The Coupling That Winded Up Under Load
We used a jaw-type (spider) coupling between a servo motor and a ball screw. On the bench, it positioned fine. On the machine, under load (the screw pressed with 500 N), the axis had a following error. The coupling winded up (twisted) under torque. The motor moved, but the screw lagged by about 0.05 mm. The problem: the jaw coupling has rubber (urethane) spider elements that twist under load. It’s good for vibration damping but not for high torsional stiffness. We switched to a bellows coupling (all metal, no rubber element). The wind-up went away. The following error dropped to 0.01 mm. The mistake was using a flexible coupling where a rigid (high-stiffness) coupling was needed.
Servo coupling selection is about torsional stiffness and misalignment. Pick the wrong type, and the axis loses precision. This article covers the choices.
What a Coupling Does
The coupling connects the motor shaft to the driven load (ball screw, belt pulley, or gearbox). It:
- Transmits torque from motor to load.
- Compensates for misalignment (the motor and screw shafts aren’t perfectly aligned).
- Affects the torsional stiffness (how much the coupling twists under torque).
For servo positioning, the coupling must be torsionally stiff (no wind-up) while still handling some misalignment.
Coupling Types
Jaw (Spider) Coupling
Two hubs with jaws, connected by a urethane (rubber) spider. Damping (absorbs vibration). But the spider twists under torque (torsional wind-up). Backlash is near zero (the spider is preloaded). Good for general motion, but not for high-precision positioning.
Best for: Steppers, conveyors, general motion (not tight precision).
Bellows Coupling
A metal bellows (thin-walled, flexible) connects two hubs. All metal (no rubber). Very high torsional stiffness (almost no wind-up). Handles small misalignment (the bellows flexes). Zero backlash. Standard for servo positioning.
Best for: Servo axes, precision positioning, low following error.
Disc (Laminate) Coupling
Stacked thin metal discs (laminates) transmit torque. High torsional stiffness. Handles angular misalignment. For high-power servo axes (large motors). Similar to bellows but for larger torque.
Best for: Large servo motors, high torque, precision.
Beam (Helical) Coupling
A single piece of metal with a helical cut (it twists along its length). Low torque, compact. For small encoders or light loads. Not for high torque.
| Type | Torsional Stiffness | Misalignment | Best For |
|---|---|---|---|
| Jaw (spider) | Low (rubber wind-up) | Good | Steppers, general, damping |
| Bellows (metal) | High | Moderate | Servo precision positioning |
| Disc (laminate) | Very high | Good (angular) | Large servo, high torque |
| Beam (helical) | Medium | Good | Encoders, light loads |
Step 1: Torsional Stiffness (For Servo)
The coupling’s torsional stiffness (N·m/rad) determines how much it twists under torque.
θ = T / K_torsional
Where T is the torque (N·m) and K_torsional is the coupling’s stiffness (N·m/rad). The angular twist θ translates to linear error at the screw:
e = θ × lead / (2π)
For a jaw coupling with K = 5,000 N·m/rad, T = 1 N·m: θ = 1/5000 = 0.0002 rad. Lead = 5 mm: e = 0.0002 × 5 / (2π) = 0.00016 mm. That’s small. But at T = 5 N·m: θ = 0.001 rad, e = 0.0008 mm. Still small. But the jaw coupling’s stiffness drops as the spider ages (softens). And under impact loads, the spider compresses.
For a bellows coupling with K = 50,000 N·m/rad (10× stiffer): e = 0.000016 mm at T = 1 N·m. Essentially zero wind-up.
The coupling rule: For servo positioning, use a bellows or disc coupling (high torsional stiffness). Use a jaw (spider) coupling only for steppers or general motion where precision isn’t critical. The coupling that winded up under load was a jaw coupling on a precision servo axis — switch to bellows.
Step 2: Misalignment Compensation
The coupling must handle misalignment between the motor and screw shafts. No installation is perfect. There are three misalignment types:
- Parallel (offset): The shafts are parallel but offset (not on the same centerline). Bellows and disc couplings handle this.
- Angular: The shafts are at an angle to each other. Disc couplings handle this well.
- Axial (end float): The shafts move axially (thermal expansion). Bellows couplings handle this.
Align the shafts as well as possible (use an alignment tool). The coupling handles the residual misalignment, not the gross error. Don’t rely on the coupling to fix bad alignment — it loads the bearings.
Step 3: Torque Rating
The coupling must transmit the motor’s torque without slipping or breaking.
- Continuous torque: The coupling’s rated torque must exceed the motor’s continuous torque.
- Peak torque: The coupling must handle the motor’s peak torque (acceleration) without failure.
For a 400 W servo motor (rated 1.3 N·m, peak 4 N·m), pick a coupling rated for at least 5 N·m (with margin). A coupling rated for 2 N·m will fail under peak.
Step 4: Inertia
The coupling adds its own inertia to the system. For high-acceleration axes, the coupling inertia adds to the motor’s load. Pick a low-inertia coupling (aluminum hubs, not steel).
For most applications, the coupling inertia is small compared to the load. But for ultra-high acceleration (gantry pick-and-place), use a lightweight bellows coupling.
Bore Sizes (Shaft Sizes)
The coupling has two bores (one for the motor shaft, one for the screw shaft). These are different sizes (motor Ø14 mm, screw Ø12 mm). Pick a coupling with the right bore sizes (or a clamp-style coupling with a range of bores).
Clamp-style couplings (split hubs with screws) are standard. They grip the shaft without keyways (no backlash). Avoid set-screw couplings (they mark the shaft and can slip).
A Coupling Selection Checklist
- What is the motor torque? (Continuous and peak.)
- What is the required precision? (Following error?)
- Servo or stepper? (Servo = bellows/disc. Stepper = jaw.)
- Torsional stiffness: is the wind-up acceptable? (θ = T/K.)
- What misalignment? (Parallel, angular, axial?)
- Coupling rated torque > motor peak? (With margin.)
- Bore sizes match? (Motor and screw shafts.)
- Clamp style (not set screw)?
- Is the coupling lightweight? (For high accel.)
- Is alignment checked? (Don’t rely on coupling to fix misalignment.)
- Is there a keyway? (For high torque? Or clamp-only?)
- Is the coupling accessible? (For installation/alignment?)
The Bottom Line
Servo coupling selection is torsional stiffness for precision. The coupling that winded up under load was a jaw (spider) coupling on a precision servo. Switch to a bellows or disc coupling (all metal, high stiffness). Size the torque for the motor’s peak, match the bores to the shaft sizes, and use clamp-style hubs. Align the shafts properly — the coupling handles residual misalignment, not gross error. The axis that positions tightly wasn’t the biggest coupling — it was the stiffest one for the application.
