Expansion

[Harry WilkesParticipant @harrywilkes58467]

Gent's below is a photo of the slide valve and I believe the term is 'buckle' out of my 3" traction engine, the slide valve is badly scored and I was advised not to use cast on cast so I have made a new slide valve from PB. So I'm concerned that they may expand a different rates on the cast parts there is .004" clearance and I have gone with the same clearance so any comments would be welcome.

H

Edited By Harry Wilkes on 02/09/2020 15:20:18

[Harry WilkesParticipant @harrywilkes58467]

[Clive Brown 1Participant @clivebrown1]

By "cast" do you mean cast-iron? Is the buckle iron? Iron expands at, very approximately, half the rate of PB, so for a PB valve and a CI buckle, a clearance will close up at approx .001"/ inch/100 deg. temp. rise so your valve should be OK.

[SillyOldDufferModerator @sillyoldduffer]

I did much the same sum in metric as Clive and am delighted to report the same answer! (I'm not good at maths.)

Clive's imperial answer reveals roughly how big a 4 thou clearance valve would need to be before thermal expansion caused a problem. It's about 4 inches. So, Harry's combination of metals is OK in a small model valve, but would jamb a full size slide valve of the same design.

If the metals were used the other way round Phosphor Bronze would increase the gap by expanding away from the Cast Iron. No jamb at any scale, but the valve would leak more as it heated up. I guess full-size slide valves were designed using the cubical expansion formula for a good seal at operating temperature, ie with both metals expanded, and it was accepted a cold slide valve would leak slightly until it warmed up.

Dave

Edited By SillyOldDuffer on 02/09/2020 16:39:45

[duncan webster 1Participant @duncanwebster1]

different expansion twixt valve and buckle won't make it leak, but clearance will introduce lost motion giving delayed valve events.

[Harry WilkesParticipant @harrywilkes58467]

Thanks for your answers and yes Clive should have written CI instead of cast

H

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