The short answer is yes: ball screws can be lubricated with either grease or oil. Grease is more commonly used for ball screw lubrication because it doesn’t entail complicated delivery methods, and it requires less frequent re-lubrication intervals. But despite the simplicity of using grease, in many applications, oil lubrication is the better choice for ball screw assemblies.
First, oil provides cooling and minimizes heat buildup within the ball screw assembly. Heat causes thermal expansion of the screw shaft, which changes the dimensions of the screw threads and negatively affects positioning accuracy, so reducing heat can be critical in applications that require high speeds and/or high precision. And if the oil is circulated through an external lubrication system, the cooling effect of the oil can be significant. Circulation systems also allow for debris to be filtered from the oil, which extends the life of the lubrication and reduces wear on the screw assembly. The drawback is that an external circulation system adds complexity and cost to the assembly.
When using oil for ball screw lubrication, it’s important to use the right viscosity oil in the proper amount. If the oil viscosity is too high, or if there’s too much oil, excessive heat can be generated. On the other hand, if the oil viscosity is too low, or if there’s not enough oil lubrication, the screw will experience additional friction and wear will be accelerated. The proper oil viscosity is based on the screw’s average speed, diameter, and operating temperature. If the applied load is high – typically greater than 15 to 20 percent of the dynamic load capacity – manufacturers often recommend using oil with extreme pressure (EP) additives for extra protection against wear.
With oil lubrication, it’s also critical that the application conditions are sufficient to allow an elastohydrodynamic (EHD) lubrication film to form. This film separates the load-carrying balls from the raceways and prevents metal-on-metal contact.
There are three primary conditions that determine whether a lubrication film will develop: the lubricant’s viscosity, the screw’s speed, and the pressure between the balls and the raceway. Because of speed’s influence on EHD lubrication, low-speed applications generally require grease, which provides better protection under the conditions of boundary lubrication (essentially metal-to-metal contact) or mixed-lubrication (a combination of metal-to-metal contact and lubrication support).
Ball screws with built-in lubricators typically work by supplying oil directly to the raceways of the screw. This oil supplements grease within the ball nut and extends lubrication intervals significantly, sometimes allowing a screw to operate for several years without re-lubrication.
Feature image credit: Hiwin Corporation