Bolted joint
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Bolted joints are one of the most common elements in construction and machine design. They consist of cap screws or studs 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).
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).
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Theory
The clamp load, also called preload, 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 property class or 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 does not see any increased load. This model is only valid when the members under compression are much stiffer than the capscrew.
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 compressed parts' stiffness with respect to the hardware's stiffness.
The results of this type of joint design are:
- Greater preloads in bolted joints reduce the fatigue loading of the hardware.
- For cyclic loads, the bolt does not see the full amplitude of the load. As a result, fatigue life can be increased or, if the material exhibits an endurance limit, extended indefinitely.
- As long as the external loads on a joint don't exceed the clamp load, the hardware doesn't see any motion and will not come loose (no locking mechanisms are required).
In the case of the compressed member being less stiff than the hardware (soft, compressed gaskets for example) this analogy doesn't hold true. The load seen by the hardware is the preload plus the external load.
In some applications joints are designed so that the screw or bolt will intentionally fail before more expensive components. 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 fasteners with the same grade originally installed.
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 will fail before the threads, but for this to hold true a minimum thread engagement must be used. The following equations defines this minimum thread engagement:
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 not then the following equations can be used to determine the addition thread length required:
Where Le2 is the new required thread engagement.
Setting the torque
Engineered joints require the torque to be accurately set. Setting the torque for cap screws is commonly achieved using a torque wrench. The required torque value for a particular screw 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. To achieve the benefits of the pre-loading, the clamping force in the screw must be higher than the joint separation load. For some joints a number of screws 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 between the threads and beneath the bolt or nut head, this friction can be affected by the application of a lubricant or any plating (e.g. cadmium or zinc) applied to the screw threads. The screw standard will define whether the torque value is for a dry or lubricated screw thread. If a screw is torqued rather than the nut then the torque value should be increased to compensate for the additional friction - screws should only be torqued if they are fitted in clearance holes.
Lubrication can reduce the torque value by 15 – 25%, so lubricating a screw designed to be torqued dry could over tighten it. Over tightening may cause the bolt to fail, it could damage the screw thread or stretch the bolt. A bolt stretched beyond its elastic limit may no longer adequately clamp the joint.
Torque wrenches do not give a direct measurement of the clamping force in the screw - much of the force applied is lost in overcoming friction. Factors affecting the tightening friction: dirt, surface finish, lubrication, etc. can result in a deviation in the clamping force.
More accurate methods for setting the screw clamping force rely on defining or measuring the bolt extension. The screw extension can be defined by measuring the angular rotation of the screw (turn of the nut method) which gives a screw extension based on thread pitch. 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 bolt tail, using a strain gauge or ultrasonic length measurement.
There is no simple method to measure the tension of a bolt already in place other than to tighten it and identify at which point the bolt starts moving. This is known as re-torqueing. An electronic torque wrench is used on the bolt under test, and the torque applied is constantly measured. When the bolt starts moving (tightening) the torque briefly drops sharply - this drop-off point is considered the measure of tension.
Recent developments enable bolt tensions to be estimated by using ultrasonic testing. Another way to ensure correct bolt 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 will 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 and the joint works itself loose, or fails catastrophically.
Over torquing will cause failure by damaging the threads and deforming the hardware, the failure might not occur until long afterward. 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, leading to a loss of clamp load and failure of the joint.
Corrosion, embedment and exceeding the shear stress limit are other modes of failure.
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.
- two nuts, tightened on each other.
- Locknut (prevailing torque nuts)
- lock washer
- thread adhesive
- lock wire, castellated nuts/capscrews (common in the aircraft industry)
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 bolt and tied to the projection. The amount of load is increased gradually till the bolt 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 bolt head radius.
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 two sides of the gripper. Externally a coil is wound in which AC (alternating current) current is passed. As the magnetic field from the permanent magnet interacts with the field created by the coil, a torque is generated which would try to rotate the magnet, thus rotating the nut. This is quite similar to the construction of the motor, and hence a motor can be directly used to provide the torque. Stepper motor can be used so that the torque is provided in steps, as desired, each time giving a small angular displacement. The torque provided by the motor can be known at each discrete angular displacement of Δθ. The process is repeated until the nut has traversed to the desired length of the bolt. The discrete torques can be added to get the net torque consumed in displacing the nut from one end of the bolt to the desirable point. This is the torque that is required to overcome the friction between the threads.
Bolt banging
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Please help improve this article by expanding it. Further information might be found on the talk page. (September 2008) |
Bolt banging occurs in buildings when structural members that are bolted together slip.
International standards
See also
References
Notes
- ^ Collins, p. 481.
- ^ a b Minimum Thread Engagement Formula and Calculation ISO, http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm, retrieved 2010-02-08.
- ^ a b Oberg et al. 2004, p. 1495.
- ^ AIPS 01-02-008: "Bolt Torque"
- ^ Oberg et al. 2004, p. 1499}.
Bibliography
- Collins, Jack A.; Staab, George H.; Busby, Henry R. (2002), Mechanical Design of Machine Elements and Machines, Wiley, ISBN 0471033073.
- Oberg, Erik; Jones, Franklin D.; McCauley, Christopher J.; Heald, Ricardo M. (2004), Machinery's Handbook (27th ed.), Industrial Press, ISBN 978-0831127008.
External links
- Bolts - AISC | Home
- AISC THE BANGING BOLT SYNDROME
- AISC BANGING BOLTS— ANOTHER PERSPECTIVE
- Threaded Fasteners - Tightening to Proper Tension, US Department of Defense document MIL-HDBK-60, 2.6MB pdf.
- NASA Reference Publication 1228 Fastener Design Manual
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