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Un-standard Gear module and Pressure angle

spe4ker

Plastic
Joined
Aug 15, 2017
I have a 2 stage planetary gearbox for which I need to know the geometric specs of each of the gears, the problem is that the formulas don't seem to give me standard results.

For one sun gear of the first stage I measured the outer diameter with a caliper and got:

OD 16mm
Number Teeth 24

Then, applying the formulas I got:

PCD 14.76923077
MOD 0.615384615

For which the closest standard module to this result would be 0.6, but I'm getting 0.615, I don't know if there are non-standard gears for which the module can be this result.

Then, for calculating the base and root circle diameters I tried the 2 most common pressure angles, but when I drew them on a scaled picture I took of the gear, they don't seem to be correct.
Alpha 20, BCD20 13.87853717
Alpha 14.5, BCD14.5 14.29879592

RCD 13.23076923
Sun24gear.jpg

My main question is that of the module of the gear, which I'm not getting a standard result, how could I find out? Thanks.
 
.6 mod and 24 teeth have a standard OD of 15.6mm. My 1st wild guess is the gear was designed with some profile shift to satisfy a center distance requirement or to improve the tooth contact. Not all gears are designed to standard ODs etc. It's actually quite common to see this in real world applications.
 
profile shift

.6 mod and 24 teeth have a standard OD of 15.6mm. My 1st wild guess is the gear was designed with some profile shift to satisfy a center distance requirement or to improve the tooth contact. Not all gears are designed to standard ODs etc. It's actually quite common to see this in real world applications.

thanks! actually you are right, i contacted the manufacturer of the gearbox and they were kind enough to give the next details
Tip Diameter 12.039
Number of Teeth 17
MOD 0.6
Preasure Angle 20

For which tip diameter I assume they mean the outside diameter? also, using the normal formulas I still can´t correlate the results with what I measure with a caliper, so I think you are also right about the profile pitch. How can I calculate all the other dimensiones from this data?
 
.6 mod and 24 teeth have a standard OD of 15.6mm. My 1st wild guess is the gear was designed with some profile shift to satisfy a center distance requirement or to improve the tooth contact. Not all gears are designed to standard ODs etc. It's actually quite common to see this in real world applications.

This. It's actually more common than not, in Planetaries. It is also exceedingly common to modify the Ring, as well. Planetaries are RARELY comprised of anything in stock form. We have done ( and continue to do ) a lot of multi stage Planetary trains and it would surprise most people just how much effort actually goes into them in contrast to just about any other train. Especially when designing them and prototyping them.

Frankly, I'm quite surprised that the manufacturer actually gave you the specifications. Consider yourself extremely lucky and blessed. Maybe even go play the lottery. I have honestly never seen that happen in all the time that I have been doing this.

Good luck.
 
Further to long addendum.....

Normal pitch dia of 17 tooth .6 mod is 10.2 .....and normal tip dia is 1.2 added on to that...because standard addendum is .6

Your 12.039 is 1.839 over the 10.2.... making the addendum .9195

The far out part here is that the 24 tooth ALSO has a long addendum
 
The far out part here is that the 24 tooth ALSO has a long addendum

Likely not so far out, John. OP doesn't say which gear that is. Could be a Planet. Could be another Sun. ( he did say it is multi stage ) Further, there are definitely times where both Sun and Planets benefit from positive Profile Shift. It really just depends on the how all Three mesh, and application's requirements. It is almost always necessary to do so in effort to maintain Contact Ratios and control Approach and Recess Actions. One of the reasons that for single direction boxes, Asymmetrical Tooth Forms really shine and pay off the efforts.

Dudley really is the go-to on this.
icon14.png
 
Yes, measuring the od of a gear to get the module is often incorrect as you are assuming you know the addendum before hand. Not sure about planetary gears, but the correct way is to measure the center distance, count the total number of teeth and calculate the module.
 
Yes, I think I was quite lucky that they gave me the specs, although they didn't give me all the dimensions, unfortunately. SO I'm still trying to figure them out, since I'm doing vibration study on planetary gears, and the gearbox I bought is a two stage, so I plan to 3d print the ring gear so I can try and measure the vibrations of each set as an individual planetary single stage set. That's why I need the exact dimensions to try and replicate it and print it, even if the mechanical properties will obviously not be the same, without even considering the geometrical errors, but is part of my current project. What´s the best software to design them? I know solidworks has a toolbox...
 
So far...

