Coupling bolts

Elongation of a bar produces a related reduction in cross sectional area. A bar with the same elastic properties in all directions will have a constant relationship between axial strain and lateral strain. This is termed the Poissons Ratio and given by the symbol n.
A bolt when tightened similarly causes a loss in area and diameter. In a clearance hole this is not a problem. With a fitted bolt however, the positive contact or 'fit' between the accurately machined bolt and reamed hole would be affected.
Shaft coupling bolts are tightened to force the faces of the flange together so the friction between the faces will provide some proportion of the drive. However, fitted bolt shanks are also designed to take a proportion of the drive. A clearance bolt could provide the first requirement but not the second. A fitted bolt when tightened and subject to reduction in cross section would also fail on the second count and probably be damaged by fretting. A tapered bolt may be used instead of a conventional coupling bolt to obtain a good fit and required tightening.

Taper fit bolt

Parallel shank fitted bolts

have Interference fit in holes so that in the event of loss of frictional grip between flanges then each bolt will take on equal share of the shear stress due to torque transmission. Parallel bolts become slack after one or two refits. Therefore taper shank bolts have been used. An alternative is the sleeved coupling bolts.

The fit of the bolt is achieved by the tensioning of the taper shank bolt. Should wear occur in the sleeve then this can be renewed, reusing the rest of the assembly.

Hydraulically fitted bolts.

The pilgrim or margrip hydraulic bolt uses the principal embodied by poissons ratio to provide a calculated and definite fitting force between bolt and hole. Center load rod fitted into hollow coupling bolt and hydraulic head fitted. High pressure oil pumped into head pushing down, seal, piston and rod .This action stretches the bolt ( within its elastic limit ) and reduces its diameter sufficiently for a sliding fit into the hole. Fluid pressure is released allowing bolt to expand and tightly grip within the hole with a radial grip of about 2.36 Kg/mm2 . Simultaneously longitudinal contraction of the bolt having already fitted the nut hand tight, exerts considerable compressive force which is about 2 1/2 x greater than that which can be achieved by normal torque tightening.
Hydraulic head and loading rod now removed and a protective cap and seal screwed back on

Advantages

Reduction in fitting and dismantling time, bolts can be used repeatedly, no replacements required , known loads applied