The role of friction in non-colloidal suspensions is examined with a model which splits the viscosity into a frictionless component (*) plus a frictional component which depends on the ratio of the particle pressure (P) to the shear stress (). The model needs the input by computation of * and P and a suitable choice of particle friction coefficient (). It can be extended to elongational flows and cases where sphere roughness is important; volume fractions up to 0.5 are considered. It is shown that friction acts in a feedback or bootstrap manner to increase the suspension viscosity. The analysis is also useful for deducing the friction coefficient in suspensions from experimental data. It was applied to several sets of experimental data and reasonable correlations of the viscosities were demonstrated. An example of the correlation for spheres in a silicone oil is shown for volume fractions 0.1-0.5.