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Encyclopedia > Bolted joint

There are three kinds of fastener given the name bolt.

  • Nowadays, the huge majority are screw bolts. Most of this article is about using those.
  • The thread of a screw can be seen as a wedge wrapped around a rod. The cotter bolt or pin bolt has a tapered hole forged through the end away from the head and into it is knocked a tapered pin or cotter. The matched tapers are in effect, a straight thread. In conjunction with a washer, this draws the bolt tight as a nut does on a screw bolt.
  • The oldest form of bolt is a clench bolt or clinch bolt, (see Clinker (boat building)). For a shipwright's use, it is of copper. A head is formed by clenching it over a washer in a swage plate. It is then knocked through a hole bored through the work to be fastened. The head is held up with a dolly and the other end is formed over a washer, in the same way as the head but without the swage plate. Until the nineteenth century was well advanced, this is what held the great ships of the world together, though some may have used iron. Until about 1960, the centre-line assembly of British Admiralty twenty-five foot motor cutters was fastened this way.
 Bolted joints
Bolted joints

Bolted joints are one of the most common elements of machine design. They consist of cap screws or studs that capture and join other parts, and are secured with the mating of threads. This article refers to the threaded fastener. ... Technically a portable double inclined plane, a wedge is a simple machine used to separate two objects, or portions of objects, through the application of force, perpendicular to the inclined surfaces, developed by conversion of force applied to the blunt end. ... Assorted washers: flat, split, star and insulated A washer is a thin disk with a hole, usually in the middle. ... Clinker boat building is a method of constructing hulls for boats and ships by fixing wooden planks to a frame so that the planks overlap each other gaining support from the frame and from adjacent planks. ... Swaging is a metal forming technique in which the metal is plastically deformed to its final shape using high pressures. ... Dolly can refer to several things: dolly, a familiar term for a childs toy Dolly the sheep, the first cloned mammal Dolly Parton, American actress and singer Hello, Dolly, a 1969 musical Steadicam, Film-making equipment camera dolly, Film-making equipment Motion compensation, camera motion, back and forth movement... 1960 was a leap year starting on Friday (link will take you to calendar). ... Old Admiralty House, Whitehall, London, Thomas Ripley, architect, 1723-26, was not admired by his contemporaries and earned him some scathing couplets from Alexander Pope The Admiralty was historically the authority in the United Kingdom responsible for the command of the Royal Navy. ... Bolted joint This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... Bolted joint This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This article refers to the threaded fastener. ...


There are two main types of bolted joint designs. In one method the bolt is tightened to a calculated torque, producing a clamp load. The joint will be designed such that the clamp load is never overcome by the forces acting on the joint (and therefore the joined parts see no relative motion). The concept of torque in physics, also called moment or couple, originated with the work of Archimedes on levers. ... In physics, as defined by Asimov, a force is that which can impose a change of velocity on a material body. ...


The other type of bolted joint does not have a designed clamp load but relies on the shear strength of the bolt shaft. This may include clevis linkages, joints that can move, and joints that rely on locking mechanism (like lock washers, thread adhesives, and lock nuts). In physics and mechanics, shear refers to a deformation that causes parallel surfaces to slide past one another (as opposed to compression and tension, which cause parallel surfaces to move towards or away from one another). ... An adhesive is a compound that adheres or bonds two items together. ... A nut is a type of hardware fastener with a threaded hole. ...

Contents


Theory

Clamp load

The clamp load of a cap screw is created when a torque is applied, and is generally a percentage of the cap screw's proof strength. Cap screws are manufactured to various standards that define, among other things, their strength and clamp load. Torque charts are available that identify the required torque for cap screws based on their grade.


When a cap screw is tightened it is stretched, and the parts that are captured are compressed. The result is a spring like assembly. External forces are designed to act on the parts that have been compressed, and not on the cap screw.


The result is a non-intuitive distribution of strain; in this engineering model, as long as the forces acting on the compressed parts do not exceed the clamp load, the cap screw doesn't 'see' any increased load. Imagine a compressed spring, captured by a cap screw. If an external force tries to separate the joint (acting on the spring), the spring will not budge unless the external force exceeds its force of compression, and if it does not budge, then the cap screw will not see any of the external force.


Image File history File links Bolted joint spring analogy Source: me, (created with Sodipodi); Pud 08:44, 27 Aug 2004 (UTC) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...


This is a simplified model. In reality the bolt will see a small fraction of the external load prior to it exceeding the clamp load, depending on the joint's geometry.


Thread strength

Nut threads are designed to support the rated clamp load of their respective bolts. If tapped threads are used instead of a nut, then their strength needs to be calculated. Steel hardware into tapped steel threads require a depth of 1.5× thread diameter to support the full clamp load. A thread is a large number of very thin fibers spun together, used in the manufacture of textiles and in sewing. ... This article refers to the threaded fastener. ...


