Question on interference fit for grid-type shaft coupling.

Kevin Q · Apr 23, 2016

I removed a coupling half from a reducer output shaft and noticed the rust color which immediately brings to mind a looseness condition. The key also had a slight offset, about .015" and it appeared to be from torque. It wasn't from wear, both offset sides still measured 2.500" in width. I measured the shaft and it was 10.500 inches exactly, the coupling bore was 10.494". There was .006" interference, still from the coloring on the shaft and the displacement on the keystock I have to assume that .006" of shrink wasn't enough for the torque load present.

I have a new coupling coming to bore and key. I am going to try a .012" interference on the bore. It is a Falk 1220 T10 grid-type shaft hub. Does anyone here have experience with the proper interference needed for a coupling that shows looseness even with .006" of interference? Just throwing it out there in hopeful anticipation...Thanks, KQ

atomarc · Apr 23, 2016

That is one big gear reducer! I have serviced quite a few Falk grid style couplings over the years and they all had a rusty tinge to the lube. I always attributed it to the fretting of the cage and grid components under less that perfect alignment. If you miss fire on your .012 shrink fit as you install it...you're going to need another new coupling in the wings.

I'll bet the rust color you found is from the grid, not the shaft to coupler interface. That is a huge gear...I don't blame the key for taking a hit when something bound up, at least it didn't shear clean off.

If I remember correctly, the factory Falk lube is almost a brown, rust color right out of the box.

Stuart

Hbjj · Apr 23, 2016

+1 on the .012 shrink fit
IIRC those couplers are 1045
go easy with the heat

IronReb · Apr 23, 2016

.0005" per inch is what I have always heard from the old timers as the go to ratio of how much interference a hub needs.

The mass of the hub matter very much too of course.

At 10.500" and a flange that's what. 20 something inches? will hold a lot of heat and grow a lot.
Same size bore but with only say a couple inches wall it will not hold as much heat and not grow very much, well it will but the cold shaft will suck the heat right out.

I have no doubt you can get it to expand .012" but I would have some serious concerns that if you babbled just the least little bit your gonna have a coupling
stuck and the only way off will be with an arc gouger.

Maybe if you rigged up a styrofoam box around the shaft to soak it with liquid nitrogen AND heat coupling you could get away with it. A big bottle of nitrogen is not that expensive. If that cant be done pack some dry ice around the shaft for about an hour, that big of shaft even with dry ice temp should shrink pretty good.

If it were me, I would look for documentation for a recommended "by the book" amount of interference to reference to if it all goes south.

Personally, I would shoot for .006"-.007" tight.
We have many a coupling out there in the wild at those numbers running and making money.

At .006" you are gonna have to get it out to .012 at the least and hurry the F up, you have just a minute before it shrinks once it hits that cold shaft.
Now if you want to double that amount you just raised the chances of sticking that coupling about half way on there.
Then you get to tell customer the great news that you have a new one on a plane and he will only have to wait another 24 hours or so.

One thing to consider, in my mind its better to slip a coupling and shear the key than to transfer that blow to a gear set that's more expensive to replace.

HuFlungDung · Apr 23, 2016

I don't deal with stuff that big at all, but I'm suspecting that the hub wall thickness is not enough to provide the needed clamping effect. What is the net thickness of the hub above the key?

motion guru · Apr 24, 2016

The coupling manufacturer will have very specific bore / shaft interference fit instructions as well as assembly instructions. We have worked with a few shafts / couplings of that size and given the expense of the coupling and no-doubt the expense of the process being run by it - it simply doesn't pay to do this without understanding what the coupling manufacturer recommends for use.

JS · Apr 24, 2016

Marks mechanical engineers hand book has a place that shows you how to figure the clamping stresses as well as the force required in tons move the fit.

I've seen some stupid fits on couplings here at the mine. The key is your safety, if it shears it limits damage.

I would use Min .006 - Max .007 and a tight key fit.

when heating to install use an oven if possible, if not a two to three rosebuds.

D&T face for all thread , I think I used 1.25" last time I did one that big.

