Hot "Pressing" with a mallet

I need to hot press a bush into an awkward object which is too big for a working space of my bar press. Bush is cast iron, casing is a steel. Hole is 1.5".

Parts are made to medium interrupted fit. I may go to some place where they have a bigger press but I am tempted to stuff bigger part to the oven, smaller one to freezer and then bang them together with a mallet or with a hammer over a plank of wood.

Alternatively I could smile to my friend, get 2-3 liters of liquid nitrogen, cool down the smaller part and do the same. Unfortunately with the second method which I know would work I cannot use anaerobic retainer because it will solidyfy upon contact with cold part and I would like to use it.

So I am left either with hot "percussion" pressing or with going to engineering firm which have bigger press.

What would you do?

Martin

Martin, if you have access to both ends can you pull the bush in using a suitable piece of studding and some clamps? you may need to make a guide or stepped washers to keep thins square.

Chris Gunn

Martin if the fit of the components is such a tight interrupted fit and that difficult to get the pieces together are you going to need anaerobic retainer as well?

Dave W

How much interference? 1 thou per inch* is 100°C with steel, 3 thou per inch is ambitious. Both should allow a slip fit with heating and cooling. More than that is generally unproductive.

*Other measurement systems are available e.g. 1mm/m; 5' 3"/mile etc.

Rule of thumb for shrink fits is 1 thou interference per inch diameter. Much more than this you risk the hub cracking if it is cast, or yielding if ductile. Having tried and failed to get railway wheels off (narrow gauge) which had been shrunk on to this rule I can vouch for its effectiveness.

As Mark says you don't need that much heat, but you need to be quick as the hub transfers heat to the axle very quickly, and if it nips before it is home it's a boring out job unless you're handy with a round nose chisel. This is why liquid nitrogen is popular as the heat transfer between very cold and room temp is less than from hot to room temp.

In either case I don't see the need for retaining compound, if you want to use that just make the bush a slide fit.

Don't even bother putting the bush in the freezer. Temp difference from room temp of 25C to freezer of -5C makes little difference to the size on a small item like this. But the bigger the temperature differential between the bush and the housing, the faster the heat will rush out of the hot object into the cooler object, thus shrinking the former and expanding the latter.

If you make your bush with the usual one thou per inch of interference fit and then heat the housing to 150 to 200C with a propane torch, it should just about fall in. (The amount of expansion of steel being roughly one thou per inch per 100C.) And you can put hi-temp loctite on it too if you feel you must.

Being prepared with a suitable stepped drift and large hammer, plus an even chamfer on the leading edge of the bush (and the hole it goes into) to guide it in would be good preparation. Or stepped plug with a large nut and bolt through it to a flat plate on t'other side of the housing would pull it in nice and even too.  For something the size of your job, I use a 9/16" UNF grade 8 bolt and nut. Those fine threads will exert A LOT of force with a large ring spanner or socket and breaker bar on the nut, with plenty of anti-seize on the threads.

If movement is a critical concern -- and once again we have been given no idea of the application for the parts in question so we can only guess -- you can always drill and dowel or drill and screw one or more places around the OD of the bushing to mechanically lock it in position. Very rare that this needs done in this age of Loctite though.

Edited By Hopper on 28/03/2018 05:57:13

I believe they don't actually recommend the use of Locktite with an interference fit. There needs to actually be some retainer in the gap, if the fit is too tight it gets squeezed out.

If you use an interference fit, it does not need to be too tight. If it is, it will just crush the bush down anyway, or stretch the housing if that is weaker. Also galling can occur as you press them on, so both surfaces get damaged. You can end up with it stuck half on, and unable to get it off again easily.

So choose one way or the other. Loctite or the equivalent can provide an excellent job done properly. I've just been making 5/8 inch brass balls for a friend, they are locktited onto a 1/4 inch mandrel for turning and they are quite hard to get off afterwards, heat is needed.

If the hole is not blind, you can pull the bush in with a bolt and nut and washers.

John

If it were me doing this i would use a bigger press - if you have ever had a bush freeze half way in no manner of hammering will move it . I wouldn't bother with retaining compound as a properly designed press fit won't move once it is in and with some bushes you have to use an anti galling compound on the surfaces , you could champfer the bush and stake it in if you really want to .

Edited By XD 351 on 28/03/2018 07:03:24

I'm afraid they do, from their own guide:.

"Clearance
FACTOR 3
The best performance for slip fits is achieved using
clearances between 0.025 mm and 0.075 mm
(0.001 in. and 0.003 in.), or with interference fits.
Performance is reduced as the clearance is increased."

Tunnel vision here, I reckon. If liquid nitrogen is available, it is obviously the best way to install interference fitting parts. Warming the housing would likewise increase the clearance for fitting.

But, without being provided with all the details it is not clear of the loading on the bush in use, etc. How long is the bush? What is the ID? Axial or radial loading? Plain bush or lipped? Through bore or blind? Will the bore be machined after installation?

Quite likely a slip fit with a suitable grade of loctite, or similar, would suffice. Quite likely a couple of grubscrews, on the joint line, would add further security.

