On faster servo systems even small power-train flexibility variations can compromise positioning accuracy. While many factors are involved, the stiffness of couplings has huge effects. That’s why metal bellows couplings are often used in servo systems: They possess high torsional stiffness without sacrificing misalignment compensation. Still, popping these couplings into systems is no longer a cure-all.

Today’s rapid starts, stops, and directional cycling are pushing the limits of bellows coupling torsional stiffness — something unusual even ten years ago. For example, the couplings might appear in circuit-board drilling machines to connect servomotors and leadscrews used for microsecond-positioning of 1,000-lb masses. Only if properly specified can bellows couplings quicken settling time (sometimes by 50% for a 10% improvement in throughput.) For that, it’s important to know exactly how much torsional stiffness a coupling really has.