An Oldham Coupling is a three-piece shaft coupling made of two outer hubs and a centre disc with two perpendicular tongues that slide in matching slots cut into each hub. It solves the problem of transmitting torque between two parallel shafts whose axes are offset — something a rigid coupling cannot do without bending the shafts. The centre disc slides radially during each revolution, absorbing the offset while keeping the input and output rotations exactly synchronised. You get zero backlash, a constant 1:1 velocity ratio, and offset capacity up to about 5% of the hub diameter — common in servo drives, encoder mounts, and stepper-driven leadscrews.
Oldham Coupling Interactive Calculator
Vary shaft offset and RPM to see Oldham disc sliding speed, stroke, PV wear index, and estimated acetal disc life.
Equation Used
The offset e makes the Oldham center disc slide in each slot once per revolution. Mean sliding speed is modeled as four offset lengths per revolution. The life estimate is fitted to the article examples for an acetal center disc, so use it as a teaching estimate rather than a replacement for manufacturer PV ratings.
- Parallel shaft offset only; angular misalignment is not included.
- Ideal Oldham coupling keeps a 1:1 velocity ratio with zero backlash.
- Life estimate is an empirical fit to the article's acetal center examples at 3000 RPM.
- PV index assumes the same contact pressure and center disc material.
Operating Principle of the Oldham Coupling
The Oldham Coupling, also called the Oldham coupling (K-series) in most industrial catalogues, works on a simple principle: a floating middle disc has two tongues, one on each face, and those tongues are oriented 90° apart. Each tongue slides freely inside a slot machined into the corresponding hub. As the assembly rotates, the centre disc slides back and forth radially relative to each hub, and the geometry forces the output hub to rotate at the same instantaneous angular velocity as the input hub. No backlash, no velocity error, no second-harmonic ripple like you get with a Hooke's joint.
The reason it tolerates parallel shaft misalignment is that the centre disc is free to translate in a plane perpendicular to the shaft axis. If the input shaft sits 0.5 mm off from the output shaft, the centre disc simply orbits with that offset, sliding 0.5 mm one way then the other across each tongue interface every revolution. That sliding is also where the Oldham earns its limits — it is a sliding contact, not a rolling one, so PV (pressure × velocity) at the tongue face is what kills the centre disc. Run an acetal centre at 3,000 RPM with 0.3 mm of misalignment and you'll see disc wear in 500 hours. Drop the misalignment to 0.05 mm and the same disc lasts 20,000+ hours.
Get the tolerances wrong and the failure modes are predictable. Slot-to-tongue clearance below 0.02 mm causes the disc to bind under thermal expansion and fracture. Clearance above 0.08 mm produces audible chatter and torsional backlash — exactly what you bought the coupling to avoid. Run it past its angular misalignment rating (typically 0.5° max) and the tongues hammer the slot ends and shear the disc. The centre disc is intentionally the sacrificial element — replace it, keep the hubs.
Key Components
- Driving Hub: The input-side hub, keyed or clamped to the motor shaft. Contains a single rectangular slot cut across one face. Slot width tolerance is typically H8 against the centre-disc tongue at h7, giving 0.03-0.06 mm sliding clearance on a 25 mm hub.
- Driven Hub: The output-side hub, identical geometry to the driving hub but with its slot oriented 90° relative to the driving hub's slot. This 90° orthogonality is what creates the kinematic constraint that locks input and output to a 1:1 velocity ratio.
- Centre Disc (Floating Element): The sacrificial middle piece carrying two perpendicular tongues, one per face. Made from acetal (Delrin), PEEK, bronze, or hardened steel depending on torque and temperature. Acetal handles up to 80°C continuous and absorbs minor shock; PEEK pushes that to 150°C.
- Hub Clamping System: Either a single-screw clamp, a split clamp, or keyway plus setscrew. For servo applications use a clamp-style hub — setscrews mark the shaft and reduce concentricity, which then loads the centre disc more heavily.
Real-World Applications of the Oldham Coupling
You see the Oldham Coupling anywhere two parallel shafts need to transmit torque with zero backlash and the designer cannot guarantee the shafts will be perfectly collinear. It is the default choice for servo-to-leadscrew connections in CNC retrofits, encoder feedback couplings, small pump drives, and lab equipment. The K-series Oldham is particularly common in semiconductor handling and medical pump drives because the acetal centre disc is electrically insulating and chemically inert.
- CNC Machine Tools: Tormach PCNC 1100 X-axis ballscrew drive — Oldham coupling between the NEMA 34 stepper and the C5 ballscrew end, tolerating the 0.1-0.2 mm parallel offset that comes from bracket-mount stepper installations.
