Thursday 16th May 2013

Life Expectancy

Abssac is often asked about the life expectancy of the ball screw products it supplies within applications. Whilst this can sometimes be a difficult thing to accurately calculate, as parameters within an application can change during its life, it can nevertheless be achieved.
During a recent design brief with one of our packaging machinery customers, a new range of machines were being designed with exceptionally long MTBS. (mean time between service). In fact, the machines unique selling point was to be its reliability and long service intervals. The ball screw to be used would also provide the primary linear movement required within the design, so was a critical part of the design guarantee.

Typically our ball screws are rated for millions of mm’s of travel at the rated dynamic load. This is the load at which 90% of a group of identical ball screws will run without flaking for their lifetime. However, they will travel farther than this at lower limits. These load-life relationships are analogous to the B10 rating common in the ball bearing industry. The relationship of load to life is an inverse cube relation. For example, by reducing the customers load by half would increase the life by eight times. (Conversely doubling the load would decrease life by 1/8).

Armed with this information Abssac’s customer then made every attempt to reduce the load within the design, which after a great deal of design change was finally achieved. The end result was a ball screw assembly with the life expectancy required by the customer.