Grease Lubrication of Ball Screws
The principles of EHD theory generally also apply to grease lubrication. Grease offers good protection against corrosion and the penetration of impurities. Grease lubrication is relatively simple and low cost compared to oil lubrication.
Lubricating Greases
A grease is to be chosen while taking the load and the speed into account according to the NLGI class as defined in DIN 51878. The following lubricating greases were tested in use with AM ball screws and are approved by us!
Item | Manufacturer | Designation | NLGI class | Base oil viscosity mm2/s at 40° |
1 | Shell | Shell-Alvania EP2 | 2 | 186 |
2 | Optimol Industrie GmbH | Optimol Longtime PD2 | 2 | 94.9 |
3 | Lubricant Consult GmbH | Turmogrease P 1002 BDY | 2 | 110 |
4 | Exxon Mobil | Mobilith SHC 100 | 2 | 100 |
5 | Klüber | Klüberplex BEM 34-132 | 2 | 130 |
6 | - | - | - | - |
7 | Interflon | Fin Grease MP 00 | 00 | 105 |
8 | Klüber | Klübersynth G 46-1200 | 00 | 120 |
9 | Castrol/Tribol | Tribol 3020/1000 | 00 | 186 |
10 | - | - | - | - |
Grease Service Life
The grease service life is the time in which the tribological functionality of the ball screw is maintained without new grease being added or losing existing grease. This characteristic value of the lubricating grease describes the aging of the grease and, among other things, depends greatly on the type of lubricating grease. With the greases normally used to lubricate ball screws, the service life is between 5,000 and 20,000 hours of operation, which is thus often shorter than the fatigue life resulting from the load duty cycle and the dynamic load capacity of the ball screw.
The grease service life is not as important for ball screws as is the case for life-long lubricated rotary bearings, for example. This is due to the unavoidable loss of the grease fill quantity due to the continuous axial stroke of the nut. At the same time that the lubricant is transported to one side of the nut, impurities and pollutants adhering on the other side are transported into the nut. That is why the use of wipers at the ends of the nuts is absolutely necessary, especially when using grease lubrication, since these make both transport effects difficult. Nevertheless, new grease must be added cyclically to compensate for the lost amount and due to the grease contamination.
Initial Filling
The nut of the ball screw is normally not filled with grease in the delivered state, but rather is provided with a preservative oil. Since the preservative oil is compatible and mixable with the lubricating greases we recommend, this does not have to be removed before the initial grease filling.
The initial filling with lubricating grease then usually is done by the user and should not exceed 50% of the free nut volume, as otherwise (especially at high speeds) there is a risk of overheating and thus damage. The required amount of grease depends on the size and design (individual or double nut) of the ball screw. So that the grease can distribute better in the nut, it should be filled in three partial amounts with a time delay. Between the partial fillings, the ball screw should be moved at low speed.
Re-Lubrication Intervals
The re-lubrication intervals are usually to be determined individually in practice, since they are dictated by influencing factors such as load, speed, temperature, ambient influences, installation position and pollutants, among other things. Under normal operating conditions, however, re-lubrication cycles of 200 to 300 hours (corresponding to a mileage of about 150 km) are quite sufficient. In practice, the re-lubrication intervals (per hour) are realized with very small amounts of grease (e.g. 0.1 cm3) in significantly shorter time periods.
If no detailed operating conditions are known, the re-lubrication amount for double nuts can also be approximately determined as 0.010 cm3 (at high load) or 0.003 cm3 (at normal load) per 10 mm spindle diameter. This re-lubrication amount may only serve as a reference value and may deviate significantly from the actual required amount.
a ≤ 0.15 Cam) without EP additives | Lithium soap greases (Fa > 0.15 Cam) with EP additives | Synthetic special greases | ||
0 | 355-385 (semi-liquid, liquid grease) | - | heavily loaded up to 800 rpm | High-speed application up to 4,000 rpm |
1 | 310-340 (very soft) | low load up to 800 rpm | - | High-speed application up to 4,000 rpm |
2 | 265-295 (soft) | normal load up to 600 rpm | very heavy load up to 600 rpm | High-speed application up to 4,000 rpm |
3 | 220-250 (medium-strength) | heavily loaded up to 400 rpm | - | - |