Description
EP6-25 Set Screw Oldham Coupling — 25 mm Three-Piece Flexible Disc for Optical Stages and Metrology Drives
Step up from desktop-class drives into the world of precision-instrument actuation, and the next coupling size that earns its keep is the EP6-25 set-screw Oldham coupling. With a 25 mm outside diameter, a 28 mm overall length, and four catalogue bore options (6.35, 8, 9.525, 10 mm), the EP6-25 is sized to span the breakpoint between Φ8 mm stepper outputs and Φ10 mm ball-screw inputs that anchors the majority of mid-range linear stages, optical positioners, and benchtop metrology gauges. Its 3.0 N·m rated torque and 5 800 rpm speed envelope give it a generous safety margin in the typical 1.0–2.0 N·m duty band of these applications.
What sets the EP6-25 apart from its EP6-20 sibling — beyond the obvious torque uplift — is the substantially higher torsional stiffness (125 N·m/rad versus 58) and the wider tolerance for parallel misalignment (1.9 mm). For a metrology machine being assembled to a tolerance budget of ±0.5 mm per stack-up, that 1.9 mm allowance is the difference between effortless installation and shimming each mount block. And the additional stiffness means that closed-loop velocity bandwidth can be cranked up without the coupling appearing as a torsional pole in the open-loop transfer function.
Full Specifications
| Parameter | Value |
|---|---|
| Model | EP6-25 |
| Outside Diameter (ΦD) | 25 mm |
| Overall Length (L) | 28 mm |
| Bore Range (d1 / d2) | 6.35, 8, 9.525, 10 mm |
| Rated Torque | 3.0 N·m |
| Maximum Torque | 6.0 N·m |
| Maximum Rotational Speed | 5 800 rpm |
| Moment of Inertia | 2.8 × 10⁻⁶ kg·m² |
| Static Torsional Stiffness | 125 N·m/rad |
| Parallel Misalignment | 1.9 mm |
| Angular Misalignment | 3.0° |
| Set-screw Thread | M5 |
| Wrench Tightening Torque | 4.0 N·m |
| Mass (max bore) | 24 g |
| Backlash | Zero |
Material Composition
Aluminum-Alloy Body
The two hubs are CNC-turned from A2017-T4 bar, hard-anodised to a uniform matt-grey finish that is non-reflective — an underrated feature in optical applications where stray reflections off rotating hardware can degrade alignment laser performance. The hub face is precision-flycut to keep slot parallelism within ±0.02 mm.
Reinforced POM Centre Disc
Centre slider is moulded from POM-H homopolymer, a denser grade than the POM-C used in the smaller EP6 series. The choice is deliberate at 3.0 N·m duty: POM-H provides ~12 % higher tensile strength and noticeably better creep resistance under sustained load. Carbon-loaded and glass-filled variants are available where the duty pushes the upper limit of the catalogue rating.
High-Tensile Set Screws
M5 cup-point set screws to ISO 4029, grade A2-70 stainless. Two per hub at 90°. The 4.0 N·m tightening specification gives sufficient pre-load to prevent slip at maximum momentary torque, while remaining within the screw’s elastic range — repeated removal and refitting will not strip the threads in the aluminum hub.
Why the Three-Piece Design Wins in Precision Stages
In a high-precision linear stage — say, a microscope auto-focus stage or a coordinate measuring machine probe carrier — the dominant error sources are the manufacturing tolerances of the mount plates, the perpendicularity of the motor mounting face, and any thermal growth of the housing during a long measurement cycle. A rigid coupling would force all of these errors into the lead-screw bearings and ultimately into the metrology output. A flexure coupling such as a beam or bellows absorbs them, but stores energy as a torsional spring; that stored energy oscillates back through the loop and shows up as a position error proportional to deceleration profile.
The EP6-25’s three-piece Oldham slider coupling architecture transforms misalignment into pure linear slip at two orthogonal interfaces. No torsional spring, no energy storage, no second-harmonic ripple. Equally important, the centre disc decouples lateral and angular degrees of freedom from the rotational degree of freedom in such a way that the load shaft sees an essentially infinite-stiffness 1:1 angular link, while the motor shaft sees an infinitely-compliant lateral link. That asymmetry is exactly what a metrology system needs.
For background reading on how Oldham couplings compare to bellows, jaw, and beam alternatives, see the Oldham coupling overview.
Industry Applications
Coordinate Measuring Machines
Driving Z-axis probe heads on bridge-type CMMs. The constant-velocity transmission preserves the metrology accuracy that determines the entire machine’s MPE certification number.
Telescope Mount Drives
Hour-angle and declination drives in small research telescopes. Zero backlash is essential for sidereal tracking accuracy.
