How to avoid ball screw buckling?

Determining the buckling load is an important step in ball screw selection.The buckling(compression)load can be easily calculated based on the root diameter of the screw,the overhead length and the end bearing configuration.

Where:

Fc=maximum compressive load(N)

fb =end bearing factor

d1=root diameter of screw(mm)

L=unsupported length(mm)

Application Note:The end bearing factor,fb,depends on the end fixity of the screw.Ball screw manufacturers recommend a safety factor of at least 2 for maximum applied buckling loads,which some manufacturers include in this published end bearing factor.To avoid oversizing or undersizing,be sure to check that the manufacturer has built in a safety factor value into the published fb values.

As shown in the equation,for a given diameter and length of ball screw,the allowable buckling load can be increased by changing the support bearing configuration.The more rigid the support bearing,the higher the bearing factor,ranging from approximately 2.5 for a fixed-fixed configuration to 40 for a fixed-fixed configuration.this means that the allowable buckling load for a fixed-fixed bearing configuration is approximately sixteen times that of a fixed-free configuration on the same screw assembly.

In horizontal applications,the axial force is usually equal to the load multiplied by the friction coefficient of the screw assembly,so buckling is usually less of an issue than other performance factors,such as critical speed or drive torque.However,some horizontal applications,such as pressing and injection,introduce additional axial loads on the screw assembly,and buckling loads can easily become a constraint on screw selection.

When a ball screw is used in a vertical application,it treats the full load as an axial force,which makes buckling a more important issue.A screw assembly with the most rigid support(usually a fixed bearing)mounted on top puts the screw in tension rather than compression and counteracts the effects of axial loads.In the unusual case of high reversal forces (e.g.,pushing the plunger into the bearing housing on the upward stroke of the screw),the most favorable bearing configuration is to orient the fixed bearing at the bottom of the screw assembly.

Orienting the fixed bearing on top puts the ball screw in tension.

Application Tip:Most,but not all,screw-driven linear actuators use a fixed bearing on the motor side and a floating bearing on the non-driven end.For vertical applications,be sure to check which end of the actuator the fixed bearing is on to avoid excessive compression forces on the screw.

Buckling is a relatively simple but sometimes overlooked factor in ball screw sizing.Important things to remember are to check the process forces in horizontal applications and to check the position of the fixed bearings in vertical applications.

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