Re-thinking the Helices
May 18, 2012 by markpaulson
A helix is a spiral of roadbed that allows a model train to ascend or descend in elevation, usually out of sight of the operators or viewers. The factors affecting the grade within the helix are the thickness of the combined plywood, track bed, track, support hardware and clearance needed for the trains, and the diameter of the helix. The smaller the diameter, the steeper the grade that is necessary to maintain the clearance needed for the trains. Think of taking a “slinky” and pulling it apart or compressing it. The distance between the rings of the slinky is your clearance. The clearance has to be high enough so that your trains don’t get stuck! In my case that height is just a little under 4.5 inches. With the clearance kept constant, the grade will vary based on the diameter of the helix. The larger the helix, the smaller the grade, and vice versa.
Helices are uncommon, but not unheard of, in real railroading…the Tehachapi Loop in California being one very famous example, that qualifies as a single circle helix. A good friend of mine at The Minnesota Transportation Museum has been helping me with my helix planning. In addition to some very useful suggestions on what kind of wood to use, he has been urging me to make the diameter of the helices larger to reduce the grade within the spiral. Locomotives are limited in their ability to pull loads uphill by the steepness of the hill (grade), the weight of cars they are pulling, and the tractive effort (pulling power) of the engine. Working uphill on a curve adds considerably to the tractive effort needed to “make the grade”, as they say. I must also credit an old friend in the Industrial Arts Department at Hill-Murray School, where I used to work before retirement, for suggesting that I use Baltic Birch plywood in the helices. I’ll talk more about this as I get into the actual construction of the helices. My friend at MTM has convinced me that I should not try to build a helix with a 3% grade, as I was originally planning. That helix would have had a diameter of about 48 inches. My problem was that to increase the size of the helices would force me to cut into some of my aisle space, perhaps even to the point of reducing room for, and hence the numbers of, my operating crew. I may now have to limit operators, or visitors, to three others beside myself.
Double-click on photos to enlarge. I need to keep the helix under Durango Yard in its original position. I could move it to the corner under the town of Durango, but I would lose my lift-out, and block my emergency exit door. The second helix was to have gone under the Silverton turntable, but putting a larger circle there would create reaching problems above in Silverton Yard. After some playing with it, I have decided to try the areas indicated by the two dark circles in this photo. I only push the Durango fascia out about four more inches, and in Silverton, I locate the helix where there is less reaching needed.
Permit me a little math, now. A 57” diameter track center-line (28.5” radius) on roadbed that is 3” wide, will make the edges of the helix 60” apart. 57” times Pi (3.14), according to the formula for circumference (Circumference = Pi times Diameter), gives me a distance around one circle of the helix of 14.915 feet. If that were 15.000 feet, at a grade of 2.5%, the separation between layers of the helix would be 4.5”, so that’s close enough for me. So, I am building two five foot wide helixes, with a grade within the helix of about 2.5%, and a clearance for trains of about 4.5 inches. This should work.
The number of circles within the two helices will not be the same because the helix under Silverton will have longer approaching tracks, whose grade can alleviate the need for as many circles in the helix.
This photo shows the new location of the fascia (brown lines), and it isn’t too bad, but I still think it will be difficult to get more than two operators into the area on the right beyond the “roll-under”.
One fortuitous outcome; I now have a nice location for my sluice mining scene. If you look closely at this photo, you’ll see that I have also added a run-around track in the Durango trolley line. I now have two O scale horse-drawn trolleys, one to run each way between the depot and the upper reaches of the town, so they need to be able to pass each other. The other addition is the indication for a smelter along the wall to the east (left) of the Durango turntable. Historically, there was an ore smelter in Durango, and if I make it a very low-relief building, with no visible servicing tracks, I can work it into that corner.
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