Ball Screws: Explaining Load Capacity

Many engineers are confused about this topic, especially if they don’t work frequently with ball screws.  So let’s take a moment to explain the terminology.

There are generally two load capacities given for a particular ball screw: Dynamic and Static.

Dynamic Load Capacity

The dynamic load capacity (DLC) is simply a load rating.  It’s the load for a (theoretical) life of 1 million revolutions (ISO/JIS standard) or for 1 million inches of travel (ANSI standard).  The DLC is critical for making lifetime calculations, since the expected life goes as the cube of the ratio of DLC to actual load.  So, if the load is only 10% of DLC, then the expected life is 1 billion revolutions.

But that still doesn’t mean you could run the screw with this kind of load and expect a to achieve that life!  Why?  The normal maximum operating load of a general use ball screw is about 30% of the DLC.  (Above that, the elastic deformation of the balls and races is too large, which may cause excessive wear.) So, if you run a ball screw with a load equal to its DLC, you may not get as much life as you expect.

Static Load Capacity

The static load capacity (SLC) indicates the load above which the screw may be damaged.  Staying below SLC ensures that balls and races don’t suffer brinelling (or plastic deformation).  This is critical!  But bear in mind that a ball screw may be damaged in other ways with loading below the SLC: you could easily tear off the flange of the nut, snap the bearing or drive journal of the screw, or even collapse it from exceeding the buckling load.

Please consult a Steinmeyer engineer for an assessment of your application.

781-273-6220
infoUSA@steinmeyer.com

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