# When to Choose a Linear Motor

DN Staff

May 15, 2006

For rotary or linear motors, motion is produced in the same way and the same basic physics and electromagnetic forces apply. The theory that produces "torque" in a rotary motor produces "force" in a linear motor. But a closer look reveals that linear motors provide unique advantages for motion applications.

## Linear Force

To understand how a linear motor produces force, imagine taking the rotary motor, cutting it open radially along the axis of rotation and flattening it out. Instead of a rotating shaft producing torque, the result is a flat or linear motor producing "linear work" or "linear force". This force can be significant, and small linear motors can produce 0.5 lbs (2 N) of continuous force and larger linear motors can provide up to 3150 lbs (13800 N) of peak force.

When reviewing rotary vs. linear motion, often-overlooked items to consider include maintenance and cost over the machine's life where linear motors have an advantage. Linear motors also bring specific applications advantages including direct coupling, accuracy, repeatability, speed and acceleration.

Direct load coupling removes a major source of positioning error by eliminating the conversion of rotary (ball screws, timing belts, etc.) to linear motion and provides improvements in accuracy and repeatability.

Accuracy for linear motors is ultimately determined by linear encoder feedback accuracy and up to 0.0004 inch/ft (10 micrometers/305mm).

Repeatability is the difference between two approaches to the target position, and is typically within a few encoder counts. Repeatability for linear motors will be 0.00004 inch (1 micrometer) or 100 times smaller than the diameter of your hair to assure that parts are produced accurately and identically.

Much higher speeds are possible with linear motors. While rotary motors can attain speed of 50 inches per second (converting rotary to linear motion), linear motor speeds reaching 400 inches per second (10 m/s) are possible.

Faster acceleration rates are possible as well. While a rotary motor and ball screws can accelerate at 3 g's, linear motors can attain up to 10 g's (98 m/s), which helps shorten machine cycle times and improve productivity.

## Wide Variety of Linear Motors

Linear motors available include brushless, iron core, ac induction, steppers and specialty or custom designed products. Choosing among these technologies depends on features and operating conditions, such as accelerations and speeds, needed for a specific application.

Linear motors integrate easily into a machine, and replace the ball screw, nut, end bearings, motor mount, couplings and rotary motor. Linear motors already build into a linear stage assembly are also available.

Linear motors provide consistent motion and precision positioning for hundreds of millions of cycles, and the same performance when mounted vertically, upside-down or horizontally.

For performance specifications on linear motors, visit: http://rbi.ims.ca/4922-527.

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