Need pointers for calculating 6061 shrink fit
Need pointers for calculating 6061 shrink fit
(OP)
Hi! I'm new here, and a shade-tree mechanic.
My current project is to shrink fit a sleeve over a drive pulley hub. Both parts are 6061 T6. The hub has an I.D of 1.6" and an O.D. of 2.1". The sleeve will have an O.D. of 2.88" with 0.10" deep serpentine belt grooves in it.
In my copy of Machinery's Handbook are formulas for calculating the shrink fit allowance for steel and cast iron components, but not for aluminum.
Can someone please point me in the right direction? Thanks!
My current project is to shrink fit a sleeve over a drive pulley hub. Both parts are 6061 T6. The hub has an I.D of 1.6" and an O.D. of 2.1". The sleeve will have an O.D. of 2.88" with 0.10" deep serpentine belt grooves in it.
In my copy of Machinery's Handbook are formulas for calculating the shrink fit allowance for steel and cast iron components, but not for aluminum.
Can someone please point me in the right direction? Thanks!





RE: Need pointers for calculating 6061 shrink fit
aluminium expands about twice as much as steel, and the pressure after the shrinking for a given interference will be about 1/3 as much.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Need pointers for calculating 6061 shrink fit
I'll go check the FAQ.
RE: Need pointers for calculating 6061 shrink fit
The FAQ suggests that I don't know enough to be here, so I'll just lurk until I get smarter. :)
Thanks again.
Jim
RE: Need pointers for calculating 6061 shrink fit
Rather than relying on the press fit to transmit torque, I'd make it a light press or transitional joint, and Dutch pin it.
It takes a long time to get smart by lurking. Asking your own questions is much faster. Go ahead; nobody here will intentionally insult you.
Mike Halloran
Pembroke Pines, FL, USA
RE: Need pointers for calculating 6061 shrink fit
The reason I don't want to make it as one piece is that it's a single complicated part that I have to turn on a manual-control lathe. There are two multi-groove serpentine belt sheaves and a very precise press fit onto a 5/8" driven shaft. That bore is deep in the part and would be very difficult to do with my machines and tools.
My plan is to turn off one set of grooves, shrink fit the sleeve over it and cut a new set of grooves in the sleeve. That's going to be significantly less work than cutting two sets of grooves, boring the shaft hole, and making all the other, non-critical features of the part.
By 'Dutch pin' do you mean pins inserted axially into the joint? Good idea! But why not shrink-fit and then pin it? Either a shrink fit or a light press fit is of similar difficulty with my machines.
I could also TIG weld it, but don't want to mess up the metallurgy. The last couple of grooves would be in the HAZ, and I don't want them annealed. The added cost of having the part heat treated would price it into the range of making a complete part from scratch.
This is a drive for a supercharger. It's a tight press fit onto the alternator shaft with an axial retaining bolt. The driven sheave is next to the alternator. The sheave that drives the supercharger is outboard of the driven sheave, and is currently the same diameter. It can't transmit quite enough power, so I'm increasing its diameter by 5/8".
The only redeeming feature of the setup is that the belt tensions are applied in opposite directions, so that the load on the alternator shaft and bearing is minimized. The arrangement is far less than ideal, but it's what my customer has to work with.
Thanks again!
RE: Need pointers for calculating 6061 shrink fit
Yes, I mean axial pins through the joint, so you're not carrying torque with the shrink alone.
Instead of smooth pins, I'd use setscrews, so the joint can also carry some axial force incidental to pressing the pulley on the alternator.
Next time you have a lot of machining to do, consider aluminum alloy 2024.
Mike Halloran
Pembroke Pines, FL, USA
RE: Need pointers for calculating 6061 shrink fit
I almost always use 6061 for machining because it's usually available in the configuration I need. My supplier in St. Louis carries lots of 6061, but only a few 2024 square bars and a modest inventory of sheet and plate. Round stock I would have to get mail order. For a single part, which is almost all of my business, the difference is usually not worth the bother and expense. 6061 cuts slowly but very well with the fluid and tooling I use.
When you're designing a part for production, you have a lot of options available that I, as a one-man job shop, just don't have. But then I have the luxury of seldom getting close to the limits of the material. If in doubt, I can usually add a little more.
RE: Need pointers for calculating 6061 shrink fit
RE: Need pointers for calculating 6061 shrink fit
I would think that the shrink fit would be pretty secure unless the belt slips. That would heat the sleeve more than the hub and loosen the joint.
RE: Need pointers for calculating 6061 shrink fit
Shouldn't the shrink fit be capable of taking the required
torque without slipping? you make no mention of the power and hence the torque your trying to transmit your basis for an interference fit should based on this.
Secondly will the joint be subject at any time to oil ingress which would reduce the friction in the joint and make it slip easier under load.
Regards
desertfox
RE: Need pointers for calculating 6061 shrink fit
It is. That would give you the same stress as in the steel part... so you may crack the aluminium one. Getting a good press fit in aluminium is not as straightforward as it might seem.
Here's a link to a calculator FAQ404-1230
Incidentally that comment in my sig isn't aimed at the you, it's there for everyone. Including me!
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Need pointers for calculating 6061 shrink fit
I could build a belt tensioner with a strain gauge on it and calculate torque from that, but again, that's more work than making a complete pulley.
