mount small to large shaft

If you heat it enough to silver-braze, it will wind up dead-soft annealed. If that isn't OK, then you might consider that the shear strength of industrial-grade epoxies run upwards of 5,000 psi. It might do the job.

-- Ed Huntress

Now, there's an idea. Any particular product?

I could reinforce it a bit with a custom key that is only 0.025" high into the pulley.

A tiny key and some good Locktite ought to do it. You could also cross drill and use a roll pin which would be a lot easier to disassemble when needed.

Depending on the hub design and load you might also use a tapered pin to retain the pulley. They also make a tapered pin with the small end threaded for a retaining nut.

Cheers,

Bruce (bruceinbangkokatgmaildotcom)

No specifics, because I'm not up on the latest.

But here are a few general points. You probably can get away with a room-temperature-cure product (which generally means amine-type hardeners), because your application probably doesn't justify the complication of A-B cures at two different elevated temperatures. I always try to ask an expert, like someone at 3M, before committing to something like this. You'll want to make clear to them all the details of your application -- metals being joined, etc. You need shear strength but not much peel/cleavage strength. To get the latter you generally sacrifice the former, so a general-purpose epoxy, which usually is oriented toward a compromise, is not your best choice. And you want to be wary of anything that has a fast cure time. They may have that issue solved today, but any cure with epoxy that takes less than 24 hours usually compromises performance.

Assuming both parts are steel, you may want more than extreme cleanliness. You may want to use the scratch-in method for getting the best bond.

Anyway, I'd try epoxy first, because if it fails, no harm is done to the parts.

I'd be wary of that because it could interfere with getting a good scratch-in coating on the parts. But whatever works.

-- Ed Huntress

How much torque is involved? Makes a huge difference.

Really quick & easy, and easy to undo (unlike silver solder): a tack of weld on the end, bridging the shaft & sprocket. Quick & Dirty (tm).

Roll pin or taper pin. If high torque. Whilst drilling for a pin, I assure the alignment by doing the tack weld above & drilling both at once.

Bob

Could you make and attach a separate hub?

If there is any hub, split it with a saw blade. Make or buy 2-piece shaft collar with the ID the same as the OD of the hub and clamp it. Otherwise, some green loctite.

What do you mean here? Don't know the term scratch in.

I'd have to make a hub, do-able but not easy. I started with a six inch length of timing pulley stock. green loctite or whatever Ed says will be my first run.

Clean the steel conventionally as well as you can. Then mix up a small amount of epoxy and apply a glob to a piece of sandpaper. Sand the spot you're bonding to, "wet" with the epoxy, until you've scratched into the entire surface. Keep the spot wet with epoxy; don't let air touch the metal again once you've started.

If you want to be neat about it, apply another glob of mixed epoxy to a piece of rag, and wipe the spot to wipe away the grit, oxide, etc. Again, keep it wet; air should never touch metal.

Then do the same to the other piece you're bonding. Apply another small glob of epoxy to the spot and stick the two pieces together. Let it cure.

This "scratch-in" method produces similar results to the chemical treatments used in production -- phosphoric-acid anodizing on aluminum, and some acid etches used on steel. It's used in high-grade repair work and some custom assembly. Obviously, it's too labor intensive to use in production. It works on most metals but it's particularly useful on stainless and aluminum. The mechanical bond you get from the rough surface is secondary. The primary objective is to produce a clean, high-energy surface to which the epoxy gets a true adhesive bond. Scratching also produces more surface area for the adhesive bonding.

The higher the strength of epoxy you're using, the more critical it is to have an oxide-free surface to bond to. The difference is really large. Also, keep in mind that you do not want a really tight fit in metal-to-metal bonding with epoxy. You get the strongest joint with a gap of 0.002" to

0.005", depending on the epoxy. For that reason some people leave the sanding grit in the epoxy and don't clean it off; it serves as a spacer. It's easy to starve an epoxy joint and ruin its strength.

Doing the scratch-in thing to a hole or other internal shape is a little tricky, but it works.

BTW, this is for a high-strength joint. If the torque load is light enough that green Loctite would do the job, then this is all a waste of time.

-- Ed Huntress

For a timing pulley on a 3/8" shaft, I think Loctite would more than suffice. I'd use Loctite 609 or 680. These are anaerobic one-part adhesives, not epoxies. I have both, use both and frankly can't tell any difference between them. Tawm, they're both green.

Parts should fit snugly. They should be reasonably clean but this isn't terribly fussy. A quick squirt of Brake-Kleen and a wipe or blow off is quite sufficient. Better bonds are achieved if an accelerator is used, but they work OK without it. Once assembled with these compounds, after some cure time you'd probably need heat to get the parts apart. About 400F would get it done.

I've always used starting ether for the final cleaning as the residue from other stuph can bite you back.

Comments?

I don't know how pure starting ether is, but commercial acetone, for example, usually is recycled and contains some hydrocarbons. In good fiberglass shops, they don't use the commercial grade, which is the stuff you find in paint stores.

-- Ed Huntress

I like the "booger green" for permanent installations and the "snot green" if I might have to take it apart.

I don't know about loctite these days but I was told by a race engineer that years ago they used a loctite product that seemed to be exactly what they wanted to hold hubs on a race car so would result in a lighter hub assembly. They failed with regularity, it turned out that on examination the loctite, while it had the tensile and compressive properties, had piss poor fatigue properties, so failed due to the cyclical nature of the drive shaft.

roll pin?

Press fit and green Loctite

or

Drill and roll pin

or

BOTH!

I thought I knew all there is to know about epoxy. How wrong can one be? This is another one for the library. Thanks, Ed!