Bolted joint

Bolted joints are one of the most common elements in construction and machine design. They consist of fasteners that capture and join other parts, and are secured with the mating of screw threads.

There are two main types of bolted joint designs. In one method the bolt is tightened to a calculated clamp load, usually by applying a measured torque 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).

This type of joint design provides several properties:

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 a locking mechanism (like lock washers, thread adhesives, and lock nuts).

Contents

Theory

The clamp load, also called preload, of a fastener is created when a torque is applied, and is generally a percentage of the fastener's proof strength; a fastener is manufactured to various standards that define, among other things, its strength and clamp load. Torque charts are available to identify the required torque for a fastener based on its property class or grade.

When a fastener is tightened, it is stretched and the parts being fastened are compressed; this can be modeled as a spring-like assembly that has a non-intuitive distribution of strain. External forces are designed to act on the fastened parts rather than on the fastener, and as long as the forces acting on the fastened parts do not exceed the clamp load, the fastener is not subjected to any increased load.

However, this is a simplified model that is only valid when the fastened parts are much stiffer than the fastener. In reality, the fastener is subjected to a small fraction of the external load even if that external load does not exceed the clamp load. When the fastened parts are less stiff than the fastener (soft, compressed gaskets for example), this model breaks down; the fastener is subjected to a load that is the sum of the preload and the external load.

In some applications, joints are designed so that the fastener eventually fails before more expensive components do. In this case, replacing an existing fastener with a higher strength fastener can result in equipment damage. Thus, it is generally good practice to replace old fasteners with new fasteners of the same grade.

Thread engagement

Thread engagement is the length or number of threads that are engaged between the screw and the female threads. Screws are designed so that the shank fails before the threads, but for this to hold true, a minimum thread engagement must be used. The following equation defines this minimum thread engagement:[2]

L_e = \frac{2 \times A_t}{0.5 \pi \left( D - 0.64952 p \right)}

Where Le is the thread engagement length, At is the tensile stress area, D is the major diameter of the screw, and p is the pitch. This equation only holds true if the screw and female thread materials are the same. If they are not the same, then the following equations can be used to determine the additional thread length that is required:[2]

J = \frac{\text{tensile strength of external thread material}}{\text{tensile strength of internal thread material}}
L_{e2} = J \times L_e

Where Le2 is the new required thread engagement.

While these formulas give absolute minimum thread engagement, many industries specify that bolted connections be at least fully engaged. For instance, the FAA has determined that in general cases, at least one thread must be protruding from any bolted connection. [1]

Setting the torque

Engineered joints require the torque to be accurately set. Setting the torque for fasteners is commonly achieved using a torque wrench.[3] The required torque value for a particular fastener application may be quoted in the published standard document or defined by the manufacturer.

The clamp load produced during tightening is higher than 75% of the fastener's proof load.[3] To achieve the benefits of the preloading, the clamping force must be higher than the joint separation load. For some joints, multiple fasteners are required to secure the joint; these are all hand tightened before the final torque is applied to ensure an even joint seating.

The torque value is dependent on the friction produced by the threads and by the fastened material's contact with both the fastener head and the associated nut. Moreover, this friction can be affected by the application of a lubricant or any plating (e.g. cadmium or zinc) applied to the threads, and the fastener's standard defines whether the torque value is for dry or lubricated threading, as lubrication can reduce the torque value by 15% to 25%; lubricating a fastener designed to be torqued dry could over-tighten it, which may damage threading or stretch the fastener beyond its elastic limit, thereby reducing its clamping ability.

Also, if the fastener rather than its associated nut is torqued, then the torque value should be increased[4] to compensate for the additional friction; fasteners should only be torqued if they are fitted in clearance holes.

Torque wrenches do not give a direct measurement of the clamping force in the screw, and indeed much of the force applied is lost just to overcoming friction.

