Gear Measuring

Hi. This is my first post so I hope it's in the right category!

I'm trying to rebuild a mini digger and 1 of the track drives is missing a few gears. I've searched for the digger manufacturer (CME) and the drive manufacturer (Oildrive) with absolutely no luck, so my only options seems to be, buy a gear and machine the bore to suit or make the gear, which I'd perfer not to do as it probably needs to be hardened in some way.

The gears are on the final stage reduction and it's missing the 3 planet gears. I've took the other drive apart and measured the gear, and I got a overall diameter of 62.8mm with 25 teeth, it's also 12mm thick. Plugging these numbers into some online gear calculators seem to suggest a mod number of 2.37?

I have a copy of Ivan law's "Gear and gear cutting" and now I'm even more lost! I seem to require the "PCD" dimension but I've no idea how to measure that. The sun gear is also very worn , so I'm unsure about taking any measurements off that. Any advise you could give would be greatly appreciated.

Many thanks

Graham

Graham,

You might find this data reference sheet a useful download. I use it as an "aide-memoire" from time to time as I infrequently make gears and am easily confused by the various terms. There is no guarantee that your gear is a MOD one, of course, it could be an Imperial size.

Maryland Metrics gear data

And while I'm at it, the technical data section of that web site has a whole host of data sheets - some more useful resources than others, here:

Technical data index

Alternatively, you could always have a fiddle about with a gear-generating program such as this, which I use to make dxf files for CAD:

GearDXF

John

Edited By John Hinkley on 29/06/2019 14:13:25

It's very close to 11DP (less than half a mm out on the OD) but I think that planetary drive gears are bound to be non-standard whichever way. Stick the gear on a flatbed scanner and take a greyscale scan of it at about 200dpi. Post the pic.

Edited By Pete Rimmer on 29/06/2019 14:19:22

The Wikipedia article on gears is excellent with comprehensive list of the various parameters involved.

As Pete says gears in planetary drive systems are invariably of modified shape. Primarily to ensure proper tooth clearance and meshing on the internal gear ring. Lots of ways to do the modifications. Probably the simplest is to trim the tips of the planetary gears for clearance and put the main modifications on the ring gear to ensure the contact patch is in the right place.

But there are better ways giving quieter and more efficient results. Higher the speed / torque / power to be transmitted the more likely the modifications are to be spread over the whole gear set. Only the manufacturers will know for sure what was done.

The gears will certainly be hardened.

Generally the changes are fairly subtle so nearest standard gear trimmed to fit will probably work for a while. Will almost certainly be noisy and probably won't last very long in objective terms. But how much work will the digger do? Might be good for a couple of hundred hours but then things will be comprehensively trashed!

You have to wonder why the gears were removed in the first place. Pity plastic gears won't take the load as they might well be flexible enough to cope with the shape changes.

Clive

John -

Imperial or Metric?

A good guide would be to measure the easier dimensions on the gears, like thicknesses and shaft diameters, and also establish what system the machine generally was designed to. I'd think it unlikely the manufacturers would have mixed measurements systems, though I suppose it's possible.

Many thanks for the replies so far. I have had a quick look at some of the web pages suggested, very helpfull, but I think I need a more in depth look with a a bit of digest time. The drive has a sticker stating "Oildrive Modena" on it so I assume it's Italian of mid nineties vintage, and as the gear is 12mm thick I rather suspect it's metric, all the bolts are.

I have attached a picture of the gear, I don't have a flatbed scanner so I printed a 1mm grid and placed it around it. I've also attached a picture of the drive to show where they go.

I take it that I'm unlikely to be able buy a straight replacement? Is this something that I can get made? Laser cut? Small specialist gear manufacturer? A new drive would be over £1000 so I would really like to get this working.

As to why they are missing, I suspect that the drive actually has another fault somewhere and these gears were removed to allow the track to roll, why they then didn't stash them away on the machine somewhere I don't know! I also haven't tested it because it's pointless unless I can get these gears, and the engine needs a few bits sorting, it hasn't run for 6 months.

Many thanks

Sorry forgot the pictures...

Armed with the dimensions, you could contact specialist gear suppliers like Davall, HPC or Reliance, to see what they have to offer. But have a chair close by when they quote the price, just in case.

