An apparatus, consisting of a rolling element (46), which is resting on a surface (48), and a force-sensing device (50), which is coupled to the surface. The force-sensing device is configured to make a first measurement indicative of a force exerted in a direction perpendicular to the surface. The force is exerted by a force-sensing probe (26) pressing against the rolling element so as to hold the rolling element stationary. The apparatus further includes a calibration processor, which is configured to collect the first measurement from the sensing device, to collect a second measurement indicative of the force from the force-sensing probe, and to calibrate the force-sensing probe based on the first and second measurements.