Wire rope to choke it and the all thread helps guide it. Plus if you get it stuck a strong back and a 50 - 100 ton hydraulic ram will pop it off. White lead or copper anti seize on the shaft.

Heavey Metal · Apr 24, 2016

If it shows rust and no frett I would suspect a crack in the coupling
https://www.google.com/url?sa=t&sou...iBkmYHjzEOvDx9B8g&sig2=O8PRlbkGt0BhpoD8fsUi6Q

In my opinion 6 is correct 10 takes some special treatment and 12 will make for a bad day

Ln2 is your friend also try a key in the heated coupling to find out how much it closes up

muckalee · Apr 24, 2016

I am interested in reading about how you removed the hub from the shaft.

Tyrone Shoelaces · Apr 24, 2016

A lot of good advice here. In my working career I've fitted plenty of similar couplings. 0.006" interference on a 10" dia is a fair amount. 0.010" is a lot and for 0.012" you better have a really good set up and work real FAST. The heat transfers RAPIDLY. Big oxy torches would be my weapons of choice.
If the shaft keyway is open ended we left the real key out and fitted the coupling without the key with what we called a " mouse ". That's a short piece of key exactly the correct size with a long rod welded to it so you don't get your fingers burnt and it allows you to line up the coupling and shaft.

Bottom line - Do Not Dawdle !

Regards Tyrone.

Tyrone Shoelaces · Apr 24, 2016

muckalee said:
I am interested in reading about how you removed the hub from the shaft.

Most of the couplings we fitted we drilled and tapped for two large threaded rods before assembly. Set up a strong back off the two threaded rods and a 50 ton or 100 ton hydraulic " Enerpac " jack. Get the oxy torches on the coupling and start then pumping the jack. You'd get maybe 2" before the heat transferred and the coupling stopped moving.

Cool the coupling down with compressed air guns and soaking wet rags. When it was cool repeat the process. You'd get maybe 2.5" next time.

I've known it take a day and a night to remove big couplings but - " Who Dares Wins " as we say over here.

You might say - " Failure Is Not An Option " .

Regards Tyrone.

keithmech · Apr 24, 2016

another possibility for coupling connections is using a bikon lock ring.
Home
I have used these on large couplings with good success.Costly but
easy to install and remove.

IronReb · Apr 24, 2016

Tyrone Shoelaces said:
Most of the couplings we fitted we drilled and tapped for two large threaded rods before assembly. Set up a strong back off the two threaded rods and a 50 ton or 100 ton hydraulic " Enerpac " jack. Get the oxy torches on the coupling and start then pumping the jack. You'd get maybe 2" before the heat transferred and the coupling stopped moving.

Cool the coupling down with compressed air guns and soaking wet rags. When it was cool repeat the process. You'd get maybe 2.5" next time.

I've known it take a day and a night to remove big couplings but - " Who Dares Wins " as we say over here.

You might say - " Failure Is Not An Option " .

Regards Tyrone.

If the coupling is to be replaced, which often it is, a big washing tip for the O/A rig gets deployed and the coupling gets split.
Now if unit is in the middle of a coal processing area with coal dust 3" thick on everything, well...its gonna be a long day with the portapower.

We have one customer who wants slip fit coupling with two set screws for the simple reason of no open flames or sparks of any kind in the area were coupling is used. (refinery)
None of them are nearly this large though. Biggest one I can recall off hand is a 6" bore.

JS · Apr 24, 2016

If you have a hydraulic ram long enough, it comes off easily in one try.

The ring fetter as we call them are a blessing and a curse. People who don't read directions usually screw them up.

They get in a hurry and remove the screws all at one time. You have tighten and loosen following procedure.

Kevin Q · Apr 24, 2016

I should have been more clear on the discoloration...it was on the shaft/coupling bore I.D. The O.D. of the coupling body is 22.25". That would be 6.125 wall thickness -1.250 for the key on one side. The grid portion of the coupling is larger than that. Ditto on the drilled and tapped face for the puller bolts. They are even more important on the gear type couplings as you can't remove the cover and get a good O.D. for the puller jaws.

