Opening remarks

01. In chapter 6 and in other parts of this book hitherto I have spoken about systems of ISAs or of screws, or more simply about screw systems. The origin in physical reality for most of these spoken remarks about screws was the capacity for instantaneous motion of some rigid body whose freedom to move at the instant was being restricted in some way. Relationships exist between these systems of screws about which small twists or rates of twisting of one body relative to another may occur, namely the systems of ISAs, and identical kinds of systems of screws about which wrenches and reaction wrenches between the same two bodies may act. An investigation of these two sets of systems of screws will reveal, at the end of this chapter 10, (a) an insight into the power expended in friction at working joints in mechanism, (b) an amplified meaning for the somewhat narrow term joint as defined for example at § 1.11, and (c) the beginnings of a method for calculating the forces at work at the joints of mechanism where mass and the consequent inertia of links is an important consideration.

02. With regard to (b) above I can mean by joint, as I shall show, the joint between for example the piston and the connecting rod of an engine designed for the transmission of power in the absence of loss, or the joint between for example a bulldozer blade and its one-off job where power is being releases in spurts, or the joint between a ploughing tool and its sod which is in a continuous, power-releasing action.