Testing apparatus

Traction and/or friction testing apparatus is described having inter-engaging, rotating, traction surfaces, the surfaces being independently driven to generate traction or friction forces therebetween. A force measuring means is provided to measure the resulting traction or friction force. The apparatus is characterised in that all forces applying in the traction measurement loop are directly readable, or elastic, thus eliminating internal friction and allowing an accurate indication of the traction or friction force. The apparatus is particularly useful for measuring rolling/sliding traction and friction.

TECHNICAL FIELD 
 This invention relates to testing apparatus and, in particular, to 
 apparatus for testing rolling traction and/or friction forces. Typical 
 applications include fuel economy modelling of automotive engine oils, 
 boundary additive evaluation and friction measurements for traction 
 fluids. 
 BACKGROUND OF THE INVENTION 
 A number of forms of apparatus have been proposed, in the past, for testing
 friction and/or traction forces. However, accurate measurement is 
 difficult to achieve as known forms of testing apparatus have in-built 
 resistances, such as internal friction, which can influence the total 
 force the apparatus is attempting to measure. 
 By way of example, traction in rolling/sliding contacts is usually 
 determined by measuring torque applied to one of the rotating specimens or
 by measuring the reaction force felt by a body supporting one of the 
 rotating specimens. 
 If measuring the torque applied, it is normally necessary to position the 
 torque transducer behind bearings supporting the specimen drive shaft. 
 This means that the transducer is also measuring the torque applied to 
 overcome frictional resistance in the bearings and/or oil seal and is thus
 not giving an accurate measurement of the applied torque alone. 
 If measuring the reaction force, this is normally measured by a force 
 transducer which constrains the body supporting one of the rotating 
 specimens from moving in the direction of the traction force. Because of 
 the applied load, the body is, normally, supported by additional means. In
 order to maximise the accuracy of the traction measurement, the additional
 means of support needs to have extremely low resistance to motion in the 
 direction of the traction force. This is typically achieved by using 
 rolling element bearings or air bearings within the additional support. 
 However, whilst such bearings have very low frictional resistance, they 
 have sufficient resistance to reduce the accuracy of the traction 
 measurement. Also, such frictional resistance as they do have will 
 normally vary with variations in magnitude of the applied load. 
 For practical purposes it may not be possible to entirely eliminate 
 extraneous forces. What is therefore required is a form of apparatus in 
 which any extraneous forces are predictable and which can thus be 
 eliminated by a calibration process. 
 It is an object of this invention to provide traction and/or friction 
 testing apparatus in which any extraneous force inherent in the apparatus 
 is predictable such that any measurement of traction or friction force 
 will be directly proportional to the actual force. 
 SUMMARY OF THE INVENTION 
 Accordingly, in one aspect, the invention comprises traction or friction 
 testing apparatus, said apparatus comprising: 
 a first traction surface; 
 a second traction surface constructed and arranged to, in use, contact said
 first traction surface, said first and second traction surfaces being 
 arranged for rotational engagement therebetween; 
 a support structure constructed and arranged to support said first and 
 second traction surfaces with respect to one another whilst allowing 
 rotational movement therebetween; 
 drive means operable to effect differential rotation between said first and
 second traction surfaces and thereby to generate a traction or friction 
 force therebetween; and 
 force measuring means associated with at least said first and second 
 traction surfaces to provide a measure arising from said traction or 
 friction force, 
 said apparatus being characterised in that any force arising between said 
 first and said second traction surfaces due to traction or friction 
 therebetween is resisted solely by elastic deformation of said support 
 structure and/or said force measuring means. 
 The subject invention is constructed and arranged to measure rolling 
 traction and/or friction. To that end, the first traction surface is 
 conveniently planar in form whilst the second traction surface has a 
 circular component to allow rotating motion thereof with respect to the 
 first surface. More preferably, the second traction surface is provided by
 the surface of a spherical ball. 
 In a particularly preferred form, the first traction surface comprises a 
 planar disc adapted to be mounted for rotation about its central axis. 
