Theatre Design & Technology - Winter 2008 - 15

FT F Fg F FT Fg F Ffr F F FN Fg F F FT Ffr FN F
* Although many of our results had uncertainties of 0.02
or 0.03, we recommend that you use a value of at least
0.05 in your calculations. Minor variations in material
composition (especially for systems made from natural
materials, such as oak or lauan) can be significant. In
general, you should use the maximum possible friction
that you would have to overcome, both for safety reasons
and to make sure that your system can actually do what
you want it to do. For example, you should use the
coefficient of static friction when determining motor size
and then assume an uncertainty of 0.05, which would give
µs = 0.26 + 0.05 = 0.31.
* Painted hardboard is included in the data, despite the poor
performance of raw hardboard in the preliminary tests,
since hardboard is a common flooring material. The
painted surface held up significantly better than its raw
counterpart, but we still do not recommend hardboard
for use in skids or other circumstances where friction
is an issue. We include the finding for reference only
and suspect that any damage to the surface will cause
shredding to recur.
* Just because the base material needs to look good onstage
does not mean that you have to use the same material for
the whole move. You may have the opportunity to use a
different material when the skid starts to move than when
it is at speed. For example, if the skid started off stage,
out of sight, you could start the move on raw MDF or
UHMW then, as you reached the sightline, change the floor
material to something more aesthetically pleasing. In that
case, it is best to consider the motion as comprised of
three pieces-the first at the start of the move overcoming
static friction, then while moving on the first material,
subject to kinetic friction, and lastly on the second
material combination, again with kinetic friction. You
should perform the same three calculations for the return
move. (A longer acceleration time or lower velocity for
resetting will help you keep your motor size down). Use
the highest of these results to determine horsepower.
* Considerations beyond the coefficient of friction should be
factored in when choosing skid materials. Wood products
vary widely in quality from one region to another. You may
have to do some preliminary tests to make sure that your
particular material can withstand repeated moves.
* We were startled by the exceptional performance of MDF
in these tests. We had no prior evidence that MDF skids
work well, although, due to its performance in these tests,
we will be experimenting with MDF in the future for these
applications.
* UHMW is an industrial material that has very consistent
properties from sample to sample, stores well, and is durable
for repeated use. Although this material is more expensive

14

THEATRE

DESIGN

&

TECHNOLOGY

W I N T E R

2008

than MDF and, in some cases, has higher µ values, we
believe that it is the best material for skid runners. To
cut down on the cost, you can use UHMW strips rather
than a single sheet-the frictional force is independent
of surface area. This can be explained by looking at what
causes friction, namely the interaction between molecules
in the surfaces of the two materials, an interaction which
is dependent on how hard those surfaces are pushed
together (the normal force). If the contact area is large,
the force pushing the surfaces together is dispersed,
causing a lower pressure. If you have a smaller contact
area, the pressure goes up. Since force is equal to the
pressure times the area, the relationship between these
properties cancels, except in extreme cases, such as trying
to drag something on an edge. Note: You will need to use
strips that fully support the surface. The surface material
should not sag or deflect noticeably between strips.
* In all of these tests the skid materials were attached with
fasteners that were carefully countersunk. Screw heads
and other fasteners must not protrude from either the skid
or the base material surface. Additionally, skid runners
were rounded off on the leading edge.

AN EXAMPLE
As an example of how to apply this information, consider the
following situation: A skid, lined on the bottom with UHMW
strips, is moved across a horizontal glazed lauan platform. It
weighs 400 lbs, moves at a speed of 6 ft/s, and has 4 s to get
up to speed. How powerful a motor do you need to accomplish
the task?
* The motor has to accomplish two tasks: get the skid moving,
and move it fast enough to travel the necessary distance.
* In the first case, the skid is stationary, which means that it is
subject to the static frictional force (see figure 7). In
this case, F net = 0, so Fnet, y = FN - Fg = 0 and Fnet, x
= FT - Ffr, s = 0. Rearranging, we then get FN = Fg and
FT = Ffr, s . Substituting into the definition of the static
frictional force (Ffr, s = µs FN) given earlier, we get FT
= µs Fg. The coefficient of static friction between UHMW
and glazed lauan is 0.24 ± 0.02. As mentioned above, we
will use 0.24 + 0.05 = 0.29, and thus find that we need to
apply a drive force of FT = 0.29 * 400 lbs = 116 lbs.
→

Figure 7. Free body diagram showing forces on skid in example.



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
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https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1970May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1970Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1969Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1968Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1967Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966May
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1966Feb
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965Dec
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965Oct
https://www.nxtbook.com/nxtbooks/hickmanbrady/tdt_1965May
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