Wear life of Ball Screws
The wear life of Ball Screws is much more predictable than Power Screws due to the large body of research and testing that has been conducted on ball bearings and bearing balls.
Assuming that a Ball Screw is a ball bearing arranged with helical inner and outer races, the listed operating loads have been determined.
The operating load ratings are based upon a theoretical 90% survival rate of Ball Screws at 1,000,000 in. of travel. Ratings also assume pure axial loading of the screw and nut with no side loads or moment loads, and a clean, well lubricated, room temperature environment. The presence of unfavorable loading, dirt, dust, lack of lubricant and external heat will dramatically reduce the service life.
Identifying Ball Screws Wear
Ball Screw life is proportional to the inverse cube of the load. If the load is cut in half, the life increases by 2 cubed or a factor of 8.
- For example, a 1 x .250 Ball Screw is to be operated at 1,000 lbs.
- The expected travel life of the Ball Screw with a 90% survival rate would be 4,100,000 inches of travel.
- Dividing the operating load rating of 1,600 lbs. for this size Ball Screw (from Table 15) by the actual load of 1,000 lbs., cubing the result and multiplying by 1,000,000 inches yields the expected life: (1,600/1,000)3 x 1,000,000 = 4,100,000 inches.
- The formula for Ball Screw wear life can be found in Useful Formulas.
Every application is unique in terms of loads, environment, duty cycle, etc. Operational and life testing of prototype Ball Screws is recommended especially for OEMs anticipating large volume production. Customers are encouraged to contact Roton’s application engineers who are available for consultation and to discuss wear life objectives for specific applications.