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phosphor bronze interference fit

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petro1head14/11/2017 09:15:31
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440 forum posts
84 photos

I have been given a job to do from a pal of mine.

He has a phosphor bronze casting with a steel pin in it. The holes for the pin have become worn and he wants me to make a couple of bushes, machine the casing and fit the bushes.

What sort of interference fit should I go for?

Emgee14/11/2017 09:46:51
648 forum posts
151 photos

If the casting is not subjected to heat when in use you could go for a shrink fit of the new bushes into the casting.

The fit of the pin in the new bushes will be determined by the use, if they have to be removed during use then possibly just a sliding fit is required.

Emgee

petro1head14/11/2017 10:29:03
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440 forum posts
84 photos

Hi

The pin is a sliding fit.

Its the bush thats an interference fit. However what tollerance?

Hopper14/11/2017 11:44:00
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2047 forum posts
23 photos

Rough rule of thumb is one thou interference per inch of diameter.

Depends on how big the job is and how thick the casting is and what its application is though. If it is something delicate you might make it a neat sliding fit and Loctite the bushes in place. If it's a big lump of a thing, a thou of interference and pull the bush into position with a nut and bolt etc or heat the casting and expand it to drop the bushing in and let job shrink to fit. Beware of cracking old castings with too much interference or heating though.

A few details on what the casting is would help.

Edited By Hopper on 14/11/2017 11:49:32

Mick B114/11/2017 12:07:57
187 forum posts
8 photos
Posted by Hopper on 14/11/2017 11:44:00:

Rough rule of thumb is one thou interference per inch of diameter.

...

A few details on what the casting is would help.

Edited By Hopper on 14/11/2017 11:49:32

That would be my understanding too. A few degees taper on the first 5mm or so of the bush o/d would aid location. Many folk use a hydraulic press for fitting if available and if the parent part isn't too thin around the 'ole - you don't get the variables you do with heat, plus you can handle it straight away.

KWIL14/11/2017 12:14:19
2800 forum posts
50 photos

Make the bush a tight fit size and then leave the bush in freezer before fitting, should drop in, but do not hang around doing it

petro1head14/11/2017 12:34:22
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440 forum posts
84 photos

Its a brush assembly for an electric traction motor off a train,

20171114_121553.jpg

20171114_121602_001.jpg

20171114_121615.jpg

In the second photo you can see how the pin has warn into a repair that was done a few years ago.

The third photo is the pin which holds the spring in place and tensions it.

I was thinking about putting the bush in the freezer

Hope this helps

Edited By petro1head on 14/11/2017 12:38:15

duncan webster14/11/2017 13:53:02
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1240 forum posts
22 photos

There is a useful table at

**LINK**

Not suggesting you need to work to these tolerances on bush and hole, but together they give the clearance/interference for different classes of fit.

Edited By duncan webster on 14/11/2017 13:53:16

Tony Pratt 114/11/2017 14:03:51
703 forum posts
2 photos

I would use Loctite.

Tony

Tim Stevens14/11/2017 14:24:21
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698 forum posts

The 'right' fit will depend on the thickness of the bush, as well as other factors. I think that a plain bush with Loctite would do the job. If you go for an oilite bush, too much interference is likely to collapse the bush inwards - a fault which is difficult to cure as a reamer will not serve. And Loctite won't do for oilite as it fills in the spaces for the oil.

It appears that the whole assembly has been sand-blasted (or similar). This might reduce the life of the springs, unless the abrasive used was 'friendly' - ie rounded chips rather than sharp ones. So, tell your friend to keep a careful eye on this detail in service.

Regards, Tim

dave train14/11/2017 15:03:10
22 forum posts

The springs are replaced

The bush OD is 20mm and the ID is 16mm

PS Thanks for the replies

Edited By dave train on 14/11/2017 15:03:47

not done it yet14/11/2017 15:55:41
1225 forum posts
9 photos

A minimal shrink fit looks easy enough, as the housing and bush have, near as dammit is to swearing, the same expansion coefficient. So bush in the freezer and housing in the oven on low heat.