I use 2 Propylene torches for heat. They are 475,000 BTU's per torch. They have long handles so we can get back away from the heat. The end of the torch looks like the afterburner on a jet engine. Sounds like it too. The end of the torch is larger than a silver dollar, perhaps 2 1/4" in diameter.

Something I do before installing a coupling is to check the key in the broached hub to make sure it fits. I have seen more couplings stuck from too shallow keyways or keyways in hubs which aren't straight to the hub bore than from having undersized bores. Also, I use light oil (EZ-1) on the shaft rather than anti-seize. The anti-seize gets hot and it piles up on the shaft like fish scales and the coupling invariably sticks.

One of the problems I have had on removing couplings is the galling of metal while the coupling is coming off. Sometimes it will really tear the shaft. A work-around for that is to put the puller on but put just enough pressure on the jaws to keep them from coming off. Then heat the coupling as quickly as possible over the keyway. When it is smoking hot you can then apply the hyd. pressure. If there is a lot of pressure applied early and the coupling moves prematurely it really seems to tear.

I also use an inside mic. and set it to about .006" larger than the shaft diameter. I will check the bore I.D. during the heating process on both ends and the middle and at at different points like 12 o'clock, 3 o'clock, etc. Sometimes heat isn't evenly applied. A little care here can save a lot of grief.

A lot of the couplings here are shot and can't be salvaged. On those we will make an axial cut with a Metabo over the key and then pulling the coupling is a snap.

OBTW: I will cut the keyway with the big draw type key cutter we picked up used from an equipment dealer last fall. D.C. Morrison retro-fit the draw bar to take their cutters. It will use their 4-bolt cutter, and they included in the job an insert that will hold one of their 2-bolt cutters. This machine will cut a keyway up to 4.5" wide. It will do 3.5" wide with a single cutter, the 4.5" will need two passes. This 'little' one at 2.5" wide should be a snap. It also will cut up to 30" in length.

Thanks for all the ideas and I'll keep you posted on how things go...KQ

atomarc · Apr 25, 2016

It certainly appears you have all the experience and tools to get the job done properly. It's interesting to see the scale some folks work in.

Stuart

Rock6.3 · Apr 25, 2016

Kevin Q said:
I should have been more clear on the discoloration...it was on the shaft/coupling bore I.D. The O.D. of the coupling body is 22.25". That would be 6.125 wall thickness -1.250 for the key on one side. The grid portion of the coupling is larger than that. Ditto on the drilled and tapped face for the puller bolts. They are even more important on the gear type couplings as you can't remove the cover and get a good O.D. for the puller jaws.

I use 2 Propylene torches for heat. They are 475,000 BTU's per torch. They have long handles so we can get back away from the heat. The end of the torch looks like the afterburner on a jet engine. Sounds like it too. The end of the torch is larger than a silver dollar, perhaps 2 1/4" in diameter.

Something I do before installing a coupling is to check the key in the broached hub to make sure it fits. I have seen more couplings stuck from too shallow keyways or keyways in hubs which aren't straight to the hub bore than from having undersized bores. Also, I use light oil (EZ-1) on the shaft rather than anti-seize. The anti-seize gets hot and it piles up on the shaft like fish scales and the coupling invariably sticks.

One of the problems I have had on removing couplings is the galling of metal while the coupling is coming off. Sometimes it will really tear the shaft. A work-around for that is to put the puller on but put just enough pressure on the jaws to keep them from coming off. Then heat the coupling as quickly as possible over the keyway. When it is smoking hot you can then apply the hyd. pressure. If there is a lot of pressure applied early and the coupling moves prematurely it really seems to tear.

I also use an inside mic. and set it to about .006" larger than the shaft diameter. I will check the bore I.D. during the heating process on both ends and the middle and at at different points like 12 o'clock, 3 o'clock, etc. Sometimes heat isn't evenly applied. A little care here can save a lot of grief.

A lot of the couplings here are shot and can't be salvaged. On those we will make an axial cut with a Metabo over the key and then pulling the coupling is a snap.

