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.
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
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.
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 X 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.
A highly skilled mechanic can out-perform a torque wrench by sensing when the bolt starts to stretch (ref. Machinery's Handbook).
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.
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.
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)
- lock washer
- thread adhesive
- lock wire, castleated nuts/capscrews (common in the aircraft industry)
See also
External links
Last updated: 10-23-2005 05:07:38
