The right rigidity

If backlash is looseness between components, torsional compliance is looseness within components. An expression of material rigidity, torsional compliance is how much mechanical parts wind up under torque. Here though, some give can be beneficial. For example, servo-insert couplings are actually designed for lower torsional stiffness. It serves to damp impact loads and reduce their potentially harmful effects on the mechanical system. "Insert couplings do in fact store energy, and spring back during fast indexing," explains Lechner. Why doesn't this storing and releasing of energy increase shock then? "Any impact involves a rapid velocity change of one element relative to another," says Lechner, "but low torsional stiffness tends to increase the load's rotational acceleration and deceleration time."

That's how insert couplings decrease force of impact resulting from torsional compliance. Proof is that insert couplings allow loads to oscillate, however slightly and briefly, once the motor has stopped: "This persistence of motion subsequent to the motor reaching zero speed is evidence that the energy is being transferred more slowly than in systems lacking torsional compliance," continues Lechner. One caveat: Reconfiguring mechanical component geometry can reduce material, inertia, and potential imbalances, but the challenge here is that a level of rigidity must be maintained. "That's why replacing metal components with lighter, high-strength plastics will most certainly be a future trend in power transmission design," says Lechner.

If compliance doesn't serve a specific design function, it should be minimized as much as possible. "Compliance in drive shafts, belts, and gear trains limit overall system response because it introduces resonant points and nonlinear effects difficult to correct with control loops. Even if control loops can compensate, often the torsional windup of the mechanics, from a positioning standpoint, cannot be tolerated," Amendolea explains.

Because cams roll smoothly through reversals with one-way rotation, they've always been a natural for indexing. It's like doing laps around a track versus running back and forth between two points — their reversals are inherently easier. Cams are also a low-compliance option. "Not only do cams produce complex motion, they're also rigid — making them particularly suitable for high-speed indexing applications," explains Mark Combs, automation sales manager for Sankyo Inc., Sydney Ohio. Modern cam profiles are synthesized from motion curves calculated to generate optimized movement. This technique allows one well-designed cam to control an entire machine — with more sophisticated performance than one might expect by just eyeballing the cam profile. "For this reason, mechanical cams can actually have shorter cycling times than electric and fluid-power actuators," — with cycles in excess of 1,000/min. in some cases — continues Combs. That's because cams avoid the other flavors of compliance — namely, electrical saturation delay and decay, and soft response due to air compressibility.