Sourcing Worm and Wheel Gear Arrangement

Does anyone know where I can get a pair of worm and wheel gears for this sort of arrangement? The reduction ratio needs to be 2:1 (i.e. for a 4 stroke engine) and the diameter of the crank shaft will be of the order 10mm to 12mm ? I’ve spent a while trying to find them on Google, but haven’t managed to come up with anything yet.

Thanks in advance,

David

Edited By David Cambridge on 16/02/2016 09:11:43

Hi David,
A worm and wheel cannot give such a small reduction as 2:1 I think what you are looking for are skew gears. (Also called helical gears.)

Les.

Ah - yes. This image is on wikipedia for helical gears. The lower section is exactly what I'm after. Are they difficult to cut? (I'll have a look in my copy of Ivan Laws book this evening!).

David

Now I now what I'm looking for http://hpcgears.com looks like a good option!

Correct they are called skew gears which are just helicals with the same hand.

Normally helicals operating in the same plane need to be opposite hand but if mounted in the a 90 degree plane they need to be same hand.

Normal plane.

Opposite plane.

David,

Sadly, Ivan law doesn't go into helical gears in his book. I made a pair of gears for my Wyvern engine which uses a similar arrangement on a 1/2" crankshaft and a 1/4" sideshaft. I think Hemingway do the gears. Diane has an article on how I made mine which may be suitable for publication. We had quite a discussion on an earlier thread

HTH,

Rod

Hi David

There was in the Model engineer some time ago an interesting
method of producing these type of skew gears .
The author was Jock Smith and can be found from 1995
Me Vol 174 issue 3989 page 336 and 3991 page 468.

Seen in album " gears " 3 photos 2 are from article MEW 193
Gear hobbing in the mill and shows a skew gear being hobbed
and the other some completed gears .The hobs were from Arc Euro
I think these are now all sold out and they now don't seem to sell
them anymore which is a shame as they were a very good cutting tool.
Obviously there is quite a lot to make to be able to produce these
type of gears in this way and to some extent depends on how many
gears you need ,i seem to make more and more of these as time
goes by the third photo some gears and some toothed belt pullies
made recently.

John

There is a person in the Ukraine selling various hobs pretty cheaply on ebay uk and usa. He has 0.8mod, other sizes and some of the usual gear cutters as well. Russian work at it's finest going on some I bought.

John

-

One aspect of skew gears to remember - especially on drives like camshafts where the load is high and variable - is to ensure copious lubrication. Another is to ensure that end float of either gear is eliminated (so far as poss).

On my 1928 car with a skew gear to drive the magneto (a light load) the gears are trapped between thrust bearings, and the pressure relief from the oil pump blows all the 'spare' oil onto the gear just ahead of engagement.

Cheers, Tim

I agree with Tim, under much of a load the wear is quite high, its best if the gears are of different materials, ie. steel for one, and bronze for the other. Reason is that the action is a sliding one, and unless they are cast iron, two like metals can tend to pick up on each other, although the brass Meccano ones seem to do all right. I suppose you could liken a skew gear to a multi start worm.

Ian S C

I think there is still something fundamental going on that I don’t understand.

If I want a 2:1 ratio between the crank and cam shaft, and I look at helical gears on hpcgears.com the diameter of the large gear (I’d use this for the cam shaft) is twice that of the small gear(Id use this for the crank shaft). Since PD = N/DP that seems fair enough. But, when I look at the photos like the image of a Find Hansen engine at the top of this thread, the two gears look either the same size or the opposite way round to what I was expecting ?

David

David,

It depends on the helix angle. For helical gears, if you want a right angle drive then the helix angles just have to add up to 90 degrees. The pitch diameter of a helical gear is given by N divided by (DP x Cos(A)) where N is the number of teeth, DP is the Diametrical Pitch and A is the helix angle. For 45 degree helix angles Cos (A) =1 so the relative diameters depend soley on the number of teeth - 36 teeth will be twice the diameter of an 18 tooth. If you choose 26.6 degrees and 63.4 degrees for the 2 helix angles then you end up with 2:1 gears that have the same OD - which is what the Wyvern uses:

HTH,

Rod

Edited By Roderick Jenkins on 18/02/2016 13:20:47  To correct the error MichaelG pointed out below

Edited By Roderick Jenkins on 18/02/2016 13:33:04

.

Rod,

Forgive me if I am being very stupid today, but:

36.6 + 63.4 = 100

Does it not ?

MichaelG.

You can see the angle on this Domestic that I made, the shallow angle on the crank gear lets it be larger than the cam gear where you would normally find the opposite with spur gears. This allows the larger dia crank to pass through without such a thin wall as in Rod's photo

Better shot where you can see the helix of both gears

Edited By JasonB on 18/02/2016 13:28:57

Edited By JasonB on 18/02/2016 13:30:56

Not as stupid as me! Sorry, typo - should be 26.6 . Rushing to go out to my guitar lesson - really must go now!

Rod

Rod

Thanks everyone. The missing part of my understanding was the cos(A) in the formula

David

If we are picking holes in this then cos(45) does not equal 1, Tan(45) equals 1 and cos(45) is 0.707 (square root of 2).

Martin

Further to this 2:1 ratio in similar sized diameters for skew gears:

The angles in the example above are found from the inverse tan of 0.5 and 2 (half the speed one way and double the speed the other).

Tan(63.435)=2

Tan(26.565)=0.5

The angles have been rounded to 3 significant decimals. If they are rounded to 1 decimal you get the angles stated earlier.

Hope this helps anyone considering other possible ratios.

Martin

Pick holes we must - got to get it right. The point I should have been trying to make was that with both gears at 45 degrees you use the same divisor, so that the relative diameters stay in proportion to the teeth number.

Sorry again - must try not to rush things.

Rod