There are five ways a ball screw may fail due to overload:
- Excessive dynamic loading, which means the screw makes too many revolutions under a certain load resulting in material fatigue. This can be avoided by selecting a ball screw with sufficient dynamic load capacity (or by reducing the number of revolutions and/or reducing the load). This is the subject of the load capacity discussion.
- Exceeding the static load capacity, which causes instant and permanent damage to the ball screw due to brinelling of balls and races, and prevents any further normal operation of the ball screw. Static load capacities are listed as technical data.
- Buckling of the shaft under compressive load. Buckling load value depends on bearing method and free length of the loaded ball screw shaft.
- Failure of the nut body or of the bolts that connect it to the slide. This may happen even before the static capacity is reached. Safe loads are discussed on the following pages.
- Radial loads. It means the load capacities given in this catalogue apply only to pure axial loading. As there are always tolerances in the alignment of bearings and linear guideways, there may be a small amount of radial force, which should be minimized. Under normal conditions, a radial load less than 5% of the minimum axial load will not cause any problems. When considering a ball screw for use under radial load, please consult Steinmeyer engineers.
There are several analytical ways to demonstrate safety from buckling. In machine design, the most frequently used is a simple calculation using formulas based on Euler equations.
Other, more accurate methods include non-linear FEM analysis and more involved mathematics. These methods are normally used in aerospace applications, where excess safety margins are not possible due to weight limitations. Please contact us if you require such an analysis.
On the following page we describe a simple form of buckling analysis.
Some ball screws cannot be loaded all the way to their static capacity. Screws with high dynamic load capacity (which might be selected to obtain a long enough service life at a much lighter load) will necessarily have a high static capacity. But the term "static capacity" is misleading, since the ball screw may actually fail due to fracture of the nut flange, nut body, or connecting bolts before reaching this load!
On the following page are the maximum safe loads.
A reasonable load for a ballscrew, which may be sustained for significant travel, is about 10% of its dynamic capacity.
A mean load of 10% of its dynamic capacity results in a theoretical life of 1 billion revolutions, which is the upper limit of the range where the life equation is valid. Mean loads of a reasonably sized ball screw will therefore be somewhat higher than this, but normally not exceed 20% of its dynamic capacity.
For short peak loads, the loading may be higher, but normally the loading of a ball nut with 2-point contact should not exceed 2.8 times the preload. And preload is around 5% - 10% of dynamic capacity.
As a rule of thumb, this all means the load range for a ball screw, to be used in a machine tool application, is really about 10% to 30% of its dynamic capacity. However Steinmeyer ball screws are used in many applications as force actuators where the loading is up to 100% of dynamic capacity but with low speed and acceleration. For example, they may be used to power injection molding machines, where high forces occur at reduced speed. Please consult our engineers for details.