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MECHANICS. 315
Solution
:
The coefficient of friction for the slide is 0.10, therefore the
axial pressure on the screw will be 16,000 X Ho = 1600 pounds.
The allowable force on a 1 ^-inch screw will be found in the
table to be 1742 pounds ; therefore, select a screw of 1% inches
diameter and a length of nut of 2^ inches. Assuming the
friction due to the reaction of the screw against its collar and
bearing to be equal to the friction in the thread, and using the
table, we have
:
Force per pound at one foot radius = 0.0112
Force absorbed by friction in collar = 0.0074
Total force per pound of load at one foot radius = 0.0186
The leverage of a 20-inch pulley is 10 inches = 1
%2 foot,
and the axial force is 1600 pounds ; therefore, the tangential
force required at the rim of the pulley will be :
F= ^Ol^XJ^ =36. 7pounds>
10
/l2
36.7 pounds is really the force required to keep the body in
motion after it is started. To start the body from rest requires
somewhat additional force, depending on the time used in over-
coming its inertia. It is not certain that the friction due to the
reaction of the screw against the collar is equal to the friction
in the screw. It may be more or it may be less ; this will, to a
certain extent, depend on the size of the collar, and also on the
finish of its surfaces, its means of lubrication, etc. Therefore,
instead of assuming this resistance to be equal to the friction of
the thread as found in column 16, it may be calculated for
each individual case by assuming a proper coefficient of friction
and assuming that this friction acts as resistance at a radius
equal to the middle radius of the collar. If a screw is acting
under the circumstances illustrated in Fig. 18, there is no collar
to absorb any of the force by friction ; but whenever the screw
acts against a shoulder this friction must never be forgotten in
calculation. Ball bearings may be used to very good advan-
tage in the thrust collar on a screw. If a screw works a load
continuously up and down, and the weight of the load always
rests on the screw, it is necessary to be very careful and allow
only a limited load on the screw (only a fraction of what is given
in the table), because the pressure of the load always acts on
the same side of the thread, and this is very disadvantageous
for lubrication, as it does not give the oil a good chance to get
onto the surfaces which rub against each other ; but when the
screw works a slide with an alternate push and pull, the wear
comes on both sides of the thread, which gives a good chance
for lubrication, and an axial pressure of 400 pounds per square
inch of projected area of bearing surface in the thread will be
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