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Servo couplings—including bellows, elastomer, distance, and safety varieties—join rotating parts while correcting for misalignment. (GAM.)
When engineers assess servo performance, the motor, drive, and gearbox typically receive the most focus. Yet the link between these components also plays a key role in how precisely motion is transferred and how long the system will last.
A coupling connects a servo motor shaft to a gearbox, a servo motor to the drive mechanism, or the gearbox to the drive mechanism. It passes load from one part to the next while making up for misalignment, letting connected components work correctly without damage.
“These parts don’t usually fit together on their own,” explains Matt Ruggles, senior design engineer at GAM, a U.S.-based maker of servo gear reducers, servo couplings, and other motion control parts. “If you can’t get it perfect, make it adjustable.”
Shaft misalignment can bring several issues into a servo system. Noise and vibration are often the earliest clues that components are working against each other, resulting in system inefficiency. Misalignment also puts unexpected stress on bearings, leading to early wear that can cut short the lifespan of the motor, gearbox, and nearby parts.
Misalignment generally comes in three forms. Axial misalignment is in-and-out movement along the rotation axis, often triggered by temperature shifts. Angular misalignment happens when shafts sit at an angle to one another. Lateral misalignment occurs when shafts are parallel but not lined up.
Axial, angular, and lateral are the three most common forms of shaft misalignment. (GAM.)
“How much misalignment a coupling can handle and still last a reasonable amount of time depends on the type of coupling,” says Ruggles. “There are as many coupling varieties as there are stars in the sky, and some are built specifically for certain kinds of misalignment.”
Take the universal joint in automobiles—it’s engineered to handle angular misalignment as the axle travels up and down over road bumps, even across thousands of miles. It all comes down to the coupling’s long-term alternating-stress tolerance; if those limits are surpassed, the resulting fatigue can eventually cause the coupling to break down.
While misalignment compensation helps extend component life, stiffness determines how accurately motion moves between parts. A coupling with greater torsional stiffness helps minimize backlash and lost motion, taking the sponginess out of the system.
“The stiffer the coupling, the less compliance in your system, which means your motion setup will respond faster and position more precisely,” says Ruggles.
The balance between misalignment tolerance and stiffness becomes clear when comparing GAM’s two primary servo coupling types: bellows and elastomer. Both are zero-backlash, low-inertia couplings built to handle shaft misalignment, but they achieve this through different compensating mechanisms.
A bellows coupling features stainless steel bellows joined to the clamping hubs via a press-fit brass wire or a welded joint. This construction delivers very high torsional stiffness while still providing flexibility for misalignment, particularly angular misalignment. As a result, it excels at preserving precision and responsiveness but is less tolerant when alignment is off.
Bellows couplings also offer some adjustability—changing the number of corrugations affects both stiffness and how much misalignment the coupling can absorb. They are well suited for faster, higher-torque, or cyclic-duty applications, as well as high-temperature settings where a maintenance-free design is preferred. (Depending on the model, bellows couplings can withstand temperatures up to 300°C and torque up to 4,000 Nm.)
An elastomer coupling uses an elastomer spider positioned between two clamping hubs. Because the two halves are separable and the elastomer sits between them, it handles misalignment more readily, adds vibration dampening, and can offer electrical insulation. The separable design also makes plug-in assembly and repairs straightforward.
Its stiffness is determined by the spider’s Shore hardness but is typically lower than that of a bellows coupling, so it can twist slightly like a spring and introduce a small delay in positioning. Elastomer couplings are generally a more affordable choice for less demanding continuous-duty uses, with temperature capability up to 120°C and torque capacity up to 2,000 Nm.
GAM provides a broad selection of servo couplings, including elastomer and bellows designs in multiple sizes. Distance couplings pair elastomer or bellows couplings at each end of a long drive shaft to bridge spans up to 20 feet without external bearings, while safety couplings offer adjustable torque settings and single-point 360° re-engagement.
“We have couplings that mount to hollow shafts,” says Ruggles. “We have ones with shrink discs. On our bellows coupling, we can go full stainless steel for food-grade or other specialized environments. We have high-speed couplings that are precision-balanced to run at significantly higher speeds than our standard catalog couplings. And we offer a wide size range—from small couplings about the size of my thumb to large ones the size of a pumpkin. It’s a broad set of options to match whatever the customer needs for their application.”
Visit GAM to discover more about servo couplings for motion control applications.
