Abstract:
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.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of the invention will now be described with reference to the accompanying drawings in which: 
     FIG.  1 : shows a schematic plan view of testing apparatus embodying the invention; 
     FIG.  2 : shows a schematic side view, partly in section, of the testing apparatus shown in FIG. 1; 
     FIG.  3 : shows (in a larger scale) a schematic end view, from the left as shown in FIGS. 1 &amp; 2, of part of the testing apparatus shown in FIGS. 1 &amp; 2 with the second traction surface in a raised position; 
     FIG.  4 : shows various Stribeck Curves at 50% slide/roll ratio derived from use of the apparatus shown in FIGS. 1 to  3 ; and 
     FIG.  5 : shows plots of traction coefficient against slide/roll ratio at various different temperatures, derived from use of the apparatus shown in FIGS. 1 to  3 . 
    
    
     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          2        (       V   1     -     V   2       )           V   1     +     V   2                              
     Rolling speed is the average of the speeds of first and second traction surfaces, i.e.            V   1     +     V   2       2                          
     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 &amp; 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 &amp; 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.