Wear Life - Power Screws

The wear life of power screws is a function of load, speed, lubrication, contamination, heat and other factors. The operating loads listed in the Screw/Nut Engineering section for each screw series provide acceptable wear life for most applications.

Wear in a power screw is generally in proportion to usage. Each movement of the screw surface against the mating nut surface removes a microscopic amount of material, usually from the softer nut material. As these wear increments add up over time, and backlash increases, the nut threads become thinner. When the shear strength of the remaining threads is exceeded by the load, failure occurs.

Although their wear life is not as predictable as Ballscrews, well lubricated power screws, without side loads or moment loads, can provide excellent service lives for many applications. Heavy loads and duty cycles which generate significant amounts of heat will cause material and lubricant breakdown and should be avoided.

Every power screw application is unique in terms of loads, environment, duty cycle, etc. Operational and life testing of prototypes is highly 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. Often, a short evaluation early in the application development can save many hours of design revision and testing.

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Wear Life - Ballscrews

The wear life of Ballscrews 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 Ballscrew 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 Ballscrews 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.

Ballscrew 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 Ballscrew is to be operated at 1,000 lbs. The expected travel life of the Ballscrew 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 Ballscrew (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 Ballscrew 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 Ballscrews 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.

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Cost Considerations

The products in this catalog are arranged in increasing cost order from front to back. Acmes are the least expensive and are the most widely used. Hileads(r), Torqsplines(r) and Ballscrews offer increased performance at increased costs.

The final choice depends upon the user’s economics, the market for the end product, reliability objectives, and many other factors. Bear in mind that initial cost is only one element in the cost equation. Installed cost, maintenance, consequences of failure and many other items need to be weighed before finalizing any design.

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