J Hancock14/11/2017 17:05:04
143 forum posts

It seems as though an awful lot of up and down movement of the brushes may be taking place to cause

that much wear.

Is the commutator running true ?

dave train14/11/2017 17:06:45
22 forum posts

Wrong forum

Edited By dave train on 14/11/2017 17:25:06

dave train14/11/2017 17:16:44
22 forum posts

Oh bugger

 

Edited By dave train on 14/11/2017 17:25:37

petro1head14/11/2017 17:27:24
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440 forum posts
84 photos

Question........s

Managed to snap a couple of drills, any idea why? [EDIT] Found this helpfull videp - https://www.youtube.com/watch?v=pAngKHIZgyA

I bored out the bushes using drills and finished off with a 5/8 2 flute end mill which was fine however I was then asked to bore them out to 16mm so used one of the 3 flute mills that I normally use for alloy and it chattered and ended up creating a poor finish.

So, as this is the first time I have worked with phospher bronze should the drill tips be at a different angle?

Should I be using different end mills?

RRMBK14/11/2017 17:37:59
106 forum posts
15 photos

Hi.

I have just put a 2" od 1/8 wall 4 2 long Phosphor bronze bush into a cast iron cylinder. I machined the bush to 2 thou oversize. As my lathe turns a slight taper over this length I had 1 thou over at one end and 2 thou at the other.

I put the bush in the freezer and heated the casting bore for 10 mins with a hair dryer. The bush slid in and very soon grabbed fast. It was slightly beyond where I wanted it at one end and I had to put some serious effort into the puller I made to shift it just 5 thou.

What is important is that you get your location right first time. From the look of your application there seems to be no reason why you cant make the bushes "top hat " type so they will positively locate against the frame. However if you have the room to make the "top hat " why not make the bushes a dead fit and secure them in place with three screws into threaded holes at 120 degrees. It would then be an easy matter to replace the bushes at a future date when they wear.

hope this is helpful

kind regards

BK

Speedy Builder514/11/2017 17:45:03
1284 forum posts
92 photos

Petro1head's link for grinding drills would be useful for answering another forum question "Runout drilling 2mm hole" ?
BobH

Andrew Johnston14/11/2017 17:46:08
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3576 forum posts
421 photos

Errr, you don't use drills to bore holes?

Here are a few pointers:

When drilling bronze it can "grab" the drill, like brass only worse. The general advice is to stone a flat on the cutting edge of the drill. That's never worked for me, so I use slow helix drills.

Tooling for bronze needs to be sharp, sharp, sharp!

Bronze doesn't like shallow depths of cut, especially if the tool isn't very sharp. The cutting edge seems to ride over the surface, until you add a little more cut, at which point the tool bites, and you end up with over, or under, size work. sad

Bronze also has a tendency to close up slightly after machining. So if you ream a hole and then try the reamer by hand it won't go in without a lot of force, if at all.

Using endmills as a "drill" does work, but you need a rigid setup to avoid chatter. Basically lock everything that isn't moving.

I suspect that the bronze "closed up" on your drills and the resulting torque was enough to snap them, especially if you weren't using coolant.

Andrew

Edited By Andrew Johnston on 14/11/2017 17:47:35

petro1head14/11/2017 18:26:27
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440 forum posts
84 photos

Maybe I have not given enought info.

The bushes and made from a round bar of PB, the bar is machines using a lathe. I use drills to get close to where I want to be and finish off with an end mill, more for good finish.

Tollerance for the pin in the bush is not tight so easy enough job.

I often use an end mill to fish a hole off if I dont have a drill big enough. This is ok for me as I don't do any high presicion work

Anyway we have gone off topic and maybe I would be better starting a new topic about working with Bronze?

Edited By petro1head on 14/11/2017 18:27:29

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