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Feb. 1, 2009
Get your next project off to a fast start, no matter what it entails, by collecting design-oriented information now. Send in your response card (stitched

Get your next project off to a fast start, no matter what it entails, by collecting design-oriented information now. Send in your response card (stitched into your copy of MSD) or log on to www.motionsystemdesign.com and click on Reader Service under RESOURCES to get the latest literature on these products and solutions to make your next design a success.

CONNECTIVITY: Cable carriers

Carriers organize and protect power cables, communication lines, and hose from crushing, crimping, and environmental degradation. Exceeding geometric bending limits can fatigue cables, so carriers must be properly sized. Plastic carriers are most common because they are less expensive and quieter. Metal carriers are required in harsher environments where solvents and abrasives exist.

PNEUMATICS: Air motors

Compressed air pushes pistons, vanes, gerotors, or turbines for continuous rotary power.

Useful in volatile atmospheres, air motors have higher power densities than electric counterparts, but at higher operating costs due to the need for compressed air.

MOTORS: Servos and steppers

A moving magnetic field produced in stator coils opposes current or magnet-induced fields surrounding the moving rotor, causing it to turn. Servo-driven ac and dc motors provide self-correcting closed-loop control, using work as the controlled variable. A stepper-driven motor doesn't operate continuously; rather, it usually pulses in a meter as you go open-loop motion.

All dc motors ride smoothly down to zero, even when switching directions. Generally speaking, ac induction motors are more powerful constant-speed motors suitable for industrial use. Widely used dc brush motors (and their more expensive, reliable brushless cousins) are appropriate for closed-loop servo use.

LINEAR MOTION: Actuators, bearings, and rails

Threaded nut or ball circuits travel a threaded screw. Ballscrews convert more than 90% of motor torque to thrust, allowing use of small gears, clutches, and drive motors. Maximum speed depends on critical screw speed and ball recirculation speed.

Jackscrews have lower efficiency, converting 30 to 50% of motor torque to thrust. Remaining energy dissipates as heat, with duty cycles up to 50%. On the other hand, jackscrews are locking, quiet, and have a high tolerance for shock loads. Actuators incorporate motors and rotary-to-linear mechanisms.


Pressurized fluid floods a barrel, causing a piston and attached rod (or load) to advance or retract. Fluid pressure and flow is converted into force and velocity.

The drawbacks of more complex installation and possible leakage are offset by high-force cylinder motion — and 20,000 psi and greater at rated system pressures.

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