quenching a shrink fit--good idea?

Why would you want to quench it anyway? It serves no purpose. Just let it cool normally.

You'll get 4 thou expansion of a 1.25" hole in aluminium at 150C. 200C would be plenty so just leaving the disk in a hot oven for a bit would be safer than getting an unknown temperature on a gas burner. Alternatively keep trying the hub carefully while you heat the disk on a very low flame and stop heating when the hole is big enough.

To be honest that isn't a very tight fit on a hole that size anyway. I routinely fit steel valve seats into aluminium cylinder heads at about the same fit by just tapping them in cold. If it has to cope with much torque I'd be looking at a tighter fit or a more positive way of securing it.

-- Dave Baker - Puma Race Engines

I'm doing them in batches, and don't want a pile of hot metal on the kitchen counter.

Entirely different application. My guess is that you have to taper the end of the valve seat to get it started, and tapping it in has to expand the aluminum of course--but I can't see how you get as good of a bond as a shrink fit. I'll take your advice and try a tighter fit, I was just playing it safe to avoid a freezeup of the hub halfway in the hole. I got the number from the RCM archives, and remember coming across a few of your posts on the subject of shrink fitting. I should do a torque test on these units--bolt the plate to a heavy welder's table, and get a torque wrench on the hub. But really, the torque on these in their intended usage will be pretty low--how does one compute ft-lbs of force from a sharp chisel on a 3-4" dia. piece of wood turning at 1200 rpm? Anyway, there are weaker points in the chain of this setup than that joint.

thanks,

Ken Grunke

I can think of more pressing needs for quenching something. Put them on a steel tray and they'll be cool in no time. A thin aluminium section cools pretty fast.

You have a radiused lead in yes otherwise it just digs into the aluminium. But you have that radius on all valve seats anyway whether you press or shrink them in. They come that way from the factory. Even when people fit them by heating the head or cooling the insert (liquid nitrogen) they don't always achieve positive clearance and even if they do that can dissapear when the insert is only partly in and it needs a last tap to seat it.

and tapping it in has to expand

There's no difference between a press fit and a shrink fit (providing the act of pressing didn't physically gall or distort either component). Both achieve elastic, not permanent, deformation of the components and end up with the outer in tension and the inner in compression at the same values. The main reason shrinking is used is on large components when the pressure required for a press fit exceeds the capacity of the machinery available.

If you had a small rad on the end of the hub you could press the two together very easily. I still think you need some sort of rad even if you shrink them together because sod's law says you'll end up with some not seated fully that need a last tweak to get them flush.

Probably not a very high torque then or it would rip the chisel out of the operator's hands. I just know that a 4 thou fit on steel into aluminium at that sort of diameter goes in pretty easily with medium weight hammer taps. How much torque it would transmit depends on the length of fit and the rigidity of the two components though. I guess you can proceed by trial and error until you find what works.

-- Dave Baker - Puma Race Engines

dry ice internal heat outer air cool

snip

"william_b_noble" wrote: (clip) I can pretty easily stall a 2 hp motor if I want to (clip) ^^^^^^^^^^^^^^ Bill, I think you are proving that the press fit is unlikely to slip. More likely, as you say, the motor will stall, or the belt will slip. Or, what I think is also very likely--the screw holding the wood will tighten or tear out before the shrink fit moves.

Coming back to the original question: Ken, the hoop stress in the aluminum will be the same at room temperature whether you quench it or cool it slowly. So, I don't think it makes any difference.

You could probably fit a dozen of these in your oven. So why not fill up the oven with the aluminum turnings, and, following Dave Baker's suggestion, bring them all up to temperature? Then, using an Ov-glove, take them out, one at a time, insert the cores, and put them back. When you have them all assembled, turn off the oven, and go away until they're cool.

I'm not sure that's an entirely accurate assumption.

I've built a couple of die sets. The pin to bushing clearance is roughly .0005" (or less) total, and the pins (typically more than 6" center to center) are pressed into steel die shoes.

Installing the pins without care (.0005" total press fit) causes them to press in while being out of square. For a die set with a maximum clearance of .001", this pretty much makes for scrap (or rework). This is easy to do with an arbor press.

The die pins for our large dies (pins are about Ø100mm or larger) are shrunk using liquid nitrogen before intallation. They can be pounded in using a

I saw a show on the discovery channel about rebuilding aircraft landing gear. It's been a while but I believe they install the bearings using liquid nitrogen.

I know the OP's application probably doesn't require this kind of accuracy...

Regards,

Robin

I haven't seen the design details, but are you relying on on the shrink fit of the steel spindle into the alumium hub to resist the side forces of offcenter wood turning?

That hub looked like it was only about a half inch thick. I would be real worried about a rip-out unless the backside of the steel spindle were larger in diameter than the through-hole. Ie, a stepped through hole.

Jim

While working for a company that repaired jet turbine engines (mostly outer cases) We used dry ice on the enter sleeve, and heated the outer case, and then let air cool.

I've also used that method for fitting alum. hubs to small electric motors to kept from using set screws.