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twopintsplease
05-20-2013, 06:36 AM
While checking out a gear on the chart on page56 of the Hercus cataloge

http://www.hercus.com.au/uploads/29161/ufiles/Hercus_Engineering_Cataloge_2008.pdf

the gears i was looking at appear 10DP or 2.5 MOD laying them on the chart

out of interest is there any real differance in strength or function or is this just going to start another metric verses imperial slanging match ?

Ed P
05-20-2013, 07:41 AM
While checking out a gear on the chart on page56 of the Hercus cataloge

http://www.hercus.com.au/uploads/29161/ufiles/Hercus_Engineering_Cataloge_2008.pdf

the gears i was looking at appear 10DP or 2.5 MOD laying them on the chart

out of interest is there any real differance in strength or function or is this just going to start another metric verses imperial slanging match ?

What does strength have to do with the decision? Either way I'm sure the gear(s) will be strong enough. It would seem to me that cost and availability would be the concern.

Ed P

kitno455
05-20-2013, 07:52 AM
Uh- meshing with what you already have would be the difference :) For that, you don't lay them on the chart- how do you know your printer made things exactly the right size? You need to measure your gear.

(number of teeth + 2) / outside diameter in inches = DP

Oh- and don't forget to check the pressure angle...

allan

Zahnrad Kopf
05-20-2013, 01:17 PM
They are similar, but not the same. 10DP would be 2.54Mod.

twopintsplease
05-20-2013, 05:15 PM
the gears i have are a pair i dont want to mix and match,

thanks Zahnrad "They are similar, but not the same. 10DP would be 2.54Mod. " explains why i could not conferm one or the other by laying on the chart

"For that, you don't lay them on the chart- how do you know your printer made things exactly the right size? "

the chart has a scale on it that you can compare ofter it has been printed to see if it is the correct size, i think its intended for a quick referance only not for precision measurements

"Oh- and don't forget to check the pressure angle..." the chart is for 20 Deg how is the pressure angle measured ? what is its affect on the gear ?

Thanks all for the help

Paul Alciatore
05-21-2013, 02:52 PM
I agree that with a small difference in DP or Mod you can not accurately determine which it is by just laying them on a chart. Using the equivalent Mod figure of 2.54 the difference is 0.04 or only 1.6%. This small difference is hard to see, but it WILL make a difference in the mesh so you do want to get the correct size for mating gears.

You are going to have to make an accurate measurement of some dimension, probably the OD if they are cut gears as there should be little wear there. Hopefully it will match or come very close to matching one or the other. The problem is, if the gears were made by some other process, like rolling, then the OD may not be accurately formed. If OD measurements are not conclusive then you will need to measure the teeth. These dimensions are given in tables.

I searched for "gear tooth measurement". Here are some measurement ideas that I found:

http://books.google.com/books?id=eVQPSr8nWekC&pg=PA480&lpg=PA480&dq=gear+tooth+measurement&source=bl&ots=5ZVy3e-XGR&sig=45ja3XZisGscemOkRgb3vw20MyA&hl=en&sa=X&ei=1rqbUZmgLoiNqwGTp4HQAg&ved=0CHQQ6AEwBg#v=onepage&q=gear%20tooth%20measurement&f=false

http://www.salemcompany.com/cgi-bin/Store/pdfs/InvGrsClSect8ed2.pdf

And many more results were returned in the search.

The problem with measurements across one or more teeth of a used gear is wear will change these numbers. Also, there are many cheats in gear making and these can also change many of these measurements, even for new gears. Perhaps the most accurate measurement you could make would be to the same face of two adjacent teeth as the wear would be close to the same on them and this dimension would be less sensitive to gear makers' "cheats". The first source I showed above has a mechanism for doing just this.

On the other hand, the 1.6% difference in size between the 10DP and the 2.5Mod MAY be small enough for the two different sized gears to mesh in a satisfactory manner in some circumstances. I would emphasize the last three words of the last sentence, "in some circumstances". This comes under the heading of gear making "cheats" as I talked about above. This would be for slow speed, low power, etc. situations where you could adjust the center to center distance between the gears for the best apparent mesh. Another example of this kind of "cheat" would be making three gears with different tooth counts (like 23, 24, and 25) that have the exact same OD and they do mesh. This kind of thing is possible, but of course, not ideal. It would lead to very confusing measurements of any sort on the gear teeth.

As for the pressure angle, as long as we are talking about full sized gear teeth, not stub gears, there is an easily apparent difference between 14.5 and 20 degree PAs and you should be able to determine that from the chart. The sizes of the 10DP and 2.5Mod gears are close enough that the difference in pressure angle should be obvious, even if you are comparing a gear of one standard against the image of the a gear of the other standard. You do want to use an image of a gear with the same number of teeth or as close to that as possible because the shape of the face WILL change significantly with the number of teeth.