Eccentric drive materials

[WearyParticipant @weary]

Experience, advice, thoughts, and/or recommendations sought:-

The application is for a steam engine using Stephenson's Link Valve Gear.

The Eccentric Straps fitting around the eccentric and transmitting the drive to the eccentric rods will be (black) mild steel. A wearing surface liner cannot be fitted.

Would it be preferable to make the Eccentric Sheaves themselves from Cast Iron or Phosphor Bronze (Pb102), or ……..????

Or is it a case that there is no significant difference 'in real life'?

Phil

Edited By Weary on 28/11/2016 09:41:59

[WearyParticipant @weary]

Stephenson’s Link Valve eccentric materials question

[LambtonParticipant @lambton]

Either cast iron or bronze will be fine for the sheaves. Phosphor bronze is not necessary unless you have some in stock.

If you use black mild steel for the strap take care to get the good accurate finish where it runs against the sheave.

[Ady1Participant @ady1]

At the back of my mind I recall stellite welding rods which would let you put a hard coating onto almost any steel surface

[Nigel McBurney 1Participant @nigelmcburney1]

blacksteel is not very good for bearing surfaces,unless a very good surface finish can be achieved,cast iron is best for the eccentric,phosphor bronze is not really suitable unless the steel is hardened,leaded bronze or cast gunmetal is the far better.

[WearyParticipant @weary]

Thank you for the replies.

From the above: Eccentric Sheave to be in cast iron then. (Unless cast gunmetal is significantly better in this usage with black steel eccentric straps).

 

As regards eccentric straps and thus bearing surface of black mild steel:

Unfortunately the (original) eccentric strap design is very thin, an intricate shape, and highly visible. It was also left as polished steel in the original. My choice of black steel was dictated because of the difficulty in cutting this item out of bright steel as the stresses were removed and deformation resulted.. The thin eccentric strap means that there is not enough room for a bearing surface to be fitted on the inside wearing surface and for structural strength to be retained. A cast gunmetal eccentric strap would need joining to the eccentric rod with a bit of deft silver-soldering, and then tinning to look like steel – but could be achieved. Tho' I wonder whether the additional work would result in such a significantly better wearing surface and thus end result mechanically as to be worth-while.

 

Your reservations over the black steel as a bearing surface mean that an alternative that I might be able to sort out would be both eccentric sheave and eccentric strap in cast-iron.  Probably the best solution.

Thanks again for the 'food for thought',

Phil

Edited By Weary on 28/11/2016 13:46:39

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