Piston-type pumps, compressors and engines usually are driven by a crank and connecting rod. During the stroke, flow rates don't follow the sinusoidal curve that textbooks and manufacturers' literature often cite. Peak rates are up to 10% higher than predicted by a sinusoidal curve and don't occur at 90° crank rotation. These higher rates increase values of net positive suction head required (NPSHR) by as much as 20% and cause greater stresses and possibly added maintenance. The actual curve is a distorted bell curve controlled by the length ratio of the stroke and connecting rod. Rods shorter than the stroke but longer than the connecting rod need special crossheads to avoid metal interference but create amazing distortion. Knowing the length ratio of a pump and its relation to flow rate provides insights on dynamic and hydraulic features that affect performance. In reciprocating pumps, compressors, etc., the crankshaft and crank move a connecting rod and piston in a cylinder (Figure 1). The crankshaft center is placed at 90° on the 0°–180° x axis; the crank's rod bearing is shown at 45°. Clockwise rotation of the shaft will cause the crank bearing to generate a sinusoidal x-y curve between 0° and 360°. Figure 2 shows this sinusoidal velocity curve between 0° and 180°. It also shows the actual velocity curve for a piston when the rod length divided by crank length is 2.1. It is not sinusoidal. Flow recorders could provide a pump flow profile but it's much easier to use simple trigonometry to calculate piston positions versus crank angles, then calculate piston travel per degree of crank rotation and plot the results. They are not sinusoidal. (Compressors and piston engines would follow similar curves.)