Introduction

The best accuracy number on a datasheet is worthless if the encoder is mounted wrong. Across service calls and commissioning delays, a large share of "encoder accuracy problems" are never sensor problems — they are mounting problems: a shaft that is not concentric, a coupling that winds up under load, a bracket that flexes at operating temperature. The sensor reports exactly what it sees at its own measuring reference, and if that reference is shifted or wobbling relative to the true axis, every arc-second of calibrated precision is measured against a moving target.

This encoder mounting and installation guide gives machine builders and integrators a structured path: how to choose shaft vs hollow-shaft form, what concentricity and runout actually cost you, and a step-by-step checklist anchored to real TIMSENS time-grating mechanical specifications.

Shaft Form: Solid Shaft vs Hollow Shaft

Rotary encoders mount in two basic ways, and the choice is set by the machine, not the sensor.

• Solid (clamped) shaft — the encoder shaft is coupled to the driven shaft via a flexible coupling. It suits applications where the encoder is a separate module on a stub shaft or bearing block. TIMSENS SEF angle encoders in this class span outer diameters of 90–375 mm with hollow bores of 35–285 mm, covering everything from compact servo feedback to large metrology rotary tables.
• Hollow-shaft (through-bore) — the encoder slides directly over the motor or rotary-table shaft, eliminating a coupling and one alignment interface. The reduced part count is a reliability win in high-vibration environments. Ultra-thin families such as TIMSENS UEF (≤81 mm outer) are built for space-limited smart-manufacturing axes where a solid-shaft coupling simply will not fit.

For economy industrial axes, TIMSENS TEF units offer outer diameters of 106–170 mm, a middle ground between the precision SEF line and the ultra-compact UEF.

The rule of thumb: if you can put the encoder directly on the load shaft, a hollow-shaft mount removes a failure point; if the encoder must sit on a parallel or offset axis, a flexible coupling with a solid-shaft encoder is the pragmatic route.

Concentricity, Radial Runout and Why They Matter

Once mounted, two mechanical errors dominate the realized accuracy:

• Concentricity (eccentricity) — the offset between the encoder's measuring center and the true rotation axis. Even a few tenths of a millimeter of eccentricity produces a once-per-revolution sinusoidal error that no internal calibration can remove, because it is a geometric mismatch between the sensor and the machine, not a sensor defect.
• Radial runout — the axial wobble of the shaft during rotation. Runout couples directly into angle error and is amplified on large-diameter scales.

Time grating's advantage here is structural: because the measurement is derived from coupled traveling fields rather than a physically printed grating read by optics, the sensor is less hostage to precision bearings and sealed optics — but it is not immune to gross mechanical misalignment. Concentricity and runout are still set by how the encoder is installed on the machine. Treat the mounting bore, shaft, and coupling as part of the measurement chain.

Coupling and Bracket Tolerances

For solid-shaft installs, the flexible coupling is the most common source of error:

• Use a coupling rated for the application's torsional stiffness; a wind-up coupling drifts under load reversals and shows up as hysteresis in the position loop.
• Keep the coupling as short as the layout allows; long, slender couplings amplify angular misalignment.
• Mount the encoder on a rigid bracket — a bracket that deflects under thermal or cutting loads moves the measuring reference, re-introducing exactly the error you bought precision to avoid.

Hollow-shaft installs trade coupling error for bore-fit tolerance. The shaft-through fit must be concentric to the rotary axis within the machine's own spec; an interference or slip fit that is off-center simply relocates the eccentricity into the bore.

Linear Encoder Mounting

Rotary rules do not transfer directly to linear measurement, and the mounting geometry changes entirely.

• TIMSENS LSA linear scale — delivered in a closed metal housing with precision bearings, strokes from 140 mm to 1840 mm, scanning speed up to 180 m/min, accuracy ±3 μm/m at 10 nm resolution. Mount the scale to the machine bed (the reference member) and the readhead to the moving member, with the housing isolated from coolant ingress; the closed housing is what keeps the precision bearings and scale clean on a working machine.
• TIMSENS GED displacement sensor — a point-measurement gauge with ranges of 12 / 32 / 50 mm, accuracy 1 μm (P-P) at 0.1 μm resolution, IP67 sealed. Its "direct PLC link, no subdivision box" design means fewer inline electronics to mount and align — but the gauge body still needs a stable, vibration-isolated fixture, because a vibrating fixture vibrates the measurement.

Step-by-Step Installation Checklist

1. Confirm form factor first — hollow-shaft direct mount vs solid-shaft coupling, sized to OD/bore (SEF 90–375 mm OD / 35–285 mm bore; UEF ≤81 mm; TEF 106–170 mm).
2. Clean and inspect the shaft/bore, coupling, and mounting face for burrs and debris.
3. Set concentricity — align the encoder measuring center to the true axis within the machine tolerance; verify with a dial indicator on the shaft.
4. Check runout — rotate the bare shaft and confirm radial runout is within spec before the encoder is fixed.
5. Torque to spec — uneven clamp torque on a hollow-shaft collar can ovalize the bore and induce runout; follow the torque sequence.
6. Stabilize the bracket — verify no measurable deflection under simulated load and at operating temperature.
7. Cable and interface — route the BISS-C / SSI / ABZ cable away from power and VFD runs; differential serial tolerates industrial noise far better than open-collector pulses.
8. Verify, don't assume — after power-up, confirm absolute position against a known reference before the first cycle.

Real Mechanical Specifications: TIMSENS Time-Grating Families

Source: High-Precision Displacement Time Grating Sensor Product Manual (2025.04), TIMSENS + encodersensor.com product pages.