Gears: Addendum modification

You need to specify the center distance between the two gears, this will define the addendum modification sum (sigma x). Once the sum has been defined, then you will still need to choose how to divide the sum over both gears.

If unsure about center distance and/or distribution, any addendum modification between -1 till 1 will do, but then your center distance will alter likewise.

Specifying your addendum modification will also have an influence on tooth bending, pitting and bearing loads as adding addendum will also change your working pressure angle. A lower addendum modification value may result in undercutting, but you probably know the theory around this.

So there is no straighforward answer to your problem as the definition of addendum modification coëfficients is a complex matter for which a lot of software is available (but rather expensive).

My recommendation would be to start out with a value of the addendum modification for pinion and wheel and check whether the gear you end up with answers the bending strength and pitting requirements of your problem. Also, check the resulting forces (Ft, Fr and Fa) and make sure your bearings are OK with these loads.

Thank you for that helpful response.

Is it possible to only have the addendum modification for the gear? since the pinion is also used for the other models we have(and it has to mesh with another gear for which is in good mesh at its current condition w/o the addendum modification). And the center distance is fixed at 21mm.

Thanks in advance!

Let's see...

u = 60/18 = 3.33333... = ratio

dw1 = 2*a / (1+u) = 9.692 mm;

dw2 = 2*a*u / (1+u) = 32.308 mm;

d1 = m*z1 = 9 mm;

d2 = m*z2 = 30 mm;

then alfawt = arccos(d1 / dw1) = arccos(d2 / dw2) = 21.787 deg;

then sum_of_addendum = sumX = (inv(alfawt) - inv(alfa)) * (z1 + z2) / (2 * tan (alfa)) = 0.487.

I presume your gears have an addendum of 0, so then the addendum of the wheel will be 0.487 - 0 = 0.487

These formulae can easily be found in any decent textbook on gear design (Roloff Matek, Niemann-Winter, Linke...).

I presume the PINION to have an addendum modification of 0, of course. I wrote "the gears", which is incorrect.

..ok..thanks really..

..so once you have the sum of addenda,the next thing to do is allocate/divide the value between the gears in mesh right?(in cases where one does not a zero addendum modification).

..thanks! great help..

one more question:

where did you get the formula in computing the working pressure angle?(alfawt)

because the other formulae i got gives a value of 29.4 and not 21.7..

Thanks in advance!

ok..disregard my last inquiry..

already found it..those that give 29.4 only make use of the change in center distance and not incorporate the shifted coefficients..

Thanks!

Damned... I forgot a factor. You are right, there is an error in the above.

alfawt = arccos(d1 * cos(alfa) / dw1) = arccos(d2 * cos(alfa) / dw2) = 29.241 deg;

then sum_of_addendum = sumX = (inv(alfawt) - inv(alfa)) * (z1 + z2) / (2 * tan (alfa)) = 3.704.

I presume your gears have an addendum of 0, so then the addendum of the wheel will be 3.704 - 0 = 3.704

This puzzles me as this is a very big profile shift for one gear (even for the sum of both).

I hope I didn't forget something else. d1 * cos(alfa) = db = the base circle. This is indeed the needed diameter to calculate the working pressure angle.

Sorry for the mixup, entirely my mistake.

..that's the same value we're getting..and we are having trouble with it since it is really a large value..even if it will be split between the gear and pinion, the values for each would still be large..

Well then, you only have few options left: change the number of teeth or change the center distance.

You might want to keep the number of the pinion at 18, but increase the number on the wheel to a value of 63 or 64. That will alter your ratio a bit, as will the diameters and more important in this case: profile shift. 64 teeth on the wheel will give a ratio of 3.556 (instead of 3.3333, that's not a big difference...) and gives a profile shift sum of 1.085.

Decreasing center distance will also be a big help. With 18 and 60 teeth, changing from 21 to 20 mm will bring profile shift back to a value of 1.089, that's a quite normal figure for the sum of profile shift.

