Rigging
Math
(Made
Simple)
A
Primer by
Delbert
L.
Hall, Ph.D.
ETCP
Certified
Rigger
ETCP
Recognized Trainer
Lesson
5:
Tension on a Horizontal Breastline
A breastline is a rope or cable that runs horizontally and is used to
swing a
hanging object out from directly underneath its suspension
point(s). For
example, you might breast an electric upstage or downstage in order to
keep
another piece of scenery away from the lighting instruments.
Because this
line (or lines) runs horizontally, it is not lifting the load, just
pulling it
out of alignment. To compute the tension on a breastline, we
use this
simple equation:
Horizontal
Force = Load (H1/V1)
Below
is a diagram of this configuration.
Example: Calculate the horizontal force on a breastline where V1 is 30', H1 is 2', and the Load = 700 lbs.
Before we start, you should note that in the earlier equations where
you wanted
to find the tension on the bridle, we took the length of the bridle (L)
and
divided it by the length of one of the other sides. Since in
this problem
we want to find the tension on the horizontal line, we start with the
length of
the horizontal distance (H1) and then divide it by the length of the
other side
(V1). This rule for knowing which side to start your division
with, can
help make it easier to learn the equation. So, plugging the
numbers
into our equation, we getÉ
Horizontal
Force = Load (2/30)
or
Horizontal
Force = Load
(.0666) or
Horizontal
Force = 700 x
.0666
or
Horizontal
Force = 46.62 lbs.
As you can see in this example, the horizontal force on a breastline
tends to
be fairly low, compared to the tension on bridles.
