Mechanical wear changes key travel and damping feel gradually, rather than failing all at once

What it is

The key still lights; the knob still emits pulses; the detent is shallow, the damping is gone, travel feels like a sponge. Wear changes feel before hard failure. Cycle life gives the hard-fail point. This layer says that before that point, the confirmation channel and the quantizing channel are already closing. People lose eyes-free use first, and meet the day it will not press at all later.

Why it happens

Dome fatigue shortens peak force and the drop; the tactile-to-make relation drifts too. Grease is pumped out or dries; a knob goes from viscous to free-spinning and starts to walk under vibration. Shaft play grows, the center band widens, a stick starts to crawl. These are continuous quantities, not binary good/bad. Users compare only to yesterday and barely notice; across months they find “this key only counts if I slam it.” Hard failure (open circuit) often arrives long after feel has left the usable window. Acceptance that only tests make/break will pass a key that can no longer be a receipt.

Studying it

In a cycle-life test, treat force–displacement, damping torque, and center residual as failure criteria alongside electrical make.

Independent variables: cycle count, environment, whether force matches the field. Dependent measures: curves of peak force, drop amplitude, damping torque, center-band width versus cycles; eyes-covered confirmation rate.

An eyes-free confirm at several cycle points is more useful than a teardown only at the end. Draw “feel out of window” as a life-end line next to electrical end.

Where it stops holding

Linear keys that never had a detent must judge feel fade on travel and return, not on a drop. Well-sealed, rarely cycled keys may harden with age before they wear; the curve differs. When users file fade as “it got smoother,” lost damping looks like an improvement—until the knob walks in a car. Swappable caps that leave the switch body do not fix feel.

Applying it

• Put peak force, drop, damping, and center band in the life window, not only make/break counts.
• For eyes-free keys, take life-end as the earlier of feel escape and electrical failure.
• Verify: at mid-cycles, run eyes-covered confirm and unattended walk. A drop in confirm rate or the start of walk is end-of-life, even if it still makes. Give those curves to maintenance for replacement before hard failure.

Related

• Same group: C10.16.1 A physical control’s life is usually measured in presses or turns it can survive · C10.16.3 High-use controls need a higher rated life than low-use ones · C10.16.4 The gradualness of mechanical failure lets it affect operation before it is noticed
• Adjacent: C10.08 Key Travel, Tactile Detent, and Confirmation · C10.11 Levers, Joysticks, and Directional Control
• Search: feel degradation · dome fatigue · damping fade