Theatre Design & Technology - Fall 1981 - 11

Stress Testing and Failure Loads of Eight Varieties of

by Ronald Fedoruk
As this study points out, all of these clamps will do exactly what the
manufacturer claims-they will adequately and safely support lighting
instruments. This study should not be taken as a rating of clamps for that
use. However, many theatre technicians use 'C' clamps for purposes
other than which they were intended. This report, then, may act as a
guide to the relative strengths of various clamps. ---eds.

A 'C' clamp may not seem like a very exciting piece of
hardware to merit investigation, but the truth is that for all the
uses we find for 'C' clamps, we do not know exactly how much
weight they will support. Considering that manufacturers have
never intended that 'C' clamps be used for anything but hanging
lights-and most lights weigh less than 50 Ibs.-this information
should not be crucial. Some catalogs, however, do list other
hardware such as boom arms and ladders that can carry a much
greater potential load. At the University of Calgary, we have
begun to explore the potential of the 'C' clamp in rigging: This
would give us a quick and versatile system, but would it give us
a safe system? There is a natural and justifiable mistrust of the
maleable iron clamp for rigging, a mistrust born of the fact that
no quantitative information exists on the strength of any 'C'
clamp. A knowledge of capabilities, limitations and predictability
is imperative for a confident use of any hardware. That is the
main concern here. It is not to discover whether a specific clamp
is safe or unsafe for hanging lights, it is rather to provide some
information for those who might be tempted to use a 'C' clamp
for loads beyond its intended purpose. Or perhaps to discourage
what is, after all, improper use of a piece of hardware.
What is given here is the ultimate or failure load, and should
not be interpreted as a safe working limit. Neither is it intended
that these results, gathered from a small sample group, be taken
as necessarily representative of all clamps. The extreme loads
described here represent a potential only. There is no guarantee
of consistency from one casting lot to another, and quality
control varies among manufacturers. This is a readily acknowledged problem, the answer to which is, of course, the safety
chain.
There is already some existing terminology for the various
component parts of a clamp, but it seems to be neither complete
nor consistent. Figure 1, therefore, shows the annotated 'C'
clamp so that it may be referred to with some degree of clarity.
The clamps used in the test were those commonly supplied by
Altman, Berkey-Colortran, J. R. Clancy (the Sure-Clamp), Delta
Industries (one of two aluminum samples), Electro-Controls,
Kliegl, and Strand Century. (The Little Stage Lighting clamp was
tested separately and is discussed later.) An attempt was made
to select a random cross-section and, wherever possible, old,
used clamps were included in the samples along with new,
unused ones.
Testing was conducted on February 17, 1981, in the Mechanical Engineering Laboratories at the University of Calgary. The
test consisted of straining each of ten samples of seven different
types of clamp to destruction on a closed-loop hydraulic testing
facility known as the Materials Testing System (MTS) (Fig. 2).
While the force is applied hydraulically, the control desk is
digital. This allows for a printout in the form of a graph, as well as
for a programmed application of the load. In this case, the load
was increased at a uniform rate as long as the clamp allowed

such treatment. At any failure or even movement (i.e. slipping)
of the clamp, the load was automatically and immediately
reduced to a level that the clamp could sustain. The MTS was
tooled such that the clamp could be mounted on a 1.9" pin (this
would compare to a 1W' ID pipe). Each clamp was secured to
the pin and the grip screw tightened with a pretorque of 100
inch-Ibs. That is more force than would normally be applied in
tightening a clamp, but it is not unusual to find some zealot who
will reef on a clamp that hard. The force was applied directly
downward on the suspension bolt until the clamp failed.
~

CJ2yE

Figure 1
The
"annotated"
'C' clamp

?'h1"EMaJ

,)/

r:t

ea.:I'

Failure is defined as the point where a visible or macroscopic
fracture occurred, or where the clamp was deformed or elongated enough that the pin could pass through the throat. This would
be comparable to the clamp stretching open to fall off the pipe.
Included here are the graphs as they were described by the
MTS. Load is the vertical component and elongation that can be
measured along the horizontal axis. The linear portion of any
graph indicates elasticity. Although the load is stretching the
clamp slightly, this sort of regular and predictable stretching is
not abnormal, and were the load removed, the clamp would

M TS

.!!!!!!!!!!.

II. e

Figure 2

'Funding for this project was made possible through the offices of Physical Plant and Development, University of Calgary. Many thanks to Dr. William White of the
Department of Mechanical Engineering at the University of Calgary, who designed and supervised the test and analysed the results; to Murray Palmer of Creative
Concepts, Calgary; Mike MacPhee of MacPhon Industries, Calgary; Jim Smart of Delta Industries, Toronto; Ron Olson of Kliegl Bros.; and William LiNle of the LiNle Stage
Lighting Co., Dallas, from whom we borrowed the sample clamps.
' 1981 by Ronald Fedoruk

Table of Contents for the Digital Edition of Theatre Design & Technology - Fall 1981