Mitre gear

[Boiler BriParticipant @boilerbri]

A fellow member of our club asked me if I could get him a mitre gear for his battery loco as one had stripped the teeth from it. It is a double drive with one motor on each bogie set each with a pair of matching of gears . One gear is steel and the matching gear, bronze.

I was pretty confident that I could get him one off the shelf to match. How wrong I was, a matching off the shelf gear would be of such fine pitch that it would probably go the same way as the one I am trying to replace.

From the good one that is left I can get the basic measurement to make a new one. 18 tooth etc. The problem is the tooth form? How do I measure it to get the type of cutter I need!

Your help please gentlemen would be appreciated.

Brian

[Boiler BriParticipant @boilerbri]

[Peter G. ShawParticipant @peterg-shaw75338]

Hi Brian,

I'm certainly no expert on gears of any sort so I expect others will offer more and better information. However, there was an article in ME 18 July 1986 by G. Tardrew of South Africa titled Parallel Depth Bevel Gears. This may help you.

Good luck,

Peter G. Shaw

[Neil WyattModerator @neilwyatt]

Well give us as many measurements as you can and we will see what we can do.

If he doesn't mind changing the larger gear (and it fits) this is a cheap and excellent solution:

Spiral Bevel Gears

It's what I used and I have a spare pair ready for the next loco.

Neil

[Boiler BriParticipant @boilerbri]

Peter, thank you for the pointer.

Neil, I will post the sizes later on today. The gears are 1:1.

Brian

[Boiler BriParticipant @boilerbri]

I turned the blank up today.

Major o/d is 37.75mm

Face to the stub end is 45 degrees

The outward face is 55 degrees

Boss 19.05 diameter

Bore 1/2"

18 teeth.

It must be to some sort of standard?

Brian

[Boiler BriParticipant @boilerbri]

[Neil WyattModerator @neilwyatt]

The outward face is 55 degrees

If it's 1:1 but the angle is greater than 45 degrees then it isn't a constant – depth bevel, so making one isn't going to be easy

You might be better just making a pair of constant depth bevels?

Neil

[Jeff DaymanParticipant @jeffdayman43397]

Might be a good idea to make one in steel and one in cast iron rather than bronze, if the existing bronze one got chewed up. Pretty tough service to chew up a bronze gear. Probably a few more teeth would spread the load a little better. Alignment is critical in mitre gears too.

Good luck, JD

[Boiler BriParticipant @boilerbri]

I think that the meshing was not very good.

So how do I know what the teeth are?

Brian

[KWILParticipant @kwil]

Judging by the "actual" measurements you quote, the imperial "equivalents" are the more likely starting point, 1/2" bore, 5/8" boss diameter and 1 1/2" diameter overall (worn down or deburred a little). ??

[Anonymous]

The tooth form will almost certainly be based on the involute of a circle. The precise form of the involute will depend on the number of teeth on the gear. However, it is not quite that simple for straight tooth bevel gears. Bevel gears are normally designed using parameters on the outer face. So an 8DP gear will be 8DP on the outer face. However, the tooth form is lofted to a point where the axes of the two gears intersect. Thus the tooth form preserves its shape, but gets progressively smaller. It is not possible to cut a true bevel gear using equipment normally found in a home workshop. However, there are two approximate methods that can be used.

The first uses an involute cutter based on the DP at the outer edge, as standard. One pass is made, followed by two more, where the dividing head is offset and rotated slightly. This has the effect of widening the gap between teeth at the outer edge while leaving the gap at the inner edge the same. The DP of the required involute cutter is set by the parameters at the outer edge of the gear. The number of teeth used to select the cutter is not determined by the number of teeth on the gear, but by the number of teeth divided by the cosine of the pitch angle, 45º for mitre gears. This method gives the correct tooth form at the outer edge, but at the inner edge the tooth form does not have enough curvature. This has to be corrected afterwards with a file, or similar. So far so good, but the involute cutter is not standard. It has the same profile as a cutter used for an equivalent spur gear, but is narrower, so that it fits through the gap at the inner face of the bevel gear. These cutters are normally stamped 'Bevel'. As far as I'm aware they are no longer commercially available.

The second approximation, often used in the modelling world, is the parallel depth method, where the tooth depth is constant. Three passes are still required, but the critical point is that these gears are designed using the DP at the inner face of the gear, so standard involute cutters can be used. A significant disadvantage of the method when trying to retrofit gears is that starting with outer face dimensions usually leads to a non-integer DP at the inner face. Or, starting with an integer DP leads to outer face dimensions that are not 'correct' for the application. If you want to use the parallel depth method both gears will need replacing.

A third, less likely, method is to use a 4-axis CNC mill, which is what I did when I cut the bevel gears for my traction engines.

Andrew

Edited By Andrew Johnston on 06/09/2015 11:34:59

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