Personally, I don’t see any particular problem given the scanty information provided.

Hi, Thanks for comments and advise.

Here is the part already made:

As you can see, it is nothing particularly sophisticated and I bet you already know, how it was fitted...

@Samarsanda, Duncan, John Olsen:

Once brass bush from my ML 7 pulley fitted in old Myford (Nottingham) gone loose I have decided to add Loctite as an additional insurance in power transmission systems whenever practical.

@Mark Rand, Chris Gunn & XD351

Actually it went in nicely at 250*C @ 2thou per inch of interference. All what was needed was a mallet and wooden block, which was not really critical but applied to bottom it down to shoulder.

@Neil

I agree. Loctite is just an extra security here and worth using.

@not done it yet & Duncan,

I ofen use liquid nitrogen for precision parts but this one seemed unsophisticated and liquid nitrogen would not be compatible with Loctite - it would freeze it. I will one day test liquid nitrogen with loctite to confirm this belief, eg I will find out, how fast is freezes.

Duncan, I am not sure about about slower heat transfer in low temperatures - when you look on thermal conductivity tables it seems to go drastically up in very low temperatures. There must be something more to it.

@Hopper,

My freezer goes to -20*C so I can win 50*C, eg 0.5 thou per inch. Every little helps, particularly if you have plenty of time to waste and patient wife.

Anyway, excitement is over and job is done. I hate spoil my work and I have seen professional guy stuffed half way with hot pressed bush. I have seen some obviously messed up job in corner if his shop, asked what have happened (what was already rude perhaps). After bitching for a while he explained that cold bush didnt drop in like it usually does and press didn't square... more bitching followed.

Martin

Edited By Martin Dowing on 28/03/2018 22:56:17

Obviously it will work with that large interference, but 250C will have seen off your loctite, The difference in using liquid nitrogen rather that heat is in radiant heating150C to 15C gives (150+273)^4-(15+273)^4 = 2.5e10 to drive the rdaiant heat, but 15C to -120C (same difference) gives (15+273)^4 - (-120+273)^4 = 6.3 e9, only 1/4 of the radiant transfer.

Is that a clutch pulley from an ML7? Could you have fitted a ball bearing instead and dragged it kicking and screaming into the 20th century?

If it is a clutch pulley from an ML7 and the sole purpose of the bushing is to allow the pulley to spin on the shaft under no load when freewheeling in the disengaged position, heavy press fit plus heat plus Loctite was way overkill. Light press fit (one thou total interference) or even a push fit with Loctite would have done the job.

Manufacturer of Loctite 636 suggests that at 250*C it is fluid enough to disconnect parts. Monomeric urethane acrylale which can be found in the bottle would polymerize at this temperature (that is what we want anyway), polymer should be barely stable but in liquidous phase.

Anyway thanks for radiant heat related explanation.

@Hopper, no this part does not belong to ML7/S7 lathe. Just a pulley.

Martin

Sorry, I rather thought it did after reading your post: "Once brass bush from my ML 7 pulley fitted in old Myford (Nottingham) gone loosie I have decided to add Loctite as an additional insurance in power transmission systems whenever practical."

I must have misunderstood.

So we still have no idea what this Top Secret pulley's application is. Whether it will be transmitting the power from a 50hp motor at 1400rpm or acting as a belt tensioning idler on a 2rpm hand cranked glass display cabinet, or one of the 10 million possibilities in between.

Which means there is no way for anyone to know whether they are offering you relevant advice or not, as specs and methods vary accordingly. It leaves everyone guessing in the dark and a good chance you end up with duff advice.

I'm not sure why we get so many questions about this type of Top Secret project where no specific details can be given about the application or the exact job in hand. Must be a lot of mad inventors on here trying to keep one step ahead of the competition.

Edited By Hopper on 29/03/2018 08:25:42

Yes, it is a Top Secrect project, will run uranium centrifuge at 200 000 rpm.

Realistically it will be used for a grinding setup with 1hp motor at 1450 rpm.

Martin

Dr Stangelove and the 200k centrifuge eh.

Ah well, if pulley is used for driving a grinding wheel, a good crush fit is probably appropriate tp ensure no movement. How will you attach pulley to shaft? By cutting a key way int he cast iron bush?

Yes, I need to cut keyway, tommorow I will. Have a broaching set and it will come handy. Bought it few months ago.

Btw after hot pressing bush bore have contracted a bit. Had to reopen it a bit with a reamer.

It is important not to forget about a hole for a setscrew *before* pressing such a bush. Otherwise there will be unnecessary hassle.

Martin

Yes, shrinkage like that of the ID is pretty common with a heavy press fit. Metal is not as immovable as we imagine it to be! Another way of doing it is to make the bush ID undersize, press it in, then set the whole shebang on the faceplate and bore the bush to final size in place, thus ensuring it is perfectly concentric with the OD.

Ah yes, the set screw. You might need a long series drill bit, or drill the hole in at an angle. I don't suppose the key will know any different if the screw is angled a bit.