Mechanica Technical Solutions
Home | Contact | About
Manufacturing Processes > Joining processes > Mechanical fastening

 

Mechanical fastening

 

Process description

  • A mechanical fastening system is a separate device or integral component feature that will position and hold two or more components in a desired relationship to each other. The joining of parts by mechanical fastening systems can be generally classified as:
  • Permanent: can only be separated by causing irreparable damage to the base material, functional element or characteristic of the components joined, for example, surface integrity. A permanent joint is intended for a situation where it is unlikely that a joint will be dismantled under any servicing situation.
  • Semi-permanent: can be dismantled on a limited number of occasions, but may result in loss or damage to the fastening system and/or base material. Separation may require an additional process, for example, plastic deformation. A semi-permanent joint can be used when disassembly is not performed as part of regular servicing, but for some other need.
  • Non-permanent: can be separated without special measures or damage to the fastening system and/or base material. A non-permanent joint is suited to situations where regular dismantling is required, for example, at scheduled maintenance intervals.

Materials

  • Can join most materials and combinations of materials using various processes. Metals, plastics, ceramics and wood are commonly joined.
  • Fastening elements made from most metal alloys such as ferrous (steel most common), copper, nickel, aluminum and titanium, depending on strength of joint and environmental requirements. Use of plastics for fastening methods common for low loading conditions.
  • Variety of coatings available for metal fasteners to improve corrosion resistance, commonly: zinc (electroplated and hot-dip), cadmium, chromate, phosphate and bluing.
Mechanical Fastening

Process variations

  • Permanent fastening systems:

    • Riveting: used to create a closed mechanical element spanning an assembly. The rivet is located through a previously created hole through the materials to be joined and then the rivet shank is plastically deformed (either hot or cold) on one side typically. Used for joining sheet materials of varying type and thickness by solid, tubular (both semi-tubular and eyelet), split, compression and explosive types.
    • Flanging: the plastic deformation of an amount of excess material exposed on one component to locate and hold it to an adjacent face of another component. Readily lends itself to full automation. Deformation can be performed through direct pressure, rotary or vibratory tool movement.
    • Staking: similar to flanging, but plastic deformation is localized to where the components are closely assembled through a punch mark in the center of a protrusion. Location of the parts is by friction and pressure at their interface. Low joint strengths.
    • Stapling: joins materials using U-shaped staples fed on strips to the head of a semi-automatic tool. Can join dissimilar materials of thin section and no hole prior to the operation is needed.
    • Stitching: similar to stapling, but the stitching is made by the machine itself into a U-shaped form.
    • Crimping: a pressure tight joint is created on thin section assembled components by localized plastic deformation at dimple points, by swaging or shrinkage. Also notching which shears and bends the same portion of the assembled parts to maintain location.
    • Seaming: creation of a pressure tight joint in sheet-metal assemblies by hooking together two sheets through multiple bends and pressing down the joint area. Joint strength and integrity can be further improved by soldering, adhesive bonding or brazing.
    • Nailing: uses the friction between a nail and the pierced materials to maintain location of the parts. Typically used for joining wood to wood, or wood to masonry.
  • Semi-permanent fastening systems:

    • Snap fits: integral features of the components to be joined typically hooked tabs which lock into notches on the adjacent part to be assembled with the application of a modest force. Commonly used for large volume production of plastic assemblies. Require special design attention to determine deflections and dimensional clearances.
    • Press fits: use of the negative difference in dimensions (or interference) on the components to impart an interface pressure through the force for assembly.
    • Shrink fits: use of the negative difference in component dimensions to impart an interface pressure on assembly by heating one component (usually the external) causing expansion and then allowing it to cool and contract in situ.
    • Blind rivets: located into a previously created hole in the assembly from a single direction using a special tool. The tool retracts a headed pin from the rivet body deforming it enough to hold the components. The head is left inside the rivet body on joint completion. Used for thin sheet material fabrication.
  • Non-permanent fastening systems:

    • Retaining rings: provide a removable shoulder within a groove of a bore or on the surface of a shaft to locate and lock components assembled to it. Presented either axially, radially or pushed into the groove using special tools. Self-locking, circlip, E-clip and wireformed types available for various applications. Made from spring steel typically. . Self-tapping screws: for assembling thin sheet material by passing a large pitch screw through previously created holes in the parts. Also self-drilling and thread forming types for soft materials.
    • Quick release mechanisms: for rapid securing and release of parts, e.g. doors, access panels, tooling jigs and fixtures. Various types available, such as clips, locks, latches, cams, clamps and quarter turn fastening systems.
    • Pins: for locating and retaining collars, hubs, gears and wheels on shafts, or to act as pivots in machinery or stops. Various types available, such as taper, spring, grooved, split and cotter.
    • Tapered and gib-head keys: for locating and holding gears, wheels and hubs on shafts through friction.
    • Magnetic devices: for locating or holding items such as doors and work holders for machine tools. Can be permanent type, mechanically or electrically actuated. Parts must be ferrous, nickel or cobalt based if direct magnetic attraction is required.
    • Threaded fastening systems: includes a number of standard thread forms and pitches. Variety of drive types (hexagonal head, socket head, slotted head), washers (plain, spring, double coil, toothed locking, crinkle, tab), nuts (plain, thin, nyloc, castle nut), locking mechanisms (split pin, lock plate, wiring), and bolt, screw, stud and set screw configurations.
    • Anchor and rag bolts: used for fixing structural sections and fabrications to concrete.
    • Threaded inserts: for use in brittle and flexible materials such as ceramics and plastics. Can be molded or cast in situ or inserted in previously threaded holes. Also Helicoil wire thread inserts for protecting and strengthening previously tapped threads.
    • Collets: for locating gears, hubs and wheels on shafts through friction mechanisms. Various types, such as expanding, taper and Morse.
    • Zips, studs, buttons, plastic tie-wraps, wire and Velcro are all very useful non-permanent fastening systems which have from time to time been used in engineering assemblies, particularly the last three.
  • All mechanical fastening systems can be manually or semi-automatically performed during assembly or installation, however, not all fastening systems readily lend themselves to full automation.

