Oldham sliding disc couplings transmit torque through a floating polymer disc captured between two metallic hubs, and the disc material is the single biggest determinant of stiffness, backlash, and humidity tolerance [S1][S2].
For zero-backlash motion control, acetal (POM) is the default; for vibration damping and shock absorption on conveyor and start-stop drives, nylon is specified instead, and the two resins differ by roughly an order of magnitude in moisture uptake at saturation [S2][S3].
How the Center Disc Sets Coupling Behavior
An Oldham coupling tolerates parallel shaft misalignment of roughly 5 to 10% of its outer diameter, plus small angular offsets up to about 0.5°, by letting the center disc slide laterally between the two hub faces; the disc is the only wear part in the assembly [S2].
Because the disc transmits torque through its sliding faces, its stiffness sets the coupling's torsional wind-up, and its moisture behavior sets dimensional drift in service; mis-specifying the resin is the most common root cause of premature Oldham failure, not the metallic hubs [S1][S3].
Moisture Absorption: The Decisive Property Gap
Standard nylon (PA 6/6) absorbs up to 8% water by weight at saturation versus 0.8% for acetal, and at 50% RH the equilibrium moisture content sits near 2% for nylon and 0.2% for acetal [S3].
The mechanical consequence is direct: conditioned nylon 6/6 (Zytel 101) drops from 12,000 psi dry-as-molded tensile strength to 11,200 psi, flexural modulus collapses from 410,000 psi to 175,000 psi, and elongation at break jumps from 60% to over 300% [S3]. Acetal homopolymer (Delrin 150) holds 10,000 psi tensile, 420,000 psi flexural modulus, and 75% elongation with virtually no humidity-driven swing, which is why acetal wins tight-tolerance and humid-environment jobs [S3][S4].
Mechanical and Wear Profile: Nylon vs Acetal
On the wear bench at 1 MPa and 0.03 m/s against mild steel, acetal records a specific wear rate of 2.0 × 10⁻¹⁵ m³/N·m versus 3.0 × 10⁻¹⁵ m³/N·m for nylon 6/6, giving acetal a lower steady-state wear rate under higher-pressure, lower-speed thrust-washer conditions; nylon, however, posts Taber abrasion factors two to five times lower than acetal in general abrasion testing, so the two resins trade wins depending on the tribological regime [S3].
Tensile and bending strength favor nylon: nylon delivers higher tensile strength and higher bending stiffness than acetal, tolerates higher loads, and handles higher continuous temperatures, with the caveat that humidity-conditioned nylon swells and loses tensile strength [S4]. Acetal returns higher compressive strength than nylon 6/6, higher cold resistance, and better dimensional stability across humidity swings [S4][S7]. Delrin homopolymer offers slightly better mechanical properties than acetal copolymer but can carry a low-density centerline porosity that designers occasionally have to derate for precision disc use [S10].
Decision Matrix for Oldham Center-Disc Selection
For a quick spec call, the four criteria that matter most are torsional stiffness, backlash, humidity stability, and shock damping; the table below lines up each resin against the criterion, with the source cited. [S2]
Acetal: high torsional stiffness and effectively zero backlash with the disc seated; near-zero dimensional change at 50% RH; lower steady-state wear at higher contact pressures; cold-tolerant; limited impact damping [S2][S3][S4]. Nylon: softer disc, so measurable torsional compliance and backlash under indexing load, but excellent vibration and shock absorption, plus higher tensile strength, higher impact resistance, and better Taber abrasion in dry-running service [S2][S3][S4]. For dimensional stability in humid environments, low moisture absorption, and consistent performance, choose acetal; for higher strength, impact resistance, and damping on start-stop or shock-loaded shafts, choose nylon [S8][S9].
Use Cases and Operating Limits in Practice
On conveyor lines that index packaging, sort product, or otherwise depend on repeatable position, Ruland's acetal-disk Oldham delivers zero backlash and accurate timing up to roughly 6,000 RPM, while running balanced with low vibration; the same coupling on a start-stop or shock-loaded section of the line is commonly reconfigured with a nylon disc to absorb peak torque and protect the motor bearings [S2].
For stepper drives, positioning slides, pumps, and actuators, the precision path is acetal; for OEM machine builders who need damped coupling response on long, heavy shafts, or who face humidity swings between 20% and 80% RH, acetal is also the safer pick because nylon discs swell by roughly 0.5 to 0.6% across that humidity band and can bind or introduce preload on the hubs [S1][S3]. If a design must run above the acetal continuous service ceiling or in a load case where the higher tensile strength of nylon is required, the resin switch is justified, but the engineer should account for the 2% equilibrium moisture pickup and the elongation jump from 60% to over 300% as the disc conditions in service [S3][S4].
Limits, Failure Modes, and What to Verify on the Print
Both resins are semi-crystalline thermoplastics, both are FDA and USDA compliant in food-grade grades, and both can be machined or molded into the same disc geometry, but the performance envelope is not symmetric; substituting one for the other without re-checking torsional wind-up, humidity coefficient, and wear life is a documented source of premature coupling replacement [S4]. Nylon's UV sensitivity is another print item: unless UV-stabilized grades are called out, nylon discs degrade in outdoor or UV-exposed cabinets, while acetal is generally more stable under the same lighting [S4]. For high-temperature or chemically aggressive service beyond the standard grades, PEEK center discs enter the catalog and are spec'd instead of either nylon or acetal, not as a hybrid [S5].
On an Oldham coupling, the disc is also the electrical-isolation element between the two hubs, so material choice indirectly sets the dielectric path; that is rarely the deciding factor but it should be confirmed on the print for drives that depend on shaft isolation for bearing-current control [S1]. For readers new to the geometry, an Oldham sliding disc coupling and its parent disc coupling families differ mainly in how the misalignment is absorbed, and the disc material is the main tuning knob once the geometry is locked. A deeper dive on the resin side is given in the POM (acetal) and nylon reference pages, which cover moisture, wear, and food-grade behavior in more detail.
Track these signals on the next spec review: OEM datasheets quoting torsional stiffness in N·m/rad for both disc materials (not just one), and any change in continuous service temperature or humidity window between the standard and filled grades; either one is the cue to revisit the disc call before the next machine build. For comparison shopping on related motion-control hardware, see the breakdown of One-Piece vs Two-Piece Stem Butterfly Valves: Spec Comparison and the conveyor-side decision guide on Overhead Conveyor Track: Enclosed Profile vs I-Beam Rail Compared, both of which sit in the same process-engineering track.