Why do geared locomotives like Shay, Heisler and Climax have so much greater adhesion than conventional steam locotives? That enable them to climb as much as a 10% grade? Is it only because they have more driving wheels on the rails ?
Partly. Adhesion, if I understand it reasonably well, is directly related to weight, materials, surface area, and friction. Friction is essentially negligible in steel wheels on rails, so the weight over the footprint of the loco will give us adhesion. Geared locos were generally light, though, because they were designed to be used on hastily placed, usually temporary track, often using either wooden or steel-capped wooden rails in the bush. What gave them their excellent qualities were their ability to take tight curves due to their gearing which obviated the need for driving rods that must stay in alignment with both the cylinder and the wheels. Also, their gearing gave them strong tractive effort for their boiler sizes. If you take a close look at a 4-8-4 passenger locomotive, the main rod mounting on the rod's driving wheel is not displaced very far from the centre of the driver's axle. This reduces the lashing and whipping forces on the heavy side rods by making their arcs small in relation to the diameter of the driver. Those locos were meant to run fast, but by sacrificing some traction at the outset...less leverage. If you can find a logging tank locomotive to compare the placement of the main rod, you will see that the pinion is very close to the outer edge of the smaller drivers. This affords increased leverage which tranlates into greater tractive effort applied at the wheels. For a loco that is a bit light, though, that could mean slippage at moderate to high throttle settings hauling heavy loads or on heavy grades.
In geared systems, the gears, especially where the rotation is redirected at 90 deg or more, as in the geared locomotives, there is more friction and usage of horsepower. Final drives in cross-country vehicles, for example, are heavy and take more power to turn since they take rotational energy in one axis and convert it to another direction. Still, the geared Shays and such were efficient enough that they could use their low gearing to do a steady chug up grades as steep as 13% (nope, not kidding). I have ridden in a saddle tank 2-8-2 Baldwin in which the site glass on the backhead has markings and a warning to not let boiler water levels fall below a cerain point when climbing pilot-first on grades near 9%.
I hope you will get more knowledgable replies than mine, but I think I am pretty close.
Geared locomotives actually had no greater adhesion than a conventional locomotive of the SAME WEIGHT on its drivers. Adhesion is nothing more than friction which is the combined product of the coeffecient of friction between steel on steel and the weight on each driver. That is why sand is used to prevent slippage. It creates a higher coefficient of friction than a steel wheel has on an un-sanded steel rail. As previously pointed out ALL of the weight of a geared locomotive and tender was placed on powered wheels vs a conventional engine and tender part of whose total was on un powered pilot, trailing and tender trucks. Because of their low gear ratio these engines could easilly be started with just enough power to keep their wheels from slipping. A conventional locomotive required a lot more finese by the engineer to prevent wheel slip particularly when starting a heavy train on wet rails. Open the throttle a bit too wide and too much power will be transmitted to the drivers causing them to slip.
As a boy I lived along the IC's main line about a mile south of Markham Yard Freight trains coming out of the yard were only up to about 15mph as they passed my house and many a hogger would notch the throttle open a bit too wide in their haste to get their train up to speed causing their drivers to slip. They then had to cut off steam altogether until the wheels stop spinning and reopen it as adhesion was regained.
Mark
While it may seem that any locomotive with a given weight on drivers will have the same tractive effort as any other locomotive with the same weight on drivers, this is not entirerly true.
On paper, yes. All locomotives have steel wheels which run on the same steel track, and so they all have the same slipping points as far as the material goes, but factors in reality go beyond simple adheasion.
Conventional side-rod locomotives have about the lowest practical adhesion of any locomotive, and there is a simple reason for this. On every stroke of the piston, power surges from 0% to 100% to 0% each time the piston reaches the end of the stroke and reverses direction. This upsets the adhesion, because it is like the wheels are being struck forward with a hammer at the peak of each stroke. This takes on anther affect at speed. Because the force of the pistons cannot be transfered to the wheels in a straight line (it always pushes or pulles above ore below the level of the piston), it has a tendancy to push up or down on the whole locomotive. These forces cannot be counter balanced without crippling the stability of the locomotive at low speeds, or completely reinventing the locomotive. This hammering motion is deminisied when the number of cylinders are increased, as when you have more cylinders, you distribute the load more evenly. This is partly where a 3 cylinder shay has an advantage.
The main advantage of geared locomotives is that because the wheels are issolated from the pistons, they are not subjected to the surging upward and downward forces that are found on conventional locomotives (called Dynamic Augment). Another advantage for geared engines is that the wheels can be geared down so much that the surging action of the pistons becomes faster, but also much softer, reducing the tendancy to slip.
Another factor in adheasion is in the weight distribution on the driving wheels themselves. On conventional locomotives, with 3 or 4 pairs of drivers, where is a tendancy that one or two axles will carry less weight than the others because either the suspension may not be perfectly balanced, or irregularities in the track, or a variety of other reasons. Because shays and other geared locomotives use independantly mounted 4 wheel trucks, the weight distribution is virtually always the same.
Adheasion is a matter of smooth and steady pressure. anytime you have a surge or sudden change in the power being applied, the wheels will slip.
By the way, the reason geared locomotives became so successfull on really steep grades isn't because of the grades themselves, but rather because of what is usally found in hand with the grades: really sharp curves. They dominated the logging and mining industries because their independant bogies could negotiate the light, crude track that was used. The early shays in the logging industry were actually built to run on wooden rails made from de-limbed trees, and had big wheels with semi circular treads!
Matthew Imbrogno
Mechanical Vollenteer, Arizona Railway Museum
Matthew,
I totally agree with every point made in your last post. I have always tended to think in terms of the "AVERAGE" force applided to the drivers in a conventional steam locomotive. If perfectly balanced, when the piston on one side of the locomotive was delivering 100% of its available power the piston on the opposite side would be delivering 0% power hence my thinking of the average driving power. You are quite right in mentioning that the weight on an engines driving wheels was seldom if ever equally balanced and any uneveness in the rails could further aggravate this unbalance.
You are also right in stating that the advantage of a geared locomotive was its ability to operate on hastily laid uneven rails with curvatures that simply could not be negotiated by a conventional locomotive.
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