Procedure for Bolt Tightening

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Using standard ranges of hydraulic bolt tensioning tools, innovative ranges of direct drives, hydraulic nuts, square drive hydraulic torque wrenches, electric and pneumatic nut runners, various industry specific bolt and nut tightening solutions can be achieved, which lead to precise tightening procedures.

About torque and tension.

  • Torque-The nut is turned using a nutrunner or a hydraulic torque wrench.The turning of the nut stretches the stud, after calculating the torque required for bolt tightening. As the yield point is acquired, the torque setting translates into load on the connection. The desired clamp force is achieved once the instruments work according to specifications and the correct lubrication, coating and other factors.
  • Tension-A stretch is produced on the stud using tensioners, rather than force being applied on the nut. The nut is then tightened after acquiring pre-stretch conditions. The nut pushes the load on the bolt to withstand the load, once the tension is removed. The required clamp force is achieved, and an even load can be procured by using multiple tensioners. Thread height requirements and footprint requirements are calculated while using hydraulic tensioners.

Although the two terms are different, they are used to achieve the same result. Although tensioners are used for bolts above 20 mm in diameter, torque tightening is also used to achieve precise tightening. Methods such as heat tightening after torque bolts tightening, use the thermal properties of the bolt, by heating and expanding it, after which it is indexed using angle turn method and left to cool. This is a slow process that is applied on large bolts.

Methods for indicating tension.

  • There are certain special load indicating bolts, load indicating washers, and methods of determining the change in length of the fastener, which are included under this topic.
  • Special bolts which indicate the force in the bolt have been designed, such as Rotabolt, where bolt extension is measured using a central gauge pin which is pushed down a central hole drilled in the bolt. The head of the gauge pin conceals a free turning rota which spins in an accurately set gap. This makes the fastener stretch elastically, whereas the gauge pin remains intact. The bolt stretches as it is tightened, to the point where the rota cannot spin freely anymore, which indicates that the bolt has been tightened as per requirement.
  • Washers consist of small raised pips which deform plastically under load, and hence the preload is achieved when a pre-determined space is generated between the washer and the underhead of the bolt. They are used in civil engineering more than mechanical engineering.

Applications.

  • Bolt tightening is required in topside flanges, sub-sea flanges, well heads, valves tress/Christmas tree pumps, sand separators, HPHT pipe joints, gate/ball valves, and reactor covers in oil and gas industries.
  • Bolt tightening is required in foundation bolts, tower section bolts, nacelle mounting, hub mounting, bearing to hub, blade bearings, gearbox bed, and generator beds in wind power generation plants.
  • Other applications include high exchange covers and nozzles, boiler feed pump covers, compressor casings, boiler feed pump covers, connecting rods, diesel engines and compressors, and power presses.

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