Wanted helical gear set for model engine

[Jasonb]

They don't have to be the same size you can even have the lower tooth count gear bigger than the high tooth count gear. This is the Domestic Stovepipe where a large diameter 6T crankshaft gear drives the smaller diameter 12T cam gear

You don't see them being mass produced in "stock sizes" as there are so many variables it would be hard to cover all eventualities

 

[GreenTwin]

That is very interesting.
It does make for a very compact gear/shaft power transmission configuration.

If space were not a consideration, a more normal gear configuration would probably be used, since a more normal gear setup would be cheaper to manufacture.
.

 

[rpf]

A worm, which is a one tooth gear, can be any diameter.

 

[GreenTwin]

A worm, which is a one tooth gear, can be any diameter.

We are getting in deep now, LOL.

Apparently gears have been around for a very long time.

 

[gbritnell]

While definitely not an expert by any means I have studied and made my own helical gears in all sizes and pitches. The first thing is a little understanding of what helical gears are and aren't. They are basically spur gears. By that I mean they have the same involute curves, the same pitches but the teeth are arranged on a given angle to the axis. If you had a helical gear in hand and took a thin slice from a spur gear of the same pitch the two gears would mesh. The dimensions for helical gears are based on pitch, the number of teeth in the gear and the angle of the helix. With these numbers the pitch and outside diameters can be found. With a spur gear the pitch, calculated at the pitch diameter determines the gear spacing and therefore the diameters but with a helical gear the normal profile of the teeth and spacing is only true at right angles to the tooth itself so when you calculate the diameters you have to use the cosine of the pitch angle to determine them. The side adjacent (normal spacing of pitch and teeth at right angles to the tooth) is used to find the hypotenuse number for a given helical angle. So therefore the steeper the helical angle the longer the hypotenuse becomes and therefore the pitch diameter and outside diameter. The reason you can use 2 helical gears of approximately the same diameter and get a 2:1 ratio is because the helical angle of the 2 gears is different for both gears but ultimately they both add up to 90 degrees. With 2 helical gears with a helical angle of 45 degrees the gears can drive in both directions, meaning that one shaft will drive the other and vise versa. With 2 gears of the same hand the gears operate at 90 degrees to each other. To get them to run on parallel shafts you need 2 gears of opposite hands, on with a left helix and one with a right helix. Now in the case of a pair of helical gears close to the same diameter but running at a 2:1 ratio the gear on the crank shaft would have the teeth at lets say 30 degrees from the axis and the gear on the camshaft would be 60 degrees from the axis. (total of 90 degrees) The crank will drive the cam but the cam won't drive the crank because the angle is too steep. The one thing to keep in mind is that the tooth count is 2:1. One gear having lets say 20 teeth and the other 40 teeth.
Don1966 (Don Darbonne) who is a contributor has made spread sheets for all types of gears, spur, helical and bevel. It's neat to take the helical sheet and plug in some numbers for pitch and tooth count and see what happens to the diameters when you start changing the pitch angles.
I got interested in all this gear magic when Chuck Fellows first came out with his helical gear cutting fixture. Since then I've been hooked and have cut all sizes and pitches of helical gears.
Another thing that comes to mind is when cutting helical gears using standard involute cutters (standard diameters) there is a calculation for using the proper numbered cutter for obtaining the correct involute curve for a given pitch. This is because when using a standard involute cutter the diameter of the cutter shaves extra material from the sides of the tooth profile so to negate this a cutter of a different number (higher tooth count has a different shape) to create the proper involute curve. I make my own cutters from generally .375 diameter drill rod so I don't run into this problem. Or at least my cutter can be made for a certain tooth count without the compensation.
gbritnell

 

[GreenTwin]

Thanks for the helical gear info george.

I considered cutting some helical gears, but it is safe to say I will never be anywhere near the machinist that you, chuck, and many others are, and my equipment is not the most rigid either.

Luckily I have gotten good with the casting thing, and so what I am doing is using JasonB's virtually-machined helical gears, and casting my gears.

JasonB's gears seem to be spot-on, and the 3D prints mesh perfectly.
I guess the swept tooth section in 3D modeling does not suffer from the same drawbacks as using an actual involute cutter, luckily.

I basically got into the casting side of engine building because it allows me to do most of the machining virtually in the 3D program, and thus mistakes are easily correctable.

And a virtual 3D system does not need to be a very large, heavy and rigid machine.

Once I cast a part, I only have to lightly skim the mating surfaces, perhaps do a little buffing with the ceramic sponge, and I am finished with the part, and so my goal of minimizing the machining required is met.
The 3D program and 3D printer do the bulk of the work.

Casting gears in foundries goes back a long way in time, and so this method is nothing new.

The lost-PLA casting process is very promising, and also very accurate, and so it should be more than sufficient to cast some helical gears.
Another benefit of casting gears from 3D patterns is that the gears can be 3D printed any size, and so they don't require a particular size of gear cutter.

As george says "helical gears are basically spur gears", and I am very glad that helical gears are not any more complicated than they are.

I guess we have sort of turned this into a "how to cast your own helical gears" thread, but lots of good information is getting posted here, and I have a much better understanding of helical gears at this point.

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