Abstract: In order to get a solution for inspecting the pre-tightening state of pre-stressed frame heavy hydraulic press, the change law of the tension force in the tie rod with the pulling force at the end of the tie rod, which shows that the tension force curve is nearly a two-segment broken line with a curved transition caused by the insufficient connection stiffness of nut, is obtained by mechanics analysis. A measurement principle for obtaining the pre-tightening force of the tie rod without release of the tightening of tie rod is presented and the insensitivity of measurement precision to the stick-on orientation of the strain gauge is theoretically proved. A measurement system is developed on the virtual instrument platform, LabVIEW, of NI Company. Experiments are carried out to verify the correctness of the principle and the feasibility of the system by using a physical model. The research provides an implementation means for evaluating the pre-stress status of the pre-tightening state of heavy hydraulic press in service, as well as the necessary condition for malfunction diagnosis of the integrity of heavy hydraulic press.

Key words: Hydraulic press, Non-release measurement, Pre-stress, Pre-tightening force, Virtual instrument