length is greater than a predetermined value then the joint between input-shaft 2 and torsion bar 4 is deemed to have inadequate strength and therefore fails the test.

Whilst the abovementioned embodiments have been described with reference to a hydraulic power steering valve, it should be understood that the same method of testing the joint can be utilised on an electric power steering embodiment wherein assistance is facilitated by an electric motor. As shown in Figure 6, the input-shaft 2 and torsion bar 4 forms part of a"torque-sensing rotor"51 instead of a hydraulic valve. The torque-sensing rotor differs from a hydraulic valve in that the sleeve and the hydraulic chambers are replaced by a device 50 that measures the relative rotational displacement between the input-shaft 2 and the pinion (driven member) 5.