Fairfield gear program 4

[Windward]

This is a fine program, free to anyone. There is one small flaw in it that they didn't notice. When analyzing spur gears, if the contact ratio is greater than 2, some of the results will be nonsense. The Fairfield product engineer says:

"Unfortunately, our program doesn’t flag that condition, but AGMA formulas for stress handle contact ratios from 1 to 2. Above 2 is a good condition to have, but the stress data would be invalid."

Since contact ratio applies only to spur gears, there is no problem when analyzing helical gears.

 

[tbuelna]

Windward-

Contact ratio applies to helical gears as well. However, contact ratio in helical gear meshes is evaluated differently than with spur gear meshes. A helical gear mesh has multiple points of tooth contact across its face, so helical gear meshes consider both profile contact ratio and face contact ratio. With helical gear meshes operating at high PLV, as a general rule the face contact ratio should be 2.0 or greater to help mitigate dynamic loading effects.

With most external spur gear meshes it would be unusual to have a contact ratio above 2. To achieve a contact ratio above 2.0 with external spur gears, it would require a situation like very large numbers of teeth on both gears and/or very low pressure angle. A spur gear mesh consisting of a pinion and internal ring could also result in a contact ratio above 2.0.

In my opinion, Fairfield is a reputable and knowledgeable gear company.

Regards,
Terry

 

[Windward]

Thanks for the information, tbuelna. I was analyzing a spur gear pair, 36 teeth each, DP20 and 14.5 PA - Boston change gears - when the problem appeared. Fairfield spent the time to figure it out, even though they don't support the program anymore. It is a good outfit.

Shigley on helical gears:

"Because of the nature of contact between helical gears, the contact ratio is of only minor importance, and it is the contact area, which is proportional to the face width of the gear, that becomes significant."

Is Shigley's contact area the same as your face contact ratio? I also have Buckingham's Analytical Mechanics of Gears, so I suppose I ought to figure this out myself.

I have never had a problem analyzing helical gears with the Fairfield program. But now I not sure that it couldn't happen, since this contact business also applies to them. The Fairfield product engineer did not go into it. Maybe I will ask him about it later. Here is an a longer quote from his email:

"We aren’t really supporting the DOS or Windows versions that are available on the website but after reviewing the code, I realized that the involute contact ratio for your set is over 2. Unfortunately, our program doesn’t flag that condition, but AGMA formulas for stress handle contact ratios from 1 to 2. Above 2 is a good condition to have, but the stress data would be invalid. I think if you modify the data slightly to get contact ratio just below 2, the stress data should be consistent for you."

 

[tbuelna]

"...Because of the nature of contact between helical gears, the contact ratio is of only minor importance, and it is the contact area, which is proportional to the face width of the gear, that becomes significant...."

With all due respect to Mr. Shigley, I would disagree with this statement. With helical gear meshes, the face contact ratio is important due to the number of teeth that have some contact at any given instant, rather than the total contact area. When you have more than 2 gear flank contacts sharing the tangential force being transmitted, like that existing in most helical gear meshes, the transfer of force is much smoother. This is why helical meshes, spiral bevel meshes, etc, are much smoother and quieter in operation than spur gear meshes.

In regards to your question about contact ratio versus contact area, they are not the same thing. For a given face width, a spur gear mesh will usually have lower contact stress than a helical gear mesh.

Hope that helps.
Terry

 

[Windward]

I am beginning to see the light. I will study Buckingham's explanation of face contact ratio, which he defines as

"the ratio between the helical advance on the pitch cylinders of a pair of helical gears across their active face width and the circular pitch at the pitch radius in the plane of rotation."

You know I couldn't make that up. He continues:

"This ratio must be greater than unity to obtain continuous helical contact on a helical gear drive."

As you said. Thanks for a good discussion.

 

[Windward]

Terry, if this is too personal for this forum, I apologize. Are you the inventor of Torque Balancing Gearbox, US Pat 8,231,503?

 

[tbuelna]

Windward- Yes I am. A star for you.

Incidentally, the title of that patent does not really provide the best description of the concept behind the gearbox. But I was not involved in drafting the document. The principle behind the gearbox was using a high degree of torque distribution at the output stage to minimize weight. If you look closely at the patent images, you'll see that the output stage has close to 2 dozen planet gears. As you might imagine, it is very difficult to get almost 2 dozen planet gears to load share within a reasonable range. The gearbox concept is an extreme example of what is possible with regards to torque-to-weight. It is fairly complex and would likely be expensive to produce, but the actual transmission the patent is based on was designed for very high reliability and long service life.

 

[Windward]

Terry, this introduces two subjects, torque balancing gearboxes and continuously variable transmissions, for which you also have patents. I will start new threads on these topics after reviewing what has already been posted.

best regards,

George Fleming

 

[tbuelna]

Windward-

It's funny you mention the CVT. A while back I was working on the design of a multi-MW version of that CVT for wind turbines. I had contacted Fairfield (Lafayette) to get quotes on manufacturing some of the components for the CVT. A couple weeks later a sales engineer from Fairfield called and said he would be traveling through my area, so we set up a meeting. We spent a couple hours at a local pub drinking beer and talking gears. He was very knowledgeable about manufacturing large, high-performance gears and I learned quite a bit from our conversation.

As for a thread on CVTs, I'm pretty sure the topic has already been thoroughly discussed. I'd be happy to discuss the subject at length privately with you, but since posting personal information is not permitted on this forum you'll have to figure out how to contact me.

Regards,
Terry

 

[Windward]

Terry, I will get in touch, have a lot to discuss.

There are 171 posts on cvt's on this site, so I will not be taking the time to look through them now. I couldn't find the one I wanted. Someone asked whether there was a cvt that could convert constant 1500 motor rpm at several thousand horsepower to a range of 10,000 - 15,000 rpm. I don't believe anyone solved that one.

later,
windward