Ball screw

A threaded shaft provides a helical raceway for ball bearings which act as a precision screw.

The ball screw was invented independently by H.M. Stevenson and D. Glenn who were issued in 1898 patents 601,451 and 610,044 respectively.

[citation needed] Low friction in ball screws yields high mechanical efficiency compared to alternatives.

This, combined with their overall performance benefits and reduced power requirements, may offset the initial costs of using ball screws.

Such undesirable behavior could range from simple loss of control of the machine, such as self-feeding (the tool of the machine causing motion of the axes without the control input of the operator), to potentially dangerous cases where unexpected force could be transmitted all the way to an operator's limbs and pose a risk of injury.

Because an ordinary lead screw resists or even prohibits such reverse operation, they are inherently safer and more reliable for manual use.

An external ballnut employs a stamped tube which picks up balls from the raceway by use of a small pick-up finger.

The cap is machined to pick up balls coming out of the end of the nut and direct them down holes which are bored transversely down the ballnut.

A ball screw involves significantly more parts and surface interactions than many similar systems.

While a basic lead screw is composed of only a solid shaft and a solid nut with simple mating geometries, a ball screw requires precisely-formed curved contours and multi-part assemblies to facilitate the action of the bearing balls.

This makes them more expensive to manufacture and sometimes to maintain, and provides more potential avenues for failure if the apparatus is not properly cared for.

The correct evaluation of the curvatures of ball screw grooves allows one to accurately design the constructive parameters of this mechanism and to enhance its performance.

The formulation commonly used in literature refers to the ball bearing's geometry, ignoring the shape of the section's profile and the helix angle.

To maintain their inherent accuracy and ensure long life, great care is needed to avoid contamination with dirt and abrasive particles.

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, such as CNC machine tools and high precision motion applications (such as wire bonding).

This is accomplished by using a thread profile that has a slightly larger radius than the ball, the difference in radii being kept small (for example, 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.

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.

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 cannot be adjusted to allow for wear.

Ball screw shafts may be fabricated by rolling, yielding a less precise but inexpensive and mechanically efficient product.

[2] High-precision screw shafts are typically precise to one thousandth of an inch per foot (830 nanometers per centimeter) or better.

They are produced on precision milling machines with optical distance measuring equipment and special tooling.

Instrument screw shafts are generally made of Invar, to prevent temperature from changing tolerances too much.

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.

A ball screw is used to expand the Deployable Tower Assembly (DTA) structure on the James Webb Space Telescope.

[citation needed] A ball screw is also planned to be used in TerraPower's Natrium Reactor as part of it Control rod drive mechanism.

[5] Another form of linear actuator based on a rotating rod is the threadless ballscrew, or "rolling ring drive".

In this design, three or more rolling-ring bearings are arranged symmetrically in a housing surrounding a smooth (threadless) actuator rod or shaft.

An advantage of this design over the conventional ballscrew or leadscrew is the practical elimination of backlash and loading caused by preload nuts.