How to select ball screws?

When selecting ball screws,it is important to learn the application criteria that the components need to meet.There are several factors to consider,each of which plays a role in the decision-making process.

●Learn the following information about your application

Travel—stroke length.

Load—the mass being moved.

Speed—velocity and acceleration requirements.

Duty Cycle—the cycle rate,dwell times,and lifetime expectation.

Orientation—how the ball screw be mounted(vertically or horizontally).

Precision—what is the positional accuracy and repeatability need to be.

Environment—harsh or rugged,wash down,heat extremes,or other requirements.

●Precision and preload

The recirculation of the ball in the ballnut affects accuracy and repeatability.As a result,ballnuts have a range of preload options to reduce or eliminate axial play as they rotate around the screw.This allows for better repeatability,because the clearance in the ball does not lose motion when the ball is reengage.There are several preloading techniques.

Some common methods include oversizing the balls inside the nut housing;use the so-called "double nut" or "tension nut" method;or by using manufactured offsets in the raceway helix to change the ball engagement angle(the "lead shift" method)and deliberately force the ball into a preloaded state.

Preloading by inserting oversized balls

●Speed

When determining the size of the ball screw,two speed factors need to be considered;characteristic and critical speed.The characteristic speed depends on the limit of the bearing system,and the speed it can withstand.This is the industry standard for various bearings and indicates the speed limit that should not be exceeded in operation.The control criteria are mainly the temperature level of the bearing,vibration or other operating limits,and the inertia of the moving parts in the bearing.By optimizing specific designs,manufacturers can increase the speed factor.

Critical speed is essentially the speed at which the ball screw begins to vibrate and introduce excessive oscillations(called screw whips).This depends on the length of the screw and the mounting support of the end bearing.Critical speed is a consequence of physics.However,there are steps you can take to ensure that the screw speed stays outside the critical speed range.These include increasing the screw diameter,selecting suitable end bearings and using screw supports.

Screw whips

Formulas to help determine critical speed

●Lubrication

The next tip is to use a good lubricant.Oils and greases are common choices.Avoid products with solid additives,as they can clog the recirculation system.

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