Angular Encoder Ring Scale Mounting for Large Diameters: Tolerances, Centering and Eccentricity Control

Eccentricity: the error that installation creates

A perfectly engraved ring still measures wrong if its grating center does not coincide with the rotation axis. With eccentricity e and ring radius R, the angle error swings approximately ±e/R radians once per revolution: on a 100 mm radius, every 10 µm of centering error contributes roughly 100 µrad, about 20 arcseconds, of systematic angular error. This term usually dwarfs the engraved accuracy of a quality ring, which is why centering is not a fitting nicety but the dominant accuracy operation of the installation.

The error is diagnosable because it is structured. In an angle-error plot against rotation, eccentricity appears as a once-per-revolution sinusoid whose amplitude gives e directly; harmonic content beyond the fundamental points to ring form error or mounting stress instead. Recording this plot at commissioning creates a baseline: if the once-per-rev term grows over the machine's life, the ring has shifted, the structure has moved or the temperature behavior was underestimated—all detectable from data before accuracy is lost.

Centering procedure with a dial indicator

Centering compares the ring to the rotation axis, so the measurement must be taken while the axis rotates under its own bearings, not with the ring measured against a fixed spigot. Mount a dial indicator or electronic probe on a rigid arm referencing the machine structure, touch the grating-side reference face or the supplied centering surface, and record radial runout over a slow full revolution. Adjust through the ring's slotted holes or adjustment features, working in opposing pairs, and repeat until runout is within the specified limit—which for large precision rings is typically tens of micrometres, tightened further when the accuracy class demands it.

Two details decide the result. First, the indicator tip and reference face must be clean and free of burrs; a single particle lifts the reading by its own size. Second, axial runout is checked in the same session: tilt between ring and rotation plane changes the readhead gap across the circumference, and a gap that drifts from nominal to limit over one revolution modulates signal amplitude enough to threaten interpolation quality. Record both runout values with the installation report.

Fixing methods and thermal design for large diameters

The fixing method must survive the temperature range, not just the assembly bench. A steel ring rigidly bolted to an aluminium structure builds differential stress with every kelvin: the mismatch bends or stresses the grating, shifts centering and can push the readhead gap across its window as temperatures change. Clamped installations manage this with defined torque and stress-relief features; bonded installations rely on a matched adhesive layer and disciplined cure; large or thermally active structures benefit from elastic mounts that carry the ring radially while absorbing differential expansion.

For multi-metre diameters, segmented or multi-head configurations replace the single ring, and the mounting plan extends to segment joints and head phasing. Whichever construction applies, the principle is the same: the ring must reach working temperature stress-free, and the mounting must hold centering across the whole specified temperature range, not only at the temperature at which it was aligned.

• Confirm ring and substrate expansion coefficients in the mounting design
• Apply the supplier's torque values and tightening sequence exactly
• Prefer elastic or compliant mounts where temperature cycling is wide
• Re-verify runout after thermal cycling as part of acceptance

Readhead alignment and acceptance evidence

The readhead completes the installation. Set nominal gap and both angular alignments to the datasheet values, then verify over a full slow revolution that the gap and signal amplitude stay inside tolerance around the whole circumference—axial runout, edge burrs and interface steps all show up here. Check signal quality as-installed: amplitude, index mark strength and, where available, interpolation error indication. A head aligned to spec on a poorly mounted ring still produces marginal signals, so amplitude variation around the revolution is the fastest combined verdict on ring and head together.

Close the installation with documented evidence: runout values, gap measurements, signal amplitudes at defined positions and an angular accuracy verification against the agreed protocol. This evidence set is what allows a later accuracy drift to be attributed—to ring shift, structural movement or readhead wear—rather than debated. It also becomes the acceptance record for the purchase, the same discipline used for the machine's linear axes.

How SENFU supports ring scale installations

SENFU supplies angular encoder rings with explicit centering runout limits, gap specifications and recommended fixing constructions for each diameter class, so the mounting interface can be designed before the mechanics are frozen. When a rotary axis requirement is submitted with diameter, accuracy class, temperature range and structure material, SENFU can propose the mounting method and the tolerances the interface drawing must carry.

For large diameters and demanding accuracy classes, SENFU can also support the centering and verification work on site or through partners, from indicator procedure to the angular accuracy measurement that closes the acceptance record.