The LNER Encyclopedia

Explaining the Phrase ‘Cut Off ’

In one of my articles published recently I mentioned the phrase ‘cut off ’ relating to the driving techniques of a steam locomotives. In fact, 50 years ago, numerous railway correspondents were allowed to ride on the footplate of many well known express trains closely monitoring and recording every minute operating performance of the driver and his engine, noting especially speed, gradients, boiler pressure, water levels and that very confusing phrase, at least it was to me, ‘cut off ’. No doubt to some members it will be widely known what it refers to, whereas I’m sure many others are not the slightest interested so this article is aimed to those members who have heard the expression and hopefully, intrigued to know what it means.

Firstly I must admit that trying to explain ‘cut off ’ is rather difficult as it is a complicated mechanical technique, most essential in the working of a locomotive workings and it would be far easier to use drawings. Admittedly although there are several excellent plans to be found in various articles, unfortunately, at least as far as I’m concerned, these are copyright and not to be published without permission. However if you are really interested to understand the subject fully, I recommend that there are several plans, one in particular which includes a working model, can be found on the Internet. If on Google or similar, just type ‘Walschaerts Valve Gear’ on the internet and you will be taken to it, preferably the very first listed item.

‘Cut off ’ is operated by the driver using the reverser situated in the cab, however apart from operating this function it’s other main purpose is obviously to reverse the engine’s direction of travel i.e. forward and back. In a road vehicle, gears are used to improve engine performance and save fuel, whereas on a locomotive ‘cut off ’ is it’s equivalent, designed simply to improve efficiency and save steam and consequently and more importantly, coal. Immediately an engine begins to move and pick up speed, by turning his revereser the driver can gradually cut off the flow of steam which is controlled in the piston valve cylinder before flowing into the the main cylinders.

There are four main types types of reverser situated in British locomotive cabs and designed to operate in four different ways.

1. A screw device set at an angle slightly above horizontal. Fitted on it’s top with a pointer set in a brass plate, indicates certain numbers calibrated in forward and reverse gear. (the most common).
2. An upright screw device. (Common in most Gresley designed locomotives). The indicator plate fitted situated upright on the front of the boiler in view of the driver.
3. A round type wheel device fitted on several BR designed locomotives with calibrations set in.
4. A long upright lever fitted in most shunting engines, designed for speed of operation.

The calibrated numbers on the reverser plate range from around 70% in forward gear and 70% in reverse gear but are calibrated down gradually, varying on different types of locomotives, in degrees of 10%, with zero in the centre. This indicates the engine is out of gear. All locomotives are usually left in this state when stationary, similar to a car left ‘out of gear’.

The amount of steam entering the main cylinder is controlled by the smaller piston vale cylinder situated either above or alongside the main cylinder and it through the latter that steam enters first when the regulator is opened. Both cylinders have two ports connecting them, situated near to the ends of each cylinder, thse ports open and close by the motion of the piston valves passing over them. (Two pistons are attached to the piston rod, one either end). When one of the ports is open it allows steam to flow into the main cylinder and push the main piston, while at the opposite end of the cylinder that port opens to exhaust allowing the spent steam to exhaust and out of the chimney. When the main piston travels to the opposite end in the cylinder the piston valves reverse the situation. Very complicated I must admit but hopefully if you are looking at the moving model on the Internet it should all fall into place.

Admittedly although the working model shows the passage of steam flow it only concentrates on showing how forward and backwards is operated and unfortunately does not dwell on ‘cut off ‘.

When the locomotive first moves off it requires the maximum amount of steam in the cylinders so the reverser will be in the 70% ‘forward’ position, indicating that the ports in the piston valve cylinders are wide open. But as the engine builds up speed, the driver gradually turns the reverser towards the neuatral position and in effect this alters the timing of the piston valves, moving in their forward and back motion,thereby cutting the amount of steam entering the main cylinder and allowing the steam to perform a fraction longer. This is the so called ‘cut off ’ phase. What he is doing is similar to driving a car and going through the gear changes until the required speed is achieved.

