Plastic Bevel Gear Design 8

[favorite53]

Hello all,
I am attempting to design a set of 2 plastic bevel gears to be injection molded. I have consulted the AGMA standards for bevel gearing and plastic gear design, but neither show how to design a gear tooth for a bevel gear. It seems to me that many bevel gears do not even have involute profiles. I have designed the main body of the gear fairly easily, and designing a basic rack is fairly straighforward, but does anyone know how to draw/design the gear teeth for a bevel gear?
Help is much appreciated.

 

[Occupant]

I would think if you have done it before - designing straight sided tooth profiles - it shouldn't be too difficult to generate a profile for a bevel gear looking at the attached, although I have never bothered to do it.

 

[3DDave]

Do you mean design or accurately model for CNC machining?

If it is the latter, then there are companies that specialize in plastic gear design. However, tooth deflection, stress distribution, and many other considerations are likely to need addressing and that's where experts come in.

Other than that they are involute, just not constant pitch across the face. I would expect that creating a curve at the smallest and largest part of the cone and linearly interpolating would be close. This may help:

http://www.atlantis-press.com/php/download_paper.php?id=2614

 

[spigor]

A spherical involute is not just a planar involute scaled along the pitch cone!
Most of the straight bevel gears do not use the spherical involute shape at all, they use the octoid shape instead.
Here is some general info:

http://www.gearsolutions.com/articl...ooth-contact-analysis-of-straight-bevel-gears

 

[favorite53]

Thank you for the input. From what I am gathering, I cannot simply apply the involute curve onto the angle of the bevel gear faces, and that there is no real standard for bevel gear teeth profiles?
If true, what is the best way to go about designing the teeth with no standards?

 

[Occupant]

I posted the picture in my previous post to show what a rack for a straight bevel gear would look like. However, in this case the rack is in circular motion as opposed to the (this time) attached picture. I have used this method to create models of regular gears and think it could also work for bevel gears with the added complication that the teeth would have to be tapered same as the circular rack.

 

[gearcutter]

There are in-fact standards for bevel gear profiles & load rating, standards are based on the brand of machine that is being used to generate the profile, eg: Gleason, Klingelnberg, etc.
Within each manufacturer; there are different machines that generate the profile in different ways.
So, as a gear designer, you need to be specific about which manufacture's profile you intend to use...............they are not interchangeable.

If producing the profile by molding; then you are free to use a theoretically correct profile..........with modifications for the molding process.
The main thing you'd need to consider is how the mold is going to be manufactured...........so speak to your manufacturer before embarking on your design.

Gear Geometry and Applied Theory, by Faydor L. Litvin, shows how to mathematically reproduce several of the more common bevel gear profiles.

 

[spigor]

I cannot simply apply the involute curve onto the angle of the bevel gear faces

It would not be mathematically correct, however, in case of plastic gears might be acceptable or even a common practice.

Here's some nice info about the shape of teeth:

https://books.google.com/books?id=i...v=onepage&q=spherical involute octoid&f=false

there is no real standard for bevel gear teeth profiles

There are three main standards- octoid, arc and spherical involute. Octoid has sub types- e.g. flat root or concave root, line and/or profile crowning, generated or formate. Arc is typically used with bigger profile angles and used for differential gears, tooth shape comes from the tooling manufacturer. Spherical involute is easy to model, general (spherical involute can be defined with few parameters, just as the planar involute) and is used by flank generators for the end milling.
The real geometry comes from the teeth making technology, the main thing is that the meshing should be correct in terms of contact pattern and transmission error.

The gear shop will typically make the electrode for EDM with the technology available to them, so the exact tooth shape is not your concern, right?

 

[favorite53]

Wow thank you for all the posts! The exact tooth shape is extremely important, because the mold maker will be making the mold directly from the resulting CAD model. This is why figuring out how to solid model the gear tooth is so critical to me. As of right now I have applied a planar involute gear tooth onto the outer angled face of my bevel gear. It seems so far that everything but the gear tooth is straightforward to model and find standards for. I think my next step will be to use Tredgold's approximation and see if the resulting teeth are noticeably different.

 

[spigor]

The Tredgold's approximation is using a planar involute to approximate the shape of the teeth of a straight bevel gear, so you should not find any difference. As already mentioned, this approach might be ok in case of plastic bevel gears.

 

[Pud]

Bear in mind you will not be able to have any undercut at the tooth root by injection moulding.

H

www.tynevalleyplastics.co.uk

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[tbuelna]

It is true that providing your tooling vendor a precise digital model of the as-molded gear tooth working surfaces is very important. But to get the best performance from your plastic gear at the lowest cost you will need to make modifications to the standard tooth surface shape, and these tweaks will be incorporated into your injection mold tooling. It doesn't cost any extra to add these mods to the initial tooling, and they can provide significant improvements in the gear's performance. But any subsequent changes to the injection mold tooling will likely be quite costly, so it is best to get the gear design right up front.

 

[Pud]

Guide to plastic gears from GE (attached, .pdf).
Might be of some use.

H

www.tynevalleyplastics.co.uk

Politicians like to panic, they need activity. It is their substitute for achievement.