Inspecting a Spur Gear

[swertel]

I'm just starting the necessary research to figure this out, eng-tips being one of my first stops so forgive me if I don't phrase all this correctly.

We have a molded plastic part with an
AGMA 20deg Full Depth Involute Fine Pitch Spur Gear
AGMA quality 8
50 Diametral Pitch
.321 Pitch Diameter

This is a first article on this part so our inspection department is checking every single feature. They came to me asking how to check the pitch diameter, quality number, and all that. "It's got a number [dimension], therefore it must be inspectable."

Well, I know what all the numbers mean in terms of size, but I don't know how to physically inspect them. I'm trying to get a copy of AGMA standards and any other spur gear references that may define inspection techniques. In the meantime, anyone have some direction for me?

Thanks,

--Scott

http://wertel.eng.pro

 

[drawoh]

swertel,

Dudley's Gear Handbook from McGraw Hill, has instructions on how to inspect gears. Verifying AGMA_8 quality could be a stretch. You need to measure the gear teeth over pins if your inspection shop is not well equipped.

JHG

 

[swertel]

One of the questions from the inspector was...

"What pins should I use?"

Does that give you a hint?

Thanks.

--Scott

http://wertel.eng.pro

 

[MintJulep]

See thread1103-208356

 

[MintJulep]

Did you know that there is a Gear and Pulley forum here on Eng-Tips?

forum406

 

[israelkk]

When you say AGMA it is not enough. Is it AGMA 390.03 (same as 2000-A88), AGMA 390.02, etc. The AGMA apec for gear accuracy was updated and changed with the years. When someone specify AGMA spec he has to specify the desired AGMA apec version. The AGMA accuracy number usually comes with modifiers. AGMA Q8X for example. The Q preceded the number means AGMA 390.03 (2000-A88). AGMA A8X where the A preceded the number refers to the newest AGMA 2015. AGMA 8X refers to AGMA 390.02. The X after the number can be A, B, C, D, E and refers to the amount of tooth thinning to assure minimum backlash at standard center distance.

All the values in the AGMA spec for measurements are alternating values around the actual average size of the gear. However, you can have any size gear with the same accuracy. This is where the dimension over pins used to come. A better and easier way is to measure with a master gear tester the "Testing radius" of the gear.

So the gear drawing either has to specify minimum and maximum testing radius or the minimum and maximum dimensions over pins. To correctly check a gear with pins it has to done on all the teeth of the gear (time consuming). With a master gear it is just one rotation of the meshed gears recording and comparing to the theoretical ideal gear dimensions.

 

[drawoh]

swertel,

You inspect gears by inserting pins between the teeth and measuring over the pins. Your inspector's question is intelligent, and it is answered in Dudley's Gear Handbook.

JHG

 

[dimjim]

I am curious why you did not select a standard 48Dp instead of 50Dp. 48 or 64 are more common for fine pitch gears. Maybe that is the reason why your inspector is asking the question. Pins for the 48Dp system might be more readily available.

 

[tbuelna]

swertel,

Since you have a molded plastic gear (and I'm assuming there are no further operations performed after molding other than removing flash, gating, etc.) then you should probably inspect a representative first article (or articles), and after that you can assume that all parts produced from that same tool are acceptable. With occasional spot checks of course.

Unfortunately, dimensionally validating a molded plastic, 50 DP, 16T spur gear to AGMA class 8 may require a little creativity on the part of your inspector. It is difficult to achieve even class 6 or 7 tolerances with a strictly molded plastic gear, but I suppose it can be done:

https://sdp-si.com/eStore/..\ss\PDF\79001084.pdf

As some have noted, pitch diameter is easily checked by measuring over gage wires. Two wires for even no.'s of teeth, and three wires for odd no.'s of teeth. A company called Van Keuren used to publish a very handy set of tables listing these measurements:

http://www.vankeuren.com/gmw.aspx

israelkk suggested using a master gear and rolling checker to inspect your gears. A single flank roll thru against a master can check for runout and tooth errors. For a large production run of machined gears this would be the way to go, because you are constantly checking samples to make sure your machining process is within spec. But for purely molded gears, the accuracy of the gear is a function of the tool, so the cost of a master gear may not be justified.

While I'm sure your QA people are good, inspecting gears is a rather specialized skill. So if your in-house inspectors don't have any experience doing it, you may wish to (tactfully) suggest that you sub the work out to a local qualified gear house. They'll have all of the necessary inspection equipment and trained personnel to do the job right.

Finally, you will need to specify exactly what finished form you need for the gear tooth geometry. Simply specifying ANSI/AGMA 2000-A88 Q8 is not really enough. Your engineering documentation should include a profile tolerance chart that defines where the active profile begins and ends, as well as any tip/root modifications. You also need to specify edge break limits and surface roughness requirements.

Be very specific and detailed in what your engineering documents require, and leave nothing open to interpretation. Otherwise, you'll likely end up paying for parts that may not be what you want.

Good luck.

 

[swertel]

Thanks for all the links and info, I'll be spending some time reading those now that I'm back from the holiday weekend.

To try to answer all your questions.

1) We recently acquired the company that makes the system this part is a component of. In order to integrate the company, and its parts, into our quality system, we need to run a first article of everything. It's just the paper trail to cover our rears in case of an audit. We're also changing some vendors around, so although we own the mold tooling, our SOPs require us to first article for a new vendor.

2) The drawing was originally done in Feb 1993 with a revision in Nov 1993. No updates since then. That may explain the AGMA callout that doesn't include all the necessary information for today's standard.

3) I should have known there was a gear/pulley forum. But I'm going to hold off on cross-posting since there is a discussion happening here.

4) The parts have worked to date, so I'm not concerned with how specialized the inspector is. I'll also have to be cautious if I revise the print to add the missing gear information. I'd hate to specify something that can't be produced by this mold, or that would require a significant rework to the mold.

--Scott

http://wertel.eng.pro

 

[israelkk]

swertel

The fact that the drawing was originally done on 1993 doesn't explain the missing modifier/s on the AGMA spec as appear in the drawing. AGMA 390.03 was the current version since 1980. According to my experience I suspect that the designer either didn't really read the spec and the AGMA design handbooks but used the data from gear design handbooks such as Dudley's GEAR DESIGN handbook that was published on 1961. One more option is that the designer used the PIC, BERG, SDP or similar catalogs as a reference were the same error of missing trailing modifier in the description of gears accuracy specifications is/was exist.

 

[swertel]

Mint, just be sure to keep your sarcasm in that thread.
(Sorry, I couldn't resist because I didn't see any sarcasm in your initial response but I loved the flame and follow-on banter.)

Is there any rhyme or reason to the standard numbering system for AGMA? For example, 1006-A97 and 1106-A97 both are titled "Tooth Proportions for Plastic Gears." What's the difference and do I need to order both? The list of specs I plan on ordering to get a better understanding of AGMA requirements is getting pretty long - both expensive and time consuming to read.

--Scott

http://wertel.eng.pro