A perfect example of how the train enthusiast can experience himself the use of ‘cut off ’ is to hear a train move off from stationary. At first there are loud sharp blasts from the engines chimney as it endeavours to build up speed with a heavy train behind it, this indicates full ‘cut off ’. This creates a very sharp blast on the fire raising it to a white hot heat for the excessive amount of steam required, but as the engine gradually picks up speed and masters the weight of the train, the driver begins ‘cut off’ and the chimney blasts slowly deaden. Another affect of all this is the fact that too much steam in the main cylinders would eventually hamper progress by not being able to exhaust it quick enough.

When a train reaches the driver’s required speed the ‘cut off’ is around 15%, but depending obviously on the trains speed and downhill gradients could in some cases go down to 10%. The engine is obviously using a mere minimum amount of steam, which can be observed by the low exhaust emitting from the chimney. However if more speed is required or the train beginning to climb, the driver decreases ‘cut off’ up to 20, 30 or 40% as required. This increases the blast on the fire, creating greater heat and the need for more coal to replace the rapidly burning coal.

Driving a locomotive required constant adjustment of reverser and regulator position. However some express drivers would drive with the regulator wide open and control steam usage solely with the reverser and using ‘cut off’. It all depended on the driver’s skill and team work with his fireman, but that was often debateable, some drivers unfortunately could be well known as heavy handed.

Locoman,
Another top post. I have always wondered if you can adjust the cut off with the regulator open, so thank you for explaining that one. Does this have an effect on the pressure in the steam chest?
Is it possible to adjust the cut off with a lever reverser with the regulator open?

Simon.

Very interesting post.

I have always explained it as follows

"imagine that you are looking at the piston travelling from one end of the cylinder from one end to the other. That is 100% of the stroke. Cut off is simply the amount of the time that piston is travelling down the cylinder when steam is being allowed to go into the cylinders. If it is 70% then for 70% of that stroke we are admitting steam, with the remaining 30% being for the steam to be expanded and exhusted, so we are getting alot of raw power but not getting much work from the steam. When we have accelerated and we now bring the reverser back to say 40% the steam is now in the cylinder much longer and can expand and is therefore more efficent"

The answer on whether you can adjust the cut off with the regulator is that it depends on the type of valves in the cylinder of which there are two types.

Earlier engines had what were called slide valves and look like a D on its side. This type of engine generally had a lever (or pole) reverser. When the regulator is open the top face is pushed down onto the face of the valve chest by the pressure of steam from the boiler meaning it would be very difficult / dangerous to try to move the reverser as all it would do is fly to full gear under the pressure of steam. Arms have been broken by those who have tried.

Later engines were fitted with piston valves where the valve is a circular valve so the pressure is spread more evenly meaning it can be moved more easily and saftley with the regulator open.

This can be best experienced if travelling behind a small engine such as a Jinty or Pannier where you hear the engine start away, there is a few chuffs, it then goes quiet for a second or two while the driver wrestles with the reverser and then steam is applied again.

There is a real knack to notching up. Do it too soon and it is like going into 4th gear in your car, there is not enough power. Leave it too long and the engine is being worked unessessarily hard. The best way is through feeling it in your feet and listening to what the engine is doing.

The other thing that has to be considered is that different types of valve gear react in different ways to be notched up. Some engines (a pannier comes to mind) will run and run when going in one notch off mid gear, where as others need more on the reverser. An 8f for example will generally accellerate nicley at about 33% where as a 5f with its bigger wheels likes a bit more. This is also partly an effect of differences between types of valve gear.

The final thing is an observation that the amount of cut off you need to get going is also dicated by the gradient, load and condition of the loco. One that is new out of shops will probably start away on the level nearer 55% where as one at the end of its ticket after about 60K miles will probably need full gear.

This reminds me of reading the about problems with Great Eastern men coasting in full gear, how does this harm the loco?

Thanks for an interesting reply Blandford1969 you obviously know the mechanics of the subject better than I.

To Simon CVR1865. Re Attempting cut off with a lever type reverser. Yes this was quite possible, but as there were only a few cogs in the bottom plate of the reverser to bolt into it was rather limited to what % could be achieved. Normally with shunting engines, which were the only engines I fired fitted with a lever reverser it wasn’t really necessary. The only time it was used was when travelling light engine to the main engine depot at the weekend or early on a Monday morning when running to the marshalling yards.

