New micro tribometer for rolling friction

To determine the rolling friction resistance in the micro rolling systems we developed a new micro tribometer consisting in a driving rotational disc in contact with 3 microballs which sustain an inertial driven rotational disc. The driving disc has a constant rotational speed and his rotational motion is transmitted to the inertial driven disc by rolling friction forces, developed in the rolling contacts between the microballs and the inertial disc. When the rotational speed of the inertial driven disc becomes constant, the driven disc is stopped and the inertial driven disc starts a deceleration process until it completely stops due to friction. A camera monitors the angular position of the inertial driven disc, from the start of the deceleration process to his completely stop (the spin-down method). An analytical model based on the integration of the inertial disc motion equation, including both rolling frictions in the rolling contacts and the friction between the inertial disc and air has been developed. The equation of the inertial disc angular position, as function of the time was obtained considering two hypothesis: (i) in the first hypothesis it was considered that rolling friction torque does not depend on rotational speed in dry contacts; (ii) in the second hypothesis it was considered a linear dependence between rolling friction torque and rotational speed for dry contacts. Experimental investigations using the new microtribometer with three 1.588 mm diameter stainless steel microballs, for a range of rotational speed between 30 rpm and 210 rpm and for normal contact loads between 8 mN and 33 mN confirm the validity of the hypothesis of constant friction torque in dry conditions. Based on the hypothesis of constant friction torque, the rolling friction torque in the contacts between the microballs and the two discs having values between 1.8 μNmm and 7.2 μNmm have been obtained with the new microtribometer. Also, rolling friction coefficients having values between 0.0002 and 0.0004 have been obtained.