Who was George Graham?

George Graham (1673-1751) of London was an English clockmaker and inventor and a member of the Royal Society. He was partner to the influential English clockmaker Thomas Tompion during the last few years of Tompion's life. Graham is credited with inventing several design improvements to the pendulum clock, inventing the mercury compensation pendulum and also the cylinder escapement for watches and the first chronograph. However, his greatest innovation was the invention of the Graham or dead beat escapement around 1715. Graham refused to patent these inventions because he felt that they should be used by other watchmakers.

 

The Graham Escapement

In 1715, George Graham is said to have modified the anchor escapement to eliminate recoil, creating the deadbeat escapement, also called the Graham escapement. This has been the escapement of choice in almost all finer pendulum clocks since then. Graham modified the arm of each steel pallet so that the lower portion of each limb was based on the arc of a circle with its center at the axis of rotation of the pallets (see Fig. 1). The tip of each limb had a surface, the angle of which, based on force directions, was designed to provide an impulse to the pallet as the escape tooth slid across the surface of each tip. The escape tooth strikes the pallet above the tip on the lower portion of the limb (see Fig. 2), where the escape wheel is rotating clockwise and is about to strike the entrance pallet on the left side, above the impulse face. The surface that the escape tooth strikes is the locking face, since it prevents the escape wheel from rotating farther.

 


Figure 1: Graham Pallets.

 


Figure 2: Graham Escapement.

 

When a pallet releases an escape tooth, the escape wheel rotates freely with about 2º of drop, until another tooth strikes the other pallet on its locking face, just beyond the tip. If the pendulum continues to swing after the drop has taken place, the escape tooth slides up the locking face until the pendulum stops. The escape wheel is not pushed backward (recoil) as the tooth slides up the locking face because each point along the locking face is at the same radial distance from the axis of rotation (pivot shaft) of the pallets. The pendulum stops at the end of each swing, to some extent because of gravitational force but mostly because of the elasticity of the suspension spring, which serves to change the direction of motion of the pendulum and start it moving again. The energy that the escape wheel provides to the pendulum is needed to maintain the motion of the pendulum. The clock is not self-starting. You must start the pendulum swinging.

 

The Working Principle



Figure 3 shows the steps that the escapement goes through in a complete cycle. Note that in the figure the circle indicates the points to be noticed and the arrow shows the rotation direction of the escape wheel. Figure 3(a) shows the 1st shock, which is the contact of a tooth on the escape wheel onto the entry pallet of the pallet fork. Figure 3(b) shows the 2nd shock, at which the pendulum reaches the farthest point and begins to move to the opposite direction. Figure 3(c) shows the 3rd shock, another tooth on escape wheel touches the exit pallet of the pallet fork. Figure 3(d) shows the 4th shock, the pendulum reaches the other farthest point. Finally, Figure 3(e) shows the 5th shock, the pallet fork and pendulum return to their original position completing a cycled.

You can see a demo of the functioning of the clock here and here:

 

Our Clock



Initially, we encountered several problems in getting our clock function correctly; one of which was the “double-clicking” of our pendulum. After much trial and error, fiddling around with the depth and angle of the pallets, we were able to position them in a way that allows the escape wheel to smoothly slide up and down the pallets, without recoil, and clearance to allow the pallets to enter between teeth as the pallets swing in and out.

 

 

Our continued work: After finally getting our clock to work as it should, we are now in the works of creating with the help of Professor Littman a more accurate animation in Matlab of how the graham escapement works.

Written by Justin Karfo '09 and Kenneth Liew '10