So far the dimensions I´ve calculated with the info I have are the following (Second Stage):
eTK8CQ

1 — imgbb.com

The problem is that since most dimensions depend on the center distance,and I don't have the correct equipment to measure it, being a quite small gear set a slight mistake in the dimensions I think still gives me a very different result, could you please advise me if my calculations are correct and what I can do to improve them? I also attach a picture of my gearset:

42case.jpg2017-08-16 16.48.10.jpg
 

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It is in the link in his post. Not of any real consequence, though as they are his own calculations. Not supplied from the manufacturer. It's all standard stuff. I will offer some kudos, along with some constructive criticism for the OP - I congratulate you on thinking far enough ahead to consider the Working Pressure Angle! Good job. However, I believe your are shooting yourself in the foot and also nullifying the very experiment you seek to employ by using a 3D printed Ring Gear. You could not do much worse if you tried. Frankly, any data you derive will be wholly irrelevant because of it. You'd be much better off removing one of the stages and taking your measurements that way.

As for what you can do to improve things, everything you need is actually right in Dudley. He did most of this work already, decades ago and specifically with regard to Plantaries in automotive transmissions. In fact, if you drive an automobile, you can likely thank him for it's smoothness and robustness.
 
It is in the link in his post. Not of any real consequence, though as they are his own calculations. Not supplied from the manufacturer. It's all standard stuff. I will offer some kudos, along with some constructive criticism for the OP - I congratulate you on thinking far enough ahead to consider the Working Pressure Angle! Good job. However, I believe your are shooting yourself in the foot and also nullifying the very experiment you seek to employ by using a 3D printed Ring Gear. You could not do much worse if you tried. Frankly, any data you derive will be wholly irrelevant because of it. You'd be much better off removing one of the stages and taking your measurements that way.

As for what you can do to improve things, everything you need is actually right in Dudley. He did most of this work already, decades ago and specifically with regard to Plantaries in automotive transmissions. In fact, if you drive an automobile, you can likely thank him for it's smoothness and robustness.

Thanks a lot for your comments! I´ll have a look at Dudley´s text. I knew my data would be somehow bad by 3d printing, but since it's so easy to get stuff printed I thought I tried anyway and see what I got. I was thinking of separating the stages and measuring like you said, and perhaps for another set of measurements only printing the ring gear.
 
thanks! actually you are right, i contacted the manufacturer of the gearbox and they were kind enough to give the next details
Tip Diameter 12.039
Number of Teeth 17
MOD 0.6
Preasure Angle 20

For which tip diameter I assume they mean the outside diameter? also, using the normal formulas I still can´t correlate the results with what I measure with a caliper, so I think you are also right about the profile pitch. How can I calculate all the other dimensiones from this data?

Hi all,

I'm still struggling to make correct dimension calculations of my gear set, there is just so many things to take into account. I also made some mistakes in the last picture I sent.
Right now, following Dudley's book, I'm calculating the dimensions of the ring gear, but as you know, the main data Is the one I gathered from caliper measurements so they are not exact, like the center distance.

I just want to confirm if for a set of planetary gears, Including sun , planets and ring, the working pressure angle should be the same for all gears, because there are formulas to calculate for external gears, and separately for internal gears, and I get different pressure angles for each case.
 
I just want to confirm if for a set of planetary gears, Including sun , planets and ring, the working pressure angle should be the same for all gears, because there are formulas to calculate for external gears, and separately for internal gears, and I get different pressure angles for each case.

The specified Pressure Angle for all three will be similar, but the Operating Pressure Angle can very easily differ between Sun/Planet and Planet/Ring combinations. Without the manufacturing data, you will be forced to measure and interpolate. This can be easily visualized by realizing that the two combinations have separate sets of calculations and Ring Gears Cery commonly have modifications to their Tooth Soaces. ( you will note that I did not say Tooth Thickness ) Good luck.
 
Differing pressure angles with the same planet gears seems like a contradiction. But you did say operating PAs. The only thing I can think of is that the gear teeth are cut to a standard PA but adjustments to the pitch diameter and therefore to the tooth spacing or thickness caused by those PD changes would cause those standard teeth to actually work/meet at a slightly different PA. Is this what you are referring to? Or what?
 
Differing pressure angles with the same planet gears seems like a contradiction.
You guys gotta get over this. "Pressure angle" the way you are using it is a fiction. Gears ONLY have a pressure angle in relation to another gear. That's why you can cut 20* PA teeth with a 14.5* PA hob, and so on. A rack has a pressure angle but a gear, by itself, does not.
 
You can cut 20 degree PA teeth with a 14.5 degree hob? HOW?

Seriously, how do you do that?



You guys gotta get over this. "Pressure angle" the way you are using it is a fiction. Gears ONLY have a pressure angle in relation to another gear. That's why you can cut 20* PA teeth with a 14.5* PA hob, and so on. A rack has a pressure angle but a gear, by itself, does not.
 








 
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