The present paper deals with a novel approach to the analysis of machine tools under load; it identifies the parameters of machine tool spindles simulating a real cutting process. The main focus of the paper is on evaluation of synchronous and asynchronous error motions and their dependency on spindle speed and load. To investigate the errors, a hydraulic load unit is used. This unit exerts a uni - or bidirectional force located between the spindle and the machine table. Behaviour of the load unit was also investigated. For the purposes of simulating a real cutting process, working conditions between 500 - 5000 rev / min and loads in the range of 50 - 500 N were analysed. Results can be used for evaluation of spindle parameters, prediction of machining accuracy and improvement of the cutting process.