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Mechanic’s Corner: Geared Locomotives

Welcome to our second installment of technical articles brought to you by the Friends of Number 9. In this edition, we will be discussing “geared locomotives” and why they found such a fitting home on Mt. Tamalpais’ scenic railroad.

When we think of a classic steam locomotive, the first image that comes to mind is large driving wheels and connecting rods flying around in circles as the locomotive speeds down the track. This instantly recognizable feature of a steam locomotive emphasizes the revolution in speed of transportation that was brought on by the development of the railroads, but sometimes power was more important than speed.

Conventional steam engine’s large driver wheel and connecting rods

The growth of the logging industry in the mountains of Northern California and elsewhere led to a need for new technologies. The felling of trees often took place in remote locations, and as our country’s need for lumber grew, the means by which logs were transported out of the forests needed a boost in efficiency. The logging industry was the catalyst for the creation of three purpose built types of steam locomotive that forever changed how efficiently and safely logs could be brought from the forest to a mill. These three steam locomotive designs are known as the Shay, Heisler, and Climax engines. The common feature among these locomotives is the use of drive shafts and gear reduction to deliver power to all of the wheels under the locomotive. The effect that this arrangement achieved is comparable to your experience riding a bike in its lowest gear only. You will find that you can’t go very fast, but you will also find how easy it is to climb a steep slope. The new technology was a great improvement for the logging industry. A track could be snaked through the forest and a long train of log cars could be loaded up to capacity, then gently maneuvered through the hills and mountains behind a slow but very powerful steam locomotive. This type of geared locomotive also turned out to be very well suited to the Crookedest Railroad in the World.

Heisler drivechain illustration shows the protected gears between the wheels of a Heisler locomotive

The Mt. Tamalpais Scenic Railway utilized both Shay and Heisler geared locomotive designs with great success. The basic designs for these engines as used on the Railway were similar. Both had two sets of four wheeled trucks under the locomotive, with beveled gears mounted at 90 degrees to one another on all four axels. These trucks could turn side to side and were steered by the rails on the ground. Driving cylinders transmitted power to the wheels through two drive shafts that were very similar to those found in modern cars. The two shafts coming out of either end of the driving cylinders were connected to the 90 degree beveled gears which powered each of the four axels at a 3:1 gear ratio. This means that for every three revolutions of the driving cylinders, the wheels under the locomotive only turned once. This design created a lot of torque, which was ideal for negotiating the grades and curves of the Crookedest Railroad.

The most significant design difference between Shays and Heislers was the arrangement of the driving cylinders. The Shays had either two or three driving cylinders mounted on the right hand side of the locomotive. Since the weight of the driving cylinders sat only on one side, the boiler was mounted offset to the left to provide a counterbalance. In the Heislers, the driving cylinders were located in the middle of the locomotive. The driving cylinders in the Heislers have a very recognizable “V-Twin” look to them which resembles the arrangement of a V-8 engine.

Mill Valley and Mt. Tamalpais Shay No. 3, showing the 3-cylinder engine at the middle, the two long driveshafts, one forward and one aft, connecting to the wheels and their beveled gears. John Zuffinetti collection.

Mt Tamalpais and Muir Woods Heisler No. 9, with its “V” twin engine in the middle and the driveshafts running down the center of the engine, beneath the boiler, instead of along the side like the Shay. The beveled gears were hidden from view between the wheels.

The design of the beveled gears arrangement also differed between the Heislers and the Shays. In the Shays, power was transmitted through beveled gears at each of the four axels directly from the drive shaft. By contrast, the Heislers had only two sets of gears, which sat at the center of the first and fourth axels. Power to the second and third axels was transmitted from the first and fourth axels by small connecting rods on the outside of the locomotives wheels. (See illustration.) This achieved the same full eight wheel drive as the the Shays.

Our locomotive, Number 9, has a fully intact drive system under it and we are eager to show off its brilliant and intricate inner workings once the engine is on display. It is truly an engineering masterpiece.

That is all for now. We encourage you to look through our photo gallery to try and pick out some of the details covered in this article. See you down the tracks!

David Waterman

Mechanic on #9


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