Frictional torque affects the free-
A bearing containing stiff grease will be difficult to spin. This means it has relatively high frictional torque. A bearing with no lubrication will spin freely meaning it has low frictional torque. The effort required to rotate a bearing depends greatly on the roundness of the bearing, the load applied, the lubrication and the closures. Better roundness and surface finish of the balls and raceways means less effort is needed to rotate the bearing. The greater the load, the greater the deformation of the bearing components leading to increased resistance.
As for lubrication, instrument oils will often produce lower torque levels especially at very low speeds but the difference between these and many low torque greases can be quite small, particularly if a low grease fill is used. High viscosity lubricants can significantly increase bearing torque due to greater lubricant drag. Torque levels for a greased bearing are briefly higher to start with as the grease takes a short time to "run in" or be distributed inside the bearing. Contact seals will greatly increase the torque figures. The effort required to rotate a bearing from rest (starting torque) is slightly greater than the effort required to keep it rotating (running torque).
You can calculate approximate figures for frictional torque using this simple formula:
Radial ball bearings: 0.5 x 0.0015 x radial load in Newtons* x bearing bore (mm)
Axial ball bearings: 0.5 x 0.0013 x axial load in Newtons* x bearing bore (mm)
This is only valid if the bearing has a standard fill of low torque lubrication, does not have contact seals and is subjected to low speed and low load. For radial ball bearings, the axial load should be less than 20 percent of the radial load while the load should be purely axial for thrust bearings. Contact us if you need more accurate figures taking into account the speed and the lubricant viscosity.
The measurements are in Newton millimetres (Nmm). This is a compound unit of torque corresponding to the torque from a force of one newton (approx 0.1 Kgf) applied over a distance arm of one millimetre.