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How, why, and when to apply the preload of the ball spline?

Views: 10     Author: Site Editor     Publish Time: 2022-12-14      Origin: Site

How, why, and when to apply the preload of the ball spline?

Like other linear motion components of the rolling elements in the raceway,the ball spline can provide preload.In the case of ball splines,the primary purpose of preload is to reduce angular clearance(clearance in the direction of rotation)when torque is applied.

ALM ball spline

Basic structure of ALM ball spline 


In the absence of preload,the ball spline will create a small amount of clearance or "clearance" when rotated due to the clearance between the ball of the spline nut and the groove in the spline shaft.Thus,when torque is applied,the assembly attached to the load(spline nut or spline shaft)will begin to rotate,but the mating assembly will not immediately follow.Backspace results in reduced stiffness and reduced positioning accuracy in the direction of rotation,especially when the direction of rotation changes.


Recall that a ball riding in a raceway can have a Gothic arch or circular arc contact geometry.While these geometrics are usually associated with the raceways of profiled guides,they also apply to the grooves of ball splines.


Typically,larger diameter ball splines have a Gothic arched groove design that provides four contact points between the ball and the raceway.This four-point contact eliminates clearance and provides higher load capacity.However,the Gothic arch design leads to higher differential slip,which increases friction.


Ball splines with Gothic arch

Ball splines with Gothic arch contact profiles between the grooves and the balls provide four points of contact,for higher load capacities and better rigidity.


Conversely,smaller diameter ball splines typically use circular arc groove geometry(providing two points of contact between the ball and the raceway)to achieve lower friction and smoother running characteristics.


Differential slip occurs because the contact area between the ball and the raceway is an ellipse.This elliptic contact area means that the diameter of the ball rolling will change,so the rolling speed will vary depending on which part of the ellipse.This causes the ball to slide rather than roll,and leads to sliding friction-called differential sliding.

 

ball and the raceway

Gothic arch designs have a greater difference in rolling diameter,hence higher differential slip and higher friction.


The greater the difference between rolling diameters,the greater the slip difference experienced by the ball bearing.Because the differences between rolling diameters in Gothic arch designs are greater than those in circular arch designs,Gothic arch designs experience higher differential slip and higher friction.


Regardless of which groove geometry is used,preload can be applied to the ball spline to ensure complete elimination of angular clearance and increase the rigidity of the assembly.The ball spline preload is usually generated by changing the diameter of the ball in the spline nut in a similar manner to a ball nut or recirculating ball or roller bearing carriage.


ball splines with normal

Rotation angle vs.torque for ball splines with normal,light (CL),and medium(CM)preload.

 

Since the primary purpose of ball spline preload is to reduce angular clearance,manufacturers typically provide "normal","light",and "medium" preload amounts(or some variation of these classifications).While most manufacturers define preload in terms of the micron of the angular clearance,the preload classified as "light preload" is usually about 2% of the basic static load rating of the spline nut.


Preload is most often recommended when the spline shaft is subjected to vibration or oscillating loads,as these loads can cause significant premature wear on the spline shaft groove and ball when there is clearance.A light preload improves positioning accuracy in the direction of rotation,while a medium preload is recommended for more severe vibrations or when high rigidity is required.Note that when two shafts are used in parallel,some manufacturers recommend a spline shaft with clearance(no preload).


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