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Interference fit of bush - PB into mild steel

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Matt Stevens 106/03/2019 21:16:52
48 forum posts
4 photos

Hi All,

The Conn rod of a Stuart Victoria calls for a Phosphur Bronze bush be fitted into the turned mild steel connection rod going to the crank.

My plan is to have a interference fit with the idea that i will heat the conn rod int he oven, cool the bush int he freezer and then simply drop it in place. Hopefully without any pounding!

Question is - what interference should i go for....i am thinking 0.001" ( a thou). Is that sufficient? The bush is 3/8" diameter so only small.

Or is a better option to loctite a sliding fit? This way it can easily be removed, but i doubt this will ever be needed...

Opinions?

Thanks

Matt

Andrew Johnston06/03/2019 21:30:01
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4699 forum posts
532 photos

No question - a sliding fit and Loctite. That's the way I did the small end bush on my traction engine connecting rods.

Andrew

Paul Kemp06/03/2019 21:37:45
283 forum posts
9 photos

Matt,

Your method will work just fine as described but a good rule of thumb is 1 thou per inch, so your thou is a bit heavy, half a thou should be enough. I have just fitted a 3.5" cast iron liner to a cast iron cylinder the same way. Had 3 thou interference, cylinder on top of wood burner and heated to 150, liner in the freezer (about minus 10), dropped in a treat. When it cooled the bore had nipped in nearly 2 thou at the points of contact so I know it's tight and isn't coming out again any time soon! If you make your fit to tight all it will do is close down the bore (you can ream it after though). Or as Andrew says, glue it! Shrink fit is a lot more fun though.

Paul.

Matt Stevens 106/03/2019 21:44:34
48 forum posts
4 photos

So if i use loctite which is probably the easiest way at least, should there be a clearance or is a nice slide fit good enough to get capillary action to work? Further - are we talking red loctite or blue....i assume red? (Yes i know there are many different types, but i am generalising to what i have at home )

Neil A06/03/2019 21:51:16
37 forum posts

I would second the sliding fit and use Loctite 641 bearing fit or its equivalent.

A 0.001" interference on a 3/8" diameter is very high, we used to reckon on 0.001" per inch of diameter as a good interference fit.

Heating one part and cooling the other works well on parts that have a reasonable mass, but small components will equalise their temperature too quickly and you may not get them together before they grab.

With a small diameter bush like this, probably with a thin wall thickness, nearly all the interference will end up reducing the bore. With the Loctite method the bush can be finished to size before fitting.

How much clearance? Check the data sheet for what you have. Loctite guide gives 0.001 to 0.003 clearance as optimum.

Neil

Edited By Neil A on 06/03/2019 21:58:06

Edited By Neil A on 06/03/2019 22:08:35

martin perman06/03/2019 21:52:04
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1591 forum posts
66 photos

I've just reamed out a 1/2 inch hole and put a 1/2" dia piece of silver steel into the hole, it was a nice sliding fit, I've used Loctite 683 and the shaft is in there solid.

Martin P

Hopper07/03/2019 00:05:05
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3651 forum posts
72 photos

On the other hand, there is no reason you can't turn your bush half a thou oversize and tap it into place with a hammer and drift. That's how it would have been done when the Stuart engines were designed. Matter of personal preference really.

Nicholas Farr07/03/2019 06:41:35
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1881 forum posts
919 photos

Hi Matt, you need a retainer compound, if you wish to use Loctite, one of these should be fine **LINK** but there are other makes and their colours may differ.

Martin, do you mean 638.

Regards Nick.

Neil Lickfold07/03/2019 06:52:47
551 forum posts
102 photos

Actually if you use castor oil, you can have a 0.001 inch diameter press fit and assemble it without too much trouble at all. No need to heat anything.

