Is oil suitable for ball screw lubrication?

The short answer is yes:Ball screws can be lubricated with grease or oil.Grease is more commonly used for ball screw lubrication because it does not require complex delivery methods and requires fewer re-lubrication intervals.But although using grease is simple,there are many applications where oil lubrication is a better choice for ball screw assemblies.

First,the oil provides cooling and minimizes heat buildup within the ball screw assembly.Heat can cause thermal expansion of the screw shaft,which changes the size of the threads and negatively affects positioning accuracy,so reducing heat is critical in applications where high speed and/or high accuracy is required.If the oil is circulated through an external lubrication system,the cooling effect of the oil is great.The circulatory system also allows debris to be filtered from the oil,thereby extending lubrication life and reducing wear on the screw assembly.The disadvantage is that the external circulatory system increases the complexity and cost of assembly.

When using oil for ball screw lubrication,it is very important to use the right amount of oil with the right viscosity.If the oil viscosity is too high,or there is too much oil,too much heat will be generated.On the other hand,if the oil viscosity is too low,or there is not enough oil lubrication,the screw will experience additional friction and wear will accelerate.The appropriate oil viscosity depends on the average screw speed,diameter and operating temperature.If the load applied is high(usually greater than 15 to 20 percent of the dynamic load capacity),manufacturers usually recommend using an oil with an extreme pressure(EP)additive to provide additional protection against wear and tear.

Manufacturers usually provide selection guidelines to help select the right ball screw oil viscosity for the screw operating temperature and average speed.

For oil lubrication,it is also important that the application conditions are sufficient to form an elastohydrodynamic(EHD)lubrication film.The film separates the load-carrying ball from the raceway and prevents metal-metal contact.

Having the right viscosity and the right amount of oil will allow the ball screw to operate in the elastohydrodynamic friction area.

There are three main conditions that determine whether a film will form:the viscosity of the lubricant,the speed of the screw,and the pressure between the ball and the raceway.Due to the effect of speed on EHD lubrication,low speed applications usually require grease that provides better protection in conditions of boundary lubrication(basically metal-to-metal contact)or mixed lubrication(a combination of metal-to-metal contact and lubrication support).

Ball screws with built-in lubricators usually work by feeding oil directly into the raceway of the screw.This oil supplements the grease within the ball nut and significantly extends the lubrication interval,sometimes allowing the screw to operate for years without re-lubrication.

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