Page:The Kinematics of Machinery.djvu/343

The crossed-chains formed from the isosceles slider-crank chains are of less importance than the foregoing. They are formed by a method : analogous to that described in § 69, and will be called isosceles crossed turning slider-crank $(C''_{2}?C''P^{\text{+}})^{d=e}$ and isosceles crossed swinging block $(C''_{2}?C''P^{\text{+}})^{b=a}$

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If we make $b$ also $=\infty$ however, we obtain some remarkable special cases. The normal cross-block then becomes oblique, or to

=just that alteration of phase which is required to change the cut-off or the direction of the engine's motion. The excessive friction in the pair 4, .when the amount of crossing is large, has prevented any great use being made of ($C''_{3}P^{\text{+}}$) for this purpose.

The mechanism ($C''_{2}P_{2}^{\angle }$), Fig. 253, might be used in the same way if it were constructed so that the angle of skew of the cross could be altered. Such an arrangement would be in some respects better than the one just described, but it presents some constructive difficulties in cases where an eccentric takes the place of the crank.