Machine & Duty Running

Operating Speed spm

Reference Speed spm

Speed Exponent n

Hours Since Service h

Drift Rates & Tolerances Limits

Timing Drift Rate ° per 1000 h

Timing Tolerance °

Tension Drift Rate cN per 1000 h

Tension Tolerance cN

Tension When Set cN

Hours to Service

— h

Set by whichever tolerance is reached first

Drift State

Timing Drift to Date: — °
Tension Drift to Date: — cN
Current Thread Tension: — cN
Speed Acceleration: — ×
Hours to Timing Limit: — h
Hours to Tension Limit: — h

Steady linear drift is a convenient fiction: real machines hold timing well and then move quickly once a bearing or a hook has genuinely worn, so a machine inside its calculated interval can still be out of specification. Both rates depend far more on thread type, fabric abrasiveness and maintenance quality than on speed alone, and must be fitted from your own service records. Skipped stitches, thread breaks and seam pucker are the symptoms that matter, and a machine showing them needs attention regardless of what the hour count says.

Sewing Machine Rotary Hook Timing & Tension Drift Modeler — free, with the formula and a worked example, at Textile School.