Adhesion

Adhesion is a measure of the resistance of friction to slippage between two parallel planes. In the case of a locomotive railwheel the parallel plane is the point on the steel railwheel where the railwheel contacts the steel rail. The maximum force or pull that a locomotive can generate in order to pull a train is limited by the weight of the locomotive and the amount of adhesion that it can maintain without wheel slippage. Once the wheel starts to slip the pulling force is lost. According to the Physics books the maximum adhesion for smooth steel on smooth steel is 42%.

If you add sand to make a sandwich between the wheel and the rail the sand can increase the maximum adhesion because it provides a rough surface that can dig into the small pores of the steel and provide a higher adhesion coefficient. This is especially important if the rail is wet. For wet rail without sand the maximum adhesion for a smooth steel wheel on smooth steel rail is about 10%. With sand on wet track the adhesion can be as high as 30%. Sand on dry track can increase the adhesion to about 40%. This is why locomotives have large sand boxes in order to apply sand on steep grades, wet track, and other slippery spots.

Adhesion is not the same as the friction coefficient because the friction coefficient is a static number. The amount of adhesion that can be maintained is also affected by the acceleration of the railwheel with respect to the steel rail. As soon as the railwheel begins to turn the amount of friction that can be maintained begins to drop. For locomotives with standard DC traction motors and conventional wheel slip systems the average adhesion is about 25%. The force required to move the train is defined by the old physics equation Force = mass x acceleration. The force derived from the acceleration can not exceed the force of derived from friction otherwise wheel slip will occur and the locomotive will lose traction. The acceleration must be controlled to prevent railwheel slippage. Acceleration on a standard locomotive is controlled by controlling the electrical power to each traction motor on a locomotive. That is normally done by controlling the throttle position on the diesel engine and letting the generator put out as much power as it can at that throttle position. It the throttle is opened too much then the power of the traction motors will over power the friction forces under a given load and cause spinning of the railwheels. When an axle begins to slip the wheel slip system will see a voltage spike and cut power to all of the traction motors for a second or two and then reapply the power. If the operator maintains a throttle position that is too high the traction motor will again over power the friction and the wheel will slip. This in turn causes the wheel slip system to cut power and the process is repeated. Unless the operator drops to a lower throttle position or lightens the load the locomotive will chatter loudly as the power to the traction motors is turned off-on-off-on-off-on, etc. This will cause damage to the locomotive and to the railroad track. The locomotive is not "smart enough" to turn down the throttle position itself, to control the rate at which throttle changes occur, or to simply stop and display an overload alarm. The success or failure of the operation is strictly in the hands of the operator or engineer controlling the locomotive. A poorly trained operator can do significant damage to the locomotive and the track system in a very short period of time. Sometimes in less than one shift.

Process locomotives put the throttle control and the control of the voltage and current to each traction motor into program that is installed on the master PLC microprocessor which in turn controls the locomotive. The PLC can make adjustments in 1/20th of a second that the operator would take at least 2 to 3 seconds to respond to. By making corrections 40 to 60 times faster than a human operator can the process locomotive can control wheel slip much more precisely. Process locomotives with AITMC or AITMC-PT automatic traction control systems can control each traction motor independently of the others on the locomotive. If an axle begins to slip the AITMC system will pick up the voltage and current changes in about 1/200th of a second. If they reach the set point then the AITMC will cut or trim power to the traction motor on the axle that is trying to slip. By reducing power it reduces railwheel surface torque below the torque necessary to exceed the friction forces necessary to maintain traction. It also starts the automatic sand time to apply sand to the rail for a preset period of time to help the locomotive regain traction. Because all of this occurs in a fraction of a second it is very unlikely that the operator will ever see a wheel slip. About the only way he would know if there were a wheel slip would be when the automatic sander applies sand or when he checks the alarm log on the HMI computer in the cab. It the operator has connected to too large of a load for the locomotive to pull the PLC program will recognize the amount of activity in the AITMC subroutine and the heating of the rectifier assembly banks and cause the locomotive to stop and display the overload alarm. The operator must then reduce the load and try to move the railcars again. Because only the axle that is slipping has the power cut or reduced the average adhesion coefficient is much higher than it would be if power were cut to all of the traction motors. Testing has shown that the average adhesion coefficient, or factor, can be increased by 20% and more depending on the type of AITMC or AIMTC-PT system employed.

Tractive effort is defined as machine weight multiplied by the average adhesion coefficient. This is true for all locomotives and mobile railcar movers. In the subject of ballast the writer explained that on process locomotives the machine is normally ballasted to the maximum allowable weight.

The second part of producing tractive effort is to maximize the average adhesion. The new AC line locomotives have what they call high adhesion control systems that use invertors to control the power to each traction motor in order to produce adhesion levels of 35% and higher. 42% is the theoretical limit according to physics for smooth steel on steel. However, by allowing the rail wheel surface speed to exceed the rail speed by up to 1.5 mph ( in other words controlled wheel slippage ) the apparent adhesion can be increased up to about 50% according to some reports. This controlled wheel slippage is not good for the locomotive or the track; but, it can be an asset for main line locomotive pulling steep grades like those found in the Western Mountain States.

Our process locomotives have two options to increase average adhesion. These are AITMC and AITMC-PT. They control standard DC traction motors and produce average adhesion levels of 30% and 32% respectively. These are below the published levels for AC locomotives but AC locomotives are so expensive that they are almost totally dedicated to main line service. Many Class I railroads can only justify the AC unit prices for a few of their routes. Standard DC traction motors are much less expensive, are readily available on existing cores or from locomotive parts suppliers in new or rebuilt, and the traction control system is much less expensive. What difference does it make if the traction motors are AC or DC as long as the traction control system can control the electrical power to them in such a way as to achieve maximum tractive effort?

A 135 ton 450 HP process locomotive with 30% adhesion can generate 81,000 lbs. of starting tractive effort. A standard 1200 HP 124 ton locomotive can generate 62,000 lbs. of starting tractive effort. Unless you need high rail speeds in your facility then you don't need high horsepower you just need sufficient tractive effort.

There are occasions where AC traction is required to meet the requirements of a certain application. In some of these cases we will supply new and new/remanufactured AC process locomotives and railtugs.

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