An assembly of two or more parts fastened together by inserting bolts through matching clearance holes in the parts and engaging nuts that are then tightened to clamp the assembly. The term bolted joint also colloquially denotes a screwed joint, for which screws are inserted through clearance holes in one part and tightened into internal threads in another part by turning the screw head. See also: Bolt
External forces on bolts
The external force acting on a bolt of a joint depends on the manner in which the joint as a whole is loaded, the geometry of the bolt pattern, and the tension induced in the bolt during installation. A bolt may be subjected to direct tension, to shear, or to simultaneous tension and shear (Fig. 1). Cylinder head bolts are in direct tension; bolts uniformly spaced around the periphery pass through the head as well as the flange that is integral with the cylinder wall (Fig. 1a). If all bolts are tightened equally and the flange and head are sufficiently rigid, all the bolts will be subjected to direct tension of equal value. The external force F applied to the bolt is due to the pressure p inside the cylinder acting on the cylinder head and to the initial tension induced in the bolt during tightening of the nut.
In a double lap joint (Fig. 1b), the bolts are in shear at the planes between the outer and inner plates. The force F in the inner plate (due to the external loading) is equilibrated by the forces F/2 in the outer plates. Because each bolt passes through two shear planes, the bolts are said to be in double shear. The bolts will be subjected to different shearing forces. The two outer bolts (in the direction of loading) will be subjected to more shear than the inner bolt because of deformations of the plates. However, for design purposes the bolts are considered to be equally loaded unless the joint is long.
If the bolts are lightly tensioned (or loose), the bolt body will bear against the plates. The plates will slide with respect to each other (the joint is said to slip) until the plates bear on the bolts. Because of these bearing forces, the bolts will be subjected to bending in addition to shear. This bending is usually neglected in design. However, if the nuts on the bolts are adequately torqued, the bolts will be pretensioned and the plates will be pressed together so that the shear force will be transmitted at the interface of the outer and inner plates by friction. If the friction is adequate, the joint will not slip and the plates will not bear on the bolts. The bolts of the tee joint (Fig. 1c) are subjected to tension due to the component V of the applied external force F and also to single shear due to the component H. A force, such as F, is a vector quantity; that is, it has magnitude and line of action. It can be replaced by two orthogonal forces, known as components, according to the parallelogram law (Fig. 1c). The bolts are in single shear because they pass through only one shear plane. If the bolts are not adequately pretensioned and the joint slips, the bolts may bear on the sides of the holes because of the component H.
Bolts of machinery joints that are gasketed (gaskets are soft materials used to seal joints that need to be disassembled) are tightened to minimal initial tension to avoid damaging the gasket material. However, bolts in most rigid metal-to-metal machinery joints are tightened to about 75% of the yield strength of the fastener. The bolts are tightened to high initial tension so that the bolt tension remains constant as the external tensile load (Fig. 1a and c) fluctuates, assuming that the force applied to the bolt by the external load is less than the initial tension in the bolt. In this way the bolt is not subjected to fluctuating stresses that could lead to fatigue of the bolt. Fatigue is progressive, localized damage to the bolt material caused by fluctuating stress. The damage starts with cracks that may lead to fracture after a sufficient number of stress fluctuations. The time required to fracture depends on the character, magnitude, and rate of stress fluctuation as well as the bolt metallurgy. Bolts are also tensioned at joints subjected to shear (transverse loading) so that the shear will be transmitted between the assembled parts by friction, and the bolt body will not be subjected to shear and bending. Transverse loading of bolts is usually avoided in machinery design.
Connections of structural steel members (angles, channels, plates, and wide flanged beams) may be made with unfinished bolts of low carbon steel that are tightened with manual wrenches. Because the amount of pretension is uncertain, they are not considered pretensioned for design purposes and hence are denoted as bearing-type connections. For design, these joints are treated similarly to riveted connections, because the magnitude of the clamping force of hot-driven rivets is also variable.
High-strength structural bolts are normally tightened to a specific installation tension, which should be at least 70% of the tensile strength of the bolt. For a specific condition of the faying surface and the controlled clamping force, the force needed to cause slip of a shear joint may be computed. For certain service conditions the connection is designed such that the shear force acting on the most severely loaded fastener does not exceed that causing slip. Such joints are known as slip-critical joints.
The design philosophy for bolted machinery joints (either permanent connections or those that are designed for disassembly) is that, generally, the fastener should pass through clearance holes and not be subjected to shear. Machinery joints subjected to shear are designed so that the shear is transferred between the parts by other mechanical means such as dowel pins, keys, and bearing surfaces configured into the joint. An exception is machinery mounting to structural steel (in some industries), where turned bolts in interference holes are utilized to resist shear and tension (Fig. 1c). These turned interference bolts may or may not be pretensioned. The general policy for bolt installation is to tighten the bolt to a specific minimum torque that is an index of the probable bolt pretension.
Lightly loaded beam-to-column connections in building frames are often made with unfinished bolts in clearance holes. The connections are usually configured so that the fasteners are in shear due to gravity forces. These joints are bearing-type without controlled pretension, and the connection is permitted to slip until the clearance is overcome and the bolt bodies bear against the structural steel parts.
High-strength bolts are used for connections in bridges and for building frame connections subjected to vibration and reversal of loading. The service condition is such that the connections are considered slip-critical. They are designed to transfer shear by friction, requiring controlled tensioning of the fasteners at installation.
For heavily loaded building frame joints that are not slip-critical, A325 and A490 bolts are designed as bearing-type connections. The allowable shear load per fastener is greater for the bearing-type connection than for the slip-critical connection, for the same fastener type and size. Hence, fewer bolts are required. High-strength bolts are required to be pretensioned for some bearing-type connections but need only be made snug tight for others. For example, a structural connection of a roof truss (Fig. 2) has high-strength bolts in double shear to transmit forces between the truss members. See also: Structural connections
High-strength bolt tightening
High-strength bolts for structural connections need to be tightened to at least 70% of their tensile strength in order to develop sufficient friction in the joint to be allowed loadings specified in building codes. The installation tensioning must be monitored. Three procedures for controlling the induced pretension for A325 and A490 bolts are the turn-of-the-nut-method, calibrated-wrench tightening, and direct-tension indicators (washers or ultrasonic transducers). For certain critical structural connections, and for special situations in machinery assembly, these three procedures are not considered sufficiently reliable, and the tension induced is monitored by measuring the bolt extension during tightening. See also: Joint (structures)