If an appropriate depth of threads are not available, or they are in a weaker material than the cap screw, then the clamp load (and torque) needs to be de-rated appropriately.


Threads are usually created on a thread rolling machine. They may also be cut with a lathe, tap or die. Rolled threads are about 40% stronger than cut threads. This article is about a lathe as a tool. ...


Setting the torque

Engineered joints require the torque to be accurately set. The clamp load produced during tightening is about 75% of the fastener's proof load. Over tightening will damage threads and stretch the bolt, ruining the joint's strength, see Hooke's law. In physics, Hookes erection while having a bj of elasticity states that if a force (F) is applied to an elastic spring or prismatic rod (with length L and cross section A), its extension is linearly proportional to its tensile stress σ and modulus of elasticity (E): ΔL = 1...


If the hardware is Cadmium plated, or lubricated (or both) the torque is reduced by 15-25% to achieve the same clamp load. Specialty coatings exists that allow for a reduction of 50% in torque (compared to non-plated, non-lubricated hardware) to achieve the designed clamp load. General Name, Symbol, Number cadmium, Cd, 48 Chemical series transition metals Group, Period, Block 12, 5, d Appearance silvery gray metallic Atomic mass 112. ...


Torquing the bolt is notoriously inaccurate. Even with a calibrated Torque wrench large errors are caused by dirt, surface finish, lubrication, etc ... The turn of the nut method is more accurate, but requires additional calculations and tests for each application. A torque wrench is a device used to precisely set the torque of a fastening such as a nut or bolt. ...


There are more expensive tools for accurate torque setting, like ultrasonic meters, but they are out of reach of most shops.


A highly skilled mechanic can out-perform a torque wrench by sensing when the bolt starts to stretch (ref. Machinery's Handbook). Machinerys Encyclopedia, 1917 Boiler, Machinerys Encyclopedia, 1917 Machinerys Handbook for machine shop and drafting-room; a reference book on machine design and shop practice for the mechanical engineer, draftsman, toolmaker, and machinist (the full title of the 1st edition) is a classic reference work in mechanical engineering...


Grades

There are many different grades of cap screws. The most common are listed below.

Grade Material Tensile Strength Yield Strength Proof Load
SAE
2 Low or Med. Carbon Steel 74,000 psi 57,000 psi 55,000 psi
5 Med. Carbon steel Q&T 120,000 psi 92,000 psi 85,000 psi
8 Alloy steel Q&T 150,000 psi 130,000 psi 120,000 psi
Metric
5.8 Low or Med. Carbon Steel 520 MPa 420 MPa 380 MPa
8.8 Med. Carbon steel Q&T 830 MPa 660 MPa 600 MPa
10.9 Alloy steel Q&T 1040 MPa (150,000psi) 940 MPa 830 MPa

Failure modes

The most common mode of failure is overloading. Operating forces of the machine produce loads that exceed the clamp load and the joint works itself loose, or fails catastrophically. Structural failure occurs when a structure breaks in such a way that it can no longer carry as much load as it could before the failure. ...


Over torquing will cause failure by damaging the threads and deforming the hardware, the failure might not occur until long afterwards. Under torquing can cause failures by allowing a joint to come loose. It may also allow the joint to flex and thus fail under fatigue.


Brinelling may occur, with poor quality washers, that leads to a loss of clamp load and failure of the joint. The Brinell scale characterises the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. ...


Corrosion and exceeding the shear stress limit are other modes of failure. Corrosion - Wikipedia /**/ @import /w/skins-1. ... Shear stress is a stress state where the shape of a material tends to change (usually by sliding forces - torque by transversely-acting forces) without particular volume change. ...


Types of bolts

  • cap screw
  • machine screw
  • stud

Locking mechanisms

Locking mechanisms keep bolted joints from coming loose. They are required where the clamp load is low or non existent, where inexpensive hardware is used, or where additional safety is warranted.

  • two nuts, tightened on each other.
  • lock nut (prevailing torque nuts)
    • polymer insert
    • oval lock
  • lock washer
  • thread adhesive
  • lock wire, castleated nuts/capscrews (common in the aircraft industry)

See also

This article refers to the threaded fastener. ... A rivetted buffer beam on a steam locomotive A rivet is a mechanical fastener consisting of a smooth cylindrical shaft with heads on either end. ...

External links


  Results from FactBites:
 
A Numerical Approach on Load Sharing Analysis and Optimization of Bolted Joint Efficiency (1646 words)
The influence of stress concentration on the efficiency of the joint was estimated numerically.
Mechanical joints require that bolt or rivet holes are drilled into the composite that reduced the net cross sectional area of the structure and introduce localized stress concentration.
Joint efficiency is the ratio of the gross section stress at failure to the strength of the laminate in the gross section.
  More results at FactBites »

 
 

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