Reciprocating examples
Two common examples of reciprocating motion are conveyor diverters and flying dies.

On conveyor diverters, smooth-reversing actuators rapidly push packages from one conveyer to another without damaging or throwing them in an un-controlled manner. "The smooth accurate motion lets conveyers run at higher speeds," Hollingsworth says.

On flying dies, actuators accelerate dies — for example, up to the speed of a vinyl-siding extrusion line. The die can then cut the siding to an accurate length at higher speeds than other technologies allow.

"Sometimes, however, reciprocating motion can imply fixed travel with no ability to adjust travel distance," says Froud. "On the other hand, pick-and-place applications involve back-and-forth motion with a travel distance variable under computer command at every move. So in that sense, certain actuators can be too sophisticated for use in simpler reciprocating motion." Here, reciprocating motion is best achieved with a simple rotary motor driving a crankshaft, in turn, producing a fixed-distance, back-and-forth action.

Other examples of reciprocating motion include piston pumps and compressors, roller pressure and tensioning systems, material testing devices, and insertion machines.

Backlash
Backlash is a particularly harmful consequence of reciprocating systems. It imparts shock to loads and actuators, damaging product and limiting equipment life. "To minimize this effect, motion is often slowed, which in turn slows the entire machine — and throughput," says Hollingsworth. "By contrast, direct-drive linear motors are inherently free of backlash," he adds. The moving member connects directly to the stationary member through a relatively stiff magnetic field, and the entire mechanism can be programmed to have very smooth starts and stops — much like cam motion, but without the limitations of follower liftoff. Smooth starts and stops also improve machine life and are gentle on transported loads.