Author Topic: Metal choices for sliding parts  (Read 2411 times)

Offline smfr

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Metal choices for sliding parts
« on: November 15, 2013, 06:48:31 AM »
I'm starting to think about the crossheads and slides for my Muncaster's engine, and wondering what the best choice of metals is for the crosshead slides? The crosshead itself has a bolted-on plate which runs on the slides:



I assume the plate is bolted onto the crosshead to allow for replacement due to wear.

The slides would have been cast into the bed, so cast iron is the obvious choice, but would ordinary mild steel (1018) also work, if the crosshead plate is made of the same material?

Of course this is a display model that probably won't get a lot of running, so maybe I should whatever bits of scrap I have to hand   :Lol:

Simon

Offline Jasonb

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Re: Metal choices for sliding parts
« Reply #1 on: November 15, 2013, 07:13:25 AM »
Case hardening the slides would be one option to stop the two similar metals picking up or you could intriduce bronze slippers to the crosshead or as you say mill from iron.

But a syou say for limited use if kept well oiled it should be fine. In which case think about adding small oilcups or at least an oil hole to the slide bars.

Regarding your exhaust in the other thread, given the scale of this engine its unlikely the exhaust would have been copper, it may look nice polished up but something to look like flanged iron pipe would be more true to the full size engines.

J
« Last Edit: November 15, 2013, 07:17:14 AM by Jasonb »

Offline Florian Eberhard

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Re: Metal choices for sliding parts
« Reply #2 on: November 15, 2013, 10:19:17 AM »
Hi Simon

As long as the parts are oiled well, there wont be much wear. The surface load of those crosshead plates wont be that great.

I would recommend to machine some oil grooves into the top and the bottom of the plate. (and not into the slide as sometimes seen. That is not good because the oil grooves can then catch up dirt, dust and whatever is flying around with the air.)

These oil grooves don't need to be wide, they can be made with a ball cutter with 1mm radius (or even less) or with a v-shaped engraving cutter though that one should not be too sharp (i would say at least 60 tip angle - better 90)

And don't get too near to the end of the plate (I would leave at least 3mm to the edges because that would cause a faster loss of oil.

Cheers Florian



Offline steamer

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Re: Metal choices for sliding parts
« Reply #3 on: November 15, 2013, 11:29:57 AM »
Hey Simon

Often with an arrangement such as that, there is a wedge or shim in the cross head for taking up wear.  For our small engines, it's a nice scale detail, but isn't really needed.     By the way, my launch engine cross heads are very similar, and I did make a shim plate, but in 10 years, 3 or which it's a people mover...there isn't any appreciable wear.   The cross head shoe is brass, and the guide is cast iron.   I scraped them to fit, designed them to be low bearing pressure ....like a bearing PV of 200 and then keep them well oiled.

Dave
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Damned ijjit!

Offline Ian S C

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Re: Metal choices for sliding parts
« Reply #4 on: November 15, 2013, 01:25:00 PM »
The ideal bearing surfaces would be cast iron on cast iron, next would be cast iron  on steel.  Steel on steel, two different grades, or one piece case hardened, and the other left untreated.  In your case, with little use, plain steel of what ever you have in the junk box/ stores department should be perfect. 
       On older machinery, the fitting was done by scraping, this leaves little dents (I suppose you could call them that), all over the surface, these retain oil, if things all go right the two surfaces virtually float on each other.       Ian S C

Offline smfr

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Re: Metal choices for sliding parts
« Reply #5 on: November 15, 2013, 03:46:36 PM »
Thanks for all the suggestions, folks! I think I'll go with cast iron for the slides, steel for the crosshead. If I decide to scrape, this would be my first scraping attempt, but I'm not sure I want to invest in scraping tools just for this small job. Are there small-scale scraping tools for small parts? Also, I was planning on the slides being L-section, but I suspect that scraping into a corner is hard, or should I just add some corner relief?

Simon

Offline Jasonb

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Re: Metal choices for sliding parts
« Reply #6 on: November 15, 2013, 03:50:05 PM »
Its such a small area that scraping won't really be needed.

I have ground the end of an old file in the past to use as a scraper.

J