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loose nut
08-01-2015, 08:03 PM
I need bevel gear sets 48 Diametral Pitch x .375" & .5" Pitch diam. Works out to 1/1.33 ratio and cannot be changed. I need these sizes to fit into a "spot".

I can get a .375PD gear in metal but the .5PD only come in Molded nylon (Boston Gear). The .5 gear needs to be modified so that the existing hub is removed and a new hub is on the opposite side compared to a normal gear. How can I fasten the nylon gear to a brass hub. There is no room for a screw etc.

Will any type of glue work?
If the nylon gear is bored out can a brass hub be pressed into the bore and stay tight?

Any other suggestions. This is becoming a "pulling hair out" situation.

Joe_B
08-01-2015, 08:25 PM
Knurl it, heat it, press it in.

elf
08-01-2015, 11:40 PM
Probably depends on the unspecified load...

michigan doug
08-02-2015, 08:53 AM
Not too much in the way of glues and adhesives make a reliable joint to nylon.

J&B Plastic Weld is reported to get the job done:

http://info.craftechind.com/blog/bid/323475/Stick-to-it-A-Guide-to-the-Best-Glue-for-Plastic


I like the knurled/heated/pressed idea.

firbikrhd1
08-02-2015, 10:45 AM
If the gear is to turn in only one direction perhaps you could use an appropriate thread, right or left hand depending upon direction of rotation, on the brass shaft and inside the nylon gear, to a shoulder on the brass shaft. This would require a shaft larger than the threaded hole in the gear, of course, at least att he point of the shoulder after which the shaft could be necked down if necessary.

loose nut
08-02-2015, 11:37 AM
No it turns in goth directions. Not much of a load, connects to a tiny hand wheel, part of the elevating gear on a artillery model but it dose need to work.

How much heat, to soften it I'm assuming and how much of an interference should I use.

Gary Paine
08-02-2015, 12:23 PM
A small pin pressed into a through hole in the shaft and sticking out on each side that fits in a slot you would cut in the hub?

loose nut
08-02-2015, 04:26 PM
The hub is going to be removed so no pinning.

oxford
08-02-2015, 04:37 PM
When you say no room for a set screw, I am guessing that you mean from the side to lock the two together. Can you press them together and then drill and tap at the seam for a set screw to lock them together that way?

darryl
08-02-2015, 06:08 PM
It would seem like the method of choice is to use splines. Just pressing a plastic gear onto a hub is likely to put too much stress on it if it's to not slip- unless the application requires very low torque. You could raise splines on a hub by knurling with the right wheels- I think a pattern like typical knurling would not be the best, but straight lines would be good. If a plastic gear is a light press fit onto a hub before the splining takes place, that should be an appropriate fit for the gear over the raised splines. JMO

Mike Burch
08-02-2015, 06:53 PM
Nylon is slightly hygroscopic (I read somewhere that it can absorb up to 6% of its own mass in water), and swells a little when immersed in water. I've never tried this, but I wonder if you could effect a kind-of shrink fit by making the brass a thou or two over size, then soaking the nylon for a week, putting the two together and letting it dry out.
(This swelling, incidentally, is the reason some boats have stiff steering. If the Ertalon rudder bushings are bored by an engineer unaware of this phenomenon to a minimal clearance on the rudder stock, then when held around the outside by the rudder tube they can't swell outward in the usual fashion, so they swell inward when immersed, and grip the stock.)

ahidley
08-02-2015, 08:15 PM
Put a pin straight between the teeth. But make the pin short enough that is won't stick out past the root dia .

Paul Alciatore
08-02-2015, 08:26 PM
I have also seen gears with set screws run in through the teeth. Drill at the root. You did say a light load.




When you say no room for a set screw, I am guessing that you mean from the side to lock the two together. Can you press them together and then drill and tap at the seam for a set screw to lock them together that way?

elf
08-02-2015, 10:29 PM
Where on a bevel gear would you put the pin?

Puckdropper
08-03-2015, 01:51 AM
The gears I've seen that aren't press fits use a straight knurl. With something as small as you're talking about, it may not be necessary to heat the part before you install the knurled axle. I'd probably try a press fit first.

