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Rik Shaw12/02/2019 10:48:36
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1241 forum posts
336 photos

I'll soon be ready to machine a pair of eccentrics for my ongoing build. The two gunmetal castings are missing from the set of second hand castings I bought. I could order a pair from Reeves or alternatively machine the eccentrics (which wrap around a 1" dia. sheave) from some 1/4" thick brass plate I just happen to have.

I am reasonably certain that the brass should be OK but what will the judges think?

Rik

mechman4812/02/2019 11:00:24
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2299 forum posts
387 photos

FWIW; brass should work ok, have you considered machining some from bronze bar, or sintered bronze bar where you could machine them then let them soak overnight in oil to absorb lubrication ?

George.

Thor12/02/2019 11:03:51
1032 forum posts
23 photos

Hi Rik,

If you only run your machines for a short time and not 24/7 I think brass should last for a long time. I assume you will provide proper lubrication. I often use brass instead of gunmetal, and have had no problems, but I only run my small engines for a short time - less than 30 min. - and always oil moving parts.

Thor

Lambton12/02/2019 11:05:53
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669 forum posts
2 photos

Rik,

I think that you are referring to the strap that goes around the eccentric sheave that is mounted on the crankshaft.

What is the sheave made of ?

What will the engine be used for light running or driving a real load|?

Brass / steel combination should be OK but as I suspect you know Bronze/ steel would be better giving a longer life.

If the sheave is cast iron you would be better off making the strap from steel.

Obviously make provision for adequate lubrication what ever metals you use.

Eric

JasonB12/02/2019 12:16:26
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15042 forum posts
1536 photos

I've used brass on several of the small 24mm bore engines that I have made running on steel eccentrics and they are fine for the light use I give them

Ian S C12/02/2019 12:26:17
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7262 forum posts
227 photos

When I rebuilt a rather battered Stuart Turner S9 I made the new eccentric strap from cast iron.

Ian S C

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Hopper12/02/2019 13:23:24
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3402 forum posts
66 photos

Last little mill engine I built used brass for the eccentric strap, on a steel eccentric. No problems.

JasonB12/02/2019 13:41:14
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Moderator
15042 forum posts
1536 photos

The strap casting for the Stuart 10 series engines is brass and there are one or two of those about that run OK with that combination of metalssmiley

Rik Shaw12/02/2019 13:44:10
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1241 forum posts
336 photos

Brass it is then chaps. I like the idea of cast iron and steel but I'll go with brass on the basis that it is one thing less to rust on the engine. Thanks for your comments.

Rik

Nigel Graham 216/03/2019 23:16:05
85 forum posts

Mechman48:

I would not recommend using sintered bar if you need to machine what will be the bearing surface, as the cutting action is likely to burnish the grain closed, so rather defeating the object. It is however possible to buy sintered-bronze bushes, and there may be a size available to suit (or so close you can modify the diameters without compromising the engine as a whole)..

In full-size, the eccentric sheaves and straps were usually both of cast-iron. Mild-steel and cast-iron and happy together too, with good lubrication: my old IXL lathe countershaft used this pairing, as does my little EW lathe headstock.

I've an idea mild-steel and phosphor-bronze are not so, with the steel wearing more.

Ian S C16/03/2019 23:31:54
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7262 forum posts
227 photos

Don't use sintered metal of any kind, it is for bearing that will be encased in a housing as the sintered metal has very little strength on its own.

Ian S C

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