 The support structure preferably includes first support means constructed 
 and arranged to rotatably mount said first traction surface; and second 
 support means constructed and arranged to rotatably support said second 
 traction surface, said first and second support means being relatively 
 displaceable to allow said first and second traction surfaces to be moved 
 into contact with their respective axes of rotation lying in a 
 substantially common plane. 
 Conveniently, the first support means mounts said first traction surface 
 for rotation about a substantially vertical axis. The second support means
 is mounted for pivotal movement about a substantially horizontal axis to 
 permit said second traction surface to be displaced into contact with said
 first traction surface. 
 Preferably said second support means further includes elastic flexure means
 constructed and arranged to permit elastic movement of said second 
 traction surface with respect to said first traction surface in the 
 direction of the resulting traction or friction force, yet resist movement
 of said second traction surface in orthogonal directions. 
 The drive means may include a first drive motor to rotate said first 
 traction surface; and a second drive motor to rotate said second traction 
 surface. The drive means may further include displacement means to 
 variably displace said second traction surface into contact with said 
 first traction surface in a direction normal to said first traction 
 surface. This displacement means is conveniently provided, in part, by a 
 stepper motor. 
 The force measuring means preferably comprises a linear force transducer 
 mounted to detect movement of said second traction surface due to a 
 traction or friction force being generated between said first and second 
 traction surfaces. 
 Many variations in the way the invention may be performed will present 
 themselves to those skilled in the art. The only limitations on the scope 
 of the invention should be imposed by the appended claims and not by the 
 description of one preferred embodiment which follows.

DESCRIPTION OF PREFERRED EMBODIMENT 
 Before commencing with a description of the apparatus and its operation, it
 is useful to define a few terms: 
 Slide/roll ratio is intended to mean the difference between the 2 speeds of
 the traction surfaces divided by their average. In other words 
 ##EQU1## 
 Rolling speed is the average of the speeds of first and second traction 
 surfaces, i.e. 
 ##EQU2## 
 Turning to FIGS. 1 to 3, testing apparatus 10 is provided having a first 
 traction surface 11 and a second traction surface 12, the surfaces 11 and 
 12 being arranged to contact one another as can be seen in FIGS. 1 & 2. A 
 support structure, generally designated 13, is provided to support the 
 surfaces 11 and 12 in their respective operating positions whilst allowing
 a certain amount of relative movement therebetween. 
 Mounted within the support structure 13 are drive means 14, 15 which are 
 operable to effect respective movement of the traction surfaces 11 and 12 
 and, thereby, to generate a friction or traction force therebetween which,
 in use, is measured by force measuring means 16. 
 The apparatus 10 is characterised in that any traction or friction forces 
 arising between the traction surfaces 11 and 12 are resisted solely by 
 elastic deformation of the support structure and/or elastic deformation of
 the force measuring means 16. 
 In the form shown, the traction surfaces 11 and 12 are respectively 
 configured to allow rolling movement with respect to one another. To this 
 end, the first traction surface 11 is preferably a planar surface whilst 
 the second surface 12 has a circular component such that, when the surface
 12 is brought into contact with surface 11, rolling traction or friction 
 forces can be generated. More advantageously, the planar first traction 
 surface 11 is provided in the form of a disc mounted for rotation about 
 its central axis by first drive means 14. 
 The second traction surface 12 is advantageously provided as the surface of
 a spherical ball mounted for rotation by second drive means 15. In one 
 particular operating embodiment, the disc 11 is 46.0 mm in diameter and 
 ball, 12, 19.05 mm in diameter. Both are formed from polished AISI 52100 
 bearing steel. They can, of course, be formed from other materials if 
 desired. Both components are designed to be single use items, after which 
 they are disposed of 
 The support structure 13 includes first support means 17 to rotatably 
 support the disc 11, and second support means 18 to rotatably support the 
 ball 12, the first support means 12 and second support means 18 being so 
 arranged with respect to each other that the axis of rotation of the ball 
 12 passes through the axis of rotation of the disc, the intersection of 
 axes coinciding with the centre of the planar contact face of the disc 11.
 Thus, under pure rolling motion, contacting surface points in the contact 
 patch will have substantially the same speed, so minimising a phenomenon 
 known as spin in the contact. The shape of the contact is circular and is 
 known as a Hertz contact. 
