The term "surging" describes an operating characteristic of some model railroad locomotives. Under some conditions, the locomotive will not run smoothly, instead it will repeatedly advance and stop. This almost always occurs running downgrade at slow speed when being pushed by a load from behind or even by it's own weight.
Videos, photos, and descriptions of the details of a surging locomotive can be found at other pages on this site.
For a locomotive to exhibit surging, three conditions must exist:
A locomotive will surge when it can be pushed or roll ahead of the motor. This most often occurs on locos with a worm drive, but it can happen on anything.
As a loco is running slowly downgrade, two forces are at work. These are the motor driving the loco downhill and the force of gravity drawing it downhill. If the acceleration due to gravity combined with the rolling resistance of the loco allow it to roll faster than the motor is driving the loco, it will surge.
When gravity takes over and caused the loco to roll, it will continue to roll until all the slop in the gear train is taken up and the gear train will bind and the loco will try to stop, hung up on it's own gears. However, the motor is still driving the loco and starts to unwind the slop and the loco can move again. Since the speed of the loco due to the acceleration of gravity increases over time, eventually the loco will start to roll faster and overtake the motor. The slop will run out again and the loco will once more come to a stop or near stop. This process will repeat over and over.
On the flat or going uphill, locos will rarely surge as gear train never gets the chance to wind and unwind unless the loco is being pushed by another loco.
Gear train slop occurs in virtually every loco, but not all of them surge. Ones with worse gear train slop can tend to surge more violently. It doesn't take much slop to create a condition where surging can occur, just a few degrees of free wheel movement is enough.
There is usually little that can be done about gearing slop, it's a function of the way that the gear train was designed and built. Sometimes, wads of grease in the gears can produce enough cushioning to reduce the slop, but this is only a partial and temporary fix.
The rolling resistance of a loco determines, to the greatest degree, if a given loco will surge or not. Newer locos with ball bearings and low drag power pickups are the most susceptible to surging. High rolling resistance prevents a loco from free rolling within the slop range of the gearing, therefore it cannot surge. Carbon brushes and track sliders are usually enough to produce sufficient rolling resistance to prevent surging. If the loco cannot free roll faster than a slow running motor is driving the loco, it won't surge by itself. It still might surge is it is pressed downhill by the weight of its train.
The Bachmann Railtruck is an example were added rolling resistance can eliminate the tendency to surge as it is never pressed by the load of a train.
Acceleration can come from the force of gravity or by another loco or motor working faster than the one that is surging. This is really common on older AristoCraft locos that buck themselves due to mismatched trucks. The AristoCraft PCC car has this same issue to some extent as the trucks may not run at the same speed, especially when running slowly.
Gravity works differently that a pushing loco, but the result is pretty much the same. The result is that the slop in the gear train is continually run in and out resulting in sudden changes in speed with the end of the slop range is reached. The PCC complains about this with a change in the gearing whine as the gear trains are loaded and unloaded.
There is little that can be done about the force of gravity except by relaxing the track grades. The force due being pushed on the flat can be fixed by locating the reason that one truck runs too slowly and fixing it or by speed matching locos that are consisted together.
This page has been accessed times since 21 Jan 11.
© 2011 George Schreyer
Created 14 Feb 11
Last Updated April 3, 2010