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Wheel slide protection and wheel slip protection are railway terms used to describe automatic systems used to detect and prevent wheel-slide during braking or wheel-slip during acceleration. This is analogous to ABS and traction control systems used on motor vehicles. It is particularly important in slippery rail conditions.
Sanding is one method of reducing wheel slip or slide. Locomotives and multiple units have sandboxes which can place dry sand on the rails, in front of the wheels. This may be initiated automatically when the Wheel Slide Protection system senses loss of adhesion, or the driver can operate it manually. Sanding may be connected to a computer system that determines the train's direction of travel and where the sand should be applied: either in front of or behind the trucks. In older locomotives there was a manual lever attached to a valve that had three positions: Off, Forward, and Aft.[ citation needed ]
Wheel Slide Protection (WSP) equipment is generally fitted to passenger trains to manage the behavior of wheel sets in “low adhesion” (reduced wheel/rail friction) conditions. It is used when braking and may be considered analogous to anti-lock braking (ABS) in cars. The system can also be used to control (or provide an input to) the traction system to control wheel spin when applying power in low adhesion conditions.
“Low adhesion” on the rail potentially causes damage to train wheels and the rails. Typically, low adhesion conditions are associated with environmental causes arising from seasonal leaf fall, or industrial pollution. Occasionally, the cause can be another less obvious factor such as light oxidation of the railhead or even swarms of insects.
When a train is braking, the low adhesion manifests as wheel slip where the wheelset is rotating at a lower velocity (speed) than the forward speed of the train. The most extreme example of this is where the wheel stops rotating altogether (wheel slide) while the train is still moving and can result in a “wheel flat” caused by the softer steel wheel being worn away by the harder steel rail.
However, the wheelset does not need to lock up completely in order for damage to be caused. If the slide is significant, heat can build up in the contact patch between the wheel and the rail, sufficiently to permanently modify the crystalline structure of the wheel's steel alloy. The steel becomes more brittle (martensite) which leads to cavities forming in the wheel. Wheel flats on railway vehicles are very evident by a distinct “bang-bang” noise made in time with the speed of the train. It is normally necessary to use a wheel lathe to remove a layer of wheel tread caused by a severe flat spot or cavity, which reduces the operational service life of the wheel and is a major operating cost to the rail industry.
In traction, low adhesion may cause a wheelset to accelerate more quickly than the train (wheel spin) to the point where it can damage the traction system or result in damage to the wheel and rail (rail burn).
WSP is generally fitted as standard to new fleets of multiple units. The primary function of the WSP is to improve the ability of a train to stop in poor adhesion conditions. However, within the rail industry it is also recognised to be valuable in protecting the wheels from damage during sliding when braking or spinning in traction. This improvement is achieved by regulation of the wheelset velocity in a controlled manner so that it maintains a relatively consistent level of slip.[ citation needed ] The controlled slip has the effect of conditioning the contamination layer on the rail (scrubbing action) thereby improving the level of friction and enhancing the ability of the train to stop. Controlled wheel slip can also have a limited cleaning action on the rail head along the length of a train. This tends to result in the vehicles at the rear having more grip than those at the front.
WSP continuously monitors the rotational speed of each axle on the locomotive or multiple unit, and intervenes whenever it detects a significant difference on any axle.
If wheelslip occurs whilst the power controller is open, WSP will shut off power to the affected traction motor(s). Despite this, most rail transport companies advise their drivers to close the power controller [1] and allow the slipping wheels to stabilize before re-opening the controller on a low setting because control of the train can be achieved more quickly.
However, when wheelslide occurs and the WSP releases the brakes on affected axles, drivers are instructed to leave the brake handle alone [1] and let the WSP control the train's braking. This is because the driver is sitting over the leading bogie of the train where wheelslide is usually most severe. [1] This wheelslide will partially clean the railhead and so further down the train the wheels will achieve better adhesion, and thus braking effect.
Driving a train under low adhesion conditions takes experience. Failing to recognize and respond correctly to railhead contamination or environmental conditions which cause low adhesion can lead to safety incidents and accidents such as a signal passed at danger, collision or station overrun.
Prior to each leaf-fall season, many train companies arrange low adhesion training [1] for their newly qualified drivers. This consists of taking over a section of line during a quiet period. Using lineside markers each driver gets his train up to speed and then makes a Full Service brake under normal adhesion conditions. The railhead is then treated with a contaminant that has a low coefficient of friction. On the second run the driver will experience the sound and sensation of the WSP activating and operating blowdown valves on the brake cylinders, and the stopping distance will be considerably greater.