 More particularly, the first support means comprises bearing block 19 
 mounted on base chassis 20, the bearing block 19 mounting first drive 
 shaft 21, in bearings 22, in a substantially vertical orientation. Disc 11
 mounts on the upper end of the drive shaft 21, whilst mounted on the lower
 end of the shaft 21 is a drive pulley 23. Drive pulley 23 receives drive 
 from a further pulley 24 mounted on the output shaft of DC servo motor 25,
 via drive belt 26. 
 Formed in the upper part of bearing block 19 is a fluid tight reservoir 27,
 the reservoir 27 being configured to retain a liquid under test, in a 
 manner such that the contact patch between the disc 11 and ball 12, is 
 immersed in the test fluid. The reservoir 27 is closed by a lid 28 along 
 interface 29 which, as can be seen, is positioned above the contact patch 
 between disc 11 and ball 12. 
 Electrical heating elements (not shown), or equivalents, are provided to 
 heat the con tents of the reservoir 27, the temperature preferably being 
 measured by platinum RTD type temperature probes 30. An external 
 refrigerated oil cooler (not shown), or equivalent, may be provided to 
 cool the contents of the reservoir 27. 
 The upper part of bearing block 19 and the lid 28, which in combination 
 define the reservoir 27, are preferably clad in a PTFE insulating jacket 
 to render the apparatus safe to touch even at the highest test 
 temperatures. 
 The support structure 13 further includes second support means 31 which 
 supports second drive means 15. The drive means 15 preferably comprises a 
 further DC servo motor, ball 12 being mounted directly on output shaft 32 
 of the motor 15 for rotation thereby. The second support means 31 is 
 arranged with respect to the first support means 17 so that the axes of 
 disc 11 and ball 12 lie in a common vertical plane as can be seen in FIG. 
 1. 
 The use of independently driven DC servo motors as the drive motors 25 and 
 15 allows high precision speed control, particularly at low slide/roll 
 ratios. 
 When in the testing configuration shown in FIGS. 1 & 2, with the ball 12 in
 contact with the disc 11, the output shaft 32 must pass through the wall 
 which defines reservoir 27. This is advantageously accommodated by 
 ensuring interface 29 is substantially coincident with the as o f shaft 32
 when the ball 12 is in the loaded position as shown in FIG. 1. The lid 28 
 and bearing block 19, are provided, adjacent the interface 29, with 
 co-operating semi-circular cavities (not shown) which, when the lid 28 is 
 place in position over the upper part of bearing block 19 to define the 
 reservoir 27, provide a clearance aperture about the shaft 32. Because 
 interface 29 is above the working liquid level in the reservoir 27, shaft 
 32 does not need to be sealed where it passes through the reservoir wall. 
 Obviously, if a shaft seal were used, such a seal would apply a resistive 
 torque and reaction force to the shaft 32. 
 Motor 15 is mounted in a gimbal arrangement 33 mounted, in turn, in 
 brackets 34 extending vertically from base chassis 20. Gimbal 33 is 
 mounted to brackets 34 through stub shafts 35 mounted on a common 
 horizontal axis. This allows the gimbal arrangement 33 to pivot about the 
 horizontal axis and thereby bring the ball 12 into and out of contact with
 the disc 11. Further, by applying a loading force on the gimbal 
 arrangement 33, about the horizontal axis of shafts 35 the force of ball 
 12 against disc 11 can be varied without varying any static interactive 
 forces between the two, in orthogonal directions. 
 Stub shafts 35 project from rigid vertical side plates 36 which form part 
 of the gimbal arrangement 33. Mounted between the upper and lower edges of
 the side plates 33 are flexures 37. Centrally located within each of the 
 flexures 37, along a common axis orthogonal to the axis of stub shafts 35,
 are further stub shafts 38. The stub shafts 38 form part of mounting 39 in
 which motor 15 is mounted. 
