Tractive Effort to raw tonnage

[LMS2968]

A dynamometer car measures several things, but mostly the force exerted on the drawbar by the locomotive and the speed at every point. Other equipment could be added to measure coal and water consumption, steam chest pressures, pressure in the cylinders over one cycle (this by means of Indicating, hence those wooden hut contraptions sometimes seen on the front of the loco to house the indicator testers), wind speed and direction, air pressure, humidity, etc., etc. The problem is that this records the force being applied to the train and not that developed by the engine since some of this is used in moving itself, more so when working against a gradient. The dynamometer car intergrates the drawbar pull and speed to give the power being applied to it (Drawbar Horsepower, of DBHP for short) but then the additional resistance of the loco against the gradient must be estimated and added to the DBHP to get the loco's true output; this is known as the Equivalent Drawbar Horsepower, or EDHP), and is generally what is shown in books about performance. But the system has built-in flaws, there are too many uncontrolled variables, including speed, out on the road and dynamometer car results fell into disfavour during the 1950s, testing station results being preferred.

 

The value of the testing station was that it removed all the variables, including wind speed and direction, and it recorded the force that the loco was putting into the rails themselves. In theory, this should have given a definitive result but life is never simple. If you tested a loco on the Rugby plant, and then repeated it at Swindon, the two results were unlikely to match, due to different techniques used at the two plants. Eventually, test plant results were followed by road tests using the Mobile Testing Units (MTUs, up to three coaches which used electric braking to accurately maintain a given speed), thus getting the best of both worlds.

[The Stationmaster]

Regarding load tables / permitted loads; there is an interesting point made in the Ais Gill accident report. During the enquiry it was suggested that a factor in the accident was that one train was heavier than the permitted load for that class of engine. In reply the Midland Railway explained they considered the permissable load was the load the engine could haul and keep time. A certain amount of overloading was permissable,  but in that case it was accepted the train would not keep to time.

 

Jeremy

That has long been a standard feature in any load tables - they are based on what can be reliably hauled in average conditions by an engine in average condition running to normal timings.  Overloading (usually up to authorised maximum for either the route or by Class of engine is rather different as it is explicitly accepted that the train would lose time.  That was even the case in BR diesel loco days until the Class 60s arrived when they were given specific loads per train at whatever timings  (instead of per section of route) but again based on various averages.

 

The best source for identifying loads - many of which you will never come near on a model railway - is as already suggested to try to find relevant load tables from old working timetables (WTTs) or Loads Books.

[lanchester]

May I be forgiven a slightly related question? I know the LNER (and possibly other railways) used a 'counter pressure locomotive' in locomotive performance testing - on the LNER it was an old B13 4-6-0. But how did this work? It presumably wasn't a case of putting the counter pressure loco into reverse and seeing who won!!

[Edwin_m]

At low speed tractive effort is normally limited by adhesion and can be calculated relatively easily from the weight on the driving wheels and the assumed coefficient of friction between the wheels and the rails.  So the zero speed tractive effort normally also applies at low actual speeds. 

 

However this assumes that the cylinders can provide enough force to exploit the adhesion.  This must be true at low speeds for main line locos, otherwise they would never slip on starting.  I don't know if it is true for industrial locos which may have much lower power and better adhesion because they generally don't have carrying wheels. 

[jamie92208]

That is very true, many locos can only utilise their full tractive effort at higher speeds due to the limits of adhesion.  Obviously shunting locos are optimised with more weight on the driving wheels to be able to apply their whole power at low speeds.  I have seen, but can't remember where, graphs of the tractive effort v the speed which follow a curve which rises quite fast then levels out.  Larger engines tended to have such things as leading and training trucks that all reduced the weight on the drivers thus limiting their adhesion.  Shunters were mainly tank engines with all their coal and water adding to the adhesive weight.

 

Jamie

[LMS2968]

Are you sure, Jamie, you don't mean power output v speed? TE tended to fall with speed, with steam anyway. The beauty of the steam engine is that it gives maximum torque (read TE) at zero rpm.

[Edwin_m]

Yes, I think Jamie means "full power" not "full tractive effort".