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306 MECHANICS.
Angle of Friction.
Suppose, instead iof using the string and the weight d (see
Fig. 12), that one end of the plane is lifted until a commences to
slide ; the angle between b and the horizontal line, when a com-
mences to move, is called the angle of friction. The coefficient
of friction may also be calculated from the angle of friction, thus :
If the body commences to slide under an angle of a degrees, the
coefficient of friction will be sm"
a
= tang. a. Thus, the coefficient
of friction is always equal to tangent of the angle of friction.
Rules for Friction.
1. Friction is in direct proportion to the pressure with
which the bodies are bearing against each other.
2. Friction is dependent upon the qualities of the surfaces
of contact.
3. The velocity has, within ordinary limits, no influence on
the value of the coefficient of friction.
4. Sliding friction is greater than rolling friction.
5. Friction offers greater resistance against starting a body
than it does after it is set in motion.
6. The area of surfaces of contact has, within ordinary
limits, no influence upon the value of the coefficient of friction,
but if they are unproportionally large or small the friction will
increase.
TABLE No. 36.—Coefficient of Friction.
Slides.
i
Bearings.
1
Materials. Well
Lubri-
cated.
Not well
Lubri-
cated.
Well
Lubri-
cated.
Not well
Lubri-
cated.
Cast-iron on wrought iron ....
Cast-iron on cast-iron
Wrought iron on brass
Wrought iron on wrought iron . .
0.08
0.08
0.08
0.10
0.16
0.16
0.20
0.20 ]
0.05
0.05
0.05
0.05
0.075
0.075
0.075
0.075
Friction in Machinery.
When the surfaces are good the frictional resistance for
slides may be assumed as 10 per cent., more or less, according
to the conditions of the surfaces. It is always well not to take
the coefficient of friction too small; it is better to be on the
safe side and allow power enough for friction. In bearings for
machinery, the frictional resistance ought not to absorb over
six per cent. If more is wasted in friction, there is a chance
for improvement.
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