As for oil entry, that won't be a problem; it will probably never be exposed to oil, but it will get water spray, and might get detergent or other cleaning agents.
That calculator is interesting. I'll run the dimensions through it later and see what it says. BTW, so far my experience tells me that I want .002" to .003" interference. Now's my big chance to splatter egg all over my face.
RE: Need pointers for calculating 6061 shrink fit
i think greg's later post is quite correct, in that if you install an Al fitting with the same interference as a Steel fitting, i think you'll likely see plastic stresses in the Al, which are probably undesirable. note, the 1/3 ratio applies only if both parts are elastic.
0.002" to 0.003" interference on a 2" diameter doesn't sound like much.
i'd worry about the groves, the wall of the sleeve is 0.39" thick, with a 0.1" grove in it, with a bunch (that's not a mathematical term) of tension stress from the interference.
RE: Need pointers for calculating 6061 shrink fit
It also says that I need about 44 degrees K differential to get it together. I think. It shows the inner part at -44 degrees K and the outer part at +44 degrees. If it's 'or' that's about what I got earlier by completely unrelated calculation. If it's 'and' I'm totally screwed up.
As for the mechanical implications, I don't believe that crushing the inner part is a significant concern; it will see only compressive forces and is well shaped to resist them.
Deforming the outer part is a concern, though. The calculator shows stresses on it from 6 to 9 MPa, or a max value of 11.7K psi. My reference shows yield strength of 40K psi for 6061T6.
This sure has been a fun learning experience, and the numbers make sense to my feeble mind. But if anyone would care to check my work I'd appreciate it.
RE: Need pointers for calculating 6061 shrink fit
does 150% mean your motor generates 200 in.lbs *1.5 = 300 in.lbs or 100 in.lbs (200 in.lbs is 100% more than 100 in.bs; 150% more is 300 in.lbs) ?
i don't know that the calculator stresses take the groove into account.
my data (mil hdbk 5) says ftu = 40 ksi, fty = 36 ksi for 6061T6.
is 9 Mpa = 12 ksi ? i always go back to 14.7psi = 0.101325 MPa so that 1ksi = 7 MPa ... maybe there's a decimal out of place ?
RE: Need pointers for calculating 6061 shrink fit
And I meant foot pounds of torque. Dope slap self-administered. I've been translating metric and American values back and forth until my head spins. I deal with metric terms too little to have a feel for what a million Pascals or a hundred Newton-meters is.
The engine has put out 430 foot pounds on a chassis dyno, and there's about a 3:1 ratio between the crank pulley and the driven alternator pulley. Allowing for the loss between the engine and the chassis dyno, 150% is close enough. In any case there is no way that the sleeve will see enough torque to make it slip; the drive belts can only transmit a fraction of what's available anyway.
For the stress calculation I used the minor dimension of the grooves, so the those numbers should be conservative.
As for being a decimal off, absolutely a possibility; as I said, I'm not used to dealing with these units, and could have very easily dropped a 10 in the translation.
RE: Need pointers for calculating 6061 shrink fit
RE: Need pointers for calculating 6061 shrink fit
Good point! Its max speed could be 18,000 RPM or a bit more. I'll need to sheck the engine data and confirm the drive pulley size; a 3:1 ratio is just a reasonable guess. The section should be about .26" thick, not counting the larger diameter of the ribs.
I think I know where to look for a calculator.
Thanks!
RE: Need pointers for calculating 6061 shrink fit
a word of caution, calculating the stress based on the net area (the cross-section through the base of the groove) is not as conservative as it appears. the stress concentration peak at the base of the groove will be higher than the thickness ratio, if you can see what i mean. but it's probably not a problem, given the very loaw stresses you're anticipating.
if i read the posts correctly, i think you've considered adding shear pins ... this would be a much better way of transmitting torque, but harder to accomodate with your groove.
thinking about your stress, maybe you don't have enough interference ?
as i understand things, you've got a 450 ft.lb motor working at 6000 rpm. there is a step-down so that the hub is expected to see 150 ft.lbs of torque at 18,000 rpm. 150 ft.lbs is equivalent to a couple of 1200 lbs (acting on an arm of 2"). the shear area supporting this force is pi*d*w, about 6w, a shear stress of 200/w psi, w maybe 1" (or more), shear stress = 200 psi, not much.
RE: Need pointers for calculating 6061 shrink fit
My customer's main concern is that the mechanism will slip, but I think that's covered. Once I get down the the final dimensions I'll have to see what room I have for Dutch pins. I'm hoping that the metal they displace won't cause a problem, being in a relaticely high-stress location.
RE: Need pointers for calculating 6061 shrink fit
RE: Need pointers for calculating 6061 shrink fit
RE: Need pointers for calculating 6061 shrink fit
Mike Halloran
Pembroke Pines, FL, USA
RE: Need pointers for calculating 6061 shrink fit
We chilled the hub to about 0F in a deep freeze and boiled the collar. That should have given an assembly clearance of .0045. We had excellent surface finish, so it should have worked.
The assembly started well, but about a third of the way on, of about an inch total, it got too tight to push by hand. I quickly put it in the press but it was too late. The next movement apparently galled the pieces, and the hub subsequently collapsed.
I knew I should have gotten some dry ice to chill the hub. Drat! So now we'll just machine a complete pulley.
Thanks for all the help, guys! Even though it didn't work out, it was a nice education.