Thread: How much interference for a press fit?

Making some new blade drive plates for my mower as the old ones were flogged out.
Shaft is 20mm diam with a 4mm key but since I've got the opportunity I'd like to make the new ones a press fit to help stop them moving.
New plates are 16mm mild steel (old ones were cast aluminium).

I've looked at the fits and tolerances in my Engineers Black Book and think I've managed to get my head around it.
If I've got it right, for a "Medium Drive Fit (Press Fit)" it works out to be from 0.014mm to 0.048mm interference between the shaft and bore.
But the "Force Fit" range only changes by 0.006mm more - from 0.020-0.054mm interference.
I'm thinking I aim for 0.03mm, but they're just numbers to me - I've got no feel for how tight it will be.

I want them to be as tight as I can practically get them, but I've only got a 6T supercrap press so I don't want to snooker myself by making them TOO tight and getting them stuck half way on.

Can anyone give me some feedback on whether 0.03mm interference is overdoing it or not?

Steve

.03mm is only .0011", for 20mm dia an interference of .0025" might take 10 ton to push on.
But a .001" fit won't take much of a push, you could go .0015" and your press should handle it no probs

Awesome - thanks Shed

Steve

If you spoke nicely to the little lady, she might just let you put the shaft in the freezer to shrink it a touch, if it's small enough that is. That way your little supercrap press will handle it easily.
Kryn

Hi Steve, Guys,

I agree with John, 1.5 to 2 thou will be a tight press fit ! Heating the hole or shrinking the shaft will allow say another quarter to half thou if you needed it to be particularly tight.

Reminds me of watching the men at the railway works pressing the wagon axles into the wheels !

.001" per 1" of diameter is the rule we use at work. More than that and i have found the parts need to be shrunk together, because they will gall up when you press them together.

It never ceases to amaze me the small numbers required to go from a sliding fit to a slight interference fit and then to a press fit.

As mentioned above, freezing the inner and heating the outer (even to only 100 degrees C) can change it from a press fit to a sliding fit and make life very easy at assembly. Also less chance at damaging the parts. However, I suspect that may not be practical with the case at hand...

Edit: can I be indulgent and share with you the huge sense of satisfaction I receive when I remove a freshly turned part from the lathe and it press fits beautifully with its mating part in the press? God dam that's a good feeling!

Simon

Be careful with press fits on mowers, at some stage you will want to part the pieces, with all the crap the joint will be exposed to it will no doubt oxidise, no point it making it difficult.

No dramas with heating/cooling these parts for assembly Simon.
The whole spindle assy is installed from under the deck.
The pulleys are installed after the rest is bolted to the deck. That’s the next thing I’ve got to repair. Pressed steel pulleys - one separated from it’s hub, the other 2 have bent a bit from the puller - even with plenty of penetrant and some heat.

Steve

Originally Posted by snapatap

.001" per 1" of diameter is the rule we use at work. More than that and i have found the parts need to be shrunk together, because they will gall up when you press them together.

I was taught the same .001 for pushing bits together. Use .0013 if liquid nitrogen is being used.

Originally Posted by mwork

Be careful with press fits on mowers, at some stage you will want to part the pieces, with all the crap the joint will be exposed to it will no doubt oxidise, no point it making it difficult.

Good point.
I’ll definitely be using a non-press fit and some loctite for the pulleys. It will keep the crud out of the joint and with a bit of heat should actually make them easier to remove later.
For these blade plates, I reckon as long as they stay solid on the shafts the next time it comes apart the alloy spindle housings themselves will need replacing due to corrosion, so all bets are off as to the final outcome then.
I’ve seriously considered just welding them into the shafts, but that’s too much commitment for me!
Maybe Mrs Claus will buy Santa a new mower for Xmas then

Steve

I definitely would not weld them, as in my experience you will likely get cracking on such an item subjected to heavy vibration as that blade mount is going to get. It is probable that you will need to replace bearings in the spindle housings long before they corrode away, so it might pay to think just how you will get it apart again. Can you set it up with a jacking screw?

Machinerys Handbook has quite a bit on sliding to interference fits so that is probably the best place to look if looking for the standards and recommended fits.
My personal view is that all jobs are different and the interference fit needs to be suited to the thickness and type of materials and
the requirement for the fit.
For example if you need to repair a flogged out bearing bore in a cast iron housing by boring it out and fitting a sleeve then it might be advisable to make the sleeve with only .0005" interference fit so as not to induce stress into the cast iron that might break it.
Hardened steels also only require a small interference fit.
I often press up to 60mm 4140 shafts into mild steel and use a .0025" fit, I put a radius on the edges of the chamfers and use oil to lube the shaft and bore, this normally takes about 12 ton to press.
Most of the smaller jobs that i press together i give about .001" and just use the vice to press the parts together.

Originally Posted by Ropetangler

I definitely would not weld them, as in my experience you will likely get cracking on such an item subjected to heavy vibration as that blade mount is going to get. It is probable that you will need to replace bearings in the spindle housings long before they corrode away, so it might pay to think just how you will get it apart again. Can you set it up with a jacking screw?

The drive plate is on the larger end of the shaft, and the shaft passes all the way through - so should be easy enough to press off later.

A photo or two probably makes it clearer.







Steve.

Makes it so much easier to give recomendations when we have pics to understand and work from.
Kryn