Repairing damaged back gear teeth?

together.

damaged

finding

Rob,

If you know the spec of the broken gear, buy a plain gear from HPC or some such place. Turn off the broken gear and pin or dog the new one in place.

AWEM

Likely as not these will be 14.5 degree pressure angle, DP-sized (as opposed to Module) gears so suppliers like HPC are unlikely to stock them - their catalogue only lists 20 dgree PA items - so via that route it would be a special at a special price.

Other possibility - work out what DP the gears are, buy a gear cutter in that size from the usual suspects, and cut one yourself. Or of course there may be a suitable cutter lurking in the workshops of the occupants of this list...

Regards, Tony

I would have thought that welding the damaged area and having someone try to cut just those two teeth back on would be more fiddly and time consuming for them than just cutting a complete set of teeth on virgin steel. Not to mention that if any part of the weld area has become too hard they'll not be able to cut through it.

The easiest way might just be to get a chunk of half decent material, turn the basic shape yourself and let someone cut both sets of teeth onto it. It's the set up time that will determine the cost rather than the machining time and I doubt if cutting two complete sets of teeth on decent material will be any more expensive than messing about trying to align one set of old teeth to the cutter and cutting through weld material without everything pulling out of whack again.

Similarly, trying to affix one new gear onto the remains of the old pair is going to take as long or longer than just making a complete item from scratch and is never going to be as strong whatever you do.

Sometimes it just ain't worth trying to repair something. I went there many years ago with a broken gear on my mill and in the end just turned a blank and had teeth cut on it for me.

As to material, this isn't an F1 gearbox so I doubt if you need to be too fussy. Anything half decent that's better than mild steel ought to last as long as you're likely to need it to.

Of course if this was in India someone would squat in the dust with a stick welder, fill up the hole, file the new tooth shapes back in by hand until you'd never know it had ever been broken and charge you 50p. I just don't think we're that clever over here.

-- Dave Baker Puma Race Engines

What's the OD of the 28 tooth gear.?

-- Regards,

John Stevenson Nottingham, England.

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A slightly random 47.5mm, 1.87" diameter, 5/8" thick.

Dave Baker wrote:

Yeah, I had kind of expect the engineers to say that today rather than a complete "No can't do, need a gear hob, we don't know any gear cutters around here."

As for pressure angles, ouch you're making my head hurt, it's been nearly 20 years since I drew any gears in Tech Drawing at school, so I'm definitely a little hazy on the subject these days.

Cheers, Rob

Ah, just found the HPC website, bit of nosing around and under spur gears, heavy duty we have a YG16-28, 16 DP gear, 28T 1.875" OD, but with a 20 degree pressure angle. So other than the pressure angle thing, that would be suitable. Any easy/obvious way to measure/confirm the pressure angle?

Cheers, Rob

Saw a beautiful solution in ME a few years back - some old-time mechanic had just chiselled a dovetail in the hub where the broken teeth were, then filed up a chunka steel to fit the dovetail and carved the top end to tooth shape. Slide her in and, nowadays, Loctite is your friend. Easy does her. Mike in BC

I once filled up the hole with brazing rod and hand filed the teeth. Lasted as long as I needed it. Mike

The first thing we need to know is what is the DP of the gear. (almost certainly DP, not module)

(Number of teeth -2)/diameter in inches.

Conveniently someone has already done the subtraction for you!

To cut new teeth onto a welded up bit is possible (I recently did it for my shaper table traverse gears). However, in this case, it couldn't be done with a hob or brown & Sharpe type cutter because the large gear is in the way. It would be possible on a shaper (got to do that soon for an odd gear replacement on the Hardinge) but is liable to be a pain. Finding someone who can cut a gear for you (Ping JS) then cutting the old teeth off and fitting the replacement might be simpler. I would recommend Loctite 603 or similar rather than pinning.

So get the measuring stick out and tell us the DP or the diameter and see if anyone can help.

