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Ball screws convert rotary motion into smooth, accurate, and reversible linear motion and can often serve as the most efficient and cost-effective choice for linear drive systems. These anti-friction devices enable precise positioning of moving parts in a wide range of applications for the automotive, aerospace, injection molding, instrumentation, medical, and machine tool industries, among others.
Over the years ball screws have given competing technologies a run for their money. In general, ball screws will perform more efficiently than sliding screws (which generate more friction); cost less than complex hydraulic or pneumatic systems; and offer longer life, higher load capabilities, and maintained precision compared with belt, cable, or chain drives.
Innovations in ball screw technology have expanded interest and designers are taking notice.
Ball Screw Basics
Ball screws consist of a screw shaft, nut, balls, and a ball recirculating system. Components are usually made from various hardened or stainless steels.
Their configuration consists of a shaft with precision ground or rolled concave helical groove (acting as the inner race) and nut with internal grooves (acting as the outer race). Circuits of precision steel balls circulate in the grooves between the screw shaft and nut.
Depending on the application, either a rotating screw shaft or nut will then translate in a linear direction. The ball screw has a natural ability with a high efficiency to convert about 90 percent of a motor's torque into thrust, which results in minimal mechanical wear and constant performance throughout the life.
Ball returns are designed to create smooth and efficient recirculation of the balls from the end of their load-carrying path back to the beginning to complete the circuit.
Ball screws have been developed both in inch and metric dimensions. Preloaded and non-standard sizes, configurations, and special materials, including composite inserts, extend the possibilities.
Application and Operating Issues
When evaluating ball screws for an application, the first rule of thumb is to understand that a solution for one application may be inappropriate for another.
Beyond the ball screw's standard life calculation, torque requirement, and output force, many other parameters must be considered, such as noise level, smoothness, repeatability, speed and acceleration, lubrication, and coatings. All should be addressed during the design phase so that the product will operate with the expected reliable performance. (Perfect traceability during the manufacturing process is also a "must" for all high-tech applications.)
In specifying ball screw to application, designers may further want to pay special attention to backlash, contamination, operating temperature, and the type of support bearings.
Backlash (with a maximum of 70 m and even less if required) is the relative axial motion between the screw and the nut when the motor is not turning. In a vertical motion application, where the load constantly pushes down on the nut, backlash is not so much of an issue. In non-vertical motion applications, backlash may result in positioning errors if load direction changes.
Backlash can be avoided by specifying ball screws with preloaded nuts. Preload can be applied with plus-size rolling elements or with an axial force applied to a split/tandem nut. The applied preload eliminates any axial play and increases the rigidity and stiffness of an assembly. (In addition, preloaded nuts are subject to less elastic deformation than non-preloaded nuts to provide reliable and accurate positioning under load.)
Figure 1: There are many types of ball screws.
Contaminants can adversely affect critical internal ball-screw components and impact the reliability and uptime of machinery. Wipers for ball screw assemblies have been developed with machined nylon, molded polyurethane, or felt seals to help prevent damage from contaminants and keep lubricants suitably applied. (Wipers can be mounted internally or externally.)
Screws made from standard steel and operating under normal loads can sustain temperatures in the range "20 C to +110 C. But higher operating temperatures can lower the hardness of the steel, alter the accuracy of the thread, and may increase chances of material oxidation or change lubricant properties. If the operating temperature range will be higher than 110 C, special steels should be selected.
The degree of support on each end of a ball screw will determine how fast the screw can spin and how much load can be handled. Simple supports (typified by deep groove ball bearings) offer good radial stiffness but poor axial stiffness, while fixed supports utilizing pairs of angular contact bearings will provide stiffness in both directions. Shafts can also be left "free" when coupled with a fixed configuration on the other end, which means no support. Application demands will help guide designers to make the appropriate choices.
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