The rotary clamp consisting of a pair of gears is a basic construction element for the design of various types of extensional rheometers for polymer melts. With the most recent model of the uniaxial extensional rheometer, built with rotary clamps, homogeneous deformations of Hencky strains up toε=7have been performed with a low‐density polyethylene melt. For the evaluation and discussion of results from tests with large deformations, it is vital that parameters which characterize the quality of each test be defined and measured. Otherwise, the measured force and the rotational speed of the clamps might be of little relevance for the stress, strain, and strain rate within the sample. The biaxial extensional rheometer consists of eight rotary clamps in a circular arrangement. With this new instrument, extensional Hencky strains up toε11=ε22=2.2can be achieved for a polyisobutylene sample. Such a high deformation corresponds to a compression in the third direction ofε33=−4.4.The material within the samples is deformed homogeneously. The samples are shaped as flat circular disks. During the test they are cut periodically at the rim between the clamps in order to keep the “active” diameter constant within a narrow range. Hence, from the forces measured at the clamps, the tensile stress can be determined.