lap, lead when using pistons with rings

[johan]

Ok, this might be the most stupid question ever but I cannot find an answer in the books I have.

When using slide valves (obviously without rings) then lap and lead is simple: it are those parts of the valve that stick out over the opening in the port face.

But when using a piston valve with rings, do you start counting at the end of the piston or at the edge of the farmost ring? After all it is the ring that makes the seal.

[davewoo]

Certainly not a stupid question, I've been through the same dilemma in the past, i was advised to use the edge of the rings as the point to time the valves to. I used a reduced diameter on the end of the bobbin and a large chamfer so the ring was the point of timing. Difficult to explain in words ( for me anyway!) but it worked the engine had sharp crisp exhaust beats and performed well, I sold the engine but still see it occassionally and it still goes ok with the original cast iron rings and squared off ports.
Theres good information on John Baguleys website although he uses PTFE rings he shows how he uses the ring as the point that the valve opens and closes.
Good Luck

Dave

As an illustration if you have a look at Chris Vine's excellent youtube film on making Clupet rings at the very end he shows the ring fitted to the piston valve and you can clearly see the chamfer and reduced diameter at the each end of the bobbins. The ones I made had plain cast iron rings but the principle is the same, even if using PTFE or Fluorosint.

[chris vine]

Hi Johan,
Again, not a silly question at all, especially as most model engineer designs don't show any details on this.

In some designs, the piston valve is made to a smaller, clearance, size in the liners. The valve is then guided by the rod.
In other designs, the valve is made a good fit in the liners, but there is a clearance in the hole so that it floats on the rod. The idea in both schemes is for the rod and liner not to be fighting each other.

Kind of Dave to mention my video on Clupets rings. You don't need to make that complicated sort for the valve to seal perfectly well. I made mine because the liner bores have a touch of porosity, so it was easier to make some super duper piston rings than to try to remove and replace the liners!!

You can see that the diameter of the valve is reduced and also chamfered quite considerably so that the edges of the rings are exposed to the steam and exhaust sides. It seems to work well and makes for a free steaming engine.

I once tried measuring the drawbar pull on Bongo and reached 26kg at around 12 mph with the wheels spinning/slipping!

Hope that helps.
Chris.

[ettingtonliam]

I've read that full size practice was to relieve or chamfer the end of the valve and then set from the end ring.

[andyhigham]

You could always use Dykes rings

[simon6200]

Just to clarify, the part of a slide valve that goes beyond the ports is the lap. Lead is not a physical thing you can measure on the valve. Lead is the amount the valve has already opened to live steam when the piston is at the dead centre.

[ettingtonliam]

Jun 21, 2024 18:51:15 GMT andyhigham said:

You could always use Dykes rings

I should know this, but please remind me what a Dykes ring is?

[andyhigham]

It's an "L" section ring, the groove in the piston is stepped so the end of the axial leg of the ring is flush with the end of the piston
mecoa.com/faq/rings/ringtypes.htm

[davewoo]

That's an interesting link,thanks for posting it, had heard of Dykes rings but never seen them or an explanation of them before, out of interest has anyone tried them or read about them being used for piston valves in a loco? Can't remember ever seeing an article or mention in M.E or E.I.M about their use in loco's.
Still learning something new every day!
Dave
Mind you I painfully did re-learn something yeaterday, when you have just welded something together don't pick it up three minutes later it's still hot!

[44767]

I used Dykes rings on the piston valves on my Black 5. It works a treat!

[44767]

Jun 21, 2024 18:39:07 GMT ettingtonliam said:

I've read that full size practice was to relieve or chamfer the end of the valve and then set from the end ring.

This is how I have designed the valve and its rings for my Class 3 tanks. It's virtually as per full size except in full size there were six rings per valve head!



Cheers, Mike