Joining is the process of connecting individual parts to create an assembly capable of performing a useful mechanical function. The appropriate joining method depends not only on strength, but also on whether the assembly must be manufactured economically, inspected, maintained, adjusted, dismantled or repaired during its working life.
Engineering joining methods range from welded, brazed and adhesive joints to bolts, screws, rivets, pins, keys, interference fits and retaining rings. Some provide effectively permanent connections, while others are specifically selected to permit repeated assembly and dismantling.
At its simplest, joining is not confined to conventional metal fasteners. Sewing, buttons, press studs, zips, clips, laces, pegs and interlocking features are all examples of joining principles used to connect components.
The same broad principles appear throughout engineering. Furniture uses dowels, screws and interlocking fittings. Timber structures use shaped joints and pegs. Mechanical equipment uses bolts, pins, keys and retaining rings. Structures and pressure-containing equipment may use welding, riveting or mechanical fasteners.
A responsible designer should therefore consider the function of the joint before selecting the technology used to produce it. Sometimes the simplest joining principle provides the most reliable solution.
Joining methods considerably pre-date modern mechanical engineering. Timber pegs, lashings, stitching and shaped wooden joints were used in structures, vehicles and watercraft long before modern threaded fasteners or welding processes existed.
Traditional coracles provide a particularly simple example. A lightweight framework was formed from woven or interlaced material and covered with a water-resistant skin or fabric. The effectiveness of the structure comes from the complete joining and construction method rather than from any single sophisticated fastener.
Early iron structures also borrowed established ideas from timber construction. The Iron Bridge at Ironbridge in Shropshire, England, completed in the eighteenth century, incorporated jointing principles derived from carpentry, including mortise-and-tenon and dovetail-type connections adapted for cast-iron components.
Riveting subsequently became extremely important for joining metal structures, boilers, bridges, ships and other fabricated products. The development of practical welding processes provided another major route for producing permanent metallic structures and assemblies.
For machines which must be assembled, maintained and dismantled, threaded fasteners remain among the most important engineering joining devices.
A useful first distinction is whether the joint is intended to be dismantled during normal service. In practice, however, joining methods form a spectrum rather than fitting perfectly into two categories.
| Joining Type | Typical Methods | Service Consideration |
|---|---|---|
| Permanent joints | Welding, brazing, soldering, riveting and many adhesive joints | Normally not dismantled without damaging the joint or one of the components. |
| Removable joints | Bolts, screws, nuts and some pins | Designed to permit assembly and dismantling without destroying the components. |
| Location and retention | Dowels, taper pins, cotter pins, circlips and other retaining devices | Used to locate, constrain or retain components and may permit disassembly. |
| Torque-transmitting joints | Keys, splines, pins and interference fits | Used particularly for connecting shafts, hubs and rotating components. |
| Semi-permanent or application-dependent joints | Adhesives, press fits and some locking systems | Removal may be possible but can require special tooling, heating or replacement of one of the joining elements. |
Welding produces a permanent joint by establishing metallurgical continuity between components. A wide range of welding processes is available and the correct process depends on the materials, section thickness, geometry, required joint properties and production conditions.
Welded structures can eliminate separate mechanical fasteners and can provide strong and efficient joints. The designer must nevertheless consider weld geometry, accessibility, distortion, residual stress, fatigue, inspection and the effects of welding heat on the parent material.
RoyMech contains separate sections covering welding processes and the engineering strength of welded joints.
A rivet provides a permanent mechanical connection by passing through prepared holes in the components and being formed or otherwise retained so that the parts are clamped together.
Riveting has historically been important in structural, boiler, ship and aircraft construction and remains useful where its particular manufacturing, inspection or material characteristics are advantageous.
Design considerations include rivet material and diameter, hole size, edge distance, pitch, load distribution and whether the joint is subjected to shear, tension or a combination of loading.
Structural adhesives can join similar or dissimilar materials without introducing holes or the concentrated loading associated with some mechanical fasteners.
Adhesive joints can distribute load over a relatively large bonded area, but their performance depends strongly on joint geometry, surface preparation, adhesive selection, curing and the service environment. Temperature, moisture, chemicals and long-term loading may all influence joint behaviour.
Adhesive joints are generally most effective when designed to load the bond predominantly in shear rather than relying on peel or cleavage loading.
An interference fit joins components by making the mating dimensions overlap before assembly. Assembly produces contact pressure between the surfaces, allowing axial load, torque or both to be transmitted by friction.
Press fits may be assembled mechanically, while shrink or expansion methods can use controlled temperature differences to provide temporary assembly clearance.
The required interference must be selected carefully. Too little interference can permit movement or fretting, while excessive interference can produce damaging stresses, distortion or assembly difficulties.
Threaded fasteners provide one of the most versatile methods of creating removable engineering joints. Bolts, screws, studs and nuts are available in a very wide range of materials, sizes, strength classes and head forms.
In a correctly designed preloaded bolted joint, tightening establishes clamping force between the joined components. Joint behaviour therefore depends on preload, stiffness, friction and external loading rather than simply on the nominal strength of the bolt.
RoyMech provides a substantial separate section covering screw threads, bolts, preload and bolted-joint design.
Pins can be used for location, retention, articulation or load transfer. The required pin type depends on whether accurate positioning, easy removal, positive retention or load transmission is the principal function.
Dowels are commonly used to establish repeatable location between mating components, while taper pins can provide accurate self-locking location. Cotter and related pins are often used primarily as retaining devices.
Keys and splines are commonly used to transmit torque between shafts and hubs. A key provides local engagement between matching keyways, while a spline distributes engagement around a number of teeth or projections.
The selection should consider transmitted torque, shaft and hub strength, stress concentration, axial location, manufacturing method and whether the assembly must slide axially during operation or maintenance.
Circlips and retaining rings provide axial retention of components on shafts or within bores. They are compact and convenient where a component must be retained but subsequently removable.
The groove and surrounding component must be designed to withstand the required axial load. Installation and removal access should also be considered during assembly design.
The joining process should be selected as part of the overall assembly design. Important considerations include:
Engineering assemblies do not necessarily rely on a single joining principle. Different methods can perform different functions within the same joint.
Dowels may provide accurate location while bolts provide clamping force. An adhesive may provide load distribution while mechanical fasteners provide positioning or retention. A key may transmit torque while a nut, circlip or shoulder provides axial location.
Understanding the function allocated to each feature can often produce a simpler and more reliable assembly than requiring one fastener to perform every function.
The following external resources were included on the original RoyMech page and provide further information relating to fasteners, assembly and joining technologies.