Does Ball Screw Smoothness Always Mean Higher Accuracy?

Or to put it another way – the old adage was “if you want a really smooth ball screw with zero backlash you have to specify and pay for a ball screw with higher accuracy”.  But is this true today? 

The short answer is a decided “NO!”  Years ago, even ball screw manufacturers believed that a more accurate ball screw, e.g. grade 3 or grade 1, was inherently smoother than a grade 5 (for a clearer definition of what the grades actually mean, please go to our web site):

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Steinmeyer has been manufacturing ball screws longer than anyone – since the mid 60’s.  Over the years we have perfected the thread grinding process and in so doing have discovered that the subjective characteristic – smoothness – can be measured and optimized.  That is, by controlling friction torque.  Furthermore we have determined that variation in friction torque (which after all is where smoothness, or lack thereof, emanates) is more a function of shaft diameter variation rather than thread lead error.  A preloaded ball nut, whether the preload is 2 point contact (double nut) or 4 point contact (single nut using ball oversize), always has higher friction torque compared to a ball nut with play.  DIN standards actually define a friction torque variation for a conventional double nut – but not for a single nut.  Once again our web site offers such values:

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However, and this is a key point, Steinmeyer is unique in the ball screw world in offering a grade 5 ball screw but with tighter friction torque variation (e.g. 20% or even lower).  We can do this because we have found a better way to control the thread BCD (ball circle diameter) – which again is the prime factor in determining variation in friction torque.  So – the message is – control the ball screw shaft diameter and you’ve gone a long way towards achieving a smoother running ball screw.  As an example, suppose you have an application where a grade 5 ball screw is suitable but you want reduced friction torque variation because your axis is scanning, which is very sensitive to this parameter.  By defining a lower than normal friction torque variation for say a grade 5 ball screw, you can get a unit that runs smoother without having to pay the full difference for a much higher grade product.

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