- Semiconductor Equipment: Brooks Automation wafer transport stages use Ruland MWS-series Oldham couplings (a K-series Oldham coupling variant) on their lead-screw drives where particle generation must stay below Class 100 cleanroom limits — the acetal disc generates no metallic debris.
- Servo Motion Systems: Yaskawa Sigma-7 servo to ground ballscrew on a Parker XR linear stage, where the Ruland MOC-25 absorbs the 0.05 mm bracket misalignment without loading the servo bearings.
- Laboratory & Medical Pumps: Watson-Marlow 520 peristaltic pump head drive — Oldham coupling between the gearmotor output and the rotor shaft, isolating the rotor bearing from gearmotor side loads.
- Encoder Feedback Drives: Heidenhain ROD 426 rotary encoder mounted to a machine spindle through a miniature Oldham coupling, where any radial load through a rigid coupling would destroy the encoder's own bearing within months.
- 3D Printing: Prusa i3 MK3S Z-axis used Oldham couplings between the 5 mm stepper shaft and the 8 mm leadscrew specifically to eliminate the wobble that rigid couplings transferred into layer-line artifacts.
The Formula Behind the Oldham Coupling
The defining equation for sizing an Oldham Coupling is the maximum sliding velocity at the tongue-slot interface, because that is what determines centre-disc life. The formula gives you a PV (pressure-times-velocity) input that you then check against the disc material's rated PV. At the low end of the typical range — say 0.02 mm offset at 500 RPM — sliding velocity is trivial and the coupling will outlast the machine. At the nominal sweet spot of 0.1-0.2 mm offset at 1500-3000 RPM the disc sees real wear but lasts 10,000+ hours in acetal. Push past 0.5 mm offset at 3000 RPM and you are in territory where you should be specifying PEEK or bronze, not acetal.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| vslide | Peak sliding velocity at the tongue-slot interface | m/s | in/s |
| e | Parallel shaft offset (eccentricity) | m | in |
| N | Rotational speed | RPM | RPM |
| PV | Pressure-velocity product on disc face (compare to material rating) | MPa·m/s | psi·ft/min |
Worked Example: Oldham Coupling in a benchtop pick-and-place gantry
You are designing the X-axis drive on a benchtop SMT pick-and-place gantry. A NEMA 23 servo with a 1/4 in (6.35 mm) shaft drives a 10 mm ground ballscrew through a flange-mount bracket. Bracket machining tolerances give you a parallel offset budget of 0.15 mm at install, and the ballscrew runs at a nominal 1500 RPM during rapid moves, with a low-end of 500 RPM during placement and a high-end of 3000 RPM during long X-axis traverses. You need to confirm a 25 mm OD acetal-centre Oldham coupling will survive.
Given
- e = 0.15 mm
- Nnom = 1500 RPM
- Nlow = 500 RPM
- Nhigh = 3000 RPM
- Acetal PV rating = 0.10 MPa·m/s
Solution
Step 1 — convert offset to metres and compute peak sliding velocity at the nominal 1500 RPM operating point:
That is 23.6 mm/s of sliding motion at the tongue face every revolution. For acetal at moderate face pressure (~1 MPa typical for a 25 mm hub at 2 Nm torque), PV works out to ~0.024 MPa·m/s — well under the 0.10 MPa·m/s acetal rating, so disc life is in the 15,000-20,000 hour range.
Step 2 — check the low end of the typical operating range, 500 RPM during fine placement:
At 500 RPM the sliding velocity drops to under 8 mm/s. PV is around 0.008 MPa·m/s, essentially negligible — the coupling will not be the life-limiting component on the gantry by any margin.
Step 3 — check the high end, 3000 RPM during rapid traverses:
At 3000 RPM you are pushing 47 mm/s of sliding velocity. PV climbs to ~0.047 MPa·m/s — still inside the acetal envelope but you have lost roughly half your safety margin. If your bracket tolerance opens up to 0.30 mm in production, you will double vslide at 3000 RPM and land at ~0.094 MPa·m/s, right at the acetal limit. At that point you specify a PEEK centre disc or you tighten the bracket bore tolerance.
Result
Peak sliding velocity at the nominal 1500 RPM design point is 0. 0236 m/s, giving an acetal PV of about 0.024 MPa·m/s — roughly a quarter of the material rating, so the coupling is correctly sized. At 500 RPM the disc barely moves; at 3000 RPM you are at half your safety margin and any production bracket drift will eat the rest. If your machine measures excessive backlash at the leadscrew within 1000 hours, the most likely causes are: (1) hub-clamp slippage on the servo shaft because someone used setscrews instead of a clamp hub and the screw flat is rounding off, (2) thermal swelling of the acetal disc against an under-spec slot clearance, causing the disc to crack at the tongue root rather than wear normally, or (3) a hidden angular misalignment from a non-flat motor mount face exceeding the 0.5° rating and hammering the tongue ends.
