Self-tightening and self-aligning grips. The initial grip or clamping of the sample is provided by the action of the spring. The clamping force ratio of the grips with the tensile force changes according to the opening of the jaws. The greater clamping force on the samples is obtained with the greater opening due to the effect of the spring. The jaws apply self-tightening on the specimen that is tested during the whole tensile process.