Factor of adhesion

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Factor of adhesion

In railroad engineering, the factor of adhesion of a locomotive

Locomotive

A locomotive is a Rail transport vehicle that provides the motive power for a train. The word originates from the Latin language loco - "from a place", Ablative case of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine,....
is the weight on the driving wheels divided by the starting tractive effort

Tractive effort

Tractive effort is the pulling Force exerted, by a locomotive or other vehicle. The term is used specifically in railway terminology.The tractive effort value can be either a theorectical or experimentally obtained value, and will usually be quoted under normal operating conditions....
. This is an important statistic, because obviously extra power is pointless if there's not enough traction

Traction (engineering)

Traction is defined by dictionaries as adhesive friction, another name for Friction#Static_friction . Traction is never properly used to mean Friction#Kinetic_friction ....
to put it to useful work. A common rule is that for a steam locomotive

Steam locomotive

A steam locomotive is a locomotive powered by steam. The term usually refers to its use on railways, but can also refer to a "road locomotive" such as a traction engine or steamroller....
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 and thus it relies on the friction between a steel wheel and a steel rail....
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.

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Encyclopedia

In railroad engineering, the factor of adhesion of a locomotive

Locomotive

Tractive effort

Traction (engineering)

Steam locomotive

Rail adhesion

Diesel locomotive

A Diesel locomotive is a type of railroad locomotive in which the prime mover is a Diesel engine. Several types of Diesel locomotive have been developed, the principal distinction being in the means by which the prime mover's mechanical power is conveyed to the driving wheels ....
and electric locomotive

Electric locomotive

An electric locomotive is a locomotive powered by electricity from an external source. Sources include overhead lines, third rail, or an on-board electricity storage device such as a battery or flywheel energy storage system....
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 gear in the drivetrain, as opposed to the common directly-driven design....
can deliver a smooth torque

Torque

Torque is the tendency of a force to rotate an object about an axis . 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 its weight for adequate 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 and thus it relies on the friction between a steel wheel and a steel rail....
. This explains why steam locomotive designers were never that interested in saving weight. Since a steam locomotive must have leading

Leading wheel

The leading wheel or leading axle of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located in a truck ....
and trailing wheel

Trailing wheel

On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle located behind the driving wheels. The axle of the trailing wheels were usually located on a trailing Bogie....
s for stability if it intends to travel at any speed, and these wheels must have some weight on them for good tracking, the steam locomotive's weight increases further.

As weight on the driving wheels increases, the number of driven axle

Axle

An axle is a central shaft for a rotation wheel or gear. In some cases the axle may be fixed in position with a bearing or bushing sitting inside the hole in the wheel or gear to allow the wheel or gear to rotate around the axle....
s must also increase; maximum axle load

Axle load

On railways, the axle load is the maximum weight of a train per pair of wheels allowable for a given section of Rail tracks. The maximum axle load is related to the strength of the track, which is determined by weight of rails, density of railroad ties and fixtures, train speeds, amount of track ballast, and strength of bridges....
is limited by the railroad's civil engineer

Civil engineer

A civil engineer is a person who practices civil engineering, one of the many engineering professions. Originally a civil engineer worked on public works projects and was contrasted with the military engineer, who worked on armaments and defenses....
, since bridges and other structures are built to take only a certain loading. Thus, 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.