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Shaft Assembly

October 9th, 2010 admin No comments

The Ball Screw and the Ball Screw Shaft

A ball screw is a mechanical device for translating rotational motion to linear motion with little friction. They are huger when compared to conventional leadscrews because they need to have a mechanism to re-circulate the balls.

They can be used in many applications, especially in the places which need high precision. For example, they are usually applied in the aircraft and missiles to move control surfaces. Besides, they can also be used in machine tools, robots and some precision assembly equipments. Ball screws can be back-driven because of their low internal friction which requires high mechanical efficiency when compared to alternatives. Typically, ball screws may be 90 percent efficient.

It is very necessary to take care of ball screws so as to maintain the inherent accuracy and ensure long life. In order to achieve this goal, dirty things and abrasive particles should be avoided which can be gained through using a positive pressure of filtered air when they are used in a semi-sealed or open enclosure or using rubber or leather bellows to completely or partially enclose the working surfaces.

As for the ball screw assembly, it consists of a screw and a nut, each with matching helical grooves, and balls. When the nut or screw is rotating, the balls will roll between the grooves that between the nut and the screw. The ball nut determines the load and life of the ball screw assembly. It is manufactured with two types of return systems. One is the external ball return system in which the ball is returned to the opposite end of the nut through a ball return tube which outstretches above the outside diameter of the ball nut. The ratio of the number of threads in the ball nut circuit to the number of threads on the ball screw determines how much sooner the ball nut will wear out.

Since we have seen something about the ball screw , I think it is rather necessary to talk about the ball screw shafts. They are fitted by rolling, yielding a less precise, but inexpensive and mechanically efficient product. The 1st precise screwshafts were produced by starting with a low precision screwshaft, and then lapping the shaft with several spring-loaded nut laps. After that, the very repeatable shaft's pitch is measured against a distance standard.

All in all, the ball screw is a high-efficiency feed screw. Compared with a conventional sliding screw, this product has drive torque of one-third or less, making it most suitable for saving drive motor power.

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