Ball screws --they come in all sizes, shapes, and capacities. You'll find
them in planes, trains, and automobiles. Not to mention boats, ABS antilock
braking systems, trailer jacks for RVs, positioning systems for satellite
dishes, passenger boarding bridges, and machine tools.
Yet, despite the versatility of ball screws, engineers seem to know little
about these mechanical actuatorseven though the technology has been around since
the 1930s. So says David Lange, director of product engineering for aerospace,
government, and defense at Thomson Saginaw (Saginaw, MI), a maker of ball screws
and linear guide rails.
Times are changing, however. Advances in materials and design, as well as
safety and environmental concerns are giving engineers a reason to learn more.
In fact, the ball screw continues to gain popularity in spite of competition
from hydraulics and pneumatics, and a relative newcomer, direct-drive linear
One reason is their strength capacity. Two 3-inch ball screws, for example,
can lift approximately 80,000 lbs on the vertical lift of a passenger boarding
bridge manufactured by FMC-Jetway Systems (Odgen, UT). The company, whose
bridges are used in airports throughout the world, has incorporated ball screws
into its equipment since 1959.
"Pneumatics are not a good choice for heavy loads such as ours," says George
Dean Hone, director of bridge engineering services at FMC-Jetway. "Hydraulics
are feasible, and some manufacturers do use the technology as a source of power,
but we found it to be more expensive because it is maintenance intensive. It is
not as clean because the hydraulic fluid has a tendency to leak. Also hydraulics
don't perform well in cold weather. Because our equipment would require long
hoses, it would be difficult to keep the fluid warm." Ball screws on the other
hand require very little maintenance, he says, commenting that they only need to
be lubricated periodically.
The company's best-selling passenger bridge, found at many airports, is a
three-tunnel system. The unified portion is 58 ft long from the beginning to the
rotunda, or point where the tunnels split. When fully extended, the tunnels
stretch to 110 ft in length. "Airlines use these bridges at least 10 times a
day," says Hone. In addition to the vertical lift, ball screws act as the
actuator arms that power the aircraft closure, or the bellows, that passengers
walk through to reach the aircraft. FMC-Jetway engineers also designed in ball
screws to power the automatically controlled floor-leveling system.
And they last. Jetway pulls the bridges apart every five years or so to
visually inspect the components. "Some bridges have had the same ball screws for
40 years now," says Hone. "They are an excellent solution for this application."
Migration to ball screws.Ball screw applications continue to
grow, despite competition from linear motors, whose advantages include no
turning parts and high speeds.
Manufacturers of office automation equipment, copiers and paper feeders for
example, use small or miniature ball screws with a shaft of 1/2 inch or less in
diameter to replace chains and sprockets.
Medium-diameter ball screws, those between 0.5 inch and 4 inches, have become
the transmission technology of choice for numerically controlled equipment, says
Babinski. "We saw their use dramatically increase in the late 70s and early 80s
when designers were putting computers on machinery. They replaced
rack-and-pinion systems that didn't have sufficient accuracy, or chain and
sprocket assemblies that didn't offer enough strength. Belt drives are fast, but
they can't transmit a lot of load."
Larger ball screws are used in heavy-duty machines in paper mills, large
rolling mills, and steel mills where workers must move large, steel ingots.
Technological advances have helped promote ball screw use, including the
development of stronger materials for the ball bearings, ball nuts, and screws,
as well as variations on the ball return system. "The latter has been key in
expanding the appeal of ball screws," says Thomson's Lange. Ball bearings are
re-circulated within the ball nut in one of three wayseither through an internal
cross-over, an external tube, or an end return. With the internal cross-over
return, the ball's return path is nested right within the body of the nut. With
this design, a graceful, 3D S-shaped path picks the ball up out of one groove,
directs it over the major shaft diameter, and drops it into the previous groove.
This design creates individual turns of re-circulating ball bearing trains.
Because of its cylindrical, compact design, the internal cross-over ball-return
system is particularly applicable when an engineer wants to spin the ball nut
instead of spinning the screw, such as in aerospace applications.
Many strengths. If you're looking for any of the following characteristics in
an actuator, a ball screw might be right for your application:
Efficiency. "A ball screw is an extremely efficient, anti-friction device.
You can convey or transmit energy produced with minimal loss. Energy-out is
nearly equal to energy-in, and that's the measure of efficiency," says
Babinski. "And the device can be self-contained, unlike pneumatics or
hydraulics where pumps, motors, and other components are external."
Accuracy. The semiconductor industry uses ball screws for wafer production
because of their accuracy. "Making wafers requires high speed and extreme
precision, as well as accuracy," says Lange. "We make ball screws with 70
millionths of an inch-per-foot lead accuracy."