Re Great Eastern engine problems when coasting in full gear I’m afraid I can’t answer that one. All I can remember that normally when a driver shut off steam for coasting a long distance they would always wind the reverser into full gear. Why, again, it was never explained to me.

Finally did you manage to find the working diagram referred to in my article and did it help to explain.

Another interesting article Locoman.As you will know Gresley big engines(not sure of O2s or J39s)had a brass plate mounted near the "sector plate" saying "when coasting, gear must be set at 25%".I believe this was to do with the vacuum created in the cylinder because,if in mid gear and coasting,the connecting rod is moving the piston along the cylinder,and the inlet ports would be closed off by the piston valves for the full travel of the piston,except for"lap and lead",which is provided by the movement of the crosshead arm via the union link and the combination lever(in Walchearts gear).So,with the inlet ports open for 25% of the piston travel there is some "breathing space" provided in the cylinder.I have heard it said that when coasting,and a vacuum being created in the cylinder,smokebox ash can be sucked into the cylinder ,via the open exhaust ports, causing quite rapid wear on the cylinder bore of course.An old engineer once told me that,on certain engines with certain types of valve gear,drivers were instructed to coast with a small supply of steam on for this very reason. We are getting in a little deep here,valve events,and all that,and I am no expert,but this just the way I see it.Hope this is of interest.

Hi I think you are refering to the JP Paker method, but rather than being at full gear it is more like about 5 to 15%. The argument for this being that the valve moved a shorter distance while coasting so would wear out more slowly.

However with a slide valve engine you are right that when the regulator is closed the engine is put into full gear (best practice is always one notch of full gear to lessen the chances of rounding the ends of the die blocks in the lifting links) The reason for this is that on a slide valve engine becuase the valve comes off its face so there is a bypassing effect on the pistons as the system is not sealed and there is less of a tendancy for a vacuum to be built up. As a result of this the normal proceedure is to put the engine into full gear with the regulator closed.

On LMS engines the Drift position is marked with a D, Some less able drivers have been known to say D means Drive! - In truth they are around 40%, you generally know with a Midland engine as you hear the relief valve sort of ringing.

Your right that drifting can lead to a vacuum being created in the valve chest which could result in ash being drawn backwards down the blastpipe and causing damage to the valve liner. This is why engines were fitted with anti vacuum valves, with Gresley engines these were of course mounted on top of the boiler, with with LMS engines generally at the top of the cylinders. However if a relief valve fails you loose most of the power in that cylinder as it goes out through the relief valve still being open to atmosphere,

My understanding of drifting with a small amount of steam is more tied in with lubrication. All locos are fitted with lubrication systems to the cylinders. How they work though varies, if we take a bog standard GW loco (appologies for swearing) then the lubrication runs at a faster rate when the regulator is open slightly on the jockey valve as opposed to when it is closed. However see the above comments in relation to slide valve engines.

Hi Locoman, yes the animations are very clear.

Locoman, blandford and Roy thanks for the additonal info. It is not something I was aware of. Good to know what the vacuum relief valve does.

Thanks again guys.
Simon

we used to run our green arrows at about 8 percent and down to 25 when coasting. a whiff of steam on -the regulator just cracked open-cushioned the valve gear. Our lever reversers on the NER locos we could notch up with steam on but some GN engines we had to shut off as the lever was too heavy. Our G5s had a lever and screw reverser combined. The B16s and Q6s dead easy -they had steam reversers.
Jim Brodie

Locoman69, you gave a very lucid explanation of "cut off" which I'm sure was very helpful to a lot of people, thank you for that.

As far as calibration of the scales on reversers is concerned though not too much reliance should be placed on the numbers. For example, H G Ivatt apparently always used to "fudge" the reverser scales on his locos according to his nephew H A V Bulleid (son of Gresley's former assistant O V S Bullied) to stop drivers from thrashing them. The ones on Henry Greenly's RH&DR locos were also found to be out of kilter by Tony Crowhurst when he became Chief Engineer there (the deep machined rebate down the middle of them now shows how the old scale was removed so that a new, and allegedly accurate, one could be stamped in).