Neil

JasonB07/03/2019 07:12:05
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Moderator
15538 forum posts
1594 photos
1 articles

I usually ream or bore the little end first then turn the bush so it is a firm push fit into the hole needing a wriggle if the rod to get it to slide over the bush. Then Loctite with 648 as I have a big bottle and it takes higher temps, 638 would be OK on the Victoria. Did one only last night.

Nick Clarke 307/03/2019 07:40:36
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288 forum posts
6 photos
Posted by JasonB on 07/03/2019 07:12:05:

I usually ream or bore the little end first then turn the bush so it is a firm push fit into the hole needing a wriggle if the rod to get it to slide over the bush. Then Loctite with 648 as I have a big bottle and it takes higher temps, 638 would be OK on the Victoria. Did one only last night.

While Loctite 648 is described as high temperature and 638 is not, both have a max service temperature of 180C according to their respective data.

I have never managed to work out the difference in practice.

JasonB07/03/2019 07:46:47
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15538 forum posts
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Nick, look at the Hot Strength Curves, 648 stays near full strength for longer, 638 drops off rapidly before leveling out at just above half strength. Also heat aging is better for 648

Edited By JasonB on 07/03/2019 07:49:38

martin perman07/03/2019 08:32:28
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1591 forum posts
66 photos
Posted by Nicholas Farr on 07/03/2019 06:41:35:

Hi Matt, you need a retainer compound, if you wish to use Loctite, one of these should be fine **LINK** but there are other makes and their colours may differ.

Martin, do you mean 638.

Regards Nick.

As Eric Morecambe used to say "right numbers but not necessarily in the right order" smiley

Martin P

Paul Lousick07/03/2019 08:43:25
1099 forum posts
483 photos

Commercial bushes are normally sized for an interference fit in a H7 hole and should be pressed into the hole with a suitable mandrel. The bore in the bush is designed to compress to the correct size after pressing but could be slightly oversize if glued into a clearance fit hole. Best to refer to the data sheet from the bearing supplyer unless you are making your own.

Paul.

 

Edited By Paul Lousick on 07/03/2019 08:47:02

Nick Clarke 307/03/2019 11:42:17
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288 forum posts
6 photos

Thanks - I was using the information on my supplier's website not the Loctite data sheets.

Where I am still puzzled is that the graph for %strength at 22C for 648 shows this to be more than 100% at 22C?????.

There are lies, damn lies etc etc I think!

Edited By Nick Clarke 3 on 07/03/2019 11:49:18

Chris Gunn07/03/2019 12:03:31
275 forum posts
16 photos

Matt, if you do not have any Loctite an interference fit is free!!!

I always fit my bushes this way, just polish one end a tad so it will start, squeeze it in and pop the reamer through afterwards.

Chris Gunn

SillyOldDuffer07/03/2019 12:21:57
4421 forum posts
957 photos
Posted by Nick Clarke 3 on 07/03/2019 11:42:17:

...

Where I am still puzzled is that the graph for %strength at 22C for 648 shows this to be more than 100% at 22C?????.

There are lies, damn lies etc etc I think!

...

Do you mean this one Nick? - it looks OK to me:

hotstr.jpg

Very educational this forum! I hadn't realised the strength of a Locktite might drop rapidly over a relatively narrow range of ordinary temperatures, then hold steady from 60C to 160C, before falling off quickly again over that temperature. Using Locktite 648 on a hot machine it would be wise to expect the joint to be less than 60% of theoretical strength. No doubt dirt in the joint would weaken it further.

Dave

Nick Clarke 307/03/2019 12:26:25
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288 forum posts
6 photos
Posted by SillyOldDuffer on 07/03/2019 12:21:57:
Posted by Nick Clarke 3 on 07/03/2019 11:42:17:

...

Where I am still puzzled is that the graph for %strength at 22C for 648 shows this to be more than 100% at 22C?????.

There are lies, damn lies etc etc I think!

...

Do you mean this one Nick? - it looks OK to me:

That is the one for 638 which seems OK - it is the one for 648 that I don't get

Nick

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