Choosing the Oldham Coupling: Pros and Cons
An Oldham Coupling is not the only way to handle parallel shaft misalignment. The two most common alternatives engineers consider are the bellows coupling (a thin-wall metal bellows that flexes elastically) and the disc-pack coupling (flexible steel discs in a stack). Each has a different sweet spot — the Oldham wins on parallel offset capacity and zero backlash at moderate speed, the bellows wins at high speed and high precision, and the disc pack wins on torque density.
| Property | Oldham Coupling | Bellows Coupling | Disc-Pack Coupling |
|---|---|---|---|
| Max parallel offset (% of OD) | 3-5% | 0.2-0.5% | 0.1-0.3% |
| Max RPM (25 mm OD) | 3000-4500 | 10,000+ | 8000+ |
| Backlash | Zero (with proper clearance) | Zero (elastic) | Zero (elastic) |
| Torsional stiffness | High | Medium | Very high |
| Wear life (acetal centre, typical duty) | 10,000-20,000 hr | Lifetime (no sliding) | Lifetime (no sliding) |
| Cost (25 mm class) | $25-60 | $80-200 | $150-400 |
| Sacrificial element | Yes — replaceable centre disc | No — replace whole unit | No — replace whole unit |
| Best application fit | Servo-to-leadscrew, encoders | High-speed precision spindles | High-torque industrial drives |
Frequently Asked Questions About Oldham Coupling
The chatter is almost always slot-to-tongue clearance, not misalignment. If your hub slot is cut at H10 instead of H8 against an h7 tongue, you are running 0.10-0.15 mm radial slop. At low speed the disc has time to fully traverse that slop on each revolution, and you hear it as a ticking or rattling.
Check it with a dial indicator on the output hub while you rock the input hub by hand — if you see more than 0.03 mm of rotational lash on a 25 mm coupling, the centre disc is undersized for the slots. The fix is a tighter-tolerance disc, not a smaller offset.
Pick bellows when your nominal speed exceeds about 4000 RPM, when your duty cycle is continuous (24/7 industrial), or when your parallel offset budget is genuinely below 0.05 mm because the rest of your machine is precision-ground.
Stick with the Oldham when offset is 0.1-0.5 mm (typical for fabricated brackets), when you want a sacrificial wear element you can replace in 5 minutes, or when the coupling sits in a contaminated environment where a thin-wall bellows would fatigue from foreign-object impact. For 90% of CNC retrofits and benchtop machines, the Oldham is the right answer because the bracket tolerances dominate.
Yes. K-series is just a sizing-and-bore designation used by several catalogue manufacturers (Ruland, Lovejoy, Miki Pulley) to identify their dimensional family of Oldham couplings. The kinematics are identical to John Oldham's original 1821 patent — two slotted hubs with a perpendicular-tongued floating disc.
What varies between K-series sizes is hub OD, bore range, torque rating, and centre-disc material. The mechanism itself is unchanged.
Yes, and it usually means you have more misalignment than you think. Heat in an Oldham comes from sliding friction at the tongue interface, and that friction power scales with the square of offset times speed. A coupling at its design point dissipates almost no detectable heat.
Pull the coupling and dial-indicate the two shafts independently against a fixed reference. You are likely seeing 0.3-0.5 mm of offset where you specified 0.1 mm. Check also for angular misalignment — a non-flat motor face can give you 0.3-0.5° of angular error on top of the parallel offset, and angular load on an Oldham generates heat fast because the tongues fight the slot ends.
Catalogue ratings for a 25 mm OD acetal-centre coupling typically list 2-4 Nm continuous and 6-10 Nm peak. The continuous rating is set by face pressure on the tongue — too much torque and the acetal cold-flows into the slot, increasing backlash permanently.
Rule of thumb: stay under 50% of the catalogue continuous rating if you have any meaningful misalignment, because misalignment loads the tongues asymmetrically and the rated torque assumes near-perfect concentricity. Step up to a PEEK or aluminium-centre disc if you need the full rating with real-world offset.
Tongue-root cracking is a classic stress-concentration failure and it has three usual causes. First, you may be running angular misalignment above 0.5° — the tongue is being bent each revolution rather than purely sliding, and acetal has poor fatigue resistance under repeated bending. Second, the slot may have a sharp inside corner instead of the small radius the disc tongue expects, concentrating stress at exactly the spot you are seeing the crack. Third, you may be running shock-loaded torque pulses (servo retune oscillation, for example) that exceed the peak rating during transients even though steady-state torque looks fine.
Check angular alignment first with a precision square against both shafts, then inspect your hubs for slot-corner radius — it should be 0.3-0.5 mm minimum, not a sharp internal corner.
References & Further Reading
- Wikipedia contributors. Oldham coupling. Wikipedia
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