Comparison of inertia forces due to rotating and reciprocating masses

Now we have seen the inertia forces due to crank which undergoes pure rotation and those due to the piston that undergoes pure translation. We observe that the inertia force due to crank is always of fixed magnitude but ever directed radially i.e., continually changing direction. The inertia force due to a reciprocating mass is always directed along the line of motion but its magnitude is ever changing. This fundamental difference between the two types of inertia forces is captured in Fig. 3.4.

Figure. 3.4

Figure 3.5 Mechanical Shaker

An interesting configuration with two rotating masses (in phase opposition) is shown in Fig. 3.5 which results in a force only along one line/axis. This is effectively used as a mechanical shaker for dynamic testing of structures.