The ability to monitor the level of clamping pressure on transformer windings, that is, the degree of winding looseness, is important in ensuring a high availability of electrical power and to prevent the high costs incurred by unexpected failures. In this paper, three-dimensional computer modelling of a three-phase power transformer from its specifications and geometry was employed to estimate the 100-Hz vibration magnitude of the tank due to the winding vibrations for a particular axial clamping pressure. The experimental and calculated clamping pressures were found to be in good agreement. In order to develop the computer model, it was necessary first to determine dynamically the non-linear modulus of elasticity of the spacers by building a finite-element model of a single winding, determining its eigenfrequencies using an assumed modulus and comparing them with experimentally-determined eigenfrequencies. The actual modulus was then found by trial and error.