Investigação da influência do revestimento de fixadores e condição de lubrificação na relação torque x pré-carga na montagem de Flanges API

Abstract

This study investigates the influence of different types of coating and lubrication conditions on the relation between the torque applied and the preload generated on fasteners during the assembly of API flanges on subsea equipment. To perform such investigation, an experimental procedure was developed in which several torque tests were carried out simulating assembly conditions using fasteners of two different nominal diameter sizes (3/4” e 1.1/2”), with two types of coating (cadmium dichromate plating and molybdenum disulfide organic coating) and under two lubrication conditions (dry and with molybdenum disulfide based grease). The magnitude of the preload generated on the fastener is obtained through the measurement of the axial elongation of the fastener, using load cells developed specifically for this work. These load cells have strain gauges attached to it to determine the preload at each torque value applied to the fastener (for the 1.1/2” greased studs condition it was also used the ultrasonic method for measurement of the pre-load in order to compare its results to the preload measured by the load cell). These torque values are therefore measured by digital torque wrenches and torque transducers. After performing an analysis upon the tests results and with the torque-preload formulas provided by literature in hand, the value of the friction coefficient can be calculated, therefore allowing to determine the influence of each given factor in the torque x preload relation. Biaxial strain gauges installed to the load cells also allowed to gather data of the resistive torque generated by the friction between the fastener underhead and the load cell surface to be further investigated.