Mark Rand RTFM

Just thinking aloud, as you have already received lots of good advice. Take note of the position of the teeth of the small gear viz-a-viz the large one before you remove it.

I have no idea if it is important or not, but you can bet your life that if you don't, it will be!

You could take a look at the Boston Gear catalog. US based I think, but they may have a distributor in your area. It takes a bit to find the download on their site, but it is there. Look for the open gearing cat.

They stock 16DP 14.5 PA gears.

Cheap and dirty PA check. Lay a snake of modelling clay on the edge of a bench, and roll the gear cross it. It should leave you with a rack form, with nearly straight sides. A little time with a protractor and a straightedge will tell a bit.

A 14.5 rack has the same included angle as an acme thread, 29 deg.

A 20 deg PA rack has a 40 deg included angle.

Or you can find suitable pins and do a bunch of math..:-(

If it is off an older machine, 14.5 is almost a given. Almost.

Cheers Trevor Jones

Mark Rand writes ..............

Mark,

I know that this will spoil your rather clever joke, but surely the formula is plus two, and not minus two.

Diametral Pitch = Number of teeth + 2 / Outside Diameter.

This gives from the data provided Outside diameter 1.87 (almost certainly 1 7/8" = 1.875) Number of teeth 28

DP = 28 + 2 / 1.875

Thus DP = 16

I could be wrong.

Mike

Good Call!

If it mates to another fixed together gearset then that really could be an issue!

Not so much if it mates to two loose gears, as in a lathe backgear.

Cheers Trevor Jones

Cheap and dirty fix.

Have the area welded or brazed full. Turn off the excess on the OD and sides. Hand grind a form tool to fit the gap in the teeth still there. Mount up the gear on a faceplate, or between centers, in the lathe and use the carriage to shape the gear tooth.

Indexing can be taken directly from the other teeth, as well as using them as reference for the setting of the tool to position it.

Slow, but doable with minimum resources.

Cheers Trevor Jones

Boston Gears catalogue is here:

Regarding how it mates with the other gear, yep it's like a lathe backgear so the gear it drives is independent of the other. The large gear on the back drive is driven by a small gear under the cone pulley that's turned by the motor. The small gear on the backgear then drives a large gear on the spindle. So the relationship between the two gears on the backgear is not critical - which is handy!!

Cheers, Rob

If you only option is making a gear, I've got a 16dp 14.5pa B&S type cutter. I could rent it out for a small fee, or if you don't have the technology and are stuck I could make you a gear (for attaching to the original part), though my workshop is about to progress from almost unuseable to totally unuseable for a week or two.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

It was a brain fart caused by starting the reply at work, while waiting for SWMBO to give me a lift home, and finishing it at home. Not applying said brain at any point in the process to check the arithmetik.

Mark Rand RTFM

If you can wait until late next week I can run you a 28T gear off in

Any good ?

-- Regards,

John Stevenson Nottingham, England.

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I wrote a program (GEARPA - free from my site) as an experiment to see if pressure angle could be determined by measuring the chordal span across gear teeth which, according to MH, varies some (but not much) with pressure angle. Running the program produces:

GEAR PRESSURE ANGLE BY MEASUREMENT OF CHORDAL SPAN Number of teeth on gear [30] ? 28 Diametral pitch of gear [6] ? 16 Pressure angle = 14.50 deg...Chordal span over 3 teeth = 0.4846 in Pressure angle = 20.00 deg...Chordal span over 4 teeth = 0.6703 in

One of our club members has used the program successfully to determine PA so the program has some substantiation.

Give it a try and, if it works for you, let me know.

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things

I'm sure this is a stupid question for those of you who are experienced in the theory of gears, but *IN PRACTICE* will a 14.5deg gear not mesh with a 20deg gear well enough for our sort of applications - ie. relatively low speed, modest loads, infrequent use, limited life expectancy etc.?

Mike D.