Factor of adhesion
Encyclopedia
In railroad engineering, the factor of adhesion of a locomotive
is the weight on the driving wheels divided by the starting tractive effort
.
A common rule is that for a steam locomotive
a good factor of adhesion
equals or exceeds 4, but not by too much. A locomotive with a factor of adhesion below 4 will have a tendency to wheelslip, especially on starting, and will require careful driving and lots of sand in slippery conditions. A greater factor of adhesion than 4 is simply excess weight or insufficient power. The number 4 physically corresponds to the inverse of the coefficient of static friction for steel on steel.
Diesel
and electric locomotive
s can work with a much lower factor of adhesion than a reciprocating steam locomotive because their power is applied smoothly, unlike the latter's pulsed power delivery. The pulses can, to some extent, be smoothed in a steam locomotive by fitting 3 cylinders (with cranks at 120 degrees) or 4 cylinders (with cranks at 135 degrees). A 4-cylinder engine with cranks at 180 degrees will deliver similar pulses to a 2-cylinder engine. Alternatively, a geared steam locomotive
can deliver a smooth torque
similar to that of a diesel or electric locomotive.
The corollary of the above is that as a locomotive's available tractive effort increases, so must the weight on the driven axles increase for adequate adhesion.
The civil engineer of a railway sets the maximum load that can be placed on any one axle. Leading and trailing axles are used to carry loco weight that cannot be placed on the driven wheels because of vehicle limitations - such as position of cylinders and firebox. All available weight is placed on the driven axles, which have to be of sufficient number so as not to exceed the civil engineers limit. Leading or trailing axles are provided to ensure the weight distribution of the loco is even - with typically all driven axles having the same load and using the smallest number of trailing axles possible to take surplus weight and distribute it evenly amongst them - again limited to the civil engineer's axle load limit.
For most locomotives, the tractive effort of a locomotive can be guessed well just by knowing the number of driven wheels and the maximum axle load for which it was designed.
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...
is the weight on the driving wheels divided by the starting tractive effort
Tractive effort
As used in mechanical engineering, the term tractive force is the pulling or pushing force exerted by a vehicle on another vehicle or object. The term tractive effort is synonymous with tractive force, and is often used in railway engineering to describe the pulling or pushing capability of a...
.
A common rule is that for a steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
a good factor of adhesion
Rail adhesion
The term adhesion railway or adhesion traction describes the most common type of railway, where power is applied by driving some or all of the wheels of the locomotive. Thus, it relies on the friction between a steel wheel and a steel rail. Note that steam locomotives of old were driven only by...
equals or exceeds 4, but not by too much. A locomotive with a factor of adhesion below 4 will have a tendency to wheelslip, especially on starting, and will require careful driving and lots of sand in slippery conditions. A greater factor of adhesion than 4 is simply excess weight or insufficient power. The number 4 physically corresponds to the inverse of the coefficient of static friction for steel on steel.
Diesel
Diesel locomotive
A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...
and electric locomotive
Electric locomotive
An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device...
s can work with a much lower factor of adhesion than a reciprocating steam locomotive because their power is applied smoothly, unlike the latter's pulsed power delivery. The pulses can, to some extent, be smoothed in a steam locomotive by fitting 3 cylinders (with cranks at 120 degrees) or 4 cylinders (with cranks at 135 degrees). A 4-cylinder engine with cranks at 180 degrees will deliver similar pulses to a 2-cylinder engine. Alternatively, a geared steam locomotive
Geared steam locomotive
A geared steam locomotive is a type of steam locomotive which uses reduction gearing in the drivetrain, as opposed to the common directly driven design....
can deliver a smooth torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
similar to that of a diesel or electric locomotive.
The corollary of the above is that as a locomotive's available tractive effort increases, so must the weight on the driven axles increase for adequate adhesion.
The civil engineer of a railway sets the maximum load that can be placed on any one axle. Leading and trailing axles are used to carry loco weight that cannot be placed on the driven wheels because of vehicle limitations - such as position of cylinders and firebox. All available weight is placed on the driven axles, which have to be of sufficient number so as not to exceed the civil engineers limit. Leading or trailing axles are provided to ensure the weight distribution of the loco is even - with typically all driven axles having the same load and using the smallest number of trailing axles possible to take surplus weight and distribute it evenly amongst them - again limited to the civil engineer's axle load limit.
For most locomotives, the tractive effort of a locomotive can be guessed well just by knowing the number of driven wheels and the maximum axle load for which it was designed.