Recommended cut-off setting when coasting - Stephenson Valve gear with slide valves?

Hi

I read in a 1950s edition of Railway Executive "Instruction Book for Standard Steam Locomotives" that when coasting with piston valves, you usually have the regulator cracked and the cut-off set to anywhere between 25% and 40%, but am unclear on the procedure with slide valves.

I heard that regulator should be cracked (to counter-act any vacuum forming in the steam chest) but that the gear should set to full stroke. Is this correct?

Thanks in advance.

Jason

 

A point with slide valves is that live steam pressure above them force them against their faces, which increases friction between them. With the regulator fully shut, not only does this friction vanish but the seal with ports does too, allowing air to flow freely from one end of the cylinder to the other, again reducing the engine's rolling resistance.

 

Hi

Thanks for the information! So I guess this means that it's better to have the regulator fully closed when coasting with slide-valves.

I'm still having difficulty in understanding why it is advised to set the cutoff to 100%, though. Is that to allow freer passage of air around the valve, or does it have something to do with oil lubrication?

Cheers
Jason

 

Slides valves tend to chatter when coasting at shorter cutoffs which can then give uneven and increased wear. Dropping the loco into full gear removes this tendency. Cracking the regulator is purely for lubrication of the cylinders and valves.

 

Avonside1563's quote sums it up perfectly.
On a Western engine such as a pannier, the valves are upright between the cylinders and fall away from the valve faces when the regulator is closed. Unless you then select full forward / reverse as appropriate, you can hear them rattling in the buckles that drive them back and forth, especially on a shorter cutoff. Full forward / reverse prevents this.
"Cracking the regulator" is opening it enough for the regulator quadrant to open the "W" (jockey) valve to allow the lubricator to keep supplying oil (via the W valve) to the cylinders and valves. "Cracking" it is not opening the regulator enough to put steam pressure into the cylinders.

 

That's a great answer. Thanks for your help - everyone else who contributed also!

 

If you're in mid gear, the valve is theoretically closing off the port to the cylinder. When the piston is moving back & forth, it is tending to create a vacuum on one side and compress on the other to one side is trying to pull the valve down whilst the other one is trying to push it away. These two forces aren't equal as the volumes on each side are only equal at about mid stroke. That is why the valve chatters, as it goes from being pushed off to pulled on. Putting it in full gear opens the ports giving a free-er passage and avoids the suck-blow.
Putting it into the opposite gear makes the engine act as a compressor. In fact you can pump up a pressure in the boiler of a loco out of steam by doing this, even to the extent of having enough pressure to move the loco.

 

If you shut off a slide valve locomotive and left the gear set where you had it for power the engine makes a very uncomfortable rattling sound. This is the valves slapping on the faces, when you apply steam, the valves are forced onto the faces and this gives you your drive.

If you were to open the regulator whilst coasting in full gear (as you must with a slide valve, Stephenson engine) all you are doing is giving the engine steam in full gear. This is perfectally acceptable should the locomotive be knocking or there is play in the valve gear, this steam cushions the reciprocal forces and puts the valve gear into a minimal power setting, it is essentially just providing enough steam to make the valve gear take up its own slack, rather than providing useful power to the rail.

However if the locomotive is in reasonable condition, cracking the regulator is not necessary.
On ex GWR locomotives, although it may seem like you are cracking the regulator for lubrication purposes, you are not. You are moving the regulator handle up, you are actually opening the Jockey valve which is attached to the regulator linkage in the cab. You would open the regulator beyond that point.