Three-legged pendulum escapement
Three-legged pendulum escapement. The pallets are formed in an opening in a plate attached to the pendulum, and the three teeth of the escape-wheel operate on the upper and lower pallets alternately. One tooth is shown in operation on the upper pallet.
Only the input is driven; every other part of this model moves by physics (MuJoCo), and its ratios are measured. Run it live, change its parameters, or switch the friction off to see the output stop.
- Modelled as
- Gravity on. Built as No. 305, with the single pin replaced by a wheel of three legs. Their ends, the teeth, are at radius 1, and a constant torque turns the wheel clockwise (the weight). The pendulum hangs from a pivot 6 above the wheel's axis. The plate on the pendulum has an opening round the wheel, and the upper and lower edges of the opening form the two pallets. A tooth coming over the top lands on the upper pallet's dead top face, 40° past the top, and rests there while the pendulum swings on. When the pendulum, coming back, carries the face's end past the tooth, the tooth drives the pallet's upright left face to the right: the impulse. A sixth of a turn later the next tooth comes up under the lower pallet, rests on its dead underside, and then drives its upright right face to the left. So the three teeth work the upper and lower pallets alternately. The dead faces are arcs about the pendulum's pivot. At rest they stop just short of the teeth, so the escapement sets itself going from rest. The only input is the torque on the wheel.
- Measured
- The wheel goes a sixth of a turn per beat (1.0473 rad; 1.0472 expected) over 18 beats. The pendulum's period is 0.707 s against 0.694 s for the free compound pendulum, 1.9% slow. Its swing holds at about 6.3° either side (6.21° in the first third, 6.26° in the last). It also runs with two-thirds and with 1.5 times the torque. The wheel turns back at most 1.3° (allowed 2.3°). The deepest contact after the first second is 0.16 mm.
- Notes
- Brown gives no sizes. The pallets, the plate and the pendulum are No. 305's. The run-back comes when a tooth lands near the end of a dead face and is pushed back a little as the face slides off. A little damping on the wheel's arbor keeps the teeth from bouncing when they land. Without the pallets the wheel would run ten times as fast (30 rad/s against 3), so the escapement, not the damping, sets the rate. With half the bob's weight the pendulum runs 4% slow.
Film (MP4) · Still (PNG) · Spec (YAML) · Model (MJCF)
