Gravity Escapement (Double Three-Legged)
Also known as: Double three-legged gravity escapement, Denison escapement
Record status
Individual type · partial
Period
Developed mid-18th to mid-19th century
A gravity escapement gives impulse to the pendulum from a small weight or weak spring rather than directly from the clock's wheel train. The train only resets the weighted arms, so the push the pendulum receives does not change when the driving force varies.
In the earliest form, two arms pivoted very close to the pendulum's suspension spring hung one on each side of the pendulum, each carrying a small deadbeat pallet with an angled plane leading to it. When the pendulum lifted one arm far enough, its pallet released the escape wheel; a tooth then slid up the angled face of the other arm, lifting it until it locked. Meanwhile the first arm, still resting on the pendulum, came down to a point lower than it had started from, and that fall delivered the impulse.
The double three-legged form was first devised by a barrister named Bloxam and later improved by Lord Grimthorpe. Two three-legged escape wheels work in parallel planes, each striking only the locking block on its own gravity arm, while three lifting pins raise the weighted arms. The energy gained in lifting the arms is what is passed to the pendulum on each cycle, and every locking produces a loud, audible tick.
Evidence at a glance
- Linked sources
- 1
- Photograph views
- 1
- Catalogue references
- 0
Counts describe this entry’s documentation, not authenticity, rarity or completeness. Photographs may show one specimen; check each caption and licence.
Historical context
The gravity escapement was developed steadily from the middle of the eighteenth century to the middle of the nineteenth. It became the escapement of choice for turret clocks, whose wheel trains face large variations in drive force from big exterior hands under wind, snow and ice loads, and the double three-legged form became the standard for accurate tower clocks. In the clock of Trinity College, Cambridge, a mass of around 50 grams is lifted through 3 millimetres every 1.5 seconds.
Technical characteristics
- Type
- Gravity escapement
- Impulse
- From weighted gravity arms, independent of the train's driving force
- Typical use
- Tower and turret clocks
- Escape wheels
- Two three-legged wheels working in parallel planes
Identification
- Source note
- Background drawn from Wikipedia (“Escapement”), a tertiary source; details have not yet been checked against a standard horological reference.
- Identification
- Two weighted gravity arms beside the pendulum; paired three-legged escape wheels; turret clock movement
- How to identify
- On a turret clock movement, look for two long weighted arms hanging either side of the top of the pendulum and a pair of three-legged escape wheels working side by side, with lifting pins between them.
Catalogue references
No catalogue numbers recorded. Numbers are only published once transcribed from a verified edition of a standard catalogue — they are never inferred.
Research required
- Dates and makers of the first double three-legged installations still to be cited from a primary horological source.