Studs require a two-part assembly operation but offer several advantages. They eliminate deviations from perfect squareness in an assembly. The ability of a nut to "float" and adjust on the nut end threads is an advantage of using the stud over the bolt or screw. The thicker the cover plate, or the longer the tapped hole, the more important this becomes.
In assembling and reassembling heavy parts such as turbine casings and cylinder heads, studs can act as pilots. Also, in the automatic assembly of small, lightweight units, studs reduce assembly costs since they permit quick and easy "stack up" of gaskets or other different parts of a joint.
Studs reduce the need for the large hole clearance and close hole alignment required by a cap screw or bolt. During assembly, the runout, or eccentricity, of cap screws and bolts is doubled as the fastener is turned. Clearance holes accommodate the runout which may cause eccentric loading and bending stresses in the joint.
Studs with an interference-fit thread or proprietary-lock thread on the tap end provide a positive lock against turning and loosening. The lock facilitates assembly and disassembly of locknuts on studs and is particularly important where maintenance of prestress is required to combat fatigue failures.
Studs applied with sealant prevent leakage of fluids through holes tapped in porous materials.
There are four basic stud types:
- Class 1 has an interference fit at one end and a free-running thread at the other. This is also called a gland or tap-end stud.
- Class 2 has free-running threads at both ends. This is also called a double-end stud.
- Class 3 is a three-bolt stud, which is used in high-temperature/pressure applications.
- Class 4 is used for general-purpose applications.