Modified Gear O.D. Calculation / Confusion
Modified Gear O.D. Calculation / Confusion
(OP)
A while back I was asking questions about calculating MOW from a given test radius. Thanks to "mfgenggear" I was able to put that into a nice TI89 calc prog that people at my shop use often. Now I am back wanting to add some more info to its output. I have hit a snag. Mostly because I am still learning about gears. Some Background. We have an old DOS program that spits some info out. Some of which I would like to duplicate to the TI calc. I can calculate everything this old DOS prog can with exception of a modified Gear I.D. or O.D.
Example: 23T, 20DP, 20PA. Modified C.T.T = .06526.
My calculation ( which is suspect ) for change in O.D. gives: 1.21351
The DOS program gives a value of 1.21757
And the Manufacturing Print actually calls out a O.D. of 1.219 +- .001.
So I'm trying to See which value is correct and what formula was used.
For mine I used the formula: C.T.T. = ( CP / 2 ) - ( 2 * O.D._Change * TAN(PA) ).
I Solved for the change in OD, Multiplied that by 2 and Subtracted that from the STD OD ( 1.25 ).
Am I on the right path? And I understand that the O.D. is not all the critical, but being math, I would like to know where there are three different answers for this.
Thanks.
Example: 23T, 20DP, 20PA. Modified C.T.T = .06526.
My calculation ( which is suspect ) for change in O.D. gives: 1.21351
The DOS program gives a value of 1.21757
And the Manufacturing Print actually calls out a O.D. of 1.219 +- .001.
So I'm trying to See which value is correct and what formula was used.
For mine I used the formula: C.T.T. = ( CP / 2 ) - ( 2 * O.D._Change * TAN(PA) ).
I Solved for the change in OD, Multiplied that by 2 and Subtracted that from the STD OD ( 1.25 ).
Am I on the right path? And I understand that the O.D. is not all the critical, but being math, I would like to know where there are three different answers for this.
Thanks.





RE: Modified Gear O.D. Calculation / Confusion
The profile shift formula is:
((tooth thickness) * (diametral pitch) - Pi/2) / (2 * tan (pressure angle))
The OD formula (for external gears) is:
(standard pitch diameter) + 2 * (profile shift) / (diametral pitch) + 2 / (diametral pitch)
The OD tolerance (external) on blanks from AGMA 917 is:
+ 0.5 * (total composite error) + 0.1 / (diametral pitch)
- 0
I'd suggest looking at the specs and going from there -- that's what I do. You might arrive at the same answers, but if future generations of employees have to understand your work, working from the specs might make all the difference.
RE: Modified Gear O.D. Calculation / Confusion
It seems you have reached the same size O.D. as I have though. So the question still remains, which is correct?
is it 1.2135, 1.2175, 1.219.
Thanks again
RE: Modified Gear O.D. Calculation / Confusion
RE: Modified Gear O.D. Calculation / Confusion
RE: Modified Gear O.D. Calculation / Confusion
can you please redo the last equation.
Let me say, very well done.
changesoul
you did very well also.
Occupant
What can I say I tip my hat to you.
All
Please find attached which is freely available on the web.
Mfgenggear
RE: Modified Gear O.D. Calculation / Confusion
RE: Modified Gear O.D. Calculation / Confusion
Profile Shift, from AGMA 913 Eq B.1
Nominal gear OD, from AGMA 917 Eq 21a
Outside diameter tolerance, from AGMA 917 Eq 62
RE: Modified Gear O.D. Calculation / Confusion
I've put the equations into a PDF instead (generated with www.codecogs.com online LaTex Equation Editor).
RE: Modified Gear O.D. Calculation / Confusion
I haven't found anything covering runout of the blank, which my company's gear drawings typically have. If you find anything on that, I'd love to hear it.
RE: Modified Gear O.D. Calculation / Confusion
RE: Modified Gear O.D. Calculation / Confusion
dinjin basically answered your question.
To my opinion both are viable calculations.
which I have not confirm if it is exact.
another calculation would be to verify at tight mesh & at the specified center distance
to verify there is no tooth tip interference with the tip (major dia) & the mating gear root radii.
I would suggest to get in more depth about gears, to to purchase design gear hand books
and read all you can stand.
israelkk
I believe tpederzani is inquiring about the pitch diameter runout,
& not the major diameter.
tpederzani
can you be more specific on you question
israelkk is correct about the major dia runout.
it can generally be what the designer requires, .001-.005" runout.
the pitch diameter depends on the function of the gears,
The AGMA 2000-A88 class will general dictate that.
Mfgenggear
RE: Modified Gear O.D. Calculation / Confusion
But with all these responses, i believe my question has been answered. Thanks to you all for your help.
RE: Modified Gear O.D. Calculation / Confusion
So im reading more on differences between backlash and tooth thickness, and I think I had one of them headaches with pictures.....you know, an thought.
So for example, using the same part specs I started this thread with.
I have this part, specs are:
Pitch Diam = 1.120 ( .03 smaller from standard)
Number Of Teeth = 23
DP = 20
PA = 20
MOW = 1.23405 ( .0864 wires )
So here is my thinking, and please tell me if I am starting to grasp this.
1: They call out a pitch diameter smaller than standard which tells me the gear has been reduced.
2: I should not assume, but for the sake of this I will. Ill assume half the pitch diam of 1.120 is the Gears portion of the Gear / Pinion Operating Center Distance
3: chugging out calculations, to thin the teeth enough to obtain a working pitch diam of 1.120, the teeth need to be thinned by .01092".
4: So this reduction in gear tooth is probably the same increase in pinion to allow the pinion to mesh at this 1.120 pitch diameter with the gear( tight mesh )
5: More chugging calculations show that with a tooth thinned by .01092" with .0864 wires my MOW is 1.240388, not the 1.23405 they ask for.
6: Now leaving the tooth thinned at .01092 putting me at a 1.12 Pitch diam, I add a bit of backlash into the mix, dropping my MOW down to what the print calls for. .01092 Tooth reduction, with .002405 backlash, I get the proper MOW.
Can I say that this gear has been reduced to operate at 1.120 pitch diam, with .0024 backlash?