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Maximum Speed of Ball Screw Actuator

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mrmitche

mrmitche

Mechanical
Jul 17, 2013
11
I am verifying some specifications for technical drawings of linear ball screw actuators. One of the specs is maximum speed, which I found the equation to be the lead size (mm) * rotational speed (s-1). How come for larger models the speed becomes affected by the accuracy grade and stroke length? Below is part of the table from the THK catalog showing how the speeds decrease for lower accuracy grade and larger stroke. It kinda makes sense that speeds would decrease with really long screws, but I don't understand how the accuracy changes the speed. If anything I would expect a less accurate actuator to be able to run at higher speeds. Can anyone explain why these two variables affect the maximum speed and point me to a more accurate equation for max speed? The complete speed table is available here.

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The longer rails have lower resonant frequencies, so they experience issues with the speed exciting resonance frequencies at lower speeds. From control theory, I expect the 'precision' grade couplings hold the rail tighter (i.e. more resistance) so that helps to dampen out the resonances (i.e. narrow the resonance bands with a higher Q-factor (quality factor)). Others may be able to put some math behind this.

Z
 
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I looked at the table for the SKR models (which are the same as KR, but use a ball cage). This speed table does not show that the precision grade affects the speed. Is this then because the ball cage affects the damping of the resonances you mentioned?

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Just a guess - precision fits would allow for a more controlled bearing preload and hence a more predictable amount of power dissipated in the bearing.

Good question. I hope some others chime in.
 
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