This affects the free-running of the bearing. Spin a bearing containing stiff grease with your finger and not much happens – relatively high frictional torque. Try a bearing with no lubrication and it will spin freely – 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 is actually very small, particularly if a low grease fill is used. A standard low torque grease such as Multemp SRL grease may give an increase of only 20 percent over a Aeroshell 12 oil. This can drop to under 5 percent for very low torque greases if a low (e.g 10 to 20 percent) 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).
Approximate figures for frictional torque for can be calculated using a simple formula. This is only valid if the bearing has low torque lubrication (and the grease fill is not high), is open, shielded or has non-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 frictional torque from a force of one newton (approx 0.1 Kgf) applied over a distance arm of one millimetre.
Frictional torque (measured in Nmm or Newton millimetres)
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)
*10 Newtons = 1 Kgf