Tractive effort

From Wikipedia, the free encyclopedia

Tractive effort (abbr. TE) is the pulling force exerted, normally by a locomotive, though the term could also be used for anything else that pulls a load. It is normally understood to be the actual force on the locomotive's drawbar or rear coupler. When a bare figure for tractive effort is quoted without a speed qualification, this is normally for starting tractive effort, i.e. at a dead start with the wheels not turning.

In most cases, especially for steam locomotives, this figure is a calculated, not measured one. The normal formula used (measurements in pounds and inches) is:

$t = \frac {cPd^2s} {D}$

where

t is tractive effort
c is a constant representing losses in pressure and friction; normally 0.85 is used
P is the boiler pressure
d is the piston diameter (bore)
s is the piston stroke
D is the driving wheel diameter

The constant 0.85 was the Association of American Railroads (AAR) standard for such calculations, and certainly over-estimated the efficiency of some locomotives and underestimated that of others. Modern, roller bearing fitted locomotives were probably underestimated in this calculation.

European designers used a constant of 0.6 instead of 0.85, so the two cannot be directly compared without a conversion factor.

Tractive effort is the figure most often quoted when people are comparing the power of different steam locomotives, but the use can be misleading, because tractive effort shows the ability to start a train, not the ability to do work by hauling it. Possibly the highest figure for starting tractive effort ever recorded was for the Virginian Railway's 2-8-8-8-4 Triplex locomotive, which in simple expansion mode had a starting T.E. of 199,560 lbf (888 kN) — but this did not translate into power, for the boiler was undersized and could not produce enough steam to haul at speeds over 5 mph (8 km/h).

Of more successful large steam power, the Union Pacific's famous Big Boys had a starting T.E. of 135,375 lbf (602 kN), the Norfolk & Western's 2-8-8-2 Y6a had a starting T.E. of 152,206 lbf (677 kN) in simple expansion mode, and the Pennsylvania Railroad's freight Duplex Q2 attained 114,860 lbf (511 kN) — the highest for a rigid framed locomotive. Later two cylinder passenger locomotives were generally 70,000 to 80,000 lbf (300 to 350 kN) of T.E.

For a diesel-electric or electric locomotive, starting T.E. can be calculated from the stall torque of the traction motors (the turning force it can produce while at a dead stop), the gearing, and the wheel diameter.

A related statistic is a locomotive's factor of adhesion, which is simply the weight on the locomotive's driving wheels divided by the starting tractive effort.

For a locomotive to accelerate from a stationary position, it must apply a force to overcome the inertia of the train, along with the frictional forces in the form of mechanical friction, and wind resistance as the train accelerates. In order for this to occur a particularly high tractive effort is required, usually the maximum tractive effort of the engine is applied. This means that the engine works to produce the highest possible force that it can exert onto the wheels to cause movement or motion. Few engines can maintain work at the maximum tractive effort for very long, but neither is it usually necessary for an engine to do this. Once the train is running at a constant velocity the train no longer needs to overcome its inertia to remain at the same velocity, and hence must only provide power to compensate for frictional forces. This leads to one potential upper limit on the speed a locomotive can haul a train at, once the force due to wind resistance becomes greater than the tractive effort the locomotive can supply (fluid drag increases with the square of velocity), the locomotive cannot accelerate the train anymore (in reality the situation is more complicated than this due to a number of mechanical considerations).

A table to illustrate the speed the maximum tractive effort, continuous tractive effort and the speed at which the tractive effort should be reduced on a selection of trains operating in the United Kingdom:

Class	Type	Top speed		Maximum tractive effort	Speed to reduce tractive effort	Continuous tractive effort	Maximum power at rail	Mass
Class	Type	mph	km/h	Maximum tractive effort	Speed to reduce tractive effort	Continuous tractive effort	Maximum power at rail	Mass
Class 08	Shunter	15		156 kN	8.8 mph	49 kN	194 kW	49.6 - 50.4 t
Class 33	Passenger	85		200 kN	17.5 mph	116 kN	906 kW	77.7 t
Class 56	Light freight	80		275 kN	16.8 mph	240 kN	1790kW	125.2 t
Class 58	Light freight	80		275 kN	17.4 mph	240 kN	1780 kW	130 t
Class 59	Heavy freight	60 or 75		506 kN	14.3 mph	291 kN	1889 kW	121 t
Class 60	Heavy freight	60		500 kN	17.4 mph	336 kN	1800 kW	129-131 t
Class 66	Heavy freight	75		409 kN	15.9 mph	260 kN	1850 kW	126 t
Class 67	Light freight	125	200	141 kN	?? mph	90 kN	1860 kW	90 t

The power at rail of a train follows the equation power (kW) = force (kN) x speed (m/s)

In general, it is more common for heavy freight trains (such as Class 59, Class 60 and Class 66 locomotives) to have a high maximum tractive effort due to the mass which they haul. Light freight trains (such as Class 56, Class 58 and Class 67 locomotives) and passenger trains (such as Class 33 and Class 43 / Intercity High Speed Train locomotives) usually have much lower maximum tractive efforts.