Theatre Design & Technology - Summer 1977 - 26

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8

LEGEND
1/2"X4" SQUARE HEAD BOLT
WITH LOCK NUTS -GRIND EDGE
OF HEAD AS SHOWN
1.660"DIAMETER PIPE CAM
SHAFT
1/4"X3"CAM PLATE
SWIVEL CASTER
3" HEAVY BUTT HINGE
H/2"X 5-1/4" FRAME
3/4" PLYWOOD LID
REINFORCED STYLE CAM
PLATE WITH 1/4" STEEL
WELDED TO BOTH EDGES

FIG.4
CAM

SHAFT OPERATED
CASTER JACK WITH
HINGE PIVOT

platform (see Figures 3,4, and 51.
Generally speaking, these camshaft caster jacks performed quite well providing both mobility and a very
high degree of stabil ity. The camshaft approach was
especially useful in providing caster jack capability at
otherwise inaccessible center platform locations such as
"A" of Figure 3.
The principal limitation of this jack design was the
strength of the cam plate to resist bending under load.
However, by carefully limiting the dead weight loads to
no more than approximately 100 Ibs. per caster no substantial difficulty was encountered except as noted later.
Several smaller wagons also employed this scheme.
Another large wagon also utilized this basic arrange'
ment but inasmuch as the load per caster was considerably greater it was necessary to reinforce the offset by
welding two ]1,," th ick side plates to the basic offset
section (Figure 4, item 8).
OPERATION UNDER PERFORMANCE CONDITIONS
It should be reported that actual performance use
h ighl ighted several significant defects in th is design:
(1) From the designer's standpoint, the scheme was
restrictive as to caster placement. That is to say that it
was always necessary to place the casters at the intersection of major structural members which met at right
angles (see Figure 3).
(2) Rough calculations indicated that it would be unwise
to use a substantially smaller camshaft. Thus, a fairly
26

SUMMER,1977

large hole had to be bored in the 1%" x 5]1,," frame
members weakening the latter appreciably. This con·
sideration also meant that it would have been unwise
to place the camshafts much above the center line of
the beams which in turn limited the maximum amount
of vertical caster movement possible.
(3) The above was related to another consideration namely the clearance between the bottom of the cam·
plate and the stage floor when the wagon was jacked
up on casters as can be seen in Figure 4. The clearance
could be no more than the lift height of the jack and
even this required critical alignment of all the com·
ponents involved. If any of these should shift or benddue perhaps to accidental overload-the tendency was
for the free end of the cam plate to dig into the stage
floor.
(4) The steel on steel cam action tended to be noisy
and lubrication was only partially effective. Roller type
cams would have solved this problem but would have
been more costly and more difficult to install.
(5) Of all the difficulties encountered with this system,
the most critical was the relative mechanical fragility of
the cam plate hinge pivot. The major concern was that a
caster jack might be bent out of engagement with its cam
bolt. This situation occurred only once in performance
and is shown in Figure 6.
TWO NEW EXPERIMENTAL MODELS
With this experience as a background the author con-

THEATRE DESIGN AND TECHNOLOGY

USITT

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