Hyeon-Ju Lee

Method for estimating the clamping force of high strength bolts subjected to temperature variation

The torque control method is mainly used as a clamping method for high-strength bolts in Korea and Japan. However, the torque coefficient affects the tension that is required during the process of tightening by torque. This makes it difficult to assume the direct tension induced into the bolt. The determinant factor that affects the clamping force is the ambient temperature, as indicated by the Korean standard B 2819. This study was planned to assess the expected clamping force of two types of high-strength bolts considering a temperature range of-10 to 50°C. Based on laboratory data subjected to temperature parameters, equations to estimate the clamping force were extracted by regression analysis.

Evaluating relaxation of high-strength bolts by parameters on slip faying surfaces of bolted connections

The clamping force of a high-strength bolt reduces within a certain time period after the initial clamping force. When special treatments are used on a faying surface, the clamping force is relaxed severely. Tests were conducted for slip critical joints subjected to various faying surface parameters. Relaxation occurred for slip resistant joints with an uncoated surface that had been shot-blasted, cleaned, milled or rusted. It was observed that the initial clamping force dropped from 6.2% to 8.0% after 744 hours. For joints with a 5 mil thick zinc coating, the clamping force of the bolts decreased by 8.37%, and in the case of 4.9 mil thick red lead painted treatment, the relaxation ratio was 24.6%. The standard deviation of the slip coefficients between the time immediately after tightening and 1,000 hours after tightening was 0.223. It is particularly noteworthy that, for red lead painted treatment, the slip coefficient dropped from 2.5 times to 2.8 times when compared with the slip coefficient before relaxation. Joints with inorganic zinc should not have thickness exceeding 5 mils, and joints with a red lead painted surface should not be allowed regardless of the thickness of coating.