Bloxam’s gravity escapement separates lifting from locking. A small pinion raises two independent pallet arms, while a larger wheel is stopped alternately by their locking studs. The arms fall against the pendulum through fork pins E and F. This page follows the arrangement in Brown 312 and Hiscox 1170.
Bloxam’s Gravity Escapement · Energy and Timing
Compare arm mass, measured extra centre-of-mass rise and beat rate. The small pinion raises the arms; the larger wheel provides alternating locks.
Equation Used
- Measured extra COM rise per arm reset.
- Equal arms; one impulse opportunity per beat.
- Ideal available energy before losses.
- Eight locking arms and half-pitch advance in the illustrated sequence.
- Arm and contact geometry enlarged for explanation.
No clock-accuracy, manufacturing-tolerance or pendulum-amplitude prediction.
Small pinion lifts; large wheel locks
The original drawing shows a large wheel with eight long stopping arms and a small toothed pinion on its arbor. Stops A and B belong to separate pallet arms that pivot close to the pendulum suspension. The fork pins E and F extend from these arms to embrace the pendulum bar.
The pendulum moves a pallet far enough to unlock the large wheel. During the brief release, the small pinion lifts the opposite pallet. When the pendulum returns, a raised pallet can fall against it and supply impulse. The wheel and pinion share an arbor; the two pallet arms move independently.
This reconstruction uses a half-pitch advance of 22.5° per beat for the eight-arm wheel. Arm motion and contacts are enlarged for clarity. Their profiles are illustrative, not a drawing from which to manufacture an escapement.
What this calculator compares
Use the mass of one arm, the extra vertical rise of its centre of mass supplied by the train, and pendulum beat rate. These determine ideal available energy and average power. The calculation does not predict clock accuracy, pendulum amplitude or actual energy transferred after losses.
The pendulum also moves each arm during its swing. The energy calculation must count only the extra lift supplied by the train; counting the whole excursion would include energy borrowed from the pendulum itself.
Potential energy and beat timing
For arm mass m in grams and extra vertical COM rise h in millimetres, available energy is E = 0.00980665 mh millijoules. At b beats per minute, mean ideal power is P = Eb/60 milliwatts. A full pendulum oscillation contains two beats, so T = 120/b seconds.
With the selected eight-arm wheel advancing half a pitch per beat, it takes 16 beats per revolution: n = b/16 rpm. Daily ideal energy is E × b × 1.44 joules.
If rise is derived from an arm angle, the starting COM angle matters: h = r(cos α₁ − cos α₂), with angles measured from vertically downward. The simpler r(1 − cos θ) applies only when the lower COM position is directly below its pivot. Enter a measured extra vertical rise here instead.
A measured 0.1 mm extra rise
A 100 g arm raised an extra 0.1 mm stores 0.0980665 mJ. At 60 beats/min, that is 0.0980665 mW of ideal average available power and 8.4729456 J per day. The beat interval is one second and the full pendulum period is two seconds.
The selected locking-wheel sequence gives 3.75 rpm. Doubling arm mass or extra rise doubles the energy; doubling beat rate doubles power and wheel speed but leaves energy per beat unchanged. These inputs are an arithmetic example, not a prescribed build.
Historical description and model limits
Denison’s 1857 discussion describes Bloxam’s pallet arbors close to the pendulum suspension, a small lifting pinion, and broad fork pins E and F. It also discusses delicate locking geometry. This is different from the shared three lifting pins in a double three-legged gravity escapement.
The calculator does not infer universal lock depth, surface finish, service interval or timekeeping accuracy. Actual transfer depends on contacts, friction, rebound and the pendulum. Nor does ideal available energy establish an operating amplitude without a model or measurement of pendulum losses.
Bloxam escapement questions
Is this a three-pin escape wheel?
No. The source drawing uses a small lifting pinion and a larger locking wheel. The fork pins E and F are attached to the pallet arms.
Why enter vertical COM rise?
It gives the potential-energy change directly without assuming the arm starts vertically below its pivot.
Does the calculated energy all reach the pendulum?
No. It is ideal available energy before losses.
Why does beat rate affect power but not energy per beat?
Beat rate changes how often the same measured lift is repeated.
Primary references
Henry T. Brown, Five Hundred and Seven Mechanical Movements, no. 312: lifting pinion, larger stopping wheel and fork pins E/F. Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, no. 1170, printed page 288.
Edmund Beckett Denison, Clocks and Locks, 1857, printed pages 46–47, Bloxam’s escapement and pallet arrangement. The animated contact dimensions and example inputs are explanatory selections.
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