How to specify a ball screw

Powertransmissions - How to specify a ball screwBall screws are available in a range of types, styles, and sizes. A few tips, though, will help you match the right ball screw with the right application.

1. You will have the best success with specifying a ball screw if you know the following information about your application:

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

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

Load—the mass being moved.

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

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

Speed—velocity and acceleration requirements.

Travel—stroke length.

2. Consider a preloaded ball screw

Preloading Ball Screw 300x292

Ball screw manufacturers can increase the rigidity of the ball screw through preloading—a step that reduces axial clearance between the ball nut and the screw shaft. In addition, preloading enhances positioning precision and repeatability. One of the more common preload options is the use of oversized balls. But other methods include the use of a double-nut. Manufacturers may also offset the raceway spiral, which can force the balls into a preload condition.

3. Know your need for speed

Manufacturers offer information on the operational speed limits that their ball screw system can handle. Variables that go into this determination include temperature, operating limits of the bearing and inertia of the bearing’s moving components.

The critical speed of a ball screw is the lowest speed at which the ball screw shaft is in resonance, i.e. where screw whip occurs. It depends on screw length and the end bearing mounting support. To prevent critical speed, select a larger screw diameter, choose the right bearings, and use proper screw supports.

Ball Screws Speed Calcs 282x300

Formulas to help determine critical speed

4. And of course, the next tip is to always use a good lubricant. Oil and grease are the common choices. Avoid ones with solid additives as they can clog the recirculating system.



Source: Linearmotiontips

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