Colchester Bantam

Can anyone please tell me what the pressure angle in on !960s model Colchester Bantam change wheels ? 141/2 or 20 ? I have read confusing information Regards

Can't help with factory information but I have used the method given in the quotation below to successfully identify the pressure angle from simple measurements.

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Fortunately there is an adequately accurate DIY method for normal involute gears. I lifted this from a reliable source and its worked out fine for me

Firstly you need to know the diametrical pitch, DP, of the gear, basically number of teeth per inch of pitch circle diameter. For practical purposes pitch circle diameter runs round half way up the tooth. As its always a whole number and only a few numbers are used best guess estimation is usually good enough unless you are watch making.

Given the DP take a measurement over any reasonable small number of teeth with whatever equipment you have.

3 to 5 teeth for a chord length of around 1/8 of a circle usually does just fine.

Multiply the cosine of the 'PA' by 3.1416 and divide by the 'DP'

add this to your measurement, this should be the measurement over one more tooth if its not the same, change the the 'PA' and try again.

Example:-
10dp 14.5 pa, 30 teeth,
Measurement over 3 teeth .776,
Cos 14.5° = 0.986147

(0.986147 x 3.1416) = 3.04

3.04/10 = 0.304

0.304 + 0.776 = 1.080 which will be the measurement over 4 teeth if the gear is
14.5° PA."

In practice you only have 14.5° and 20° pressure angles to choose from for general purpose gears so the difference is usually pretty conclusive. Its not an exact to umptyfour decimal places calculation, just a nearest to result for separating sheep and goats.

I'd be very careful with gears having heavily modified addendum, dedendum and teeth shape tho'. It works, after a fashion, with stub teeth but some judgement is needed.

There is probably a direct version for module gears but substituting nearest DP is good enuf.

Heavily engineered devices such as car and motorcycle gearboxes often include gear pairs which are way outside standard formulae. Especially Japanese motorcycle gearboxes where its not uncommon to find teeth numbers up to one tooth different from what the standard formulas give for gears of that DP or Module and diameter.

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Lathe gears are almost invariably standard full form.

Hope this helps

Clive.

Phillip

I made a cutter to make some Bantam change gears, I used 14.5 degree and the gears made mesh accurately with the standard gears.

Emgee

I would install GearDXF on your PC and use it to produce DXF of both pressure angles for a gear tooth count you already have. Open the DXF in Draftsight (or other CAD drawing program), print at 1:1 scale and simply put the gear on top of the printed DXF. Closest match wins.