How to select end fixity conditions for ball screw applications?

The ball screw assembly consist of three main components:ball screw shaft,ballnut and end support bearing.The end support bearing is more than just an accessory,it has a significant impact on the stiffness,maximum speed and buckling load of the ball screw assembly.There are four combinations of end fixation:fixed-fixed,fixed-floating,floating-floating,and fixed-free.Each provides a different level of stiffness for the ball screw assembly,suitable for specific applications and assembly requirements.

 

●Fixed-Fixed: This is the strictest form of end-fixation.It provides the highest speed capability and support for buckling loads,but does not allow any axial movement of the ball screw in the case of thermal expansion.Ball Screws in tension(such as those used with driven ball nuts)are supported by fixed-fixed bearings.

●Fixed-Floating(aka Fixed-Simple):This is probably the most common type of end fix.It provides axial and radial support for the ball screw and contributes to high critical speed and buckling loads while still allowing thermal expansion of the ball screw shaft.

●Floating-Floating(aka Simply-Simple):This bearing configuration has the advantage of being easy to install while still providing radial support for the ball screw.It also allows for axial compliance.

●Fixed-Free:Leaving one end of the ball screw assembly"free"(no bearing support)simplifies installation but greatly reduces critical speed and buckling load capacity.Therefore,no fixed support is only used for short ball screw length and slow applications.

Two parameters of ball screw performance,critical speed and buckling load,are proportional to the coefficients based on the end bearing configuration.This end bearing coefficient is highest for the most rigid support(fixed-fixed)and lowest for the least rigid configuration(Fixed-free).

The buckling load and critical speed are also inversely proportional to the square of the length of the overhead ball screw.Thus,using a fixed brace at one or both ends provides a secondary benefit because it reduces the overhead length of the ball screw,reduces the buckling load and denominator of the critical velocity equation,and provides a higher value for both.

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