Tractive Effort

Can anyone tell me what locomotive tractive effort is considered necessary to move a particular trainload from standstill. ( I assume restarting the train load on a gradient of X would form part of the consideration). I do realise that there are a whole host of other things to consider as far as actual locomotives are concerned rather than just nominal TE when assessing the capability of a locomotive to take a train on a journey, I'm just interested in getting it moving.

I seem to remember reading once that with the power turned off a train loses speed (on level straight track) at quite a low rate - a few (maybe 10 ?) mph per mile travelled - which is, if true, a bit surprising but would indicate that rolling resistance is quite low, so once the initial inertia of a static load is overcome the effort required to keep it moving is low.

 

I think you may be confusing 'tractive effort' (a theoretical calculation based on cylinder dimensions,wheel dia, and boiler pressure) and drawbar pull. Ray.

 

Hi,
There is a lot on TE within the MIC threads. If you supply a train weight I should think one of the other users can answer the question and explain how they worked it out

 

Tractive Effort is a force, and to maintain a given speedit must equal the resistive forces operating on the train: the total resistance of all the bearings of the train (the train weight times the co-efficient of rolling resistance of the bearings plus that of the wheels on the track). Generally, a nominal CoR (say 0.02) is chosen times the train's total weight, plus the resistance due to gradient (zero on level track; positive against a rising gradient; negative on a falling one. The total weight of the train is divided by the gradient, so a 1000 ton train on a 1:100 gradient would need ten tons [22,400 lb]) to maintain a given speed. Aerodynamic resistance can be discounted at very low speeds.

If the engine's Tractive Effort equals the train's (and its own) total resistance, it will not move, the TE must be greater than the resistance. That excess would then determine the rate of acceleration in accordance with Newton's Second Law (Force = Mass x Acceleration, so A= F/M), i.e. the excess tractive effort divided by the total weight. This works well in SI units but Imperial requires conversion factors..

Note that the TE we're talking about is that actually delivered to the rails, and not the Nominal Tractive Effort quoted in specifications, and Ian Allan ABCs. The TE is also governed by the wheels' ability to grip the rails so adhesion plays a part.

 

Interesting calculation that, based on that calculation assuming a 400 ton train Galatea's mamoth effort may just have been possible.

45699 was ex works suppose the cylinders were say 18" diameter and the wheels were say 6' 6" the nominal tractive effort would be almost 31,000lb.

This is not the only maximum effort by this engine, I read somewhere that she did something very similar on a summer saturday.

Must have made some real Jubilee music.

Only once have I experienced an all out performance and that was Leander returning to the SVR after a visit to Scotland.

After various delays culminating i a long stopover on the stump of the Warcop branch we were allowed on our way we got checked at Kirkby Stephen and on the way up to Ais Gill I have never heard a sound like it.

You could not look out of the windows goggles or not way too many sparks, later found out from the Bahamas support guys who were doing support for the SVR that she was absolutely flat out.

After this run I find the description of the sparks thrown by Galatea entirely believable.

Cheers Dave

 

Take the full weight of the train (including the loco) and divide it by the gradient. That will give you the force required to overcome gravity.
Next, you need to consider the rolling resistance of the train (but excluding the loco. BR published details of the rolling resistance of Mk.1 stock and this varies with speed. At 25mph it is about 4 lbf/ton but increases significantly with increasing speed - they didn't publish figures at starting. The rolling resistance of the loco is quite complex and the best you can probably do is make a stab at it but, if you want a bit of quiet reading on the subject, have a look at: 'Steam Locomotive Resistance and Information on its Values' by John Knowles ( http://freespace.virgin.net/johnk.pb15/Steam Locomotive Resistance.pdf ) I'd go for 15lbf/ton as a simple figure, a bit more for a slide valve loco.
You may also need to take account of curve resistance, which again can be a bit complex but 3.5lbf/ton would be a starting point and finally wind resistance, if you need to consider starting trains in gale force conditions. Work on the cross sectional area of the loco head on and multiply it by the coeffficient for the required wind speed.
All the above should give you the force (cylinder tractive effort) required to start the train. don't forget the steam loco produces max TE at starting and the actual TE will drop off significantly as speed increases.
HTH

 

Thanks for the various replies, in particular the very detailed ones. The document referred to by Steve ( John Knowles) is very good indeed. It'll keep me quiet for a bit !

Thanks once again.

 

Big Dave - you refer to "Galatea's mamoth effort" and "Leander returning to the SVR after a visit to Scotland" as though they've been recently discussed, but I've not read those threads. Can you point me to the right places to read the full stories, please? Or if these are events well known to loco-performance geeks but not more widely discussed, perhaps you can supply the details here? What were the loads and the conditions? Thanks in advance.