If you are near to them, it may be worth taking one of the existing gears to them as a pattern.

Since it seems likely o be metric, from your figures, it could be 2.25 Mod or maybe 2.5 Mod

My very old HPC catalogue ( C23 ) quotes a heavy duty steel gear XG2.5-25 as having 25 teeth and an OD of 67.5 mm, but it is 30 mm thick. Price, then was £30.28

The standard duty gear has the same dimensions, but is listed as PG2.5 - 25 and as 25 mm thick Priced at £22.72

XG and PG are plain gears without a boss. YG and G have a boss, which you would not need.

No doubt these were ex VAT prices and need to be increased considerably.

They quote a 23T 2.5 Mod gear as having a 62.5 mm OD

They don'r seem to mention anything between 2 Mod and 2.5 Mod.

If any of the above were to be suitable, you would need to turn half of it away to 12mm thickness you quote

HTH

Howard

In 1990 OilDrive was acquired by Dinamic Oil - might be worth contact Dinamic Oil via the web site

**LINK**

Just for fun, I "fiddled" around with GearDXF and scaled your original photo in QCAD, then roughly overlaid the outline of a 25 tooth gear generated by GearDXF. The other parameters - for no good reason apart from that's what the program worked out, given the number of teeth - were pressure angle 20°, tip diameter 64.022mm, pitch diameter 59.8mm. These figures give a MOD of 2.371. It gives an approximation, within the limits of the photo, thus:

The yellow line is obviously the gear outline from GearDXF. It doesn't quite match, but it's close, if you take into account parallax, etc.

Good luck with your search.

John

Cutters can be got for both 11 dp and 2.25 mod which will be very close to Johns theoretical 2.371 MOD. With suitable attention to the addendum or dedendum one of these cutters would almost certainly fit well enough to work for a reasonable period of time. I imagine any error could be made less than that intrinsic to using B&S type cutters where one cutter covers a range of tooth numbers. The other gears will be a little worn anyway so as new fit isn't going to happen.

If you have access to a shaper any odd size gear can be cut. Slow job but it works. I imagine that using one of the other gears engaging in a short rack would work just as well as the wire on PCD usually specified to rotate the gear during feed. A row of pins ought to work just as well as a proper rack.

If you do go the shaper route maybe a rack cutter grinding pattern could be made by coating a steel stub with bodyfiller or other rigid but soft enough to shave material to make a slighly oversize rack form. Running it against the standard gear should shave it to the correct shape needed to exactly replicate the original. If that worked the exact specification of the original is irrelevant as the cutter will simply duplicate the original. I'd probably do an intermediate step with an aluminium "cutter" to verify the exact shape.

As for hardening any of the home / small shop methods ought to work well enough. Or maybe worth looking into the PVD coating services from folk like Oerlicon Balzers, Balinit et al, which are very effective and don't necessarily need an uber hard substrate. I believe that they are now quite well set up to do small orders at reasonable cost. Not cheap but probably comparable to buying commercial gears.

Clive

These may be able to help, KHK

What measure the gear?. You have two that you could use to make a single point cutter from. Then turn a blank to the same diameter out of say, 817M40 (EN24). Setup using one of the gears to set depth of cut. Cut gear blank, probably cut a sample from ally if you like. The flame harden the teeth, or take to a hardening shop with induction coils.

As the sun gear is 10 teeth, there is certainly a positive addendum modification, and certainly also for the planet gears, These gears must so be cut with a generation machine, or a special drawing made for a fly cutter.

JM

Many thanks everyone. I'm almost certainly going to end up making these gears myself, using a single point tool, but unfortunately it will have to wait for a while. I do have a milling machine but it's of the mini variety. I'm sure it would cut the gear but not to any consistency and that would lead to the drive destroying itself. I think a new addition to workshop is in order.

When I posted the original question I was rather hoping I'd missed something obvious and I could get an off the shelf item.

Once again thanks everyone, especially John for the CAD work! I shall post an update on the outcome.

You can grind a tool bit to the correct profile. Use one of the existing planet gears as a template. Use the tool bit in a flycutter. I have used that method to cut gears of nonstandard pitch or pressure angle.