Preload (engineering)

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.

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 is not 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 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 ^[1] 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^[2] 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.

Notes

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. The first number in the bolt property class indicates the nominal tensile strength, and the second number the yield stress as a proportion of the tensile strength. In other words class 8.8 means tensile strength of 800 MPa and proof stress of 0.8 x 800 MPa = 640 MPa.^[1]

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.

Types of bolts

Bolted joints in an automobile wheel. Here the outer four screws are studs that project through the brake drum and wheel, while nuts with conical locating surfaces secure the wheel. The central nut (with cotter key) secures the wheel bearing to the steering spindle. Other configurations use a bolt into threaded holes in the axle end or brake drum.

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 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 till 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.^[2]

International Standards

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