This paper presents the theory for a first approximation of force modeling for the dynamic problem of a quick pneumatic servomotor with translatory motion of the working element. With only a few assumptions, the inter-relation of motion, evolution of air parameters and mass flow have been analysed. The model has been described by six first order differential equations. A numerical solution has been carried out on a quick acting servomotor used in the hammering mechanism of pneumatic hammer without a back-kick (dead-beat) Atlas Copco RRD-57. The results obtained confirm the theory of the model giving a good qualitative evaluation but a slightly worse quantitative one, this will be the subject of further research.