Beam Engine with Pickard’s Patent Mechanism
Built by Dick Watson in October 2014
James Pickard deserves considerable credit for getting a Newcomen engine to drive a rotating shaft instead of being purely a pumping engine. He realised that a beam engine, as constructed at the time, would almost certainly stop at one or other of the dead centres (the pivot of a wooden beam was fixed to the underside making it unstable in any other position). This is an even greater problem with a single acting engine with a chain instead of a connecting rod. To overcome this, he devised the mechanism fitted to this engine and shown in the patent drawing on the back of this sheet. The two gears have a ratio of 2:1. The weight on the smaller one is arranged to rotate the engine so that its crank does not stop in a dead centre position. The patent drawing is the only remaining information about his engine so that I have assumed that the rest is similar to other Newcomen engines.
The mechanism works well, but I suspect that it was not long before it needed repair and the engineman resorted to a crowbar to turn the engine to a good starting position. It has a problem which may not have worried its user. The engine will stop in both dead centre positions, so that the direction of rotation will depend on which one it stops at (the piston can only be sucked downwards in a Newcomen engine).
Pickard’s patent presented James Watt with a problem. Because the patent drawing shows a crank, he thought that the patent might be interpreted as including the use of a crank on a steam engine (even though cranks had been used on spinning wheels for some centuries). To avoid arguments, he invented his sun and planet mechanism and used it until Pickard’s patent expired.
Making the model requires a pair of large 2:1 gears. I decided to try to use a built-up gear made of Toothed Segments. These have 27 gaps between teeth, and so there are 26 whole teeth and two, approximately three-quarter, teeth.
Fixing four to a faceplate will produce a gear with a total of 112 teeth but with four incomplete gaps. I decided deal with the gaps by fitting six segments to an Exacto faceplate with holes at 30° angles. This fills in the gaps makes it possible to get more evenly spaced teeth. The problem with this is that the meshing gear has to have 56 teeth. I thought that I could fiddle a couple of extra teeth by pushing the segments out a bit using the play in the holes. I found that I could do this, but could not rely on getting the number that I wanted.
I gave up, and made a wide faced 56-tooth gear. Fortunately, I had a 56-tooth gear with a crack in it. I removed the boss (quite hard work with the 1911 patent boss) and bolted it to a complete one. The crack is very useful for checking that I actually have a 2:1 ratio. Again, I found that I could not guarantee getting the correct number of teeth.
Having done all this, I found that I had a Stokys 76-tooth gear, hidden away with my Marklin gears which would have solved the problem but looked rather small and had a short lever arm for the counterweight.
I eventually decided that Sprocket Wheels would look better and made a wide face 3” one with a rusty nickel one.