Although this only provides an approximation of how a train will behave during low adhesion, it does ensure that the driver can recognize the onset of wheelslide and will know the correct actions to take when it occurs. [1]
Modern WSP systems are microprocessor controlled and employ two stage valves that permit fine control over the air pressure in the brake cylinders. This is essential to be able to capture and control a sliding wheel and to minimise the amount of air resource used by the WSP. When the brake is applied, the WSP first applies the dynamic brake. If that is not successful it then “blends” the friction and dynamic braking systems. [2] If control is still not established the system reverts to friction braking only where blowdown valves [2] rapidly cycle the air in the brake cylinders . Examples of this kind of equipment are manufactured by Knorr Bremse (EP compact, EP2002) Faiveley Transport (EPAC) and POLI Wabtec (ATHENA).
Manufacturers of WSP equipment include Faiveley Transport, Knorr-Bremse, Wabtec, DAKO, KES & Co GmbH, Mitsubishi, Siemens, Selectron Systems AG, ABB and LCA Ballauri.
Demonstrating the improvement provided by a WSP system is very difficult as the naturally occurring low adhesion condition occurring at the rail can be difficult to re-create in a test track environment.
For track testing, a detergent based solution has historically been used to provide low adhesion test conditions. European and international standards often refer to this test method (BS-EN 15595, UIC 541-05). In the UK, British Rail Research adopted two approaches including a laboratory simulation method to all WSP approvals from around 1992, and track testing using carefully conditioned paper tape adhered to the railhead. The paper tape method used in the UK is believed to offer a realistic representation of the challenging very low adhesion conditions encountered during the autumn leaf fall. With an increase in the privatisation of railways in Europe, track testing has become very expensive to organise and to conduct. As a consequence, simulation based testing is rapidly becoming more popular with WSP manufacturers and national bodies.
Simulation testing employs a computer representation of the train and the track conditions, and signals are provided to the WSP system that effectively deceive it into thinking it is fitted to an actual train.
A bogie is a chassis or framework that carries a wheelset, attached to a vehicle—a modular subassembly of wheels and axles. Bogies take various forms in various modes of transport. A bogie may remain normally attached or be quickly detachable. It may include a suspension component within it, or be solid and in turn be suspended ; it may be mounted on a swivel, as traditionally on a railway carriage or locomotive, additionally jointed and sprung, or held in place by other means.
An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface and allowing the driver to maintain more control over the vehicle.
A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.
A traction control system (TCS), is typically a secondary function of the electronic stability control (ESC) on production motor vehicles, designed to prevent loss of traction of the driven road wheels. TCS is activated when throttle input and engine power and torque transfer are mismatched to the road surface conditions.
In railway engineering, the term tractive effort describes the pulling or pushing capability of a locomotive. The published tractive force value for any vehicle may be theoretical—that is, calculated from known or implied mechanical properties—or obtained via testing under controlled conditions. The discussion herein covers the term's usage in mechanical applications in which the final stage of the power transmission system is one or more wheels in frictional contact with a railroad track.
Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of the vehicle to changes in steering angle associated with changes in lateral acceleration. This sensitivity is defined for a level road for a given steady state operating condition by the Society of Automotive Engineers (SAE) in document J670 and by the International Organization for Standardization (ISO) in document 8855. Whether the vehicle is understeer or oversteer depends on the rate of change of the understeer angle. The Understeer Angle is the amount of additional steering that must be added in any given steady-state maneuver beyond the Ackermann steer angle. The Ackermann Steer Angle is the steer angle at which the vehicle would travel about a curve when there is no lateral acceleration required.
Rail transport terms are a form of technical terminology applied to railways. Although many terms are uniform across different nations and companies, they are by no means universal, with differences often originating from parallel development of rail transport systems in different parts of the world, and in the national origins of the engineers and managers who built the inaugural rail infrastructure. An example is the term railroad, used in North America, and railway, generally used in English-speaking countries outside North America and by the International Union of Railways. In English-speaking countries outside the United Kingdom, a mixture of US and UK terms may exist.
A locking differential is a mechanical component, commonly used in vehicles, designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction available to either wheel individually.
Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or other wheeled vehicle occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle from responding to control inputs. If it occurs to all wheels simultaneously, the vehicle becomes, in effect, an uncontrolled sled. Aquaplaning is a different phenomenon from when water on the surface of the roadway merely acts as a lubricant. Traction is diminished on wet pavement even when aquaplaning is not occurring.
