Ball screw

For the musket tool, see Musket.

Photo showing two ball screws. Inset images are close-up photos of the ball assembly of the top screw. Left inset: recirculating tube removed showing retainer bracket, loose balls and tube. Right inset: closer view of the nut cavity.

A ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with minimum internal friction. They are made to close tolerances and are therefore suitable for use in situations in which high precision is necessary. The ball assembly acts as the nut while the threaded shaft is the screw. In contrast to conventional leadscrews, ballscrews tend to be rather bulky, due to the need to have a mechanism to re-circulate the balls.

Another form of linear actuator based on a rotating rod is the threadless ballscrew, a.k.a. "rolling ring drive". In this design, three (or more) rolling-ring bearings are arranged symmetrically in a housing surrounding a smooth (thread-less) actuator rod or shaft. The bearings are set at an angle to the rod, and this angle determines the direction and rate of linear motion per revolution of the rod. An advantage of this design over the conventional ballscrew or leadscrew is the practical elimination of backlash and loading caused by preload nuts.

Applications

Ball screws are used in aircraft and missiles to move control surfaces, especially for electric fly by wire, and in automobile power steering to translate rotary motion from an electric motor to axial motion of the steering rack. They are also used in machine tools, robots and precision assembly equipment. High precision ball screws are used in steppers for semiconductor manufacturing.

History

Historically, the first precise screwshafts were produced by starting with a low precision screwshaft, and then lapping the shaft with several spring-loaded nut laps. By rearranging and inverting the nut laps, the lengthwise errors of the nuts and shaft were averaged. Then, the very repeatable shaft's pitch is measured against a distance standard. A similar process is sometimes used today to produce reference standard screw shafts, or master manufacturing screw shafts.

Description and operation

To maintain their inherent accuracy and ensure long life, great care is needed to avoid contamination with dirt and abrasive particles. This may be achieved by using rubber or leather bellows to completely or partially enclose the working surfaces. Another solution is to use a positive pressure of filtered air when they are used in a semi-sealed or open enclosure.

While reducing friction, ball screws can operate with some preload, effectively eliminating backlash (slop) between input (rotation) and output (Linear motion). This feature is essential when they are used in computer-controlled motion-control systems, e.g., CNC machine tools and high precision motion applications (e.g., wire bonding).

Disadvantages

Depending upon their lead angle, ball screws can be back-driven due to their low internal friction (i.e., the screw shaft can be driven linearly to rotate the ball nut). They are usually undesirable for hand-fed machine tools, as the stiffness of a servo motor is required to keep the cutter from grabbing the work and self-feeding, that is, where the cutter and workpiece exceed the optimum feedrate and effectively jam or crash together, ruining the cutter and workpiece. Cost is also a major factor as Acme screws are cheaper to manufacture.

Advantages

Low friction in ball screws yields high mechanical efficiency compared to alternatives. A typical ball screw may be 90 percent efficient, versus 20 to 25 percent efficiency of an Acme lead screw of equal size. Lack of sliding friction between the nut and screw lends itself to extended lifespan of the screw assembly (especially in no-backlash systems), reducing downtime for maintenance and parts replacement, while also decreasing demand for lubrication. This, combined with their overall performance benefits and reduced power requirements, may offset the initial costs of using ball screws.

Manufacture

Ball screw shafts may be fabricated by rolling, yielding a less precise, but inexpensive and mechanically efficient product. Rolled ball screws have a positional precision of several thousandths of an inch per foot.

Accuracy

High-precision screw shafts are typically precise to one thousandth of an inch per foot (830 nanometers per centimeter) or better. They have historically been machined to gross shape, case hardened and then ground. The three step process is needed because high temperature machining distorts the work-piece. Hard whirling is a recent (2008) precision machining technique that minimizes heating of the work, and can produce precision screws from case-hardened bar stock.

Instrument quality screw shafts are typically precise to 250 nanometers per centimeter. They are produced on precision milling machines with optical distance measuring equipment and special tooling. Similar machines are used to produce optical lenses and mirrors. Instrument screw shafts are generally made of Invar, to prevent temperature from changing tolerances too much.

Ball return systems

The circulating balls travel inside the screw and nut thread. If the ball nut did not have a return mechanism the balls would fall out of the end of the ball nut when they reached the end of the nut. For this reason several different recirculation methods have been developed.

An external ballnut employs a stamped tube which picks up balls from raceway with use of small pick up finger. Balls travel inside of tube and are then replaced back in thread raceway.

An internal button ballnut employs a machined or cast button style return which allows balls to exit raceway track and move one thread and reenter raceway.

An endcap return ballnut employs a cap on the end of ball nut. The cap is machined to pick up balls out of the end of nut and direct them down holes which are bored transversely down the ballnut. The compliment cap on the other side of nut directs balls back into raceway.

Thread profile

To obtain proper rolling action of the balls, as in a standard ball bearing, it is necessary that, when loaded in one direction, the ball makes contact at one point with the nut, and one point with the screw. In practice, most ball screws are designed to be lightly preloaded, so that there is at least a slight load on the ball at four points, two in contact with the nut and two in contact with the screw. This is accomplished by using a thread profile that has a slightly larger radius than the ball, the difference in radii being kept small (e.g. a simple V thread with flat faces is unsuitable) so that elastic deformation around the point of contact allows a small, but non-zero contact area to be obtained, like any other rolling element bearing. To this end, the threads are usually machined as a "gothic arch" profile. If a simple semicircular thread profile were used, contact would only be at two points, on the outer and inner edges, which would not resist axial loading.

Preloading

To remove backlash and obtain the optimum stiffness and wear characteristics for a given application, a controlled amount of preload is usually applied. This is accomplished in some cases by machining the components such that the balls are a "tight" fit when assembled, however this gives poor control of the preload, and can not be adjusted to allow for wear. It is more common to design the ball nut as effectively two separate nuts which are tightly coupled mechanically, with adjustment by either rotation one nut with respect to the other, so creating a relative axial displacement, or by retaining both nuts tightly together axially and rotating one with respect to the other, so that its set of balls is displaced axially to create the preload.

Equations

$T=\frac{Fl}{2\pi}$

Where $\mathit{T}$ is torque applied to screw or nut, $\mathit{F}$ is linear force applied, $\mathit{l}$ is ball screw lead, and $\nu$ is ball screw efficiency.

See also

• Ball spline
• Jackscrew
• Leadscrew
• Roller screw
• Linear motion bearing
• Recirculating ball
• Linear actuator