Economic considerations

  • High production rates possible depending on the fastening system and degree of automation. Also dependent on time to ‘open’ and ‘close’ fastening system.
  • Economical for very low production runs.
  • All production quantities viable.
  • Regular use of same fastening system type on an assembly more cost effective than the use of many different types.
  • A smaller number of large fasteners may be more economical than many small ones.
  • Consideration must be given to fastener replacement costs for maintenance or service requirements.
  • Tooling costs low to moderate depending on degree of automation.
  • Equipment costs low.
  • Direct labor costs low to moderate.
  • Cost and skill of joint preparation can be high.
  • Finishing costs very low. Usually no finishing is required.
  • Little or no scrap, except where hole generation concerned.

Typical applications

  • Structures for buildings and bridges
  • Automotive, aerospace, electrical and marine assemblies
  • Domestic and office appliances
  • Machine tools
  • Pipework and ducting
  • Furniture
  • Clothing

Design aspects

  • Applicable to all levels of design complexity.
  • Identification of possible failure modes (tension, shear, bearing, fatigue) and calculation of stresses in the fastener at the design stage recommended in joints subjected to high static, impact and/or fluctuating loads.
  • Examination of the stresses in the joint area under the fastener important to determine the load bearing capability and stiffness of the parts to be joined.
  • Use of recommended torque values for bolted connections critical for obtaining correct preloads and should be indicated on assembly drawings.
  • Differentials in thermal expansion must be taken into consideration when using a fastener of different material to that of the base material.
  • Provision for anti-vibration mechanisms in the fastening system where necessary, e.g. Nyloc, lock nuts in combination with split pins, spring washers.
  • The damping characteristics of the assembled product must be considered when using a specific fastening system with fluctuating loads.
  • Can incorporate pressure tight seals with most bolted joints, e.g. gaskets.
  • Try to use standard fastener sizes, lengths and common fastening systems for a product.
  • Keep the number of fasteners to a minimum for economic reasons.
  • Design for the easy disassembly and maintenance of non-permanent fasteners, i.e. provide enough space for spanners, sockets and screwdrivers.
  • Placing fasteners too close to the edge of parts or too close to each other avoided because of assembly difficulty and reduced strength capacity, i.e. pull out and rupture.
  • Maximum operating temperatures of mechanical fastenings approximately 700°C using nickelchromium steel bolts.
  • When joining plastics it is good practice to use metal threaded inserts or plastic fasteners.
  • Minimum section thickness =0.25 mm.
  • Maximum section thickness, typically =200 mm.
  • Unequal section thicknesses commonly joined.

Quality issues

  • Galvanic corrosion between dissimilar metals requires careful consideration, e.g. aluminum and steel.
  • There is a risk of damage to joined parts or fasteners when using permanent systems or nonpermanent fasteners that have been disassembled many times.
  • Stress relaxation can cause the joint to loosen over time (especially in high temperature operating conditions over long periods). Subsequent re-torquing is recommended at regular intervals. This should be written into the service requirements for critical applications.
  • High temperature applications in combination with harsh environments accelerate creep and fatigue failure.
  • Rolled threads on bolts and screws are preferred over machined threads due to improved strength and surface integrity.
  • Variations in flatness and squareness of abutment faces in assemblies can affect joint rigidity, corrosion resistance and sealing integrity.
  • Variations in tolerances and accumulations of tolerances can result in mismatched parts and cause high assembly stresses. Dissimilar materials will also cause additional stresses, if reactions to the assembly environment result in unequal size changes.
  • Variation in bolt preload is dependent on degree of automation of torquing method and frictional conditions at the component interfaces. Both should be controlled wherever possible.
  • Lubricants and plate finishes on fasteners can help reduce torque required and improve corrosion resistance.
  • Hydrogen embrittlement in electroplated steel fasteners can be problematic and accelerates failure.
  • Stress concentrations in fastener and joint designs should be minimized by incorporating radii, gradual section changes and recesses.
  • Hole size and preparation (where required) is important. Holes can act as stress concentrations. Fatigue life can be improved by inducing compressive residual stresses in the hole, e.g. by caulking.
  • Reliability of joint and consistency of operation are improved with automation generally. Can be highly reliant on operator skill where automation not feasible.
  • Fabrication tolerances are a function of the accuracy of the component parts and the fastening system used.

costing