Application Digest

January 09, 1995

Why step up to five

Craig Ludwick, Application Engineer,
Oriental Motor USA Corp.

For many applications, five-phase step motors have a distinct advantage over
two-phase. Their increased resolution and reduced vibration make them excellent
candidates for medical products, IC manufacturing, and leadscrew applications.
When smaller step angles and low vibration are important, a five-phase step
motor should be considered.

Rotors for both two-phase motors and five-phase motors each have 50 teeth. The pitch between teeth equals (360 degrees/50) or 7.2 degrees. In a two-phase motor, the rotor turns .25 of a tooth pitch--or 1.8 degrees--as each winding is energized. This movement produces 200 steps per revolution.

By contrast, five-phase motors rotate 1/10th of a tooth pitch or 0.72 degrees for each winding energized. They increase resolution by a factor of 2.5 over two-phase, providing 500 steps per revolution.

Regardless of whether the motor is two-phase or five-phase, each phase contributes a sinusoidal torque curve with a period of 7.2 degrees. Curves for two-phase motors are displaced by 90 degrees, and torque ripple (peak-to-valley difference) measures 29%. A five-phase motor's curves are displaced by 36 degrees, and torque ripple totals just 5%. Because torque ripple contributes directly to vibration, a five-phase motor is substantially smoother.

To commnicate with an application engineer from Oriental Motors, call (310) 515-2264.

Choosing Between Acme Screws And Ball

Bernie Jacobsen, Chief Engineer, Ball Screws & Actuators

Ball screws and acme screws both provide efficient and accurate solutions to linear motion problems. Choosing between the two, however, can often be difficult.

Ball screws are very efficient devices--far more efficient than acme screws. Whereas ball screws boast efficiencies above 90%, acme screws with plastic nuts average 40% efficiency, and those with bronze nuts just 25%. And often an application using a ball screw can specify a lower-torque motor than the same application with an acme screw.

The low efficiency of bronze-nut acme screws can be exploited in vertical-travel applications to prevent back-driving. The rule is: All ball screws will back-drive under load, but acme screws with leads less than 1/3 the screw diameter won't back-drive in a vertical application.

Acme screws, especially those with plastic nuts, usually provide smoother motion than ball screws due to the lack of ball bearings in the nut. Without bearings, acme nuts must always be lubricated to prevent wear.

Predicting travel life of acme screws is more difficult than with ball screws. Engineers can calculate ball screw life using ball-bearing load data. Variations in duty cycle, friction, and heat buildup, however, make estimating life-cycles for acme screws tough.

To commnicate with an application engineer from Ball Screws & Actuators, call (408) 629-1132.

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