Fanuc spindle alarm 61 means the difference between the semi-closed side (motor-end sensor) and the closed side (spindle-end sensor) exceeded the level set in parameter 4354 — on machines using the dual position feedback function, where the control watches both the motor's sensor and a separate spindle-end sensor and cross-checks them. When the two disagree beyond the limit, something between motor and spindle — or in one of the two feedback chains — is wrong. This is a genuinely useful alarm: it is often the first hard evidence of a slipping spindle belt. Fanuc gives four causes keyed to the machine's history.
Case 1 — At machine start-up / commissioning: parameters
If AL-61 appears on a fresh installation, retrofit, or right after parameter work, the conversion between the two sensors is mis-defined:
- Check the motor-end position feedback conversion coefficient parameters: 4171 and 4172 (high gear) and 4173 and 4174 (low gear) — these tell the control how motor rotation maps to spindle rotation per gear range. Wrong coefficients make the two sensors disagree by a constant ratio, tripping AL-61 the moment the spindle turns.
- Also verify the sensor-related parameters per B-65280EN — sensor types and directions for both ends.
Case 2 — Coefficients correct, still trips: parameter 4354 too low
Every real transmission has some compliance — belts stretch, couplings wind up — so the two sensors always disagree slightly under load. If the tolerance in parameter 4354 is set tighter than the mechanics can honour, normal operation trips the alarm. Raise it only to the builder's documented value — not to whatever makes the alarm stop; an inflated 4354 blinds the machine to the real slippage this alarm exists to catch.
Case 3 — A machine that ran fine for years: slippage (the important one)
Fanuc's words: ‘there may be slippage between the motor and spindle — check the machine.’ The motor sensor says the motor turned; the spindle sensor says the spindle didn't keep up. On a previously-healthy machine that is mechanical:
- Spindle drive belt worn, glazed, oil-contaminated or under-tensioned — by far the most common finding; inspect and re-tension/replace.
- Keys/couplings/taper-lock bushings working loose on the pulleys.
- Clutch or gear stage slipping under load on geared heads.
The giveaway: AL-61 during heavy cuts or aggressive accel (when a marginal belt slips) but clean at light load.
Case 4 — Intermittent on a healthy machine: noise in a feedback signal
Random, non-load-correlated AL-61 points at count errors from electrical noise in either the motor-end or spindle-end sensor signal — the same shielding/routing discipline as the position-coder alarms: shield continuity, frame-ground clamps, separation from power leads (AL-47 covers the method).
Diagnosis checklist (in order)
- History: new/just-worked-on → verify 4171–4174 per gear and sensor parameters.
- Load-correlated on a veteran machine → inspect belt tension/condition, pulleys, keys, clutch.
- Random/intermittent → check both feedback cables' shielding, grounding and routing.
- Everything verified and the mechanics documented → only then compare 4354 against the builder's value.
- After the fix: run Cs-contour or orientation moves under load and confirm the two sensors track.
Related guides
- Fanuc spindle alarm 47 (pulse count abnormal / noise method)
- Fanuc spindle alarm 36 (error counter overflow)
- Fanuc spindle alarm 27 (position coder disconnected)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained