Novel Washer as Force Transducer for Monitoring Clamping Force of Bolted Joints Available to Purchase

Bolts are widely used in industry for joining members together. The behavior and service life of bolted joints depend on several factors like bolt material, dimensions, surface finish, surface coating and thread tolerances. However, the uppermost factor affecting the reliability and durability is the correctness of the clamping force exerted by the bolt. Thus it is very important to monitor the bolt clamping force during assembly process to ensure a proper preload. Each of the available monitoring techniques including torque control, torque-angle control, strain gaged bolt and ultrasound suffers one or more limitations rendering the methods impractical for application in a production environment. In this paper, we propose an alternative method for determining clamping force. The method utilizes a simple washer whose surface has a speckle pattern; the speckle pattern acts as a strain sensor. By measuring the deformation of the washer using an image processing technique called fast digital image correlation, the clamping force can be determined. This method, which enables real-time monitoring of clamping force during fastening, has a potential of being used for mass measurement of clamping force in a production environment. Experiments conducted on a bolted structure with washers of different sizes have demonstrated the reliability and usefulness of this new approach.