Lubrication and Wipers / Seals
Supply of fresh lubricant and wipers must be considered together. The selection not only depends on environmental conditions, but also on loads and speeds. In this section we discuss whether grease or oil is the right lubricant, and which grease or oil should be used.
If the duty cycle is such that an EHD lubrication film can build over a significant part of the motion, then oil lubrication with a properly selected oil grade and viscosity will always outperform grease in terms of wear. On the other hand, grease has an edge at slow speeds because it offers better wear protection under mixed friction or boundary friction conditions. Grease can also be used for long-term or for-life lubrication. Lubricant loss with grease is lower than with oil.
Follow this link to view a general discussion for oil vs. grease lubrication and the results of a scientific test of several commonly used lubricants by the tribology lab of CSEM at Neuchâtel/Switzerland (www.csem.ch).
Hybrid design of a ball screw means screw and nut are made from steel and balls are made from ceramic. Most suitable ceramic is Silicon Nitride in HIP-quality (not isostatic pressed).
Hybrid ball screws are designed to operate with very minimal lubrication. When adequate lubrication is available there is no difference compared to conventional ball screws equipped with steel balls.
The high hardness and Young's modulus of Silicon Nitride (Si3N4) results in a higher applied load to the steel-ball race. Hybrid ball screws however must be assembled using smaller steel spacer balls which means that only 50% of the balls are carrying load. Therefore, the application of hybrid ball screws must be carefully reviewed.
Steinmeyer gained technological insight and experience concerning hybrid ball screws by participating in the EUREKA! research program "Rocotrib". Please ask for our recommendations when considering the use of hybrid ball screws.