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13
7. Eccentric (special case of a cam) is disk or cylinder that fits
eccentrically on the shaft, i.e. with the displacement of a geometrical axis of a
disk concerning an axis of the shaft. Eccentric is a link that performs
rotational movement but not around its geometrical axis (Fig. II.6).
The mobile and motionless links can be incorporated differently, but this
connection always provides relative movement, forming kinematic pairs.
If an element of a kinematic pair (KP) is a surface (flat, cylindrical,
spherical…), such KP is called a pair of lower degree. If an element of KP is
a line or a point, such KP is called a pair of higher degree.
2. Classification of Kinematic Pairs
The possible connections of links in kinematic pairs are rather various.
They can impose restrictions on relative movements of links and determine
both the character and the quantity of movements.
The following kinds of classification of kinematic pairs are used:
1) by the relative character of movement - rotary "R" and translational "T"
kinematic pairs;
2) by the character of kinematic pair’s element (lower KP and a higher
degree one);
3) by the number of motion freedom.
It is known, that any object in space has six degrees of freedom.
The movement of any object can be imagined as sliding lengthways and
rotational around three arbitrarily chosen and mutually perpendicular axes X,
Y, Z.
Thus, generally, each object has six kinds of possible independent
motions in space − three rotational, around the axes X, Y, Z, and three
translational movements, along the same axes (Fig. II.7). Therefore, if no
conditions of constraint were not imposed on the movement of the first link
of a kinematic pair which is accepted for an absolutely rigid body, the
movement of such a link could be imagined as the movement consisting of
slider
guide
ε
Fig. II.3. Fig. II.4. Fig. II.5. Fig. II.6.
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