Method and apparatus for evaluating clamp force in a bolt by means of ultra-sonic measurements

A method of evaluating a clamp force (F) in a threaded joint between at least one bolt and a matching threaded piece during a tightening operation, comprises:

• tightening the joint by means of a torque delivering device,
• registering a rotation angle (α) between the at least one bolt and the matching threaded piece during tightening of the joint,
• measuring a torque (T) provided to the joint during tightening thereof,
• simultaneously inducing ultra-sonic pulses into the at least one bolt and measuring a time of flight (TOF) of the pulses in the at least one bolt,
• monitoring, in response to a measured increase of the torque (T), the torque (T) as a function of the rotation angle (α) in order to determine an increase rate (dT/dα) of the torque (T),
• monitoring a time of flight (TOF1) of the ultra-sonic pulses as a function of the rotation angle (α), in order to determine an increase rate (dTOF/dα) of the time of flight (TOF),
• determining, based on the determined increase rate (dT/dα) of the torque (T), an initial rotation angle (α0) of the tightening operation where the torque (T) starts to increase as a function of bolt deformation,
• defining the initial rotation angle (α0) of the tightening operation as a rotation angle where the time of flight (TOF) starts to increase,
• determining a total increase of the time of flight (ΔTOF) during the tightening operation based on the initial rotation angle (α0) of the tightening operation and the increase rate (dTOF/dα) of the time of flight (TOF), and
• evaluating an established clamp force (F) in the joint based on the total increase of the time of flight (ΔTOF), the clamp force (F) being presumed to be proportional to the total increase of the time of flight (ΔTOF) according to the equation F=k1*ΔTOF, where k1 is a bolt specific constant dependent on the geometry and material of the bolt.

A tightening apparatus for tightening at least one joint between at least one bolt and a matching threaded piece, comprising:

• a torque delivering device that comprises:
• a shaft for driving a tool implement in order to provide a mutual rotation to the at least one bolt and the matching threaded piece, and
• a motor for driving the shaft,
• a torque meter for measuring the torque as a function of a rotation angle applied to the at least one joint between the at least one bolt and the matching threaded piece during a tightening operation of the at least one joint,
• an ultra-sonic transmitter for inducing ultra-sonic pulses into the at least one bolt and measuring a time of flight (TOF) of the pulses in the at least one bolt,
• a rotation angle meter for registering a rotation angle (α) between the at least one bolt and the matching threaded piece during tightening of the at least one joint, and
• a control unit configured to:
• determine an increase rate (dT/dα) of a torque (T) as a function of the rotation angle (α) based on the measured torque (T),
• determine an increase rate (dTOF/dα) of the time of flight (TOF) as a function of the rotation angle (α) based on the measured time of flight (TOF),
• determine, based on the determined increase rate (dT/dα) of the torque (T), an initial rotation angle (α0) where the torque (T) starts to increase due to bolt deformation,
• defining the initial rotation angle (α0) as the rotation angle where the time of flight (TOF) starts to increase,
• determine a total increase of the time of flight (ΔTOF) during the tightening operation based on the initial rotation angle (α0) and on the increase rate (dTOF/dα) of the time of flight (TOF), and
• evaluate an established clamp force (F) in the at least one joint based on the total increase of the time of flight (ΔTOF), the clamp force (F) being presumed to be proportional to the total increase of the time of flight (ΔTOF) according to the equation F=k1*ΔTOF, where k1 is a bolt specific constant dependent on the geometry and material of the bolt.