This work deals in particular with variable displacement swashplate axial piston pumps which play a relevant role in feeding adjustable power for hydraulic mobile systems. In these machines the displacement changes with the inclination of the swashplate and to ensure a stable displacement setting is necessary to analyze the moments acting on the swashplate: the relevant contributions to this moment are due to the forces exerted by the pistons on the swashplate through the slipper bearings, which mainly depend on operating conditions, valve plate geometry, pistons angular positions, piston-slipper assembly inertia, viscous friction and on the force of the variable displacement actuator. The resistant moment is never equal to zero when speaking of actual machine and rises with delivery pressure up to consistent values which compromise the operation of the variable displacement mechanism. In the case of the machine studied the variable displacement mechanism is manually operated by the user, who has found significant difficulties in the displacement setting in some operating conditions.A lumped parameter model of the machine has been created which allows to determine the pressure transients within the pistons, taking into account the actual geometry of the valve plate and allowing to compute instantaneous and average moments acting on the swashplate. Comparison between the numerical average moment acting on the swashplate and experimental measurements on the machine has been done. At this point the influence of operating and design parameters has been analyzed and in particular it was found that the valve plate design together with the timing of the high and low pressure flow passages strongly influence the resistant moment arising on the swashplate. In particular, few modifications on the flow areas can lead to significant reduction of the resistant moment without compromising the machine operation.