Locomotive wheelslip is an event that affects railway motive power

Motive power

In thermodynamics, motive power is an agency, as water or steam, used to impart motion. Generally, motive power is defined as a natural agent, as water, steam, wind, electricity, etc., used to impart motion to machinery; a motor; a mover. The term may also define something, as a locomotive or a...

 when starting from stationary.

The greatest effort is required from a locomotive when starting. At this time, if the driver applies too much power to the wheels (i.e., 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...

, he opens the regulator too far) the turning force applied to the wheel will greatly exceed the opposing friction

Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

 force effected by the surface of the rail

Rail profile

The rail profile is the cross sectional shape of a railway rail, perpendicular to the length of the rail.In all but very early cast iron rails, a rail is hot rolled steel of a specific cross sectional profile designed for use as the fundamental component of railway track.Unlike some other uses of...

, and the wheel will turn without being able to move the train forward. If the driver does not take quick corrective action (i.e. by closing the regulator) the locomotive can end up stationary with its wheels spinning: this can damage both the locomotive drive mechanism, through running too fast, and the rail surface, which, in extreme cases, can be left with a dip where the spin took place. (Such a dip is dangerous if not repaired subsequently.)

Some locomotive types were more prone to this phenomenon than others. Also, the effect is much more likely to occur with a heavier train or where the rail surface is compromised, for example, during wet or icy weather, or when there is oil or leaves on the track.

Causes

The causes of locomotive wheelslip are various, but the predominant factor lies in power

Power (physics)

In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

-to-weight

Weight

In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

 ratios. Ideally, locomotive designs will have roughly equal power-to-weight ratios that enable smooth acceleration

Acceleration

In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

 from a 'cold start', or stopping position. However, if the power of a locomotive vastly exceeds its weight, then an imbalance ensues which causes the violent turning of the wheel

Wheel

A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

s through loss of traction

Traction (engineering)

Traction refers to the maximum frictional force that can be produced between surfaces without slipping.The units of traction are those of force, or if expressed as a coefficient of traction a ratio.-Traction:...

.

Other causes include the contact of oil

Oil

An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

 with the flanges and rims of wheels, which reduces adhesion with the surface of the rails, and a general loss of traction on steep gradients when pulling heavy loads.

Alleviation

Most locomotives are fitted with sandboxes

Sandbox (railways)

A sandbox is a container on most locomotives and self propelled multiple units, or trams, that run on tramways and adhesion railways...

 so that sand or Sandite

Sandite

Sandite is a substance used on railways in the UK, Ireland and the Netherlands to combat leaves on the line, which can cause train wheels to slip and become damaged with flat spots...

 can be dropped on the rails to improve adhesion. Modern diesel locomotive

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...

s 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 are fitted with electronic wheelspin detectors

Wheel speed sensor

A wheel speed sensor or vehicle speed sensor is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation...

which automatically reduce the power supplied to the wheels if wheelspin is detected.