OBTW: I will cut the keyway with the big draw type key cutter we picked up used from an equipment dealer last fall. D.C. Morrison retro-fit the draw bar to take their cutters. It will use their 4-bolt cutter, and they included in the job an insert that will hold one of their 2-bolt cutters. This machine will cut a keyway up to 4.5" wide. It will do 3.5" wide with a single cutter, the 4.5" will need two passes. This 'little' one at 2.5" wide should be a snap. It also will cut up to 30" in length.

Thanks for all the ideas and I'll keep you posted on how things go...KQ

Pictures PLEASE !!!

Tyrone Shoelaces · Apr 25, 2016

Kevin Q said:
I should have been more clear on the discoloration...it was on the shaft/coupling bore I.D. The O.D. of the coupling body is 22.25". That would be 6.125 wall thickness -1.250 for the key on one side. The grid portion of the coupling is larger than that. Ditto on the drilled and tapped face for the puller bolts. They are even more important on the gear type couplings as you can't remove the cover and get a good O.D. for the puller jaws.

I use 2 Propylene torches for heat. They are 475,000 BTU's per torch. They have long handles so we can get back away from the heat. The end of the torch looks like the afterburner on a jet engine. Sounds like it too. The end of the torch is larger than a silver dollar, perhaps 2 1/4" in diameter.

Something I do before installing a coupling is to check the key in the broached hub to make sure it fits. I have seen more couplings stuck from too shallow keyways or keyways in hubs which aren't straight to the hub bore than from having undersized bores. Also, I use light oil (EZ-1) on the shaft rather than anti-seize. The anti-seize gets hot and it piles up on the shaft like fish scales and the coupling invariably sticks.

One of the problems I have had on removing couplings is the galling of metal while the coupling is coming off. Sometimes it will really tear the shaft. A work-around for that is to put the puller on but put just enough pressure on the jaws to keep them from coming off. Then heat the coupling as quickly as possible over the keyway. When it is smoking hot you can then apply the hyd. pressure. If there is a lot of pressure applied early and the coupling moves prematurely it really seems to tear.

I also use an inside mic. and set it to about .006" larger than the shaft diameter. I will check the bore I.D. during the heating process on both ends and the middle and at at different points like 12 o'clock, 3 o'clock, etc. Sometimes heat isn't evenly applied. A little care here can save a lot of grief.

A lot of the couplings here are shot and can't be salvaged. On those we will make an axial cut with a Metabo over the key and then pulling the coupling is a snap.

OBTW: I will cut the keyway with the big draw type key cutter we picked up used from an equipment dealer last fall. D.C. Morrison retro-fit the draw bar to take their cutters. It will use their 4-bolt cutter, and they included in the job an insert that will hold one of their 2-bolt cutters. This machine will cut a keyway up to 4.5" wide. It will do 3.5" wide with a single cutter, the 4.5" will need two passes. This 'little' one at 2.5" wide should be a snap. It also will cut up to 30" in length.

Thanks for all the ideas and I'll keep you posted on how things go...KQ

You seem to have covered all bases there Kevin. I worked a lot on Rubber mixing machine that used similar sorts of couplings on the motor to gearbox and gearbox to mixing machine. 1000 HP motors were not unusual.

The techniques you recommend are the techniques we also used. Making sure the keys fit before assembly is essential.

Regards Tyrone.

Heavey Metal · Apr 25, 2016

If your dealing with both the shaft and coupling you can use a series of steps to reduce the distance neaded for

The removal.

An electric motor outfit that I sometimes bore and key for uses an induction heater to remove and replace those couplings.

Tyrone Shoelaces · Apr 25, 2016

Heavey Metal said:
If your dealing with both the shaft and coupling you can use a series of steps to reduce the distance neaded for

The removal.

An electric motor outfit that I sometimes bore and key for uses an induction heater to remove and replace those couplings.

I can see how an induction heater would work when you're installing the couplings. Will an induction heater get the heat into the coupling quick enough during removal ?

Regards Tyrone.

Question on interference fit for grid-type shaft coupling.

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