Waterbury, CTMost hollow-shaft stepper motor actuators use a bronze drive nut with a stainless steel leadscrew. The relative softness of bronze compared to the hardness of stainless steel causes nut wear even after minimal usage. Instead, Hybrid Size 17 and 23 Linear Actuators from Haydon Switch and Instrument Inc. use a nut with molded thermoplastic threads inside a metallic shell that extends actuator life beyond one million cycles.
Compared to traditional machined v-thread and bronze-nut designs, HSI's patented design not only extends life, but improves actuator efficiency, cuts maintenance, reduces noise, and allows drop-in replacement for the same cost.
The secret is a proprietary injection molding process that allows HSI to incorporate crystalline thermoplastic threads infused with lubricant in the rotor drive nut of its non-captive actuators. "Others have tried using nuts made entirely of thermoplastic before," says HSI Engineering Manager Keith Kowalski. "But it's difficult to achieve dimensionally stable threads using thermoplastics alone."
That's why HSI chose a composite nut design that uses a brass insert to provide dimensional stability for the thermoplastic threads. "The metallic insert not only provides a stable substrate for mechanical thread retention, but it's actually an integral part of the mold with built in gates for injection points in the molding process," says Kowalski.
HSI's design costs about the same as competitive hollow-shaft stepper motor actuators, according to Kowalski. "But," he says, "the reduced friction doubles the output force, reduces power consumption, and extends motor life more than ten times that of traditional bronze nut styles. In fact, stainless steel and bronze nut configurations have proven to wear considerably after minimal usage, but our design is built for long life (over 1 million cycles)."
Depending on resolution, output thrust of the size 17 actuator is up to 50 lbs using only 7W input power. With speeds of up to 10 ips attainable, resolution ranges from 0.001 inch to 0.002 inch per step. The size 23 hybrid linear actuator is available in a wide variety of resolutions ranging from 0.0003125 inch to 0.001 inch and delivers thrust of up to 200 lbs.
"The biggest advantage of this design is that there is no maintenance [application of additional lubrication] required throughout the life of the device," says Kowalski. "OEM equipment designers can appreciate how this feature will open up new applications due to the simplicity, efficiency, and reliability of the actuator."
Target applications include medical devices, piston pumps, remote valves, semiconductor and telecommunication equipment, and other areas requiring precise motion control.