A lot of the industrial machines use belt transmission for power transfer. These mechanisms often use the round belts several millimetres in diameter, which are made of the thermoplastic elastomers, usually polyurethane. The mandatory step in their production process is bonding of belt ends, which is often performed by the butt welding, using the hot plate. In general, this operation comprises two main mechanical interactions between belts and the automatic welding equipment. The first one is a compression, performed by shaped grippers during heating and joining step. Compression proceeds both along the axis of the belt and in the radial direction. The second of them is shearing, performed by cutting edges, during belt preparation and flash removing step. Shearing is present both in a plane which is tangential to the belt outer surface and also in a plane which is perpendicular to the axis of the belt. To correctly develop the model of the whole welding process, both the proportionality modulus and the shear modulus are required. The paper demonstrates the procedure of the experimental determination of these parameters, with some assumptions which are valid for the hot plate welding process. There were presented the results of the compression test in the axial and radial directions, and results of the shearing test. Finally, the values of proportionality and shear modulus as a function of test parameters are presented.