We report dissipative particle dynamics (DPD) simulations of grafted polymer brushes under shear at three separation distances. We investigate the impact of the shear on single monolayers and compressed bilayers formed by neutral and charged polymer chains. The dependence of the friction on the solvent quality is studied for different charge fractions in two compressed polymer brushes. The change in the frictional forces upon the addition of the salt is analyzed in terms of change in the interpenetration coefficient. We complete this study by a description of the brush structure including ordering, composition, tilt and local electroneutrality at different separation distances, salt concentrations, charge fractions and shear rates. Interestingly, some heterogeneities in the monomer density profiles of polyelectrolyte brushes appear under shear and vanish upon addition of salt.