MAKING COG WHEELS FOR GEARED BELTS

Jan Ridders



Introduction
For a 4-stroke engine, distribution of 1 to 2 is needed between the crankshaft and the valve system. I usually use plastic (polyurethane) geared belt for that. It can also be done with interlocking gears but timing belts offer significant advantages:
1. Much more construction freedom because the centre distance between the crankshaft and the valve system is not is determined by the dimensions of the gears;
2. A greater distance be covered with relatively small cog wheels and the geared belt can easily be stretched by a tensioning wheel;
3. A geared belt is silent and does not need to be lubricated.
4. Making your own interlocking gears is a lot harder than cog wheels for geared belts because one also must have the disposal of a special and (expensive) module cutter set. This is not the case with cog wheels for geared belts and that's what the story here is about.


Geared belts
There are timing belts for sale in all shapes and sizes and they are relatively cheap. Personally, I always use 6 or 10mm wide SYNCHROFLEX polyurethane timing belts type T2, 5 (2.5 mm pitch of the teeth) reinforced with thin steel wire; they are very flexible and strong.
There are numerous suppliers for all kinds of tooth belts on internet; type for instance “synchroflex timing belt” on Goggle Search. Here two examples:
http://www.beltingonline.com/polyurethane-timing-belts-182/synchroflex-4/synchroflex-t-series-73/?zenid=oc1nottoiqjn6l1rgo6r57qb95
and

http://www.transdev.co.uk/pages/belts/synchroflex/t2_5_main.htm

They mostly provide also associated cog wheels, but they are quite expensive in my opinion. Moreover, one must finish them with respect to the shaft hole and all other dimensions. That is why I developed a method for making this type of gears easily yourself from a good quality of brass or aluminum.


Making the cog wheels
I first turn the entire wheel with bore for the shaft and tapped holes for fixing it on the the shaft. It is crucial that the diameter of the part where the teeth must be milled is accurate to a tenth of a millimeter to make sure that the geared belt will nicely fit over it. This diameter is determined by the number of teeth that the wheel should have. On page two of my drawing plan I have put a list of some of these wheels with different number of teeth and the corresponding diameters.
The two gears that I mostly use have 36 and 72 teeth, so with a ratio of 1 to 2. The diameters of the teeth are respectively 28.10 and 56.80 mm. The angle set-up between the teeth is 10 and 5 degrees respectively because 36x10 and 72x5 is equal to 360 degrees. One must therefore be equipped with a turntable on the mill with which the angle set-up can be made accurately.


On my turntable there is even a provision for the proper angle set-up for an arbitrary number of teeth, see figure 1 above.The milling cutter is made of a HHS piercing punch nipple with 5mm diameter. The first cutting portion is grind at an angle of 35 degrees and thereafter ground half way so that there are two cutting edges, see figure 2 below.


The milling depth is 0.9 mm that must be made with two or three settings in order to get nice smooth cuts. It is recommended that the teeth on the top are somewhat rounded for example with a suitable fine key-file and then be finished with a brass brush or fine sandpaper. If all this is done well, the geared belt fit will fit nicely and this cog wheel will hardly or not differ from a purchased and original cog wheel.


Drawing plan
I have made a 2-sheet plan where you will find all dimensions and other data; click here for a request by e-mail.

 
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