Traction or friction measurement apparatus and method of calibration

A method of determining Disk Track Radius (DTR) in a traction or friction measuring apparatus, said apparatus comprising a disc including a flat disc traction surface;

• a ball including a spherical ball traction surface constructed and arranged to, in use, contact said disc traction surface;
• a support structure constructed and arranged to support said disc and ball traction surfaces with respect to one another whilst allowing relative rotational movement therebetween;
• a disc drive motor and a ball drive motor operable to drive said disc and said ball, respectively, to effect the relative movement between said disc and ball traction surfaces to generate a traction or friction force therebetween;
• disc speed measuring means and ball speed measuring means for measuring the speed of the disc and the ball, respectively;
• and force measuring means associated with at least said disc and ball traction surfaces to provide a force measurement arising from said traction or friction force and wherein measurement of ball speed and disc speed can be made at a point of pure rolling between the ball and disc in order to determine a disc track radius (DTR) based on a known ball track radius,
• said method comprising the following steps:
• establishing contact between the spherical ball traction surface of the ball and the flat disc traction surface of the disc;
• steadily increasing the disc speed and reducing the ball speed (or vice versa) in such a way as to ensure that at some point the speeds pass through a point where the disc and ball are in pure rolling contact, wherein said point where the disc and ball are in pure rolling contact is determined when said disc speed and ball speed are equal such that there is no slippage between the disc traction surface and the ball traction surface;
• measuring ball speed and disc speed at said point when said disc and ball are in pure rolling contact;
• measuring traction force between the ball and the disc and plotting traction force against a slide/roll ratio (SRR), wherein the slide/roll ratio (SRR) is calculated as a difference in speed between the disc speed and ball speed divided by the average of the disc speed and ball speed and expressed as a percentage according to the following formula in which Ud is disc speed and Ub is ball speed and:
• SRR=200*(Ud−Ub)/(Ud+Ub);
• wherein said step of measuring ball speed and disc speed and said step of measuring traction force comprise observing and recording motor speed values of the disc drive motor and the ball drive motor that correspond respectively to said disc speed and the ball speed at a point of transition from positive to negative or negative to positive traction force as the contact passes through pure rolling contact when said disc speed and ball speed are equal without slippage between the disc traction surface and the ball traction surface when SRR=0%, and,
• using said known ball track radius and the measured ball speed and the measured disc speed during pure rolling contact to determine the disc track radius (DTR) based on the formula:
• DTR=Ball speed×ball track radius/Disc speed.