Oil Lubrication of Ball Screws
The achievable service life values of oil-lubricated ball screws are superior to those with grease lubrication in principle, provided an elastohydrodynamic (EHD) lubrication can be achieved for a significant part of the work cycle.
In order to achieve an increased wear protection in the mixed friction area in the ball contact points and to effectively prevent any surface damage (such as seizure marks) in the event of an overload, it is required and recommended to use CLP lubricating oils with EP additives (wear-reducing additive) according to DIN 51517 part 3.
If the wear-reducing additive is missing, the lubricating oil will be pressed out of the lubricating gap in the event of a high Hertzian pressure (2,000 to 4,000 N/mm2 is characteristic for ball screws) and no stable lubricating film can form, even at low speeds. This inevitably leads to a higher wear rate and a reduction in service life.
CLP lubricating oils are also characterized by a good aging resistance. In addition, highly-viscous lubricating oils improve the running quality and reduce the noise development of the ball screw.
Lubricating Oil Viscosity
The lubricating oil viscosity is also very important. Highly loaded ball screws require highly viscous lubricating oils so that the elastohydrodynamic lubricating film can form between the rolling partners.
This lubricating film must not only be present at ambient temperature (up to 40 °C), but also must be reliably present at higher operating temperatures (up to about 80 °C), even though the oil viscosity is significantly lower due to the temperature. At a temperature increase of 40 °C to 50 °C, a ISO VG 220 oil loses two viscosity classes.
Lubricating Oil Recommendation
That is why we recommend using the lubricating oil CLP 220 (DIN 51517) for highly-loaded precision ball screws. It is also generally possible to use the lubricating oil CLP 150, whereby the lubricant dosage should be increased by about 30%. In order to ensure a reliable lubrication, low-viscosity lubricating oils (also due to the lower pressure stability) should not be used if possible, since the lower oil viscosity also cannot be compensated for, even with significantly more lubricating oil.
Lubricating Oil Quantity
The required lubricating oil quantity depends on the spindle data (nominal diameter, pitch, ball size, number of loaded turns and design) and the operating data (load, speed and installation position). Since the ball screw nuts are usually equipped with wiper systems that also make lubricating oil discharge difficult, the nuts may overheat if over-lubricated and at high speeds. In order to achieve a good lubricant distribution, a frequent lubricating oil supply in small quantities is more favorable than an infrequent supply in large amounts. The lubricating oil injection intervals should occur about 3 to 4 times per hour.
Oil Filtration
The purity and the temperature of the lubricating oil are important. A sufficiently effective oil filtration (mesh size < 10 µm) is advisable so that neither impurities nor solids enter the ball screw with the lubricating oil. This increases the ball screw service life and the reliability of the lubrication.
Oil Cooling
In addition to the lubrication, the lubricating oil also has the task of cooling the ball screw or at least quickly dissipating the resulting frictional heat. That is why the lubricating oil should be kept at a sufficiently low operating temperature by an oil cooler. The cooling temperature of the oil cooler (oil inlet temperature about 40 °C) should not be chosen to be too low in order to avoid an undesired formation of condensate. The oil temperature of 40 °C also makes it easier to assess the lubricating oil, since the base oil viscosity is specified at this temperature for most oils.
CPL 100 for Preservation
If no special lubricating specifications are required by the operator, the functional check and the delivery of the ball screw occurs with the lubricating oil CLP 100 (DIN 51517-3). The lubricating oil wetting upon delivery is exclusively for preservation purposes and does not replace any service lubrication. The highly-viscous lubricating oil flushes out the preservative oil already after a short time or mixes with it so that no cleaning is necessary before commissioning the ball screw.
Track and Hydraulic Oils
According to the specifications of various oil manufacturers, track oils (e.g. Mobil Vactra, BP Maccurat or Aral Deganit) can also be used for lubricating linear guides, gearboxes, bearings and ball screws in machine tools. Since most track oils lack the necessary pressure additives, we do not recommend using track oils as a lubricating oil for ball screws, even if they are suitable for "high pressure!" The same applies to hydraulic and hydrostatic oils!
Oil Mist Lubrication
An oil mist lubrication on the spindle shaft is only recommended for nut designs without a wiper (prerequisite: no or only very little contamination). An oil mist lubrication as a minimum quantity injection lubrication of the ball screw nut is only permissible and possible with a nut specially designed for this type of lubrication. Oil bath or oil splash lubrication should only be used at low travel speeds. Drip oil or loss lubrication is possible in principle, but the necessary lubricating oil quantities must be observed.