Theatre Design & Technology - Winter 2008 - 12

FT F Fg F FT Fg F Ffr F F FN Fg F F FT Ffr FN F
Figure 2. Velocity/Time Graph
of a typical trapezoidal move.
Section A is the acceleration.
During Section B the object
travels at a constant velocity.
Section C is deceleration.

Figure 1. Photo of hardboard skid from preliminary tests.

rial (the bottom of the skid). We chose a selection of the most
common materials used for scenic construction. Our base materials were UHMW, MDF, painted MDF, glazed lauan, painted
hardboard, and painted and sealed hardboard.Our skids were
UHMW, MDF, steel, lauan, oak, poplar, and pine. We included
oak because of its possible use in low profile skids and tracks.
In the data presented here, if there is no surface treatment listed
then the material was used raw-no paint, sealer, wax, flame
retardant, or treatment of any kind other than a fine sanding to
knock down the edges. For the purpose of this article, glazing
is a thinned latex paint that, when applied, highlights the natural grain in the material. Glazing is absorbed into the surface
rather than coating the surface. Painted surfaces were rolled
with a latex paint thick enough to completely coat the surface
and, where applicable, to obscure the grain. Sealed surfaces
were rolled with a water-based Varathane for Floors (polyurethane) that had been diluted 1:1. Lauan skids were run both
with and against the grain to show the effect of the grain on the
friction in the system. All other wood materials were run with
the grain only.

THE FINAL TEST STAGE - Technical Data and Set-up
To gain the most accurate information in the final testing we ran
each material combination through 100 iterations of a simple
trapezoidal move: starting from rest, accelerate at a constant
rate to a set velocity, run at that constant velocity, and then
decelerate to a stop (fig. 2). The skid was driven by a continuous chain loop that attached to the front and back of the
skid through two Cooper Instruments model LFS 260-100 load
cells, which are electrical devices used for measuring forces. A
load cell generates an electrical signal that corresponds to the
tension in the cell. In our case, those tensions were recorded
throughout each move using National Instruments LabVIEW, a
software package commonly used in scientific research and industrial applications as well as a motor control system used by
one of the authors. We were able to use this software for both
motor control and data acquisition, essentially automating the
whole process.
Figure 3. Setting up the final test
stage and configuring the control
system.

PRELIMINARY TESTING
Starting in June 2005, thanks to a grant from the United States
Institute for Theatre Technology (USITT), we set out to measure
coefficients of friction for some of the most common theatrical
materials. As a first step, we conducted some preliminary tests
of our equipment, methodology, and materials before beginning the final (and more extensive) phase of testing.
The preliminary testing included two additional materials
that were eliminated from the final testing. In the course of the
preliminary tests we discovered that both raw hardboard (Masonite) and aluminum were poor skid materials. The surface
of the unpainted, double-tempered hardboard we used began
to break down early in the testing process becoming unusable
before the testing could be finished. It deteriorated both as a
base material and as a skid material (fig. 1). Aluminum did not
make a good skid surface when combined with any wood product. The aluminum tended to leave tracks of grime on these
surfaces which increased the friction as the test progressed.
Consequently, we do not recommend aluminum or raw hardboard for skid applications and did not test them further.

Figure 4. Force/time graph of actual test data. The peak at point 1 is proportional to the static
frictional force and is the last data point before the skid begins to move. The kinetic frictional force
is the average of the data points between 2 and 3 where the skid is traveling at a constant velocity.

W I N T E R

2008

THEATRE

DESIGN

&

TECHNOLOGY

11



Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 2008

Contents
Theatre Design & Technology - Winter 2008 - 1
Theatre Design & Technology - Winter 2008 - 2
Theatre Design & Technology - Winter 2008 - 3
Theatre Design & Technology - Winter 2008 - Contents
Theatre Design & Technology - Winter 2008 - 5
Theatre Design & Technology - Winter 2008 - 6
Theatre Design & Technology - Winter 2008 - 7
Theatre Design & Technology - Winter 2008 - 8
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Theatre Design & Technology - Winter 2008 - 73
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