Bolts are tightened such that a high tension, usually above yield strength, is developed in the bolts, these are called preloaded bolts. The plates of the connection are thus clamped together and shear transfer between the plates is achieved through friction.
Preload is the tension load developed in a fastener while tightening. The tensile force thus induced in bolt, creates a compressive force in the bolted joint this is called clamp force. The clamp force in an unloaded bolted is assumed to be equal and opposite of the preload.
Benefits of using of Preloaded Bolts are:
- rigidity of joints (no slip in service)
- no loosening of bolts due to vibrations
- better fatigue performance
- tolerance for fabrication/erection (because of the use of clearance holes)
- familiarity within industry.
With minimal disadvantages, disadvantages quoted by some designers are:
- difficulty of ensuring that all bolts are adequately pre-loaded
- In double cover connections, small differences in ply thickness in plates of nominally the same thickness can result in the preload from bolts near the centre of joint being applied to the wrong side of the joint.
The variables that affect the relationship between torque and pretension include:
- The finish and tolerance on the bolt and nut threads.
- The uniformity, degree and condition of lubrication.
- The site conditions that contribute to dust and dirt or corrosion on the threads
- The friction that exists to a varying degree between the nut face or bearing area of the bolt head and the supporting surface
- The variability of the air supply parameters on impact wrenches.
- The condition, lubrication and power supply for the torque wrench, which may change within a work shift.
- The repeatability of the performance of any wrench that senses or responds to the level of the applied torque.
Pretensioned joints are required in following applications:
- Joints in which fastener pretension is required in the specification or code that invokes this Specification;
- Joints that are subject to significant load reversal;
- Joints that are subject to fatigue load with no reversal of the loading direction;
- Joints where bolts that are subject to tensile fatigue;
- Joints where bolts are subject to tension or combined shear and tension, with or without fatigue.