Both solutions are of course application dependant. Pure technically spoken, there's not really a problem on using high profile shifts apart from higher sliding velocities and severely increased friction. Also bearings will have to work a lot harder. If these effects are considered and you have sufficient amount of tooth head thickness, you could consider using the high profile shift if no other option is available.

Look at the bright side, a very high bending resilience. :-)

The downside: possibly very bad resistance against pitting, so you need to do a decent rating calculation (DIN3990, ISO6336 or AGMA2101-D04 (or AGMA2001 if you're an American citizen)).

.the problem is this..the center distance is fixed at 21 and the number of teeth for both gears must be retained. the output of the product depends on the determined gear ratio.

..we tried getting the actual specification of the gear (with the sample we have) using the tooth span method and shifted coefficient for the gear is at 0.6 and the pinion is at -0.26..

..but we can't reconcile these results on the theoretical calculations..

HI ,

i JUST NEED ANSWERS TO THE FOLLWING QUESTIONS ON GEARS ..CAN ANYONE PLAESE HELP & EXPLAIN THE ANSWERS.

1) WHY WE USE INVOLUTE FOR GEAR TOOTH?

2) WHY THE GEAR TOOTH CAN BE ASYMMETRIC MOST OF THE TIME?

3) WHERE THE GEAR TOOTH MUST BE SYMMETRICAL?

4) HOW THE BASE CIRCLE DIA. CHANGES WITH ADDENDUM MODIFICATION?

5) what is a PCD define in terms of geometrical features?

PLAESE REPLY ASAP. as i need this answers for my college presntation..

thanks in advance..

Regards

Khadar

Please understand two things

a) CR4 is not a homework site. If it is for college presentation, you have to study.

b) Do not shout. Caps lock are very impolite.

All the questions that are asked are the basics of the gears. read any book on mechanical power transmission , tha chapter on gears.

Additionally find out

What is addendum ? what is the opposite (d.... dum)

Why modify these ?

What is PCD (full form ?)

...

I am extremely sorry Mr. SB ..for being impolite with capitals & being offence. I regret for that.

well, This is nothing to do with my homework aswell. Ya may be these are the basic stuff which would have got in text books, But to be frank , these are questions by from my prof. which none of us could find answers @ books, thats the reason i have asked if any of you would kindly help searching for .(only if impossible)

Finally, PCD is pitch circle diameter

Addendum is -The radial distance from the pitch surface to the outermost point of the tooth

Deddendum - The radial distance from the depth of the tooth trough to the pitch surface

Finally, again these extremely sorry for being offence with capital letters.

Please if any body can help me get these answers.. will surely be greatful

Regards

Khadar

1) The profile of inviolute, if you study the locus, is the generation of the point of a circle. This propert means thet as the gears mesh, the velocity on the surface is such that the teeth of the gears will roll over each other and not slide. And no sliding means no wear out.

The circle on which this involute is generated is the pitch circle of the gear.

When the two gears mesh, the pitch circles of the two must be aligned. This is done by maintaining the center distance between the axes of the pairs

CD = (PCD₁ + PCD₂)/2

Chsck the following links i could get quickly by google

http://www.cs.cmu.edu/~rapidproto/mechanisms/chpt7.html

http://science.howstuffworks.com/gear8.htm

The second one has a gif image for you to understand the rolling motion.

and this one has about unequal gears (addendum modified) on page 3 along with other explanatory check

http://nptel.iitm.ac.in/courses/IIT-MADRAS/Machine_Design_II/pdf/2_4.pdf

Hope this will answer all your question and additional.

thank U mr.Sb ,

For the helpful articles..really greatful to u for the help.

Regards

Khadar

involute = continous motion (Euler)

there is sliding and friction between the 2 flanks. and it's variable depending of the contact point. otherwise, agreed.

Bogdan

your above quarries is absolutely incorrect... bcz the shifted amount of pinion is 0 & gear have been shifted,,, its create lot of confusion to design subject pear of gear..