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How to test a 12" girth gear with pinion

How to test a 12" girth gear with pinion

How to test a 12" girth gear with pinion

Hello all,

I have a bit of a problem. I have a spare girth gear that (might) belongs to a ball mill, and I need to check it with the pinion, and perform contact test, backlash, radial and axial runout (2 sections), et cetera). There is a distinct possibility the pinion does not correspond to the gear for whatever reason (cough!, cheap, cough!).

As we do not have a spare mill lying around, I am thinking of building a structure with supports for gear and pinion. And with an old motor and belts make them turn. Is there a more efficient way to do this?

Unfortunately we do not know if the gear belongs to the pinion, or even if the sections of the gear belong to each other, if it has cracks, how worn is it (we can see visually it is worn, but not as much as the one currently working), we do not know brand... In short, we do not know what we do not know... Helps would be highly appreciate it



RE: How to test a 12" girth gear with pinion

Do you have any picture of the gear and the pinion you want to check?.
are they spur gears or helical gears?

RE: How to test a 12" girth gear with pinion

Hello Oscyes,

Is a helical gear. Is actually herringbone. The pinion and gear will look something like the picture (only instead of spur gear, is helical)



RE: How to test a 12" girth gear with pinion

First, roll the pinion over the gear by hand. If it is of any good quality it will roll smoothly. If it doesn't then they don't fit together.

RE: How to test a 12" girth gear with pinion

3DDave's recommendation is a quick way to check the pinion and gear before putting any time/money into more detailed inspection.

If the new pinion rolls over the gear fairly smoothly, a next step would be to record some digital point/surface data from both gearsets and compare them using a software tool. You can hire a vendor to do this work on-site using a portable CMM or laser tracker. Either machine should provide sufficient accuracy for your application. The portable CMM is more limited in range than the laser tracker, but the laser tracker is limited to line-of-site measurements. The ball mill gear drives I have seen were accessible by removing their guard covers, so it should be possible to take measurements of the installed gear/pinion without too much trouble.

Besides checking the gear geometry, remember to also verify that the interfaces match (bolt patterns, locating features, shaft journals/faces, etc).

Hope that helps.

RE: How to test a 12" girth gear with pinion

The problem with this is the following:

The gear is 12ft in diameter. Rolling the pinion by hand will be quite difficult
The gear and pinion are spares. That is are not attached to the equipment. The piece (2 sections) are on the floor and the pinion also. I can get housings and bearings and fabricate a support, but I cannot get them to stop the mill to test with the gears.
The portable CMM idea is good, my question is if with that would we know if the pinion belongs to the gear? Even if the geometry is the same, there is the possibility (if the gear is used), that the imprint for the teeth are not from the pinion...


RE: How to test a 12" girth gear with pinion

This time of year, if you were >1000 miles North, you could lay the girth gear down on a sheet of ice, and, uh, curl the pinion around it.
Got a nice flat floor you don't mind getting greasy?

OR, and this will probably be more attractive from most perspectives,
cut cardboard templates of a sector of gear and pinion, and run them against each other,
OR, possibly better, cast replicas of sectors of gear and pinion, and mate them.
OR, rent a portable CMM and measure the tooth profiles and associated geometry,
OR, better, contract someone with CMM expertise to do the same.
OR, roll the girth gear through a puddle of wet mortar (covered with Saran wrap),
and when the mortar dries into a rack of sorts,
roll the pinion along that.
OR, find an old cranky machinist to measure the tooth geometry of each, and lay them out on CAD or on paper to see if they match.
OR, build a skeletal pseudo-mill as you propose, and try running the gears together, with Prussian Blue for a contact check, or with lapping compound if they are close, but not quite right. I'm assuming they are of zero value if they don't actually fit, so there's little downside.

Mike Halloran
Pembroke Pines, FL, USA

RE: How to test a 12" girth gear with pinion

Those are great ideas Mike!

-This time of year, if you were >1000 miles North, you could lay the girth gear down on a sheet of ice, and, uh, curl the pinion around it.
-Got a nice flat floor you don't mind getting greasy?
I am guessing you mean flat on its side... Maybe this is what Dave was referring as well. The idea about the CMM mentioned by you and Tbuelna I also like... These are all great ideas! thanks!

A follow up:

If time is not a problem (this is a spare after all), but money is tight. Would it be the same thing to cast a mortar mold as to pay for a CMM drawing?

Best regards,


RE: How to test a 12" girth gear with pinion

If you want CMM measurements contact Horsburgh & Scott in Cleveland. They are a gear company with a CMM machine with a 4 x 4 x 2 meter measurement envelope. Your 12 foot diameter gear would just fit. You can find pictures of it on their website. I don't know how close you are or how much it would cost but they could probably measure not only the tooth geometry but the interface surfaces as well.

RE: How to test a 12" girth gear with pinion

Browsing through ENG-TIPS I came across this forum problem being similar to what we have encountered here in the UK (Industrial north England) and attached is a photo of how we solved the problem relatively economically.

We are finding that some of large girth gear drives are having expensive meshing problems to overcome, as it appears probable in your case.

Our photo shows a replacement by chain device (chain not yet assembled in the photo) using inexpensive, 90% profiled segments and a cheap but solid way to do it.

This information is intended as an advisory solution to your problem.

Ron Gill

Grybrook Limited

RE: How to test a 12" girth gear with pinion


Here is a sample inspection report comparing the 3D laser scan of a part surface to its 3D CAD model. Obviously this approach requires a 3D CAD surface model. And since you indicated there was no engineering documentation available for the pinion and girth gear, what I suggested previously was taking 3D measurements of both the spare pinion/gear and the pinion/gear currently installed on the ball mill, and then compare the relative sets of 3D tooth surface data against each other to see if they match.

You stated it would not be possible to shut down the ball mill for the period of time required to fit check the spare pinion/gear. However, it wouldn't likely require more than a few minutes to 3D laser scan the existing pinion/gear in-place using a handheld device. You don't need to scan the entire pinion/gear, just a partial sector. So you might consider doing it during the next time the mill is shut down for maintenance.

Might give a vendor like these guys that performs this type of work a call to see if they have any suggestions.

Good luck to you.

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