In the preceding chapters, it was assumed that surfaces in contact
were either frictionless or rough. If they were frictionless, the force
each surface exerted on the other was normal to the surfaces and
the two surfaces could move freely with respect to each other. If they
were rough, it was assumed that tangential forces could develop to
prevent the motion of one surface with respect to the other.
This view was a simplified one. Actually, no perfectly frictionless
surface exists. When two surfaces are in contact, tangential
forces, called friction forces, will always develop if one attempts to
move one surface with respect to the other. On the other hand, these
friction forces are limited in magnitude and will not prevent motion
if sufficiently large forces are applied. The distinction between frictionless
and rough surfaces is thus a matter of degree. This will be
seen more clearly in the present chapter, which is devoted to the
study of friction and of its applications to common engineering
situations.
There are two types of friction: dry friction, sometimes called
Coulomb friction, and fluid friction. Fluid friction develops between
layers of fluid moving at different velocities. Fluid friction is of
great importance in problems involving the flow of fluids through
pipes and orifices or dealing with bodies immersed in moving
fluids. It is also basic in the analysis of the motion of lubricated
mechanisms. Such problems are considered in texts on fluid
mechanics. The present study is limited to dry friction, i.e., to problems
involving rigid bodies which are in contact along nonlubricated
surfaces.
In the first part of this chapter, the equilibrium of various rigid
bodies and structures, assuming dry friction at the surfaces of contact,
is analyzed. Later a number of specific engineering applications
where dry friction plays an important role are considered: wedges,
square-threaded screws, journal bearings, thrust bearings, rolling
resistance, and belt friction.
8.2 THE LAWS OF DRY FRICTION.
COEFFICIENTS OF FRICTION
The laws of dry friction are exemplified by the following experiment.
A block of weight W is placed on a horizontal plane surface
(Fig. 8.1a). The forces acting on the block are its weight W and the
reaction of the surface. Since the weight has no horizontal component,
the reaction of the surface also has no horizontal component; the
reaction is therefore normal to the surface and is represented by N
in Fig. 8.1a. Suppose, now, that a horizontal force P is applied to the
block (Fig. 8.1b). If P is small, the block will not move; some other
horizontal force must therefore exist, which balances P. This other
force is the static-friction force F, which is actually the resultant of
a great number of forces acting over the entire surface of contact
between the block and the plane. The nature of these forces is not
known exactly, but it is generally assumed that
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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