Читать книгу Fundamentals of Heat Engines - Jamil Ghojel - Страница 22

The moment of force F about a point 0 is the product of the force and the perpendicular distance L of its line of action from 0 (Figure 1.3a):

(1.14)

Figure 1.3 Definitions of moment, couple, and torque.

A couple is a pair of planar forces that are equal in magnitude, opposite in direction, and parallel to each other (Figure 1.3b). Since the resultant force is zero, the couple can only generate rotational motion. The moment of the couple is given by

(1.15)

Torque is also a moment and is given by Eq. (1.14), but is used mainly to describe a moment tending to turn or twist a shaft of reciprocating and gas turbine engines, motors, and other rotating machinery. In machinery such as engines, force F will be applied to the arm L at a right angle (θ = 0). In these applications, the power is often expressed in terms of the torque (see Eqs. 1.21 and 1.22 in Section 1.1.9).

Figure 1.4 Kinetics of rotating shaft: (a) accelerating shaft; (b) decelerating shaft.