More accurate methods for setting the clamping force rely on defining or measuring the screw extension; for instance, measurement of the angular rotation of the nut can serve as the basis for defining screw extension on thread pitch.[5] Measuring the screw extension directly allows the clamping force to be very accurately calculated. This can be achieved using a dial test indicator, reading deflection at the fastener tail, using a strain gauge, or ultrasonic length measurement.

There is no simple method to measure the tension of a fastener already in place other than to tighten it and identify at which point the fastener starts moving. This is known as re-torqueing. An electronic torque wrench can be used on the fastener in question, so that the torque applied can be constantly measured as it is slowly increased in magnitude; when the fastener starts moving (that is, becoming tightened) the required torque magnitude briefly drops sharply, and this drop-off point is considered the measure of tension.

Recent developments enable tensions to be estimated by using ultrasonic testing. Another way to ensure correct tension (mainly in steel erecting) involves the use of crush-washers. These are washers that have been drilled and filled with orange RTV. When the orange rubber strands appear, the tension is correct.

Large-volume users (such as auto makers) frequently use computer controlled nut drivers. With such machines, the computer in effect plots a graph of the torque exerted. Once the torque reaches a set maximum torque chosen by the designer, the machine stops. Such machines are often used to fit wheelnuts and normally tighten all the wheel nuts simultaneously.

Failure modes

The most common mode of failure is overloading: Operating forces of the application produce loads that exceed the clamp load, causing the joint to loosen over time or fail catastrophically.

Over-torquing might cause failure by damaging the threads and deforming the fastener, though this can happen over a very long time. Under-torquing can cause failures by allowing a joint to come loose, and it may also allow the joint to flex and thus fail under fatigue.

Brinelling may occur with poor quality washers, leading to a loss of clamp load and subsequent failure of the joint.

Other modes of failure include corrosion, embedment, and exceeding the shear stress limit.

Bolted joints may be used intentionally as sacrificial parts, which are intended to fail before other parts, as in a shear pin.

Locking mechanisms

Locking mechanisms keep bolted joints from coming loose. They are required when vibration or joint movement will cause loss of clamp load and joint failure, and in equipment where the security of bolted joints is essential.

Measurement of frictional torque of threads in bolt

The torque is applied by means of suspending the weights on one end of the rope and other end is wound around the head of the fastener and tied to the projection. The amount of load is increased gradually until the fastener starts rotating. The applied load is then calculated by adding up the weights. This is the load that is required to overcome the friction between the threads. Similarly, the net applied torque is calculated by multiplying the resultant load by the radius of the fastener's head.

In another method, the torque is applied to the nut by an electromagnetic force. A specially designed gripper is used to grip the nut. A bar magnet is mounted on the gripper, and the gripper is then surrounded by a coil of wire through which alternating current is passed. As the magnetic field from the permanent magnet interacts with the field created by the coil, the permanent magnet (and thus the nut) is subjected to a torque. This is quite similar to the construction of an electric motor, and hence a motor can be directly used to provide the torque. A stepper motor can be used so that the torque is provided in steps, each of which causes a small, measurable angular displacement in the nut from which the torque can be calculated. The discrete torques can be added to get the net torque consumed in displacing the nut from one end of the fastener to the desired location. This is the torque that is required to overcome the friction between the threads.

Bolt banging

Bolt banging occurs in buildings when bolted joints slip into bearing under load, thus causing a loud and potentially frightening noise resembling a rifle shot that is not, however, of structural significance and does not pose any threat to occupants.[7]

International standards

See also

References

Notes

  1. ^ Collins, p. 481.
  2. ^ a b Minimum Thread Engagement Formula and Calculation ISO, http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm, retrieved 2010-02-08. 
  3. ^ a b Oberg et al. 2004, p. 1495.
  4. ^ AIPS 01-02-008: "Bolt Torque"
  5. ^ Oberg et al. 2004, p. 1499.
  6. ^ "The use of two nuts to prevent self loosening". boltscience.com. http://www.boltscience.com/pages/twonuts.htm. 
  7. ^ "Steel Interchange: 'Banging Bolts'", MSC: Modern Steel Construction.

Bibliography

External links