BR Standard class 6 No. 72010 'Hengist' and Clan Discussion Thread

The TE is the average force available at the wheel through the whole cycle. It does vary throughout a single revolution as you say according to the angle.

So in practice, if you had a single cylinder, the torque available varies through one revolution, at certain points dropping to zero and at other points being higher than the nominal TE. If you have a two cylinder loco, the two cranks are set out of phase such that when one cylinder is at its lowest torque, the other is at its maximum, the result is the torque still varies through one revolution, but by a smaller amount. (Essentially you work out the variation with angle for each cylinder separately; and then add them to get the overall effect). A three cylinder design (or a four cylinder design with the Lord Nelson crank angles) will have the variation in torque smoothed out to an even greater extent.

Tom

 

If you look at the diagrams of turning moments produced by Barkhausem in 1912 you will find these to be helpful. The most readily found ones are for two cylinder compounds compared with two cylinder simples and for four cylinder compounds compared with four cylinder simples, cranks are conventionally set at 90 degrees.

 

At BDC/TDC the valves are not admitting steam and so no real work is going on. Well, steam is being exhausted, but that's the inertia of the rotating assembly transferring kinetic energy to a gas, as opposed to a gas transferring energy to the motion.

Its also not strictly linear as the valves do not open instantaneously. They "gradually" open in relation to the geometry of the valve gear, so we don't see maximum crank pin forces until we are many degrees past TDC/BDC.

Now, if we fitted digitally controlled valves we may see your scenario come to pass, admitting steam exactly at TDC/BDC, but steam isn't admitted until the valves open and the wheels need to have rotated some for those valves to open.

 

You are correct in saying that the crank angle has an effect on torque but it is largely irrelevant. The well-known formula gives the average torque/tractive effort, which is what people need to know. The fact that it varies slightly over a revolution is overcome on a stationary engine by having flywheel. The flywheel is there to provide the inertia necessary to smooth out the torque curve. In fact, a flywheel is so good at doing this that most stationary engines only had one cylinder and worked perfectly well, even though the torque varied from zero to maximum. Two cylinders are only really necessary where you want reversing and starting without the need to use a bar. Steam locos need two cylinders but don't have a flywheel because they don't need one. They are not stationary engines and once the loco and train starts to move it has its own inertia which overcomes the lower torque parts of the cycle. The only time that the point of peak torque becomes important is at the actual moment of starting and loco drivers generally know that this is with the cranks on the angles. However, it's a bit of theory that is very hard to put into practice as you can't really stop it in such a position. If loco won't start because it is at a position of low torque, it is easy to reverse it slightly to get it going without looking to see where the cranks are.

 

Actually, except on some shunting locos, the valves generally open before TDC. This is referred to as lead steam and, at low speed, actually has a negative torque, albeit because of the crank angle, a very low amount.

 

Another exception to the input valve being open at tdc is the standard GW locos with two outside cylinders and Stephenson gear. Due to the variable lead characteristic of the Stephenson gear it was arranged with negative lead at full gear (i.e. input valve closed) lead gradually increasing as the gear was notched up.

 

Thanks Steve, and others who replied. What I was expecting you to say was that the conventional formula logically explained the net result (and somehow factors in the crank angle issue), which may be what you are saying in a different way. I can sort of see this through a glass, darkly albeit I would not like to stand up in front of a roomful of people and take questions.

As Steve, says, the port opens to lead steam in what textbooks call the pre-admission phase before the start of the stroke. At short cut-offs the lead can account for most if not all of the valve opening to steam. The compression phase starts well before then (in respect of any remaining steam that side of the piston) as the valve closes to exhaust at the same % of stroke / angle of crank as the other valve head opens to exhaust - on a Reuleaux diagram these events are diametrically opposite. For a 43000 2-6-0 (which has 1/4" lead), at 30% cut-off for example, release (exhaust opening) and the compression phase take place/start at around 72% of the stroke. When you imply that full pressure is not available for many degrees past TDC, that assertion is not borne out by indicator diagrams - see e.g. the BR Performance and Efficiency bulletins, where the diagrams have very vertical leading edges, presumably due to lead. With Walschaerts though, the speed of the valve phases varies as in some phases the combination lever and return crank imparted motions work against each other and in some phases they are additive.

 

Interesting, I would like to see this diagram.

 

For a simpler way of presenting it, ignore how the force components change with various angles of the connecting rod and crank, and just look at the total work done by the steam on the piston during one full piston stroke from one end to the other with some effective average pressure in the cylinder (conventionally 85% of full boiler pressure). Divide that work by the distance travelled by the loco in half a revolution of the driving wheels, which is half the wheel circumference.

 

That’s an even shorter explanation than mine I post 2256

 

Whilst this has been an enthralling discussion on an important subject, can I bring this back more to the subject of the thread?

I am delighted to see that a coming together for mutual support, sharing and commonality of purpose has happened within the 'new build/restoration' mainline locomotive groups.
See https://www.theclanproject.org/Clan_News.php
A while ago it was suggested that projects needing BR tenders could put in a batch order somewhere, it does not appear to have borne fruit. I certainly hope that this initiative does, it makes very good sense, to me.

 

As one of those involved in the coming together, I can say I found it a very useful start to a forum, I think we all can see how it can help reduce our workload in some areas relating to mainline certification, and is also an opportunity to present a united front in others. Onwards and upwards!

 

Not really. How many different tender tanks need to be built. Also, the chassis structures could be different, depending on the type of tank fitted.

 

That might have well been the problem indeed, or a case of timing of build versus funding. I don't think it was envisaged to build several of any 1 type, but they are not a cheap item anyway.

 

Is there much saving to be made in a batch build? Individual components such as axle boxes etc then maybe, but I doubt there is much advantage to batch building tanks, and a few potential disadvantages in locking up capital and space before being needed.

Tom

 

Quite agree Tom. A different matter for the fabricator though, if they have a batch of similar jobs to do it makes it easier and perhaps cheaper for them. What is more important is to know the job will be done properly. A nice contract to let to an engineering company who can do everything required.

 

I'm a bit surprised that the NYMR aren't part of any BR tender group, given that one will be needed for 92134 once 73050's overhaul is complete.

 

I suspect the quest is on already.

 

NYMR are not the owner. I believe there is a substantial kit of parts already in stock.

 

I see from the website that the Project has joined the clients of Ricardo as their Assessment Body. As the only approved body in UK where else could they go?

So the 'Newbuilds' have formed a 'club' and they are basically all under one Assessor. This will ensure compliance to one set of rules but I hope it does not result in only one train operator for the special trains.