In rail transport, a derailment is a type of train wreck that occurs when a rail vehicle such as a train comes off its rails. Although many derailments are minor, all result in temporary disruption of the proper operation of the railway system and they are a potentially serious hazard.
Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body rolls on a surface. It is mainly caused by non-elastic effects; that is, not all the energy needed for deformation of the wheel, roadbed, etc., is recovered when the pressure is removed. Two forms of this are hysteresis losses, and permanent (plastic) deformation of the object or the surface. Note that the slippage between the wheel and the surface also results in energy dissipation. Although some researchers have included this term in rolling resistance, some suggest that this dissipation term should be treated separately from rolling resistance because it is due to the applied torque to the wheel and the resultant slip between the wheel and ground, which is called slip loss or slip resistance. In addition, only the so-called slip resistance involves friction, therefore the name "rolling friction" is to an extent a misnomer.
An adhesion railway relies on adhesion traction to move the train, and is the most widespread and common type of railway in the world. Adhesion traction is the friction between the drive wheels and the steel rail. Since the vast majority of railways are adhesion railways, the term adhesion railway is used only when it is necessary to distinguish adhesion railways from railways moved by other means, such as by a stationary engine pulling on a cable attached to the cars or by a pinion meshing with a rack.
Cadence braking or stutter braking is a driving technique that involves pumping the brake pedal and is used to allow a car to both steer and brake on a slippery surface. It is used to effect an emergency stop where traction is limited to reduce the effect of skidding from road wheels locking up under braking. This can be a particular problem when different tires have different traction, such as on patchy ice for example. Its use in an emergency requires a presence of mind that the situation itself might preclude. Cadence braking is supposed to maximize the time for the driver to steer around the obstacle ahead, as it allows the driver to steer while slowing. It needs to be learned and practiced. For most drivers of modern cars, it has been entirely superseded by ABS, however it is still a valuable skill for drivers of non-ABS equipped vehicles such as classic cars.
Traction, traction force or tractive force is a force used to generate motion between a body and a tangential surface, through the use of either dry friction or shear force. It has important applications in vehicles, as in tractive effort.
A sandbox is a container on most locomotives, multiple units and trams that holds sand, which is dropped on the rail in front of the driving wheels in wet and slippery conditions and on steep grades to improve traction.
Slippery rail, or low railhead adhesion, is a condition of railways (railroads) where contamination of the railhead reduces the traction between the wheel and the rail. This can lead to wheelslip when the train is taking power, and wheelslide when the train is braking. One common cause of contamination is fallen leaves that adhere to the railhead of railway tracks. The condition results in significant reduction in friction between train wheels and rails, and in extreme cases can render the track temporarily unusable. In Britain, the situation is colloquially referred to as "leaves on the line".
A wheelspin occurs when the force delivered to the tire tread exceeds that of available tread-to-surface friction and one or more tires lose traction. This leads the wheels to "spin" and causes the driver to lose control over the tires that no longer have grip on the road surface. Wheelspin can also be done intentionally such as in drifting or doing a burnout.
A flat spot, or wheel flat, also called spalling or shelling, is a fault in railroad wheel shape. A flat spot occurs when a rail vehicle's wheelset stops rotating while the train is still in motion, causing part of the wheel to ablate against the hard steel of the rails. Flat spots are usually caused by use of the emergency brake, or slippery (low-adhesion) conditions that cause wheels to lock up while the train is still moving. Flat spots are more common in the autumn and winter when the rails are slippery.
A train wheel or rail wheel is a type of wheel specially designed for use on railway tracks. The wheel acts as a rolling component, typically press fitted onto an axle and mounted directly on a railway carriage or locomotive, or indirectly on a bogie, also called a truck. The powered wheels under the locomotive are called driving wheels. Wheels are initially cast or forged and then heat-treated to have a specific hardness. New wheels are machined using a lathe to a standardized shape, called a profile, before being installed onto an axle. All wheel profiles are regularly checked to ensure proper interaction between the wheel and the rail. Incorrectly profiled wheels and worn wheels can increase rolling resistance, reduce energy efficiency and may even cause a derailment. The International Union of Railways has defined a standard wheel diameter of 920 mm (36 in), although smaller sizes are used in some rapid transit railway systems and on ro-ro carriages.
Decelostat is a wheel slide protection system developed by Westinghouse Air Brake Company that is used in railroad cars to prevent over-braking that causes wheel-slide, a condition of reduction in friction between train wheels and rails. This low wheel/rail adhesion condition reduces braking performance and causes damage to wheels and the rails.