Safety. Ball screws are gaining in popularity, particularly in aerospace
applications, because as all-metallic devices (with few exceptions), they are
not susceptible to fire or failures of equipment operating in the vicinity of
the ball screw, says Lange. (Suppose, for example, a turbine blade comes apart
in an aircraft.) That's because ball screws can be designed with enough
redundancy to resist ballistic impact and debris. "When designing the
pylon-conversion actuator for the V-22 Osprey, a vertical take-off aircraft,
we actually had ball screws shot with various shell sizes to see how well they
survived a ballistic attack," says Lange.
Rigidity. Ball screws are high-spring-rate actuators. "I'm not relying on a
column of fluid or a column of air as you would in a hydraulic or pneumatic
actuator. I'm relying on a steel component. Imagine the difference in
compressibility between those three. A ball screw is very, very stiff," says
Predictability. Given that they are similar to rolling element bearings,
and have a long performance history, it is straightforward to predict how a
ball screw will react under different loads, fatigue conditions, and how long
it will endure, says Babinski.
Small ones, tall ones. "Typically ball screws range from 1/8th
inch diameter up to 12 or 13 inches, but the sky's the limit on the large size,"
says Lange. To wit, Thomson manufactures a 4-inch-diameter, 45-ft-long ball
screw for the steel industry.
Thomson also developed a five-stage telescoping ball screw for a silo
application with a retract length of 6 ft. It extends out to 28 ft in a
telescoping fashion. The outermost stage has a diameter of 13 inches and the
innermost stage a diameter of 4 inches, with three stages between the two.
Despite their versatility, ball screws aren't right for every application.
Hydraulics and pneumatics are extremely fast and, depending on the purpose, may
be a less expensive alternative. And the market for direct-drive linear motors
continues to grow. But in any applica-tion requiring linear motion, ball screws
merit a look.
Choosing the right ball screw
There are as many different types of ball screws as there are applications. A
clear understanding of what you want to accomplish will help guide you to the
right one. Here's a list of questions to help you decide which ball screw is
best for your application:
How heavy is the load?
Anatomy of a ball screw
The ball screw is a marriage between ball bearing technology and lead screws,
says David Lange from Thomson Saginaw. "Ball screws achieve 90%-plus efficiency
in translating rotary to linear motion and visa versa."
Ball screws look like ordinary bolt and nut systems. They have only four
major components, as shown in the accompanying diagram: 1. A shaft, which is
analogous to a bolt, 2. A nut, 3. Bearing balls, 4. A ball recirculating system.
In addition to shaft diameter, ball screws are typically described in terms of
their pitch, which can ultimately determine the speed of the system. The pitch
is the distance between adjacent heads on the screw.
The helix angle is another measure of a ball screw's speed. A licorice stick,
for example, has a high helix angle. The longer the pitch, the higher the helix
angle. The smaller the pitch (and the smaller the helix angle), the slower the
ball nut travels.
Materials used for the shaft, nut, and ball bearings depend on the
application. Ball screws are made of a variety of hardened stainless steels.
Engineers may plate this base material with manganese phosphate, nickel, thin
chromium, zinc, or cadmium. When stainless steels are used, the natural
passivation layer formation eliminates the need for any further corrosion
In applications where weight is a factor but endurance is not, ball screws
may be made out of aluminum. In rocket thrust control applications, for example,
ball screws may only have to make one stroke to pull or push a component at a
prescribed moment during rocket booster flight.
Ball Screw Suppliers
Allied Devices Corp.
2365 Milburn Av.
Baldwin, NY 11510
Roller Bearing Co. of America
400 Sullivan Way, P.O. Bx.
West Trenton, NJ 08628
Ball Screws & Actuators Co.
3616 Snell Ave.
San Jose, CA
Roton Products, Inc.
660 E. Elliott Av
Duff-Norton Co. Industrial Components Div.
Charlotte, NC 28241-7010
11 DeAngelo Drive
Bedford, MA 01730
E-Drive Design Inc.
124 Hebron Av.
Star Linear Systems
14001 South Lakes Dr.
Phone: 800-828-6344 x011
Hiwin Technologies Corp.
4344 Regency Dr.
Glenview, IL 60025
Stock Drive Products/Sterling Instrument
2102 Jericho Tpke., Bx
New Hyde Park, NY 11042
Lead Screws International, Inc.
2101 Precision Dr.
City, MI, 49686
TECHNO-ISEL (Div. of Designatronics, Inc.)
2102 Jericho Tpke., Bx
New Hyde Park, NY 11042
Nook Industries, Inc.
4959 East 49th St.
Thomson Industries, Inc.
2 Channel Dr.
Port Washington, NY
NSK Corp., Precision Products Div.
Bloomingdale, IL 60108
The Torrington Co.
59 Field St., P.O. Bx 1008
Parker Hannifin Corporation
6035 Parkland Blvd.
200 E. Commerce Dr
Schaumburg, IL 60173
Pow-R-Jac (Div. Limitorque Corp.)
P.O. Bx. 11318
Warner Electric/Dana (Div. of Dana Corp.)
449 Gardner St.
Beloit, IL 61080
Raco International Inc.
3350 Industrial Blvd.
Bethel Park, PA