There are many methods for assembling various parts of a product. Bolts are widely used because they easily hold components tightly together and are simple to install and remove. However, bolts require other components to prevent loosening, which increases their cost. This study proposes equations that explain why bolts become loose and solves them using finite element analysis. Furthermore, because the number of bolts used in attaching components is often based on the empirical experience of designers in the field, the safety margin in plate heat exchangers is often too high. The equations presented in this study permit the number of bolts to be reduced. These equations take into consideration the relationship between the critical shear force acting on the screw and the normal force acting on the cooling plate induced by the working fluid.