Bolted joint

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 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).

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 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. 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 reduced 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.

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.

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.

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.

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.

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.

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

Property class

There are many different property classes (grades) of bolts and nuts. The most common are listed below. Note that each nut property class listed can support the bolt proof strength load of the bolt it is listed beside without stripping.

Bolt property class	Material	Proof strength	Tensile yield strength, min.	Tensile ultimate strength, min.	Bolt marking
ISO, per ISO 898-1
5.8	Low or med. carbon steel	380 MPa	420 MPa	520 MPa	5
8.8	Med. carbon steel Q&T	580 MPa	640 MPa	800 MPa	8
10.9	Alloy steel Q&T	830 MPa	940 MPa	1040 MPa	10
SAE, per SAE J429
2	Low or med. carbon steel	55 ksi	57 ksi	74 ksi	2
5	Med. carbon steel Q&T	85 ksi	92 ksi	120 ksi	5
8	Alloy steel Q&T	120 ksi	130 ksi	150 ksi	8

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. Something that is not considered structural failure, but nevertheless is becoming a modern annoyance in new buildings is bolt banging.

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, leading to a loss of clamp load and failure of the joint.

Corrosion and exceeding the shear stress limit are other modes of failure.

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, castellated nuts/capscrews (common in the aircraft industry)

External links

References

Jack A. Collins Mechanical Design of Machine Elements and Machines p. 481

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