http://www.youtube.com/watch?v=SygBmftr90E&NR=1
What is the pro and cons for wheelslip on a locomotive?
There is no "pro" for wheelslip on a locomotive.
Wheelslip can be very damaging to the rail and cause 'burn' spots. In an extreme case, it could probably pull the head off the rail.
A very small amount of wheelslip is actually a good thing. It increases the adhesion above the theoretical 25% and is how the current locomotives achieve the tremendous TE values. The builders call it "creep" and it requires a lot of sensors and computing to control properly.
Uncontrolled wheel slip cause a loss of adhesion, and many times damage to the rail and drive wheel. Early diesel wheel slip was basically a control stand light that flashed on when you picked up wheel slip. Another light(sort of a 'running' light) was mounted on the engine frame below the engineer's cab window. The engineer could 'see' if the train was moving at night by watching the ties move below him!
By 1966, EMD had IDAC(Individual Detection And Correction) available in the SD45 and though it was a complex 'Rube Goldberg' device, it did work quite well. Variation of it went into EMD production in later years.
In the 70's, EMD engineers were testing adhesion limits and designed an attachment they put in a testbed engine(one of the SD45X engines, IIRC) so they could produce measured wheel slip on demand and record the results back in the EMD test car. Inital testing did not go well. As they 'dialed in' wheel slip, the engine actually pulled even better! What they found was that carefully measured wheel slip 'heated' the rail/wheel 'contact patch' and the TE increased. They finally released this feature as on option on some late SD40-2 engines. It used a small radar to measure the true ground speed, and adjusted the wheel slip to ride on that perfect 'bubble' This was installed on at least one order of BN SD40-2 engines and was know as the SS(Super Slip or Super Series) option. A few years later it became standard equipment on the '50' line. Riding on an engine with 'SS' enabled and pulling hard on a grade sometimes makes one feel like there is something 'grinding' underneath him. The BN GP50's were noted for this.
GE also was developing similar adhesion controls as the cost of electonics dropped. The GE C40-8 had all of their 'bells & whistles' in it and set the standard in new engines. With AC traction, this is still being used.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
Wheelslip was a lot more common on steam than diesel, seemed to be a fact that it would just happen. Not sure if it was a good thing but always impressive to see.
Wheelslip, especially rail grind or rail burn from locomotives just standing, spinning their wheels and "making glass" as we call it, can damage the rail to the point that the rail itself is ground down to half its height. Photos of such damage abound on the web.
Moving wheel slip places stresses on the trailing tonnage, causing run in, run out and possible stringlining, or the act of pulling a train across the arc of a curve instead of through it. The ensuing derailment is not pretty. The SP train which derailed several years ago and dumped chemicals into the Sacramento River above Dunsmuir was a case of stringlining. The other thing that occurs is when the slack runs in, then power is applied again as the engines regain their footing, knuckles or even entire drawbars are pulled out by the roots. The Engineer can help avoid this by reducing the throttle when uncontrolled wheelslip occurs, thus reducing the stresses placed on the drawbars and knuckles as the engines regain their footing.
The wheel creep described earlier does enhance the horsepower placed to the rail, but the systems are so sensitive that if not maintained properly, uncontrolled wheelslip is the result and the Engineer is back to square one in his or her fight to maintain traction.
Wheelslip is the result of a complete loss of adhesion, so the better the factor of adhesion to start with, the better the chance of not encountering it. Steam locomotives encountered "slipping of the drivers" more frequently than diesels since the larger wheel diameter and less weight on drivers created a very poor factor of adhesion.
Factors which reduce adhesion on a temporary basis include lubricants on the rail, water, grain or grass and even some gypsum and cement powders.
Don't forget wet leaves, they have been producing havoc for NJTransit around here lately, more in the way of flat spots.
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