 Each of the flexures 37 is configured and arranged to provide low torsional
 stiffness about the axis of stub shafts 38 yet provide high stiffness 
 around any other rotational axis or in any translational direction. More 
 importantly, the flexures 37 are configured and arranged to ensure that 
 any resistance to movement, particularly about the axis of stub shafts 38,
 is purely elastic. In the embodiment depicted, the flexures 37 comprise 
 four beams 40 which are arranged at right angles and which are relatively 
 thin when viewed vertically as in FIG. 1. Whilst four beams 40 are 
 depicted and described, it will be appreciated, by those skilled in the 
 art, that three, or more than four, beams could be made to function 
 equally effectively. 
 The beams 40 are preferably machined from aluminium and include a central 
 hub 41 in which stub shafts 38 mount in a non-rotating manner. Thus, any 
 rotation of the motor 15 about the axis of stub shafts 38 is resisted by 
 elastic deformation of the flexure beams 40. 
 Projecting from the gimbal arrangement 33 is a loading beam 42, the outer 
 end of which is linked to stepper motor 43 and a ball screw actuator to 
 apply a displacement to the beam 42 and thereby displace the ball into and
 out of contact with disc 11 in a direction normal to the plane of disc 11.
 Once the ball has made contact with the disc, the actuator will cause the 
 load beam 42 to bend and so, progressively increase the load applied to 
 the disc. After calibration, the magnitude of the load can be precisely 
 measured from the step count on the stepper motor or through strain gauges
 mounted on the load beam. 
 Finally, the force measuring means 16 is advantageously mounted on one of 
 the vertical, rigid side plates 36 so as to contact mounting 39, supported
 in the flexures 37, which moves with the motor 15 and ball 12. Clearly, in
 this configuration, the force measuring means 16 resists movement of the 
 mounting 39 about the axis of stub shafts 38. 
 The means 16 comprises a linear force transducer which is very much stiffer
 than the flexures 37 to maximise the sensitivity of the traction/friction 
 measurement. However, the transducer 16 is also configured to ensure that 
 any displacement thereof is elastic. Accordingly, it will be appreciated 
 that because the transducer 16 and the flexures 37 form a linear elastic 
 system, the transducer signal is directly proportional to any traction or 
 friction force which arises between ball 12 and disc 11, and so can be 
 calibrated to precisely measure the traction or friction force. 
 In use, before testing is commenced, the reservoir 27 is carefully cleaned 
 and dried. Among suitable solvents for cleaning the reservoir are white 
 spirits or varsol followed by iso-octane or heptane. In general, the first
 solvent should be chosen to give good removal of the types of lubricant 
 under investigation, whilst the second solvent should be chosen to give a 
 clean, dry surface. 
 After application of the solvents, the reservoir is dried with a hot air 
 dryer or with an air or nitrogen line. Alternatively, the reservoir can be
 filled with solvent which is then vacuumed out, drying the reservoir in 
 the process. 
 The disc and ball must be carefully cleaned prior to use, with particular 
 care being taken to remove any protective surface coatings (such as 
 anti-corrosive coatings) applied to prevent deterioration of the 
 components prior to use. Cleaning can be effected using a soft tissue and 
 then the disc and ball placed in separate beakers containing iso-octane or
 another suitable solvent. Each component is then cleaned in an ultrasonic 
 cleaner for 2 minutes, the solvent then replaced with clean solvent, and 
 cleaning resumed for another 10 minutes. The solvent is then replaced 
 again and cleaning effected for a further 10 minutes, after which the 
 components are dried with a clean, dry air line or with a nitrogen line. 
 The disc 11 and ball 12 are then mounted securely on their respective drive
 shafts. 
 Once the disc 11 and ball 12 have been secured in place, a number of tests 
 can be conducted for a given lubricant under test. Each test will have a 
 given temperature, normal load, speed and slide/roll ratio. These 
 parameters may advantageously be stored in a computerised control system 
 which cycles through the various tests, collecting and storing 
 measurements from the force transducer 16 as it goes. 
 Typical test output readings are shown in FIGS. 4 and 5. 
 It will thus be appreciated that the present invention provides a form of 
 traction and/or friction testing apparatus which eliminates non-predictive
 forces from the test componentry, is compact in form and, at least in the 
 preferred embodiment described, cycles through a variety of tests with the
 minimum of human intervention.