Understanding the Rolling Offset
A **Rolling Offset** occurs when a pipe or conduit must change direction in three different planes simultaneously. It is the most complex type of offset a pipefitter or electrician encounters, as it requires calculating a single hypotenuse (the **Travel**) from two right triangles.
The Two-Step Pythagorean Theorem
The calculation is a two-step process using the Pythagorean theorem ($a^2 + b^2 = c^2$) twice, plus a final trigonometric step to find the take-off.
Step 1: Calculate the Roll (Diagonal Run)
The Roll is the combined run of the horizontal and vertical travels, which forms the base of the final right triangle. $$\text{Roll} = \sqrt{(\text{Horizontal Travel})^2 + (\text{Vertical Travel})^2}$$
Step 2: Calculate the Travel (Hypotenuse)
The Travel is the actual length of the diagonal pipe required. The sides of this final triangle are the **Roll** (calculated above) and the **Set**. $$\text{Travel} = \sqrt{(\text{Roll})^2 + (\text{Set})^2}$$
Step 3: Calculate the Take-Off
The Take-Off (or deduction) is the distance required from the center of the bend to the mark on the pipe, which accounts for the loss of material into the fitting. $$\text{Take-Off} = \text{Travel} \times \cos(\text{Angle})$$
Common Fitting Angles and Constants
When using multipliers (constants) instead of trigonometry, here are the values used for the Travel (Hypotenuse) calculation:
| Fitting Angle | Constant Multiplier | Used to find... |
|---|---|---|
| 30° | 2.0 | Travel (by multiplying the Set) |
| 45° | 1.414 | Travel (by multiplying the Set) |
| 60° | 1.155 | Travel (by multiplying the Set) |
When to Use a Rolling Offset
Application
- Connecting two pieces of equipment with misaligned nozzles.
- Bypassing structural beams or obstructions on two walls.
- Tying into existing piping systems where the final position is rotated.
Alternative
- A **Simple Offset** only moves in one plane (e.g., up and over).
- A **Parallel Offset** moves in two planes but the pipe does not roll.