Wooden teeth (cogs) on machinery!

         Wood Cut of Apple Tree, Marco Bussato 1612 Giardino d'Agricoltura.

 

Ever been to an old water mill in the UK? Hopefully so; there are quite a few working ones. Many owned and restored by the Natural Trust.  I hope you took a close look at the gears on the shafts.  The gear is made of 2 parts the wheel and the teeth or cogs. On all the ones I have visited the wheel is of cast iron but the teeth are of wood. And not just any wood but apple wood.

                Houghton Flour Mill River Ouse  Cambridgeshire

The reasons are buried in antiquity. Greeks and Romans had gears. These would have been made all of wood. However with iron smelting and the industrial revolution cast iron was used for the wheels but wood for the cogs. There are several reasons.

Firstly they are quieter than metal on metal.

Secondly they don't create sparks. This is important in a flour mill as most were to grind the wheat for bread. Flour in the air is quite a combustible mixture. Even so some mills burnt down. If there was a jam on the line the water wheel would keep turning and the wooden parts heated up.  And if there was a major jam the teeth would break and the machine stops. Metal teeth cast with the wheel would be expensive to replace, wooden cogs can be fitted easily in minutes. Held in with just one pin.

And thirdly had both gears been made of cast iron, the result, aside from noisy running, would have been a lot more vibration as two gears of equal hardness with rough and random cast surface finish ran against each other. The other result would have been very rapid gear wear and short life. In design of  gearing, it is common practice to design a pair of mating gears so there is a significant difference in hardness between the two gears. Typically, a pinion will be made a good deal harder than the bull gear it drives. The reason is to equalise wear. The combination of one gear being iron and the other having wooden teeth accomplished this.  Long before the formulas for calculating tooth pitting and service life were ever thought of!

The type of wood was important it had to be strong enough to take the strain, and water resistant close to the waterwheel. In the UK oak elm and hornbeam were stronger and water resistant but apple preferred in the Mill. There was a lot available and although its nearly as strong and fine grained, it’s quite brittle. This meant that if there was a problem with the machinery the replaceable teeth would shear before damage was caused further up the gear train.

So the apple tree had uses after it was no longer needed in the orchard.