OVSB of course produced the ultimate "fudge" with the modified LSWR steam operated reverser he put on his pacifics for the Southern. There the numbers meant absolutely nothing and it wasn't uncommon for a loco to be travelling forwards at about 70 mph with the scale showing it was in back gear

Whenever I've driven a steam loco I've never "done it by numbers", I've just adjusted the reverser to positions where the loco felt "comfortable"; the exhaust blast wasn't too harsh, and the fire stayed on the fire bars rather than jumping up and down or even shooting up the chimney If someone asked me what cut off I'd used on a particular run the only answer I'd be able to give them would be "B***ered if I know!".

I know some drivers who "do it by numbers" and will say to trainees things like "When we get to the bridge wind her back three turns" and similar nonsense. In my opinion they're just making the job more difficult than it needs to be. In exactly the same way that I don't change gear formulaically in my car - I listen to the engine and decide from the sound whether I need to - I don't fiddle with the reverser formulaically on a loco, once again I listen (and watch too, because if you've got a long cut off and a wide open regulator together then the fire will go all over the place - if it's flying out of the chimney you know you've got your settings wrong!).

No two drivers do things in the same way and the foregoing is just a description of my technique, which no one's ever criticised, and approximates to what SR drivers did with "Mr Bulleid's magnificent machines" as they became known (until Ron Jarvis butchered them and made them 6 tons heavier in the process), where the catchphrase was "If the loco likes it then leave it alone".

with regard to full regulator working, the rules governing push and pull working, certainly in the NE region, demanded that the engine regulator be wide open.

This could lead to some awkward moments if the rules were striktly observed.
At my test for push and pull working as required by the board of trade, the examiner made certain that the driver had his vacuum regulator shut and then opened the cab rgulator wide before going up front. Naturally, the bell system was coupled up and tested.

It was unfortunate that I had one of the only two push and pull fitted G5's with a straight reversing lever and we took off in fine style on a run to South Howden on the old H & B but when the driver signalled me to notch up I could not move the lever. I went to close the regulator but the examiner waved me away as the rules said the fireman did not touch the regulator.
We passed through Spring Bank crossing still in full travel and I still could not move the lever.
Finally the examiner came over and grabbed the lever and he could not move it either !

With the combined weight of the two of us we managed to notch it up just before the driver shut off for Willerby station.
The examaning inspector was in a quandary as he knew as well as I did that the fireman did all the regulator work and notching up without bells or anything else on a daily basis but he was there to test me and observe the rules.

Swearing me to secrecy he indicated that he would "Shut his eyes " when the bell came to notch up and that he would make a report as to the unsuitabilty of the two engines with lever reverse for P & P work.
I did pass.

Thanks to everyone for their contributions, although some of the later technical jargon has lost me a little until I learn more about lap and lead, crosshead arms and union links.

I think that the subject of cut off has always confused me because of the apparent contradiction in the terminology. Locoman says - "However if more speed is required or the train beginning to climb, the driver decreases ‘cut off’ up to 20, 30 or 40% as required."

It is the combination of decreasing cut off with increasing percentages that is confusing for an ancient old spotter like myself.

But, I am learning slowly thanks to the patience and detailed explanations from you all.

Strang Steel:

Try putting it another way. If the valve cut off is, say, 20% that means that the valves are open to steam for only 20% of a full travel.
If more power is needed then a decrease of cut off means going lower than 20%.
Therefore if the driver decreases from 20% to 40% that means that the valves are open to steam for 40% of a full travel. Any further could result in a fireman with a broken back !

Locoman, you're posting some material that I'm sure is of great interest to many Members besides me. But may I recommend that you post your topics in the appropriate subject area? I only came across this one by chance, as I don't go to all the topics, and for all I know may have missed other items I would have enjoyed because they're in the wrong place.

Kudu

Hi, have just read this post and have a couple of questions. I assume "using the reverser" to control cut off means that it would be used either forward of "zero" (when in forward gear) OR after zero (when in reverse gear) i.e your are "using" the reverser but you may not be in reverse?

Also, using the motor car gear analogy, sounds like you have to manipulate the lever or screw straight through the higher gear positions to get to the first gear position to get the locomotive going, either forward or reverse, and then pull back to the higher positions from there?