Rotary motor motion is usually converted to linear through rack and pinion sets, belts and pulleys, and other mechanical linkages. These are effective but limited systems.
In contrast, integrated linear actuators convert rotation to translation inside the motor itself to eliminate components and boost output force and accuracy. More specifically, some linear actuators use a stepping motor (as a rotary power source) designed with a threaded nut on a precision leadscrew instead of a rotor shaft. The rotor turns (as in a conventional stepper motor) but produces linear motion through the nut and threaded screw. This tighter integration increases mechanical advantage as well as resolution. Figure 1
This month's handy tips provided by Ray LaChance and Frank Morton at Haydon Kerk Motion Solutions Inc., Waterbury, Conn. For more information, contact [email protected] or (203) 756-7441, or visit haydonkerk.com.
Why are stepper motors used instead of conventional rotary motors?
Steppers turn a given amount for every electrical input pulse, which is useful in positioning applications. More specifically, permanent-magnet stepper motors include magnets on the rotor that align with the electromagnetic field moved by sequentially energizing and de-energizing stator coils. This causes stepping action and incrementally moves the rotor for angular motion. Depending on design, their resolution is 15 to 0.9 rotational degrees per step. Precision leadscrews downstream maintain this finer positioning. Their bearing systems also handle radial and thrust loads, and are preloaded to withstand reversing loads on the leadscrew — to prevent lost motion.
How does leadscrew and nut design affect motion?
The leadscrew provides linear force via its inclined plane. Much like a steel shaft with a ramp wrapped around it, the mechanical advantage (or force amplification) is determined by the ramp's angle — in turn, a function of lead, pitch, and screw diameter. (Lead is the axial distance a screw thread advances in a single revolution. Pitch is the axial distance between adjacent thread forms. Number of starts is the number of independent threads on a screw.) The relationship of lead to pitch depends on the starts: Lead = Pitch × Number of starts. For a single-start thread, the two are equal. Figure 2
Leadscrew threads allow small rotational force to translate large loads. A small lead provides highest force and resolution output; large leads transmit less force, but correspondingly greater linear speed from the same source of rotary power. The screw's riding nut is sometimes embedded in the stepper-motor rotor, different from other rotary-to-linear designs. Traditional nuts here are made of bearing-grade bronze, as the material lends itself to the machining required for internal threads. However, bronze is a compromise between physical stability and lubricity. One alternative is linear actuator power nuts made of lubricated thermoplastic to lower nut-to-screw thread interface friction. In fact, switching from traditional bronze nuts to those with engineered thermoplastic threads reduces friction by approximately 45%. This results in higher efficiency while allowing a smaller motor footprint for the same amount of force output.
How do plastic nuts withstand heat?
Unfortunately, though plastic is useful for threads, it is not stable enough for critical bearing journals of a hybrid stepper motor. Under continuous full-load conditions, plastic bearing journals get warm and can expand four times more than metallic journals. This expansion is unacceptable, because the stator-rotor airgap is only a few thousandths of an inch, and must be maintained for reliable operation. Designs with injection-molded plastic threads within a metallic rotor assembly address the challenge. They maintain low friction and high bearing journal stability, for quiet and efficient operation. They also improve motor life 10 to 100 times over traditional bronze nut configurations.