Bolted joint Totally Explained

Everything about Bolted Joint totally explained

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 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 other type of bolted joint doesn't 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).

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.
   When a cap screw is tightened it's 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 don't exceed the clamp load, the cap screw doesn't 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 doesn't 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 won't 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.

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 requires a depth of 1.5× thread diameter to support the full clamp load.
If an appropriate depth of threads isn't available, or the threads are in a weaker material than the cap screw, then the clamp load (and torque) needs to be derated 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.

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 dependant 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 (for example 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're 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 don't 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 using a dial test indicator, reading deflection at the bolt tail, using a strain gauge or ultrasonic length measurement.

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