What You Ought To Know About Ball Screws

By JOHN DEGENOVA
Thomson Saginaw Ball Screw Co.
Saginaw, Mich.

The next time you're faced with a design project that calls for linear actuation and you're considering "old standbys" like hydraulic or pneumatic cylinders, think a little longer. Ask yourself practical questions like, "Do I really want to deal with pneumatic noise?" Or, "If a hydraulic line breaks how much time and money will the messy cleanup demand?" If the answers to these questions lead to second thoughts about your plans, think about ball screws as well.

Ball screws should be the method of choice in linear-actuation applications. Ball screws convert rotary input to linear motion and offer several advantages over other actuators, such as Acme screws, hydraulic or pneumatic systems, and belt, cable, or chain drives. For example, ball screws are up to three times more efficient than Acme screws. This lowers system power requirements and allows using smaller gears, clutches, and motors. Ball screws cost less than hydraulic or pneumatic systems, operate more quietly, and don't require pumps, hoses, fluids, or shop air. And although belt, cable, or chain drives are often less expensive than ball screws, they also are less precise and stretch as they wear, which leads to inaccurate positioning.

BALL-SCREW BASICS
Ball screws are force and motion-transfer devices in the family of power-transmission screws. They operate like conventional power screws but the rolling friction of bearing balls replaces sliding friction. Ball screws consist of a screw, nut, and balls that operate similarly to bearing components.

The screw has a precision ground or rolled helical groove acting as the inner race. The nut has internal grooves that act as the outer race. Circuits of precision steel balls recirculate in the grooves between the screw and nut. Either the screw or nut turns while the other moves in a linear direction. This converts torque to thrust. A simple calculation determines the torque required to drive a ball screw: T = L P/5.65, where T = torque (lb-in.), L = screw lead (in.), and P = axial load, (lb).

For example, consider a ball screw with a 1.875-in. lead moving a 535-lb load. The torque required is:

T = L P/5.65
= (1.875 535)/5.65
= 177.54 lb-in.

After selecting a screw-and-nut combination, other ball-screw components are needed, such as ball returns and wipers. Ball returns either internally or externally carry balls from the end of their path back to the beginning to complete their circuit. The type of ball return often depends on space constraints and the number of redundant circuits.

Wipers keep contaminants out of critical internal ball-screw components and keep lubricants applied to them. In many applications wipers extend ball-screw life and enhance machine reliability. Wipers are either internally or externally mounted.

A RANGE OF APPLICATIONS
Ball screws are used in a variety of jobs. Their high efficiency makes them useful in high and low-load applications. Off-the-shelf models have load limits ranging from 170 to 200,000 lb.

High-precision ball screws are used heavily in commercial and military aircraft. Commercial planes use ball screws in mechanisms such as engine thrust reversers and propeller pitch controls. On many commercial aircraft several ball screws position multiple wing flaps using a single drive. Military aircraft use ball screws in horizontal stabilizers, main landing gears, and variable engine inlet and exhaust-nozzle actuators.

Though aircraft applications represent some of the most advanced technology, ball screws are also used in slightly less demanding industrial and automotive applications. Industrial ball-screw applications include milling-machine tables, robotics, and semiconductor wafer transport systems. Miniature ball screws in ABS automotive brakes rapidly open and close valves that apply and release fluid pressure to brake pads. This creates a pumping effect to help prevent brake lock-up.

DESIGN FOR RELIABILITY
Manufacturers use various design features when applications require high reliability. Aside from using wipers to prevent contaminant ingression, another way manufacturers increase reliability is with redundant load paths. An example of a load-path redundancy is the use of two or more circuits of balls within a ball nut. This increases reliability because if one circuit malfunctions the ball screw will continue to operate.

Screw threads with multiple starts also provide redundancy. Double-start screws have two threads concentric with each other allowing independent circuits of balls to operate in each path. When multiple circuits are combined with double-start architecture, many redundancies are possible. For instance, two circuits in each of two starts produce four independent load paths.

Aviation and aerospace applications often require ball screws with several redundant load paths. Although a ball screw's projected life drops with each lost circuit, structural integrity and function are preserved. A ball screw with four circuits in normal operation, for example, might be designed to operate for 209,000 cycles. The screw can still function for 2,600 cycles with just one circuit operating.

Sometimes standard ball screws won't provide the right solution for a particular application. In many cases a variation of conventional ball screws can solve multiple problems. Hollow ball screws, for instance, convert torque to thrust while allowing a path for coolant flow or wiring. Coolants running through the screw shaft can either help maintain tolerances in precision ball-screw operations or lubricate cutting tools. The hollow screws also have lower rotational inertia than solid screws, which can dramatically increase starting and stopping speeds. Cross-sectional properties of hollow ball screws also allow higher operating speeds than those of similar solid screws.

Although solid ball screws reach lengths exceeding 70 ft, sometimes space constraints limit their lengths. In applications requiring long travel in tight spaces, telescoping ball screws are a handy solution. A hollow outer screw in these mechanisms surrounds a solid or hollow inner screw. The inner screw extends from the outer screw, increasing the screw's reach. Three or more screws can be combined this way for even longer travel. Telescoping ball screws are widely used in machinery and are required on aircraft to actuate leading-edge wing flaps.

Another space-saving technique combines ball screws with electric motors. Performance Pak Actuators have a motor output shaft connected, through a gear transmission, to a ball screw. The screw is enclosed in a sealed tube with a mounting bracket that connects to the load and extends from the outer tube. Actuating the motor extends or retracts the screw to push or pull a load. The devices are used in positioning applications, such as commercial satellite dishes or hospital beds, in material handling, and in applications requiring simple lifting, opening, or closing operations. The actuators can also hold a load stationary without consuming power, or in power-off situations.

FIGHTING BACKLASH
Backlash is free axial motion of ball nuts along screw threads. Designers who need to avoid backlash use preloaded nuts instead of standard nuts. Preloading bearing balls loads them in the direction opposite the working load. This, in turn, applies opposing pressure to shaft threads and stiffens operation.