Theatre Design & Technology - Fall 1981 - 14

Century-Lot B
LOAD
lb. I

LOAD (y): 1 em.

~

200 Ibs.

ELONGATION (x)' 1em.=1mm.

4000

3000 _

1000

(

,'"

Figure 11

L

Figure 12
The results for J. R. Clancy's Sure-Clamp are completely
different from all the others tested, and thus must be analyzed in
a different manner. There are so many moving parts that it is
hard to tell when the slack is being taken up and when the clamp
is being deformed. No sudden or dramatic failure was observed
in any specimen. The clamps started stretching under comparatively low loads and just kept on stretching. Some sort of
deformation seemed to be occurring at about 300 Ibs. (Fig. 16).
The results of the first four tests were consistent, but hard to
decipher. We therefore tested a fifth specimen manually, adding
the load in 100 lb. increments, and inspecting for damage after
each increase. At 600 Ibs. a visible fracture was evident in the
hook-shaped piece that goes over the pipe (Fig. 15). Once that
fracture was opened, it reqUired steadily less force to keep the
clamp stretching.
The original investigation included only seven types of clamp,
but in the time between the execution of the tests and the
publication of the results an eighth type was tested. The Little
Stage Lighting Company was in the process of developing a
new clamp, heard of our project, and asked if we would run a
similar test on their clamp. In this instance, the manufacturer
was seeking evidence of the reliability of his product before
putting it on the market. Such information could be used to
modify and improve the design, or to determine a safe load.
Ultimately, the recommended safe working load could be displayed on the clamp as easily as is the manufacturer's name.
This sort of development is exactly the response we had hoped
for, so naturally we were more than willing to undertake the
testing. Here then, are the results of the test executed on July 9,
1981, using the same equipment and under the same conditions
as for the earlier tests.
The Little Stage Lighting clamp is of the aluminum alloy,
"Tensalloy." It is a different design from all others tested in that
the web along the spine is made more like an 'I' beam. There is
no spigot, and the suspension bolt screws directly into the
flange. All of the specimens fractured through the grip teeth just
below the second curve (Fig. 17). The average ultimate load
was 1485 Ibs., and the average proof load was 1003 Ibs. (Fig.
18). The results in the ultimate load were extremely consistent,
with only 130 Ibs. separating the lowest and the highest. The
proof loads were less consistent, and one clamp did show some
elongation at about 600 Ibs. All the clamps were new and were
of the same casting lot.
A secondary issue was incidentally raised during the course
of the tests. This is where the oddball Kliegl gets explained. We

had not noticed until it was on the test machine that this
particular specimen had been previously deformed. The grip
screw was twisted, indicating that at some time the clamp had
been overloaded and bent laterally. This clamp's behavior was
completely irrational, and the ultimate load it sustained was
substantially lower than the other specimens (Fig. 6). Some
supporting information is evident in the Century and Colortran
samples. Some of the specimens in these groups deformed
slightly and slipped, although they did not slip completely off the
pin. The load was maintained however, because it was judged
that the elongation was not enough to constitute failure. In none
of these examples, however could the clamp again sustain as
great a load as that which first deformed it. Similarly, once the
Sure-Clamp fractured, its ability to sustain a load was reduced.
The conclusion here is that a deformed 'C' clamp is not as
strong, nor will it perform as predictably as an undamaged one.
Damaged clamps should be retired immediately.
Because the clamps in this test performed under high loads is
no guarantee that all clamps will do so. These results were
obtained from a necessarily tiny portion of the total clamps in
daily use and since quality control varies greatly from one
manufacturer to another, it is impossible to say with any
certainty whether these results are indicative of the general run
of the foundry. The only assurance of safety is to proof test each
and every clamp.
Proof testing in industrial rigging usually consists of first
determining the proof load or point of deformation for a particular
piece of hardware. This necessitates sacrificing a few specimens. Once the proof load has been reliably established, all
similar pieces of hardware may be safely loaded to a point
slightly below that limit. The hardware is not damaged, but
demonstrates its potential. If any deformation is recorded, the
specimen is of course rejected. Since the safe working load
should be well below the proof load, an adequate safety margin
is thus ensured. It is a matter of knowing all the facts about every
individual piece of equipment. Proof testing may raise the cost of
the hardware, but the peace of mind is worth the cost.
The 'C' clamp, however, is not rigging hardware, and no such
proof testing exists. That may be understandable, but what this
test does indicate is that 'C' clamps have some considerable
potential for load bearing. That potential must remain unfulfilled,
however, for want of adequate quality control. Possibly manufacturers are not interested in developing a 'C' clamp beyond its
stated use of hanging lights. My only response to that is, "Why
no!?" 0

Ronald Fedoruk was Stage Manager for the Theatre Services Department, University of Calgary



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

Contents
Theatre Design & Technology - Fall 1981 - 1
Theatre Design & Technology - Fall 1981 - 2
Theatre Design & Technology - Fall 1981 - 3
Theatre Design & Technology - Fall 1981 - Contents
Theatre Design & Technology - Fall 1981 - 5
Theatre Design & Technology - Fall 1981 - 6
Theatre Design & Technology - Fall 1981 - 7
Theatre Design & Technology - Fall 1981 - 8
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http://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966May
http://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966Feb
http://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965Dec
http://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965Oct
http://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965May
http://www.nxtbookMEDIA.com