Lubrication
As a general rule, the same lubricants are used for satellite roller screws as for roller bearings, i.e. either oil or grease. The type of lubricant used mostly depends on operating and maintenance conditions. Unless the customer explicitly instructs otherwise, standard Rollvis grease is applied at the factory.
Oil lubrication
Circulating mineral oils with EP additives to enhance resistance to aging and corrosion in compliance with CL based on DIN 51517, part 2, are especially suitable for the lubrication of satellite roller screws. Speed, ambient temperature and operating temperature are determining factors in the choice of viscosity.
The amount of oil required depends on the screw diameter, the number of supporting rollers and the amount of heat to dissipate. 1 cm3/h (for small screw diameters) to 30 cm3/h (for large screw diameters) can be used as reference values.
The shortest possible lubrication intervals (...5 minutes) are recommended for high loads and longer intervals (5 minutes to 1 hr) for low loads. Automatic lubrication is recommended for high loads and speeds.
For immersion lubrication, the oil level should be such that the bottommost roller is completely submerged in the oil. The amount of oil and the oil-change intervals depend on the loading and installation.
The viscosity of the oil should be chosen to enable an adequate film of lubricant to form on the contact surfaces.
Figure a shows the operating viscosity VK to achieve for the mean speed of the satellite roller screw and screw diameter.
Viscosity VK ensures a state of lubrication to fulfil the nominal lifespan provided the lubrication system is free of foreign bodies.
Nominal viscosity can be calculated on the basis of viscosity VK with the viscosity-temperature diagram (diagram V-t, figure b) and the operating temperature. Nominal viscosity is the viscosity of oil at 40°C. Viscosity classes ISO VG (DIN 51519) are plotted in diagram V-t.
Figure a shows the nominal diameters of RV satellite roller screws. The diameters of RVR satellite roller screws are slightly different. The requisite operating viscosity values can be obtained by interpolation.
Intermediate values can be obtained from the steps. These must be rounded up to the nearest viscosity value.
The operating temperature must be known or estimated to calculate nominal viscosity. The operating temperature must be measured on the nut after stabilisation. A suitable oil can be found in oil suppliers' lists with the nominal viscosity at 40°C. In general, an operating temperature of 30°C can be taken as a basis for selecting a lubricant.
Example:Satellite roller screw RV 39 x 10
Mean operating speed: nm = 1400 min-1
Operating temperature (estimated): t = 25 °C
In figure a, for a number of revolutions nm = 1400 min-1 and a nominal diameter of 39mm, the nominal viscosity is VK = 33 mm2/s. In diagram V-t (figure b), the 25°C temperature and 34 mm2/s viscosity lines intersect between ISO VG 15 and ISO VG 22. The oil to choose will be in viscosity class VG 22.
A suitable CLP (DIN 51517) or HLP (DIN 51525) oil can be selected from this class.
Grease lubrication
The best grease lubricants to use are KP (DIN 51825, part 3) with a consistency factor of 2.
Lubrication intervals depend on the screw arrangement, size and operating conditions. Rollvis can provide recommendations for any type of application.