~ How does lubricant affect bearing torque? ~
The use of low torque ball bearings is improving energy efficiency, minimising operating costs and reducing carbon dioxide emissions. To help machine builders and plant managers leverage these benefits, Chris Johnson, managing director of miniature bearing specialist, SMB Bearings, explains the importance of one critical torque factor, bearing lubrication.
Rolling element bearings, such as ball and roller bearings, support the load required to carry out a machine’s function at a much lower friction than a fluid bearing alternative. This reduces the power required to drive the equipment, lowering the cost of moving the load and the energy required to operate the equipment.
Bearing torque is the force required to overcome internal friction to start or maintain rotation of the bearing. This frictional torque is generally increased with a larger bearing, increased load or increased lubricant drag. Let’s focus on the latter.
So, how do you get it right? Discussing lubrication options with your bearing supplier can help you to choose the correct option, as there are several oils and greases to choose from.
Many applications require the bearing to spin easily with extremely low frictional torque, without generating excess heat. Dry lubricant is one way of meeting these low torque requirements. Despite being solid, dry lubricants can reduce friction between two surfaces without the need for oil or grease. Using very low viscosity grease with a reduced fill can also achieve a similar outcome. However, this will also permit much higher running speeds.
When choosing between oils or greases, it’s important to remember that torque levels for a greased bearing are briefly higher to start with, when the equipment begins running. This is because the grease takes a short time to ‘run in’ and be distributed inside the bearing. It’s most likely not a problem for many continuous applications but this could cause problems for stop start machinery.
Instrument oils are another option for low torque requirements. They will often produce very low torque levels especially at very low speeds. However, it’s worth mentioning the difference between these and very low torque greases is actually quite small, particularly if a low grease fill is used. This combination may give an increase of only 20 percent in frictional torque over the Aeroshell 12 instrument oil.
For higher torque requirements, high viscosity lubricants can significantly increase bearing torque due to greater lubricant drag. The fill level is also a huge factor in the frictional coefficient, with higher fills resulting in stiffer bearing rollability. For example, robotics applications require precise and predictable movements of robot arms. It is often the fill level in robotics bearings that is trialled during product development, to achieve the ideal frictional coefficient, enabling smooth and precise movements.
There are many oils and greases to choose from, and there are other factors to consider aside from torque, such as contamination, speed or corrosion resistance. With the right lubrication type, correct fill level and consideration of load, you’ll be able to source bearings with the correct frictional torque for the specific application.
If you do need some more advice on bearing lubrication, call +44 (0) 1993 842555 to speak to a member of the SMB Bearings technical team.
