The escapement is the part of a typewriter that most owners never think about until it fails, at which point they think about very little else. It is a small assembly of levers and a toothed wheel, tucked behind the segment and above the front of the mainspring drum, and every keystroke passes through it. When it works, you never notice it. When it does not, every letter reminds you.

What the escapement does

On paper the job is simple: each time a key strikes, the carriage should move exactly one letter-width to the left. In practice, that requires converting a rotational impulse from the typebar linkage into a precisely metered release of a spring-loaded rack. The mechanism that does this has changed remarkably little in a hundred years of typewriter design.

At the centre is a star wheel — a small toothed gear rigidly attached to a pinion that engages the carriage rack. When the star wheel turns one tooth, the carriage moves one space. The star wheel is held stationary between keystrokes by a pair of dogs: small pivoted arms with hardened tips.

• The loose dog (sometimes called the check pawl) sits between two teeth of the star wheel. It is the resting-state block.
• The rigid dog (or release pawl) is held clear by a spring, but when a key is pressed, a lever pushes it into engagement, releasing the loose dog.
• As the key rises, the sequence reverses: the loose dog catches the next tooth, the rigid dog releases, and the star wheel has advanced one position.

The whole cycle takes perhaps forty milliseconds. Repeat it eight times a second on a fast typist and the escapement is doing about half a million cycles per novel.

The five failure modes

Nearly every escapement complaint reduces to one of five conditions.

1. Irregular spacing

The letters land at slightly different intervals — some tight, some loose, in a random pattern. Cause: dried lubricant on the dog pivots. The dogs move fractionally slower than they should, and the timing between loose-dog catch and rigid-dog release becomes inconsistent.

Fix: flush the dog pivots with naphtha or a light solvent. Do not oil afterwards; the pivots should run dry or with only a whisper of clock oil.

2. Double letters (skipping forward)

The carriage occasionally advances two spaces on a single keystroke. Cause: the loose dog is failing to catch the next tooth. Either its tip is worn, its spring is weak, or dried grease is holding it back a fraction too long.

Fix: inspect the tip under magnification. If sharp and clean, replace the spring or flush the pivot. If the tip is rounded, the dog needs professional attention — grinding to profile is possible but demanding.

3. Runaway carriage

You press a key and the carriage runs left indefinitely, sometimes to the margin bell before you can stop it. Cause: the loose dog is not engaging at all, usually because it has come off its pivot or its spring has broken.

Fix: open the ribbon cover, remove the front plate, and inspect. This is a diagnostic that takes ten minutes; the repair is usually straightforward once you can see the mechanism.

4. Sticky return

The star wheel rotates but the carriage does not move (or moves reluctantly). Cause: this is not actually an escapement problem — the mainspring or draw band is at fault. The escapement is releasing correctly; there is simply not enough spring tension to pull the carriage.

Fix: separate diagnosis. Test by moving the carriage by hand with no keys pressed. If it does not glide, the escapement is not the culprit.

5. Bell timing off

The bell rings too early, too late, or at inconsistent points. Cause: the bell trigger arm has bent, or the bell trip lever has come loose on its pivot. Not strictly an escapement fault but usually diagnosed alongside because the mechanism is adjacent.

How to inspect

You need three things: a bright light, a small mirror on a stick (dental supply houses sell these), and a spray can of a light degreasing solvent with a red tube. The escapement lives under the segment and behind the typebar rest. On most machines it is visible from below if you tip the typewriter carefully onto its back on a padded surface.

Do not disassemble anything on a first inspection. Watch the mechanism through several slow keystrokes with a finger holding a typebar down at the platen. You will see the rigid dog swing into engagement, the loose dog release, the star wheel advance one tooth, and the loose dog catch the next tooth as the key returns. If any step is sluggish, that is your target for cleaning.

The one rule

Do not over-oil an escapement. This is the single most common way that well-meaning owners destroy their machines. Heavy oil migrates into the dog pivots, dries into a varnish over months, and progressively slows the mechanism until it fails intermittently. Light clock oil in trace amounts, on pivots only, once every several years, is more than enough. If in doubt, run dry. A dry escapement is noisier than an oiled one but does not fail; a gummed escapement is silent right up until it does.