Gear Design Problem

[freerangequark]

Hi,

I spend my days designing modular buildings so this gear problem is a bit out of my realm of experience. I am looking for some help designing a gear set for an invention of mine. I know gear design basics, but this is beyond what I know. Any help would be greatly appreciated. The needed gears (to be made from plastic) are part of a kids toy that I am developing on my own time and with my own money.

For my prototype, I will need to make a 3D solid model in AutoCAD and will export it for fabrication perhaps via stereo lithography.

I need two identical gears which combine to create a beveled helical gear set. As far as I can tell, the relationship of these two gears will not permit a straight spline design.

Here are a couple of drawings (top view and front view) that will better describe what I need for this project.... (NOTE: The gears in the pics will not mesh and are only shown to describe the geometrical relationship of the two gears.

http://www.gsg.microminutes.com/images/gear

1 of 2.jpg

http://www.gsg.microminutes.com/images/gear

2 of 2.jpg

The angular dimension on the first drawing is 48 degrees.
The angular dimension on the second drawing is 12 degrees.

I found the equations for designing "Helical Gears for a given shaft angle with equal center distances", and I found equations for designing Bevel Gears. I couldn't find equations that covered a combination of the two.

I have found several programs that will help to create gear models, however none of them will handle this type of design.

So my question is how do I go about designing beveled helical gears?

Thanks for your help.


-Glenn

 

[insideman]

Could not get to the sites you suggested....
Quite interested in helping you with this. I have lots of toy and gear experience....

 

[freerangequark]

Hi,
Thanks for the offer to help.

Here are the corrected links...

http://www.gsg.microminutes.com/images/gear1of2.jpg

http://www.gsg.microminutes.com/images/gear2of2.jpg

-Glenn

 

[gearcutter]

It sounds like you need a Spiral Bevel set.

Ron.

 

[freerangequark]

Hi gearcutter,

Will this 48 degree spiral bevel set also give me the 12 degree angle between the shafts as seen in the second drawing I posted?

Thank you,
Glenn

 

[gearcutter]

I think I've misunderstood your requirements. Are you saying that there is a compound angle between the shafts?
Ron.

 

[freerangequark]

Hi Ron,

Thank you very much for helping me with this. I really appreciate it.

If viewed from above, the bevel angle is 48 degrees (drawing #1)

But as viewed from the side (drawing #2), there is a 12 degree difference in the shaft angles. (while still maintaining the 48 degree bevel)

It's hard to tell from the picture, but it looks like you may have included both of those parameters.

Do you have these as a solid model that I could manipulate in AutoCAD?

Thanks,
Glenn

 

[gearcutter]

Hi Glenn,
Sorry, can't include the 12*. Might be possible to cut though. Spiral angle on the screen shot I sent you is 30*. If perhaps we cut one at say 24* and the other at 36* it might work. Setting up spiral angles on Bevel machines is pretty much a "hit & miss" affair (on the older machines anyway). You'd have to try it and see. Can't send you a solid model, sorry.

 

[freerangequark]

Hi gearcutter,

Thanks for having a go at it. Unfortunately for my design, it is critical that the two gears be identical. I'll keep at it. This will be a relatively simple invention to prototype once I have that gear set.

Thanks again,
Glenn

 

[diamondjim]

Why don't you rotate both 12 degrees
to make it easier to simulate and
cut? You can always rotate the two
12 degrees at the final assembly.

 

[freerangequark]

diamondjim,

I am not sure I understood your suggestion, but it is important that the gears shaft axes differ by 12 degrees.

-Glenn

 

[diamondjim]

I am not certain if the 12 degrees
means that the centerlines of the
two gears are off apex and do not
meet at a common center. Is that your
intent? I could not tell if the
first was at 12 degrees and the second
at 60 degrees, ie 12 plus 48 but have
their centers on apex.

 

[freerangequark]

Hi diamondjim,

I fear that something is getting lost in translation. Since a picture is worth over 1000 words, I have posted a DWG file showing the gear geometry at

http://www.gsg.microminutes.com/images/gear_relation.dwg

NOTE: I have used circles to represent the gears so you can clearly see their point of contact.

