Often customers are a little surprised when they embark on the Geometric Dimensioning and Tolerancing (GD&T) phase of their design asking – “why must I design in such tight alignments, concentricity and squareness?” The answer is simple… so the ball screw will operate at its fullest efficiency and anticipated life!
Most critical certainly in applications with zero backlash screws, it must be understood that for every “X” micron(s) of mis-alignment there are relatively significant additional forces being applied to the balls and races. As a rule the higher the accuracy class the tighter the mounting tolerance recommended. Experience teaches that these mis-alignment issues will be most notable when the ball nut is nearest to the ends of travel… i.e. at the motor end and also the far end if the shaft is also supported by a duplex or even just a radial bearing (i.e. a fixed / fixed or fixed / supported bearing configuration).
Whenever a ball screw fails prematurely (that is significantly prior to its calculated L10 lifetime) we typically look first to possible issues relating to contamination, lubrication or perhaps sudden and severe shock. Once those concerns are eliminated, we then consider the loads, speeds and duty cycle as well as alignment and mounting tolerances.
When the screw shows evidence of overloading and the balls and races are worn uniformly we review the supporting data (i.e. anticipated loads, speeds, accel/decel, etc…). However when we find the balls are severely damaged and races are worn “non-uniformly” that is only in certain portions or quadrants of the shaft or ball nut this is usually evidence of excessive radial / side loads. This could be due to a number of reasons including mechanical deflections and/or a flexing when under load of the supporting structures, brackets between ball nut and slide, incorrectly machined motor mount face, etc.
Besides those structural possibilities a gross misalignment of the assembly is sometimes the cause. Often this can be corrected by a simple review of the assembly techniques and allowing components to float or self-align before being locked into place. Other times it might mean a full GD&T review including investigating not only those controlling surfaces related the ball screw but also the linear bearings or rails on which the axis travels. Often it is learned that an inevitable stack up of machining errors can only be addressed with the strategic use of slotted holes and/or fitting spacers, keys or dowels. By no means are we suggesting you have to grind everything – we’re simply saying you should well control and plan for any potential mis-alignments so as to get the best and longest performance possible from your premium ball screw!
Please contact a Steinmeyer representative for information on recommended mounting tolerances and permissible side loads / forces / limits.