The Coupling That Failed in a Month
We mounted a bellows coupling between a servo motor and a ball screw. On the bench, it ran fine. On the floor, after a month, the coupling cracked. The bellows (the flexible part) had a fatigue crack. The problem: the motor and screw weren’t aligned. The parallel offset (the two shafts weren’t on the same centerline) was about 0.5 mm. The bellows flexed to compensate. At every cycle (the axis moved back and forth), the bellows bent. After thousands of cycles, it fatigued. We re-aligned the shafts (laser alignment, or a dial indicator). The offset dropped to 0.05 mm. The coupling stopped failing. The mistake was mounting the shafts without aligning them — relying on the coupling to take a large offset.
Coupling misalignment and alignment determines the coupling’s life. A misaligned coupling flexes every cycle and fatigues. This article covers the alignment.
Misalignment Types
There are three misalignment types between two shafts:
- Parallel (offset): The shafts are parallel but offset (not on the same centerline). The coupling must flex sideways.
- Angular: The shafts are at an angle to each other. The coupling must flex (bend) at an angle.
- Axial (end float): The shafts move axially (thermal expansion). The coupling must allow end movement.
Every coupling has a rated misalignment (how much it can take). Don’t exceed it.
Step 1: Align the Shafts
Align the shafts as well as possible before relying on the coupling. Use:
- Dial indicator: A dial indicator on the coupling hub. Rotate the shafts, measure the runout. Adjust the motor (shim under the feet) until the runout is within tolerance.
- Laser alignment: A laser alignment tool (more accurate, faster). For high-speed or precision shafts.
For a servo axis, aim for parallel alignment under 0.05 mm (and angular under 0.5°). The coupling handles the residual misalignment, not the gross error.
The alignment rule: Align the shafts first (dial indicator or laser). The coupling that cracked had 0.5 mm offset on a bellows coupling. Bellows couplings handle 0.05–0.1 mm (not 0.5 mm). Align to within 0.05 mm. The coupling handles the residual, not the gross error.
Step 2: Coupling Misalignment Ratings
| Coupling Type | Parallel Offset | Angular |
|---|---|---|
| Bellows (metal) | 0.05–0.1 mm | 0.5° |
| Jaw (spider) | 0.2–0.5 mm | 1–2° |
| Disc (laminate) | 0.1–0.2 mm | 0.5–1° |
| Oldham (sliding disc) | 1–2 mm (parallel only) | Low |
A bellows coupling (for precision, article 89) takes small misalignment (0.05 mm). A jaw coupling (spider) takes more (0.2–0.5 mm) but has torsional wind-up. Match the coupling’s misalignment rating to the alignment you can achieve.
Step 3: Don’t Use the Coupling to Fix Bad Alignment
A common mistake: “the coupling is flexible, so we don’t need to align.” This loads the coupling (and the bearings). The coupling fatigues (cracked bellows). The bearings wear (side load). Align the shafts. The coupling handles the residual (a few hundredths of a mm), not the gross error (0.5 mm).
Step 4: Thermal Expansion (Axial)
A ball screw heats up (article 101). It grows axially (thermal expansion). The coupling must allow this end float (axial movement). A bellows coupling allows some axial float (it stretches). A rigid (clamp) coupling doesn’t (it pulls the motor bearing). For a long, hot screw, use a coupling with axial float (or a floating bearing support, article 61).
Step 5: Mounting Tolerances
The motor and screw mount to the frame. The frame’s flatness (article 107) affects alignment. If the frame isn’t flat, the motor and screw are at different heights. Machine the mounting pads (article 107) so the motor and screw are co-linear.
An Alignment Checklist
- Are the shafts parallel aligned? (Dial indicator?)
- Is the offset under 0.05 mm? (For bellows?)
- Is the angular misalignment under 0.5°?
- Is the coupling type rated for the residual misalignment?
- Does the coupling allow axial float? (Thermal expansion?)
- Are the mounting pads flat? (Machine pads, article 107?)
- Is the alignment re-checked after a week? (Frame settled?)
- Is the coupling centered? (No excessive runout?)
- Is the coupling the right size? (Torsional stiffness, article 89?)
- Are the clamp screws torqued? (Loose coupling?)
The Bottom Line
Coupling misalignment and alignment aligns the shafts, then the coupling handles the rest. The coupling that cracked had 0.5 mm offset on a bellows coupling. Align to 0.05 mm (dial indicator). The coupling’s misalignment rating is for residual error, not gross misalignment. Allow axial float for thermal expansion. The coupling that lasted wasn’t the most flexible one — it was aligned.
