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An upper bound solution for compression of viscous material between rotating plates is proposed. For many conventional constitutive equations its form has been given by Hill. In the case of viscous materials the main difficulty with the application of the upper bound theorem is that conventional friction laws are not compatible with the conditions used to prove it. A reduced version of the upper bound theorem that accounts for specific viscous constitutive equations and boundary conditions is adopted. In such a form, in contrast to the general case, the theorem determines an upper bound on the load required to deform the material. The dependence of the upper bound force based on a simple kinematically admissible velocity field on material and process parameters is illustrated. The solution is reduced to numerical integration and minimization of a function of one variable.