Theatre Design & Technology - Spring 2013 - 52

percent greater than the load being lifted, simply due to the starting and
stopping of the hoist. A 64-fpm chain hoist can create a shock load that is
200 percent greater than the load being lifted.
According to Phil Braymen, engineering manager at Laclede Chain
Manufacturing Company, "The chain industry does not test, certify, or
condone shock loading [of chain]." The National Association of Chain
Manufacturers (NACM) states, "Manufacturers do not accept any liability
for injury or damage which may result from dynamic or static loads in
excess of the working load limit."
To help understand how static loads affect chain, I ran several tensile tests using 1/4-inch Grade 30 proof coil chain. At its working load
limit-25 percent of its breaking strength-this chain stretches only
about one-half of one percent of its length. At approximately 60 percent of the breaking strength of chain-the yield point of the steel-the
chain stretches approximately 1 percent of its length. However, when
the force on the mild steel chain reaches the yield point, two things
happen: 1) the shape of the links begins to deform, causing the links
to get narrower and longer, and 2) the metal enters its "plastic stage,"
where the metal stretches significantly under slight increases in force.
During this stage, the growing tension on the chain slows dramatically
as the deforming and elongating chain absorbs the energy. The tension
grows even slower as the tension surpasses 90 percent of the breaking
strength of the chain. In the final 40 percent of the breaking strength
of the chain, the stretch of the chain is approximately 7.5 times greater
than in the first 60 percent. This nearly constant changing of the rate of
stretch under different amounts of force makes it nearly impossible to

50

theatre design

& technology

accurately calculate shock loads on chain with a mathematical equation.
The bottom line is that shock loads on chain can be quite high and
can very easily exceed the working load limit of the chain, especially
given the low design factor used for chain. Shock loading chain should
be avoided when possible.

chain hoiStS and Shock forceS
Because most chain hoists start and stop suddenly (very little "ramp up"
or "ramp down" to the time), the starting and stopping of a chain hoist
can produce a significant amount of shock load. How much? That depends on the acceleration and deceleration speed of the hoist. The starting and stopping of a 16-fpm chain hoist does not produce as much shock
load as the starting and stopping of a 64-fpm chain hoist.
As a general rule of thumb, we say that a 16-fpm chain hoist produces a shock load of between 20 and 25 percent of the weight of the load it
is lifting when it starts or stops. A 16-fpm chain hoist lifting a 2,000-pound
load would produce a shock load between 400 and 500 pounds. So, the
total force on the point would be between 2,400 and 2,500 pounds, plus
the weight of the hoist and chain. Let's test this rule of thumb using the
dynamic force formula to calculate the force of a 16-fpm chain hoist starting or stopping a 2,000-pound weight. If we assume that the acceleration/
deceleration time for the chain hoist is one second and we convert the
velocity of the chain into seconds (16 fpm / 60 = 0.266667 fps) we see
that the acceleration of the chain is 0.266667 fps². Now we can plug these
variables into the dynamic force equation:

s p r i n g 2013



Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 2013

Contents
Theatre Design & Technology - Spring 2013 - 1
Theatre Design & Technology - Spring 2013 - 2
Theatre Design & Technology - Spring 2013 - 3
Theatre Design & Technology - Spring 2013 - Contents
Theatre Design & Technology - Spring 2013 - 5
Theatre Design & Technology - Spring 2013 - 6
Theatre Design & Technology - Spring 2013 - 7
Theatre Design & Technology - Spring 2013 - 8
Theatre Design & Technology - Spring 2013 - 9
Theatre Design & Technology - Spring 2013 - 10
Theatre Design & Technology - Spring 2013 - 11
Theatre Design & Technology - Spring 2013 - 12
Theatre Design & Technology - Spring 2013 - 13
Theatre Design & Technology - Spring 2013 - 14
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