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arnfield's gravity escapement used in the BHI 150 clock realised in lego

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Published on Feb 13, 2009

A two pendulum version of the escapement used in the three pendulum clock built to mark the 150th anniversary of the British Horological Institute. The escapement, designed by James Arnfield, is, as with his single pendulum escapement, completely detached from the train torque, so should be capable of excellent consistency of rate. For videos of the actual clock see 'luddite22''s page.
The escapement works as follows. On the central vertical shaft, driven by the going train, are two arms. The lower is the locking arm, the upper is the gravity arm. As the vertical shaft rotates, the tail end of the gravity arm (unfortunately I had to use wire for this critical part), catches on the appropriate prop arm (the props are hanging from the top left and right of the frame and are arranged to hang directly in the path of the gravity arm tail), which then pushes down the tail of the gravity arm, lifting the head. Now that the gravity arm is no longer in contact with the locking arm, the latter is tail heavy and hits the locking detent after a rotation of 180 degrees. The escapement is now reset.
A pendulum now unlocks the the gravity arm by lifting the head, which lowers the tail and causes the prop arm to revert to its natural position. The gravity arm is now only held by the pendulum, and as it falls below its rest position, gives impulse to the pendulum. As it gives impulse, the arm accelerates downwards, and is suddenly arrested by contact with the head of the locking arm, which releases the detent at the tail, releasing the escapement, and allowing the vertical shaft to rotate as before.

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