Spur Gear - addedum modification/profile shift coefficient 2

[noobita]

Hi all,

If this is a repeated post, i'm sorry, but from my searches here in the fórum i couldn't find na answer to my doubt.

Just joined the forum (I'm an electrical engineer, so i don't have much experience in mechanical design) and the reason was because after reading more than 5 books about gears i couldn't find a proper answer (some are handbook of gear design and Shigley's Mechanical engineering design, and many more).

So, i need to design a gearbox of 5 stages, but my question will only be about one stage, because when i solve this i can do the rest.

Stage: 13 teeth pinion, 33 teeth gear, alpha = 20º

Of course there will be interference and from the books i read, to remove this interference the addendum modification coefficient is equal to x = (Zmin - Z)/Zmin, being Zmin = 2/sin(alpha)^2

According to the books, if Z1 + Z2 > 2* Zmin, then pinion's addendum = 1+ x and gear's addendum = 1-x. Else, if Z1 + Z2 < 2*Zmin, we need to do a shifting with center distance modification.

Accordingly to this equations, in my case the value of x would be 0.24. One exemple that i saw, would give to my x the value of 0.39. Now my question, is there any other formula, perhaps a better one (since this value was given by a program)?

Grateful

 

[tbuelna]

A 13T/33T spur gear combination is something I would not choose except as a last resort. The combination has a low contact ratio which will not be suitable for medium/high PLV. As you note a 13T spur pinion requires a fair amount of profile shift to reduce undercutting and prevent interference with the mating 33T gear. But there are also many other factors you might want to consider when optimizing your gear geometry. Normally both the pinion and gear profiles are adjusted to provide balanced bending fatigue life. Both the pinion and gear tooth profiles are also adjusted to provide balanced contact sliding for good efficiency and reduced potential for scuffing.

I don't know what rationale you used for selecting a 13T/33T combination, but you might consider a combination using greater numbers of smaller size teeth with a similar ratio. A spur pinion with at least 19T would perform much better.

 

[israelkk]

If you are an electrical engineer why are you trying to design a system that most mechanical engineer will find very challenging. To design such a system the mechanical engineer has to very experienced in many aspects of mechanical engineering subjects such as: stress analysis, dynamics, surface and heat treatments, etc. Then, he can enter the field of gear design with an experienced gear designer that will guide him. To my opinion what you are trying to do is same as learning to swim from books.

 

[noobita]

Thanks for your reply tbuelna. I need this number of teeths to keep my system small...Could you point out where i can find the formulas to calculate the coeficiente based on those parameters (improve sliding and teeth life)? From the books i read, the only formula i could find was the one i posted. I came to the forum to find those equations (i know there must be some to improve sliding and remove interference, read that in the books, but the formulas weren't there) and also to get your opinion and help

israelkk, like you said, you don't learn by books (but in this case i don't think your exemple will work, since all mechanical design is theorical and not pratical , you can construct a car without knowing nothing and just with a piece of wood and hammer - being practical), so, i'm trying to make a practical exemple - i will construct the gears and test their efficiency and durability, so i can improve in this field.

Any more help would be appreciate

Best regards

 

[israelkk]

This is the first time I hear that mechanical design is theoretical and not practical. I have 40 years of experience of R&D and design of custom designed mechanisms in my field of work off the shelf products were too bulky or unavailable. I always used calculations, analysis, dimensioning, tolerancing, defining surface and heat treatments, manufacturing and assembling but, never guessing. If you were an experienced mechanical engineer asking intelligent questions I could help you. But you don't even know what and how to ask the correct questions. Therefore, it will be a waste of time from my point of view. Wish you good luck.

 

[noobita]

Dear israelkk, from what you said, "I always used calculations, analysis, dimensioning, tolerancing, defining surface and heat treatments", I'm a little confused. In my opinion, calculations, analysis are theorical....not practical...since when math is a practical discipline? Physical Education is practical, not math (but we are only arguing about our interpretation of practic and theorical)

Like you said in your reply, i'm not an experienced mechanical engineer, and i told that in my post, the initial one. About not making the right questions or not knowing how to make the question, don't know another way to make it (from the terms i learned from the books), maybe "How to determine the profile shifting/ addedum modification coefficient (to maximize sliding and remove interference)?"
Best regards

 

[tbuelna]

AGMA standards are probably the best source for any analytical approach you need to optimize your gear designs. The AGMA standards provide all the equations necessary and instructions on how to use them. Just be aware that some of the analytical approaches used to optimize gear designs can get a bit complicated, so I hope you're good with math.

Start with AGMA 913-A98. And good luck to you.

 

[noobita]

Thanks for your reply tbuelna, math isn't a problema for me....could you point me out where i can find those docs? I'm not being able to find them at agma.org (already had searched for agma 207.07)...

 

[tbuelna]

noobita- Here is a brief discussion of the reasons for applying profile shift. Definitely worth a read.

The relevant AGMA standards you will need are AGMA 1003-H07 , AGMA 913-A98 , AGMA 901-A92 .

Since you noted that space is a concern with your gear drive design, I'd recommend starting with AGMA 901-A92 "A Rational Procedure for the Preliminary Design of Minimum Volume Gears".

Good luck with your project.
Terry

 

[noobita]

Thanks for your answer tbuelna, i'll see it and then post if i have any doubt