Laser-cutting Focus Heads
Linking Z-axis focus motors to the focal-distance compensator in fibre-laser cutting heads. POM’s electrical insulation isolates the focus motor circuit from the high-current cutting loop.
Chromatography & HPLC Pumps
Reciprocating-pump head drives in analytical liquid-chromatography systems. The combination of zero backlash and electrical isolation matches LC-MS hardware requirements precisely.
⚡ Precision XY Tables
Stacked-axis positioning tables in wire-bonders, die-bonders, and electronic-test handlers. The 1.9 mm parallel tolerance forgives generous machine-build budgets.
Cinema Motion Control
Slider, jib, and focus-pull motors on motion-control rigs. Smooth, lash-free transmission preserves on-screen motion fidelity.
Why Choose Ever-power
Quality Credentials
Production of the EP6-25 falls under an ISO 9001:2015 management framework, with first-article inspection records archived for every customer drawing. Material declarations cover RoHS, REACH SVHC list 235, and conflict-minerals (3TG) status. CE documentation accompanies machinery-directive integrations on request.
Engineering Customisation
Catalogue bores are 6.35 / 8 / 9.525 / 10 mm. Mixed bore combinations are offered without lead-time penalty. Keyway slots (typical 3 × 1.4 mm to DIN 6885) are broached on request. Special slider materials — PEEK, PTFE-filled POM, carbon-fibre filled POM — are released through engineering review.
Support & Drawing Library
Native CAD files are available in STEP, IGES, Parasolid, and SolidWorks format. Application questions are answered within 24 hours by an English-speaking engineering desk that has spent the last decade specifying couplings for metrology and optical OEMs.
Customer Reviews & Case Studies
★★★★★
Switzerland — CMM Builder
Used in: Z-axis probe drive of a bridge-type coordinate measuring machine certified to ISO 10360.
Feedback: “The EP6-25 became our standard after laboratory tests showed it added less than 0.2 µm to the indicated axis error budget. The bellows we previously used contributed roughly 1.5 µm of velocity-dependent error — a meaningful improvement in our overall machine spec sheet.”
★★★★★
Australia — Astronomy Instrumentation Lab
Used in: Declination drive of a 0.5 m research telescope.
Feedback: “Tracking jitter measured at the focal plane dropped by a factor of four after we replaced the previous Lovejoy-style flexible coupling with the EP6-25. Important note: we re-shimmed the motor mount to take full advantage of the 1.9 mm parallel allowance, which made installation noticeably easier.”
★★★★☆
United Kingdom — HPLC Manufacturer
Used in: Reciprocating-piston pump head of an analytical UPLC system.
Feedback: “The constant-velocity output kept our flow-rate ripple well within the 0.05 % specification. Service-life testing on three units crossed 18 months of continuous duty with no measurable slider wear. Four stars only because we had to specify a custom flat-point set-screw for our hardened motor shaft — that took an extra week.”
Frequently Asked Questions
What does the centre-disc replace cost? Can it be replaced in the field?
Spare POM centre discs for the EP6-25 are available in single-piece quantities and at the price of approximately 25 % of a complete coupling. Field replacement requires only loosening one hub from its shaft — the slot geometry self-aligns the new disc.
How does the EP6-25 behave under reversing-direction duty?
The Oldham architecture is symmetric — clockwise and counter-clockwise responses are mechanically identical. There is no “preferred direction”, and zero backlash means a step reversal in the host servo is faithfully reproduced at the load.
What is the recommended operating temperature range?
Standard POM slider operates from −40 °C to +100 °C without de-rating. Beyond +100 °C, a derated torque curve applies; PEEK slider variants extend the useful range to +260 °C. Consult engineering for documented torque-vs-temperature plots.
Can two EP6-25 couplings be stacked for greater misalignment compensation?
Yes — a “double Oldham” arrangement with an intermediate spacer extends parallel compensation roughly proportional to the spacer length. This is a recognised technique in stages where motor-to-load offset exceeds the 1.9 mm single-coupling allowance. Ask engineering for a tandem-mount drawing.
Is keyway-broaching available on the standard 10 mm bore?
Yes. A 3 × 1.4 mm keyway to DIN 6885 / JIS B 1301 is the most common; alternative keyway depths and widths are produced regularly. Add roughly 5 working days to standard lead time for a keyway-broached order.
Ready to Specify the EP6-25?
For OEMs designing precision stages, scientific instruments, and metrology hardware, the EP6-25 is one of the most-specified mid-range Oldham couplings on the market. Explore our complete flexible coupling catalogue, read about Ever-power’s precision-manufacturing background, or submit a B2B enquiry below.
