Sagitta Rule: Difference between revisions
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== Use == | == Use == | ||
Using a [[Sagitta Rule]] is as simple as following these steps. | |||
<gallery widths=300px heights=300px perrow=2> | |||
File:2020-12-03 10.42.26.jpg|Step 1: Place the ends of the rail against a wall or other straight edge. Place the Sagitta Rule at the midpoint of the rail, where the distance from the wall is greatest. | |||
File:2020-12-03 10.42.41.jpg|Step 2: Read the radius of the rail from the Sagitta Rule. In this photo the rail has a curve approximating a 50 foot radius. | |||
</gallery> |
Revision as of 17:13, 3 December 2020
- In geometry, the sagitta of a circular arc is the distance from the center of the arc to the center of its base. It is used extensively in architecture when calculating the arc necessary to span a certain height and distance.
In Live Steam a Sagitta Rule is used to directly measure the radius of a curved section of track.
Calculating
The following spreadsheet is provided to calculate the sagitta value for different radius curves for 10 foot and 20 foot rail lengths.
Here are the values provided by the spreadsheet above.
Rail Length (feet) | Radius (feet) | Sagitta (inches) |
---|---|---|
10 | 30 | 4.988 |
10 | 35 | 4.278 |
10 | 40 | 3.745 |
10 | 45 | 3.330 |
10 | 50 | 2.998 |
10 | 55 | 2.725 |
10 | 60 | 2.499 |
10 | 65 | 2.307 |
10 | 70 | 2.142 |
10 | 75 | 1.999 |
10 | 80 | 1.874 |
10 | 85 | 1.764 |
10 | 90 | 1.666 |
10 | 95 | 1.579 |
10 | 100 | 1.500 |
10 | 105 | 1.428 |
10 | 110 | 1.363 |
10 | 115 | 1.304 |
10 | 120 | 1.250 |
20 | 30 | 19.815 |
20 | 35 | 17.027 |
20 | 40 | 14.922 |
20 | 45 | 13.279 |
20 | 50 | 11.960 |
20 | 55 | 10.879 |
20 | 60 | 9.977 |
20 | 65 | 9.213 |
20 | 70 | 8.557 |
20 | 75 | 7.988 |
20 | 80 | 7.490 |
20 | 85 | 7.051 |
20 | 90 | 6.660 |
20 | 95 | 6.310 |
20 | 100 | 5.995 |
20 | 105 | 5.710 |
20 | 110 | 5.451 |
20 | 115 | 5.214 |
20 | 120 | 4.997 |
Construction
It is easy to construct a Sagitta Rule. Obtain a large paint stir stick (for 5 gallon buckets of paint) from a big box store or where paint is sold. Print the following paper rule. Cut out and tape or glue to the end of the paint stir stick.
Use
Using a Sagitta Rule is as simple as following these steps.