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innovative hobbing

It is quite an interesting youtube. I agree: the larger gear was likely shrunk on in the same manner as a steam locomotive's driver tire. On some shrink fitted parts such as this gear "rim", a mechanical engagement is used. I wonder if the rim was pinned to the hub after shrinking into place. This is often done by tapping holes on the joint line, so half the tapped hole is on the rim, and the other half on the hub. The holes are then plugged with set screws or alloy-steel studs on larger sizes. These setscrews or studs are then faced off flush. Possibly, this was done and with the facing done, the pinning was not visible.

In designing this sort of assembly, I agree with going for something near maximum hoop stress, but this, to me, is a kind of "loaded question" (sorry about the pun). The hobbing of the gear teeth removes material from the rim. The corners at the roots of the teeth are also places where there will be stress concentrations. If I were designing the rim for the shrink fit, I'd consider only the portion of the rim below the base circle of the gear as being stressed for the shrink fit. The teeth will see a combination of bending stress and shear stress in service. I'd likely allow a little extra metal between the base circle of the gear teeth and the "stressed zone" of the rim. The result, in actuality, would be a lower developed tensile stress (aka "hoop stress"). I've designed a number of shrink-fitted parts in my career, and prefer using shrink fits to press-fits.

As a young fellow, a good 45 years ago, I had occasion to salvage a small crankshaft. It was from a power hacksaw. The shaft and crankpin were worn and chewed up badly, but the crank throw was OK. The crank throw was an iron casting. I heated things up and drove out the shaft and crankpin. I then got the bright idea of making the new parts with shrink fits. what I failed to take into account was the fact the wall of the hub of the crank throw was relatively thin in the area 180 degrees opposite the crankpin. I heated the crank throw up with a rosebud and tapped in the shaft and crankpin. As things cooled, I could see a crack developing in the thinnest section of the hub's wall. Nothing to do but reheat, take the shaft out, and machine and shrink a steel collar around the hub, then re-fit the shaft. It was a lesson I took with me into the next 45 plus years of my career. I never take hoop stress for granted when figuring a shrink fit on anything with what seems to be a relatively thin wall. Something like the rim of this gear, once the teeth were hobbed into it, would not have as much material left in the "net tension area".

The other unanswered question from this youtube is what kind of steel and heat treating were done. Apparently, the pinion did get some kind of heat treating (oil quenching and tempering from the looks of it). The gear, bring a shrink-fitted rim, was probably left at whatever hardness the stock originally had. I know for good gearing design, a hardness difference is figured between mating gears to get better gear wear.


I am always awed by people who can setup machines like hobbers (or horizontal milling machines)to cut helical gears. While I certainly can do the math and design a set of gears, setting up a hobber or mill and doing it is one of those things I regard as something of high art. Seeing a hobber running never fails to get my mind working as to who did the calculations and the setup. The people who can do this sort of work are the real wizards in my book.
 








 
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