Torque and Angle Controlled Tightening

TORQUE AND ANGLE CONTROLLED TIGHTENING

OVER THE YIELD POINT OF A SCREW

Göran Toth

ABSTRACT

Reduced weight and fuel consumption, improved reliability and engine performance are the main requirements in the automotive industry of today. These requirements demand robust screwed joints where a high amount of preload obtained by effective utilization of the screw is essential. This work presents a method, based on Monte-Carlo simulations or Taylor’s series expansions, of utilizing a screw over its yield point, and thereby to its maximum, in a controlled fashion, even under simple tightening conditions.

The proposed torque and angle control method can be used in modern assembly plants as well as in small workshops with less sophisticated tightening tools.

The method is very useful for engineers since it defines the minimum and maximum snug torque values. The snug, or initial, torque prior to application of the tightening angle must be high enough to close gaps in multi-layered joints and stiff clevis joints, but low enough to minimize the effects of friction. An appropriate combination of snug torque and tightening angle to reach over the yield point of a screw is also determined. In addition, the elastic angle, permanent elongation, final preload and final torque, etc. are determined for a certain tightening angle. The plastic spring constant of the screw is also defined and used in the calculations.

Monte-Carlo simulations and Taylor’s series expansions both take into account the probabilistic nature of the different variables required for the calculation of screwed joints, e.g. friction and yield strength. The outputs of these two techniques are compared with the extreme-value method, which covers a worst-case scenario, as well as with practical tests. It is suggested that the Monte-Carlo simulation method is the method to prefer. The extreme-value method is too pessimistic in estimating the minimum and maximum values of the quantities of interest, and should therefore not be used when optimizing for low weight.

Keywords: Bolted/screwed joints, clevis, torque controlled tightening, torque and angle controlled tightening, turn-of-nut method, yield, snug torque, Monte-Carlo simulations, Taylor’s series expansions, extreme-value method, plastic spring constant of the screw, plastic deformation, final torque, final preload, elastic angle, momentary elongation factor, permanent elongation factor.