The ball screw keeps rollin' along

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 motors.

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 Lange.

• Environmental soundness. Heightened environmental awareness has fur- thered the ball screw's use. Because these devices are mechanical, there are no inherent fluids to contend with eliminating the potential for leaks or hazardous disposal problems.

• 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.

• Low weight. Aerospace engineers use hollow ball screws because they reduce inertia and weight, while allowing for more precise positioning. Machine tool manufacturers use the space inside hollow ball screws to accommodate electrical wiring.

Small ones, tall ones. "Typically ball screws range from 1/8^th 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?

• How fast does it need to move?

• What type of motion profile will you use to drive the system? Note: slow accelerations won't generate as much force as fast accelerations.

• How many hours per day, days per year, and years, do you want it to operate?

• How accurate do you need the operation: better than 0.001 inch/ft of movement or doesn't it matter?

• Can you live with a little backlash from your assembly? Or does it need to be preloaded?

• How will it be mounted: horizontally or vertically?

• Will you have bearing supports at one or both ends? What kind of bearings will be used?

• Is there any external force that will be exerted on the ball screw? In vertical applications, gravity is assumed to be an external force.

• .Stroke length: How far does it need to move?

• .Cycle time: How long do you want it to sit before retracting?

• .What is the overall stage length required?

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 protection.

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.

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