Threading may work as well, if you either apply thread locker or use a jam nut. For what it sounds like you're doing, red loctite will probably be fine.

FWIW, I've glued Delrin gears to steel shafts with CA glue. It seems to hold up for model railroad use. Nylon would probably do the same.

DaveM 7766
08-03-2015, 06:35 AM
Key and retaining rings

aribert
08-03-2015, 07:29 AM
loose nut:

I would heat with a propane torch - need to be above the melting point of nylon. You also need a way to fixture the shaft to keep it square to the gear while the nylon solidifies about the shaft. Once I would have the two pieces joined and aligned, I would place a damp rag against the shaft to take the heat out. Quite a number of threaded inserts into plastic are installed after the parts are molded as press fits - sometimes at elevated temps, other times at ambient. Since you are doing a one off, I would preheat the shaft.

I did something similar (hot press fit) but on a static part. I bought a 10 yr old riding mower where the junction between the fuel tank and the rubber hose was leaking. Previous owner managed to break off the hose nipple on the blow molded fuel tank (guessing fuel tank material was polypropylene) leaving just a stub to attach the fuel hose onto. I machined a brass hose barb of the appropriate diameter for the fuel hose and significantly smaller on the opposite end to fit into the fuel tank stub. I either knurled or machined concentric rings on the OD (several years ago, details fuzzy). I heated the part w/ a propane torch and pressed into the tank stub and immediately cooled the brass nipple with a wet rag. Its worked fine since, still in use. Granted my repair does not need to resist a rotational load but my yard is a bit lumpy and the repair is certainly subjected to plenty of vibration.




No it turns in goth directions. Not much of a load, connects to a tiny hand wheel, part of the elevating gear on a artillery model but it dose need to work.

How much heat, to soften it I'm assuming and how much of an interference should I use.

ironmonger
08-03-2015, 09:05 AM
The gears I've seen that aren't press fits use a straight knurl. With something as small as you're talking about, it may not be necessary to heat the part before you install the knurled axle. I'd probably try a press fit first.

Threading may work as well, if you either apply thread locker or use a jam nut. For what it sounds like you're doing, red loctite will probably be fine.

FWIW, I've glued Delrin gears to steel shafts with CA glue. It seems to hold up for model railroad use. Nylon would probably do the same.

Test the loctite on a sample you donít have much money in...

http://diydrones.com/profiles/blogs/warning-loctite-eats-plastic

I have had plastic, polycarbonate to be specific, turn to popcorn overnight exposed to anaerobic thread sealant. Who knew... I guess we could have read the instructions, but who does that... :>)

In any event, try a test piece first...

paul

elf
08-03-2015, 04:29 PM
This technique works on ABS and may be worth trying on nylon: https://www.youtube.com/watch?v=iGNXHWPFAKo

loose nut
08-03-2015, 04:48 PM
It's hard to explain what I'm trying to do but here goes.

I need to machine the existing hub off of the gear, leaving the toothed section .100" thick. the new hub will have a smaller section that needs to be inserted into a bored hole in the gear and fastened but on the opposite side of the gear compared to a stock gear. The remainder of the hub sticks out. The inside of the new hub will be threaded with a square thread (or acme if I have to) for a threaded shaft to pass through. The idea is that turning the gear by a secondary gear (miter) connected to a hand wheel by a shaft, will move the threaded rod up or down, the threaded rod will be keyed into the housing so it doesn't rotate.

Clear as mud right.

It may be evident or not, that pinning or screwing or any king of mechanical fastener just won't fit, there isn't any room. It can't be pinned because a pin cannot go through the gear, it would interfere with the threaded rod. The idea of making small grooves length wise down the part of the new hub that fits into the gear so that the nylon will dig in and grip sounds like the best idea so far.

Thanks.

oxford
08-03-2015, 06:25 PM
I still say the way I mentioned before with the set screws, but I am not sure if you have the wall thickness or not. You would drill and tap on the same axis that the threaded rod is going through. Just catch half the ID of the gear and half the OD of the bushing and don't drill all the way through so the set screw can bottom out and tighten. If you threaded the 2 pieces you would also be able to do this with just a roll pin.