The decision to build a model of a digger steam was the result of a childhood dream and a challenge to do something new .

My research of the machine began in November 2011. I realized that it would be a long and difficult task, and I was not even sure that I could find sufficient documentation. After many failed attempts, negative for both the poor quantity and quality of the documentation, I found the site Roots of Motive Power ( http://www.rootsofmotivepower.com ) and then the adventure started.

Roots of Motive Power (RMP from now on) is an association of approximately 350 members and is based in Willits, California, 240 km north of San Francisco, in an area covered by redwood forests. RMP is a real Mecca for fans of logging and possesses a number of machines: steam engines, steam winches (Steam Donkey), steam cranes, diesel locomotives, etc. Take a look at their website and you will see!

RMP has a Steam Shovel model 50-B of Bucyrus-Erie, donated in 1992 by the firm Guy F. Atkinson. The year of construction is 1932.

After an initial email exchange, they put me in contact with James, the person who follows the maintenance of the B-50 and that proved to be most helpful.

Sifting through the titles of the books in their library, I saw that there was something on the 50-B. James looked this up, and the documents provided what could be a good starting point. This was a ‘blueprint’ and a booklet of parts with several drawings, not to scale or dimensioned, but still very useful to understand how it all works.

That gave up a lot of information, but not enough to make a model. Then I learned that RMP had an annual festival where they operated the 50-B. I could not resist going to Willits for a week.

After a short course of instruction, they put the control levers in my hand and I had the thrill of manoeuvring the monster. Magical moments!

The 50-8 has three double cylinder steam engines.

The Main Engine (8 in. by 9 in.) is used to raise the dipper, to raise the boom by a winch and to drive the tracks.

The Swinging Engine (6:0 in. by 6 in.) is used to rotate the revolving frame The Thrusting Engine (6:0 In. by 6 in.) is used to move the dipper handle. Another steam cylinder opens the dipper door.

The 50-B could be set in four different ways: as shovel, crane, dragline or clamshell. So it has two winch drums connected with two clutches driven by two steam cylinders (RAM). All the actuators are driven by manual levers or foot levers and this makes things very Interesting to incorporate in a model.

I chose to build the model In 1:10 scale, both to have a dimension that will allow me to drive directly avoiding the use of odd controls and because the scaled dimensions allow me to make use of standard metric gears (that will be machined to give the right shape).

At this point I started the design of the model. After a general examination, especially as regards the principles of operation and identification of the various parts. I calculated the various gears and the related gear ratios, selecting the closest metric gears; the difference is very small.

The next step was the verification of the dimensions of the steam engines. Basically, there are two kinds of engines: the Main Engine and the Swinglng/Thrusting Engine (the two Swinging and Thrusting engines differ only in some external details, but they work in the same way).

The Main Engine was a big problem, since I didn’t have dimensioned drawings and it was impossible to take useful measurements on the 50-B of some of the  parts, such as the accessible parts that were covered with grease! The valve gear looks like that of the Stephenson engine, but with some variants that did not  allow the use of on-line simulators.

The solution I adopted was to draw, with CAD, a simplified model with which to verify the operation, gradually modifying the measures to obtain a satisfactory result.

Here is a picture of the simulator:

The engines have a three position slide valve: in the central position the engine stops, while in the others two positions the engine runs clockwise or anti-clockwise. Reverse is obtained simply by inverting the steam inlet and exhaust; the difference between the areas exposed to the steam pressure causes the two slide valves to be pressed on the valve bodies of the two cylinders. So the engine has no expansion link and has only one eccentric acting on the slide valve. Also in this case I made a simulator with CAD, as is seen in the image below. It lacks the three position slide valve for the reverse command, but this is simply constituted of a simple slide valve that exchanges inlet and exhaust.

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Editor - David Carpenter