How does a steam loco cut off/reversing lever work?

[PaxmanValenta]

Hi
I'm trying to learn more about steam locos and have never grasped how the cut off works.
I know that when a steam loco starts to move, the cut off needs to be 100% for maximum torque, but when at full speed it can be set at less than 50%.

Am I correct in thinking that the reversing/cut off lever alters the frequency at which the valve opens and closes, hence the more frequent it is then the less distance the piston can move so a low percentage is obtained for high efficiency.

By contrast if the valve opens and closes at lowest frequency then the piston gets to travel the full length of the cylinder giving 100% maximum power and torque.

If this is the case then how does the lever alter the frequency of the valves and at the furthest position operate as reverse?

Thanks in advance.

 

[neonison]

Interesting question.
The frequency is determined by the reciprocating motion so cannot be changed.

Put simply, the cut off alters the proportion of each stroke for which steam is admitted through valves into the cylinder. At 100% the maximum amount of steam is allowed in but once up and running the amount per stroke can be cut back. The additional benefit is that the apparent shortfall in steam allows what steam there is to expand, all adding to efficiency. If the engine is put under additional load, such as going uphill it becomes necessary to admit more steam per stroke. It is not quite the same but you could drive a car everywhere in first gear. You would still get there but it would be heavy on fuel.

Reverse is achieved when the valves opening to admit steam are in opposite phase to the pistons.

There are various means of adjusting the proportion of each stroke for which steam is admitted. All are mechanical and all rely of the motion of the pistons. The design is known as valve gear and may be for example, Stephenson's, Walschaert's, Lenz etc.

 

[Spagnoletti]

To further expand on the comments above, Wikipedia is your friend.

https://en.wikipedia.org/wiki/Cutoff_(steam_engine)

The best source I've found for understanding steam loco operation is the BR Enginemans handbook ('black book') last published in the 1950's.

It can be got for around a tenner off ebay, I've seen plenty for sale at pres railways and there are some modern reprints too. There are a number of scanned versions around if you google.

 

[tiptoptaff]

The reverser "cut-off" is the % of the way through the valve travel that the live steam is shut off. The piston will always travel the full distance. Most engines can be wound back to around 30% or less when travelling at speed

 

[84A]

The reverser "cut-off" is the % of the way through the valve travel that the live steam is shut off. The piston will always travel the full distance. Most engines can be wound back to around 30% or less when travelling at speed

Caprotti B.R Standard 71000 could be driven with as little as 3% cut off at full speed with the regulator in the "roof" (which is the correct method for this loco) and she would still accelerate such was it so economical with steam. Did this make it easy to fire...........from my B.R firing days on the WCML experience with her.............. A BIG NO!!!!!

 

[coppercapped]

The reverser "cut-off" is the % of the way through the valve travel that the live steam is shut off. The piston will always travel the full distance. Most engines can be wound back to around 30% or less when travelling at speed

I think you'll find it is the percentage of the piston travel when the valve cuts off the steam flow.
--- old post above --- --- new post below ---

Caprotti B.R Standard 71000 could be driven with as little as 3% cut off at full speed with the regulator in the "roof" (which is the correct method for this loco) and she would still accelerate such was it so economical with steam. Did this make it easy to fire...........from my B.R firing days on the WCML experience with her.............. A BIG NO!!!!!

According to reports Duke of Gloucester didn't steam well originally because BR didn't get the draughting right. When it was restored for main line use the draughting was correctly set up and the economy improved considerably.

 

[Elecman]

Caprotti B.R Standard 71000 could be driven with as little as 3% cut off at full speed with the regulator in the "roof" (which is the correct method for this loco) and she would still accelerate such was it so economical with steam. Did this make it easy to fire...........from my B.R firing days on the WCML experience with her.............. A BIG NO!!!!!

Did this very small cut off have any deleterious impact on the draughtiness of the fire as very little steam would be going through the blast pipe compared with a 30% cut off?

 

[84A]

Did this very small cut off have any deleterious impact on the draughtiness of the fire as very little steam would be going through the blast pipe compared with a 30% cut off?

Correct it would and did have the effect you mention with a very low cut off, it was common practice to wind her up to around 45% to draw the fire if she was not steaming too well, once nice and bright (the fire) reduce the cut off. I have fired this "Old Girl" on numerous occasions on the main line after her first restoration, make no mistake, she does not suffer fools easily! modifications, well they have made a difference HOWEVER she was thirsty and still was very hungry. Let her get away from you and you will experience one hell of a job to bring her back up towards the red line 250psi. For those interested.......I used to box her up before heading out of Crewe with a down express, baffle plate out and fill with best sized coal you could find amongst the rubbish normally loaded, the back end was filled to the maximum until it was about to fall out of the firehole door, baffle plate put back in........ the rest of the 48.6 sq ft of grate was fired as such..... down the sides (well into the front corners) across the front and then evenly across the grate with no holes......Exhaust injector went on just as we left the platform after the R/A from the guard, and was regulated to keep the boiler at least 2/3 full. I could go on for hours but at the cost of no doubt boring you all the death ha!!! All the best guys................Oh yes I have emptied the tender of nearly 10 ton of coal on numerous occasions, lodge out over night and do the whole thing again the following day heading south.......Those were the days!!!!!!!!!!!!!!

 

[MarkyT]

Did this very small cut off have any deleterious impact on the draughtiness of the fire as very little steam would be going through the blast pipe compared with a 30% cut off?

That is one phenomenon that fancy exhaust arrangements like the Kylchap double blastpipe can help with. The Kylchap was used in small numbers pre-WW2 by the LNER, was popular in France, then later became very popular with BR(ER) after the patent had expired! It helped to get more draught out of the expelled steam using a series of cowls and nozzles in the smokebox, and also allowed the blastpipe diameter to be enlarged to reduce back pressure. Fitting a double Kylchap exhaust was one of the successful measures adopted in rebuilding Duke of Gloucester.