Oldham Coupling Size Chart: How to Read Specifications and Select by Dimension

Oldham coupling catalogues present a dense matrix of numbers — outer diameters, bores, torque ratings, misalignment capacities, inertia values, and speed limits — that can be difficult to navigate without a clear understanding of what each parameter means and how the numbers relate to each other. A coupling that is too small for its application fails prematurely; one that is too large adds unnecessary inertia and cost. Reading the size chart correctly is the foundation of the entire selection process.

This article explains every dimension and parameter found in a standard Oldham coupling size chart, describes how they interrelate, and walks through a practical selection example from start to finish.

The Key Dimensions on an Oldham Coupling Drawing

OD — Outer Diameter: The maximum outside diameter of the coupling assembly, measured across the hub body. This is the dimension that determines whether the coupling fits within the available radial space around the shaft. In tight installations — inside machine housings, between adjacent components, or near shaft shoulders — verify that OD plus a clearance margin fits within the available envelope before proceeding with selection.

L — Overall Length: The total axial length of the assembled coupling from hub face to hub face with the disc in place at its nominal axial position. This dimension determines the axial space required between the motor and driven machine. Ensure sufficient clearance exists for the coupling length plus any end-float that occurs when the motor shaft extends during thermal growth.

LH — Hub Length: The axial length of each hub, which determines how far the hub engages the shaft. Longer hubs provide greater clamping area and typically higher slip torque for the same hub OD. LH also determines the minimum shaft engagement length — the shaft must protrude far enough into the hub for the hub fastener to grip within the designed clamping zone.

D — Bore Diameter: The inner diameter of the hub bore, which must match the shaft diameter. Most size charts list the available bore range for each coupling size — the minimum and maximum bore that can be machined into the standard hub body without compromising hub wall thickness or clamping capacity. If both shafts have the same diameter, one bore size applies to both hubs. If the shafts differ in diameter (common in motor-to-ballscrew connections), specify the driving bore and the driven bore separately.

Disc OD / Disc Thickness: Some datasheets include the centre disc dimensions separately. The disc outer diameter determines the maximum slot length in the hub and therefore the maximum lateral offset the coupling can accommodate. Disc thickness governs the axial float available between the hub faces.

Performance Parameters in the Size Chart

Rated Torque (T_N): The continuous torque the coupling can transmit at the reference misalignment and reference speed conditions. This is the primary performance parameter and the starting point for torque-based size selection. Important: this is the continuous rating, not the peak rating. The peak torque capacity is typically 2 to 3 times the continuous rating for short-duration loads.

Max Lateral Offset (ΔKr): The maximum permissible lateral displacement between the two shaft axes. As discussed throughout this site, this is the maximum possible value — for long service life, operate at 50 percent or less of this value. The symbol ΔKr appears in European and ISO-aligned catalogues; North American catalogues may use terms like “parallel misalignment” or simply “lateral offset.”

Max Angular Misalignment (ΔKw): The maximum angular deviation between shaft centrelines, in degrees. For Oldham couplings, this value is typically 0.5 to 1.0 degrees for all sizes — it does not increase significantly with coupling size the way lateral offset capacity does.

Max Axial Displacement (ΔKa): The maximum axial movement one shaft can make relative to the other without the disc contacting a hub face. Typically 0.5 to 2.0 mm depending on coupling size.

Max Speed (n_max): The maximum rotational speed at the reference misalignment condition and ambient temperature. As discussed in the speed article, this value decreases as operating misalignment increases — consult the combined speed-and-misalignment chart when operating above 50 percent of both the speed and misalignment ratings simultaneously.

Mass Moment of Inertia (J): The rotational inertia of the complete coupling assembly (both hubs and disc), in g·cm² or kg·m². This must be included in the total reflected inertia calculation for servo drive sizing. The value in the catalogue is for the standard hub material and disc material at the nominal bore — actual inertia changes slightly with bore size (removing hub material reduces inertia) and hub material (aluminium vs stainless steel).

Torsional Stiffness (C_T): The angular stiffness of the coupling under torsional load, in N·m/degree or N·m/rad. Higher torsional stiffness means less angular deflection per unit torque. For servo applications, torsional stiffness affects the drive system’s resonant frequency — higher stiffness raises the resonance, allowing higher servo bandwidth.

Representative Size Chart

Values shown are representative for standard aluminium-hub, acetal-disc Oldham couplings at 0.2 mm lateral offset and 25°C ambient. Always verify against the specific manufacturer’s datasheet for the coupling being selected.

Worked Selection Example

Application: Servo motor driving a ballscrew on a CNC router axis. Motor rated torque 4.0 N·m, peak torque 12.0 N·m. Motor shaft 10 mm, ballscrew shaft 12 mm. Operating speed 2,500 RPM. Measured lateral offset 0.25 mm. Angular misalignment corrected to below 0.3 degrees. Standard indoor environment, 25°C ambient.

Step 1 — Design torque: Peak torque 12.0 N·m × service factor 2.0 (servo with reversals) = 24.0 N·m design torque.

Step 2 — Bore sizes: Need 10 mm driving bore and 12 mm driven bore. Check size chart for couplings that offer this bore range.

Step 3 — Torque check: The 40 mm OD size is rated at 15.0 N·m continuous. Design torque is 24.0 N·m — this exceeds the 40 mm rating. Move to 50 mm OD, rated at 30.0 N·m continuous. Design torque 24.0 N·m falls within this rating. The 50 mm size supports bores up to 25 mm — both 10 mm and 12 mm are within range. Proceed with 50 mm OD.

Step 4 — Speed check: 50 mm rated at 3,500 RPM at 0.2 mm offset. Operating at 2,500 RPM with 0.25 mm offset. The offset is slightly above the reference condition, so effective speed limit is slightly lower — approximately 3,200 RPM at 0.25 mm. Operating speed of 2,500 RPM is well within this. Acceptable.

Step 5 — Misalignment check: 50 mm rated for 0.90 mm maximum offset. Operating at 0.25 mm = 28 percent of maximum. This is excellent — well below the 50 percent recommended operating limit. Long disc service life expected.

Step 6 — Inertia check: 50 mm coupling inertia is 22.0 g·cm². Motor rotor inertia (typical for 4 N·m servo motor) is approximately 180 g·cm². Coupling inertia is 12.2 percent of rotor inertia — slightly above the 10 percent guideline but acceptable for a standard-bandwidth CNC axis. If servo tuning proves difficult, consider the 40 mm coupling with a higher-rated disc material to reduce inertia.

Result: 50 mm OD Oldham coupling, clamp hubs, 10 mm driving bore, 12 mm driven bore, standard acetal disc, aluminium hubs. All parameters within specification with comfortable margins.

Conclusion

Reading an Oldham coupling size chart correctly means engaging with every parameter, not just bore and torque rating. OD and length determine physical fit. Bore range determines shaft compatibility. Rated torque combined with service factor determines the minimum acceptable size. Speed rating, misalignment capacity, and inertia verify that the selected size meets the application’s operational requirements. Working through each parameter systematically — as demonstrated in the selection example — produces a specification that delivers reliable, long-service-life performance. Skipping any parameter risks selecting a coupling that is physically correct but operationally mismatched.

Browse our full Oldham coupling catalogue with complete datasheets, or contact our engineering team for selection assistance on your specific application.