-Glenn

 

[insideman]

Me again...
One type of gearing that works with skewed axes is called hypoid. This is usually for ratios far from 1:1 as in an automobile rear end. The geometry is quite complex. But I think I can work up something simpler that will work in a toy context.

Some questions:

1. speed and power.
2. size- like pitch and number of teeth. Or a range that woulkd suit your needs,
3. You want both to be identical. Why? If this is so both can be made in the same mold for production, we could temporarily abandon it to simplify prototyping.
4. You want to make them of plastic, which is understandible. But. again, the proto might be different.
5. You mention rapid prototyping. My experience with this has been disastrous. I don't believe it can make usable gears.
6. Remember that toys are assembled by morons with hangovers. Design accordingly! We want to come up with a design that will not require any critical adjustments during assembly.

 

[sreid]

I'm now a gear expert so not an authoritative opinion. Gears with non-intersecting, non-parallel shafts are called skew gears These gears are commonly made for shafts at right angles but even though the gears are seemingly identical they are actually left and right handed. I think by extension that your gears can't be identical either. Also, although there may be a simi rational way to design skew gears, I believe that the usual design proceedure is "design" one gear and design the second gear to "match" the first gear.

Any chance you could use two sprockets and use a ball chain to couple them?

 

[gearcutter]

Hi Glenn,
Another option is to use straight cut bevels with shafts at 48* (you don't have to use spiral bevels), this will make the gears identical; a mitre set. Could you then have the 12* shaft conect with its gear via some sort of universal joint? I can send you a screen shot of the gears if you like.
Cheers, Ron.

 

[insideman]

Freerangequark:

Hypoid gears are what you need. These have hyperboloidal pitch surfaces. But this bothers no one since it is possible to substitute a cone for the hyperboloid.

 

[freerangequark]

Hi,

Thanks for the replies and the continued help with this problem.

Per your suggestions, I did some more research on skew gears and hypoid gears.

I'll try to go into a little more detail here about the application which will hopefully serve to explain why my gear requirements are what they are.

The toy I am designing requires the specific gears I mentioned not as internal components which are required to achieve a certain type of motion or geometry, but because the gears themselves are what will make this product unique and interesting. They will be exposed, non motorized, and rely on the symmetry and simplicity of being identical in order to achieve a unique configuration which will make this an interesting toy to play with. It is for this reason that I cannot go with gears sets that are constructed from dissimilar components, and/or have other components such as u-joints and to achieve similar motions. The gear geometry and layout is every bit as important to this toy as is the final motion of the gears.

BTW, when I say "toy" I am referring to a novelty that is a toy in the same capacity as a Hoberman Sphere (

http://www.hoberman.com/fold/main/index.htm)

is a toy. Really neat to play with and such but doesn't really accomplish much in its toy form.

I'll get some info posted here about some of the specific design parameters.

Thanks again,
Glenn

 

[insideman]

Assume 16 teeth, 16 diametral pitch on the large face of the gears. Then major diameter = 1.125. minor diameter= .850, cone angle on pd = 44.262 degrees. Cone angle on major diameter = 49.176 degrees. Distance from face to tip of cones = 1.2239. Skew agle for teeth is 8.291 degrees.
This was determined by approximating the hyperboloid as a cone. This will work if gear thickness is not too great. /8 inch looks good.

These gears can be cut using standard gear milling cutters, but the depth of cut should be somewhat deeper than normal. About.01 to .015 seems right.

For a bigger gear, simply multiply dimensions by the ratio of new diametral pitch to old diametral pitch.

This will serve for prototyping. A somewhat different approach will be needed for production parts.

I have been amusing myself by edesigning the old Hootnanny gear toy, overcoming its shortcomings. Can't get a manufacturer interested....

 

[freerangequark]

insideman,
Thanks for the info... I think I may have just found a way to avoid using a conical gear all together. It will just require me to get away from traditional involute cut splines. This should be interesting.

-Glenn