re-pinning a gear to a new shaft using a round pin

I have this old machine. It has a gear on a shaft, and the shaft had been bent. The gear was pinned to the shaft by a transverse press-fit pin which was filed so smooth I didn't even see it for a long time. I guess this was an old way to make the joint. Anyway, I removed the gear and have a new shaft. I had been going to just machine a 3/16" keyway and stroke an inside keyway in the gear, but someone on this NG said don't do that, pin the gear back on the way it was since the pin may be stronger and also may shear in the event of a collision.

I haven't had any time to work on that machine in the interim, but it's coming up in my queue shortly, so I'm starting to think about it.

Where would I get round key stock? I don't know the size offhand, might be

5/16", might be 3/8".

I'm a little leery of pressing a pin into a hole in a hole, if you know what I mean. It's tough enough to put a cotter pin through sometimes, let alone a press-fit pin! Anyone have any bright ideas? To be more specific, let's assume it's a 3/8" pin, and the hole in the gear is 0.374" and I drill and ream the shaft to 0.374" also. Then I put the gear on the shaft and look through the hole and tap it around as best I can until it looks pretty good, then I put the pin's tapered edge in the top of the top hole in the gear and press with the arbor press - what if the holes aren't aligned right? Worry, worry ..


Reply to
Grant Erwin
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My guess would be that at original assembly the gear has a spot drill mark or a smaller through hole in the gear hub. The gear probably was lightly pressed on the shaft, then drilled and reamed for the pin.

Since you already have the reamed hole in the gear hub, make a longer pin of the appropriate diameter with a shallow long taper to move the gear and shaft holes into alignment. Then practice your filing technique to blend the pin and hub so that the next time the new owner will have a time trying to get the gear off the shaft :). Lube the shaft & pin with moly never-seize.

John Miller

Reply to
John Miller

Roll pins have tapered leading edges for easy installation, and the coiled shape absorbs minor differences in diameter, tolerates impact from driving and use, plus is easier to get out. If you haven't used them, you don't know what you're missing.

Reply to
Carl McIver

Put a healthy chamfer on the pin. That requires making it over-length, of course, and grinding it off later. It should self-align if the gear/shaft fit isn't too tight.

That's the way the bull gear was pinned to its shaft on my old shaper, which was built in the '20s. (The hole on mine was off-center, cut only half the pin diameter into the shaft, functioning like a hybrid between a cross-pin and a round key perpendicular to the normal key/keyway orientation. The boss on the gear was very thick.) I did remove it once and I was able to re-insert the old pin, but its rotational alignment in the hole was less than perfect. However, it worked.

-- Ed Huntress

Reply to
Ed Huntress

OK, this just got a whole lot more complicated. As I said earlier, it was nearly impossible to see the ends of the pin on the gear hub, and I removed the old shaft by boring it out. When it got thin enough, it collapsed, and at that point I was able to remove it while saving the gear's bore. I had never actually removed the pin stubs on either side until tonight.

The pin was *tapered*. That means for sure it wasn't designed for a shear pin, so I'm not going to worry one second more about designing it for shear. It miked .258" at the smaller end, and up near .287" at the larger end, different values in between of course. Probably a no. 6 tapered pin 5" long, both ends cut off and filed. Nor do I have a no. 6 taper pin reamer.

Now the problem is different. I have a gear which was obviously drilled and then taper reamed for a pin while installed on the old shaft. How do I install it on a new shaft?


Grant Erw> I have this old machine. It has a gear on a shaft, and the shaft had

Reply to
Grant Erwin

Get the right size taper pins and a taper pin reamer. They are not expensive. I had to buy some pins for my Sheldon lathe. I had inherited (literally) the taper pin reamer set. Getting the old pin out without damaging the shaft or hub was the tricky part for me, you've already done most of that. MSC is a good place to get these items.


Reply to
Jon Elson

Just start a new hole on the gear, drill right through your shaft and the other end of the gear and slam a rolled pin in. You will never be able to start at the old hole on the gear and come out on the other end at the right spot.

Tapered pins are reamed and assembled in place.


Reply to
Nick Mueller

Side-topic: perhaps they filed the spot where the pin went in so if it started to come loose it would be very obvious, and it could be repaired before damage occurred?


Reply to

You can try to match the original holes if you want. With a taper pin it's more likely than with a straight pin (since the reamer can be used to help fine tune the alignment). In most cases I just drill another hole in a different place (nearly always on a straight pin setup). I run into this a lot on farm machinery. It's not cost effective to try and exactly match the original holes in the piece (in some cases nearly impossible since the original hole often isn't anywhere near straight or centered).

If you insist on using the original hole the best method is to have a pin on the table that's lined up with the drill bit. Set the back side of the original hole on this pin and drill half way through from the top side. The flip and drill the rest of the way. Using the original holes as a drill guide like this will get you pretty close (though sometimes you end up with a bit of a curve in the hole where the two halves meet).

Reply to
Wayne Cook

Greetings Grant, Assuming the tapered hole in the gear is on center drill the shaft with the proper drill so that the hole will clean up completely with the tapered reamer. Then drill a shallow hole in the shaft that's the same diameter as the small end of the larger of the tapered holes in the gear. Now slide the gear on and use a straight pin to align the holes. If you twist the gear on the shaft you will be able to see by the tilt of the pin if the gear is in line with the hole in the shaft. If the holes in the shaft is zized properly you won't be able to twist the gear very much. If there is not a drill size close enough then drill both the gear and shaft with the drill that's the closest but bigger than the small end of the big hole. Clamp the gear in place and finish the hole with the proper sized reamer. If the hole in the gear is not centered then measure how far off center it is and drill the shaft accordingly. ERS

Reply to
Eric R Snow

Grant, that's not an assumption I'd automatically make. Just because the pin is tapered doesn't mean it's average diameter wasn't considered for shear service.

It's not particularly harder to extract a sheared taper pin than a straight one. You just have to make sure the alignment is correct to the right ends of the segments before knocking them out.


Reply to
Lloyd E. Sponenburgh

Drill through the hole in the gear and then use a taperpin reamer and appropiate pin. ...lew...

Reply to
Lew Hartswick

I've got a pinion like that in the apron of the Hardinge HLV that I'm rebuilding. I don't know if it was a half-assed repair or half-assed original design... the pin is in the middle of a needle roller bearing journal.

I would suggest the use of Loctite. The original manufacturer would probably have used it if it were available at the time.

Mark Rand RTFM

Reply to
Mark Rand

A couple more ideas that will work with misaligned holes: (1)Install a dummy pin in the gear and align it in the drill press chuck. Remove pin, install shaft in the gear, and drill. (2)Install shaft, mark hole locations with transfer punches. Install an alignment point on drill press table concentric with spindle. Set one shaft punch mark on lower point and align top mark with spindle. Drill half way thru, turn shaft over and drill other half. Install gear and ream with taper pin reamer.

It sometimes helps with alignment problems like this if you can freeze the shaft and/or heat the gear so they can be easily adjusted while installing the pin.

Don Young

Reply to
Don Young

If you can take the shaft out, you could turn a tapered pin that is only tapered on one end so that you could mount your fabricated pin in a drill press or mill, slide your shaft up on pin and then use whatever work holding you can come up with to hold the shaft in place.

Then it would be easy to re-drill and ream.

Wes S

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