As a general rule, the same lubricants are used for planetary roller screws as for rolling element bearings are either oil or grease. The type of lubricant used is most dependent on the operating and maintenance conditions.
Proper lubrication is essential to the proper functioning of a roller screw. The key point is in the initial lubrication of the nut so that all components have adequate lubrication.
Unless otherwise specified, CMC roller screws are shipped “dry” with only a protective coating, and the customer must properly lubricate the roller screw before usage.
The volume, spread, and frequency of re-application of the lubricant must be properly selected and monitored. At high speed the lubricant on the surface of the shaft may be thrown off by centrifugal forces. Therefore, it is important to monitor this effect when operating at high speed, and considering this when selecting a lubricant.
Monitoring the equilibrium temperature reached by the nut permits the frequency of relubrication or oil flow rates to be optimized.
The selection of lubricant and the maintenance of relubrication is the responsibility of the customer.
A centralized recirculating oil system is ideal due to its ability to continually supply filtered, temperature controlled oil at prescribed flow rates. While such systems represent the optimum, they are not always practical from a cost or size perspective and alternate solutions are available that can achieve effective results if properly configured.
Selection of Oil:
Circulating mineral oils with EP additives to enhance resistance to aging and corrosion in compliance with DIN 51517, Part 2, are particularly suitable for the lubrication of planetary roller screws. Operating speed, ambient temperature and operating temperature are all factors in determining the required viscosity of the lubricant.
The required volume of oil depends on the screw diameter, the number of supporting rollers and the amount of heat to dissipate.
For immersion lubrication, the oil level should be such that the lowest roller is completely submerged in oil. The amount of oil and the change interval depend on the intensity of the loading on the system and the details of the installation.
Figure A below shows the operating viscosity, νk (mm²/s), required for any given mean speed of the screw system based on the diameter of the shaft. The viscosity, νk, prescribed by figure A, ensures a sufficient lubrication to achieve the nominal life for the system, provided that the lubricant is properly filtered and maintained. Intermediate values can be estimated by interpolation between the curves provided in figure A.
Nominal lubricant viscosity can be determined based on the required viscosity, νk, the steady state temperature of the roller screw using the viscosity-temperature diagram in figure B, and the operating temperature of the roller screw system. Nominal viscosity is the viscosity of the lubricant at 40°C. Viscosity classes consistent with ISO VG (DIN 51517), Part 2, are plotted in figure B. The operating temperature of the roller screw must be known or estimated to calculate the required nominal viscosity using this technique.
Grease is the most common form of lubrication for CMC roller screws, and provides an effective solution for most applications. The required viscosity in the case of grease lubrication can be calculated using the same process outlined above for oil viscosity determination. The viscosity of grease is rated with ISO VG levels just as oils and this information is typically provided by grease manufacturers. Re-greasing intervals depend on the screw arrangement, size and operating conditions.