A pressurized bolted joint assembly is beginning to leak, creating an unsafe situation. A rotor and its blades split off from the nacelle before spinning through a wind generator and then crashes onto the ground.
With constant vibrations from the ocean freighter’s engine, nuts bolt on a massive part of the mining machinery to free themselves from the joints bolted together and then move around the hull, damaging the machinery.
What Causes Loose Bolts?
Bolted joints are crucial to ensure the safety of a variety of equipment used in a wide variety of uses, such as manufacturing, power generation, mining, and transportation.
In a bolted joint, tightening the bolt causes the bolt to stretch a tiny amount, much like pulling on the stiff spring. The stretching, known as tension, creates an opposing force in the clamp that holds the two segments of the joint. When the bolt is loose, the clamping force will weaken.
The long coach bolts are more than an inconvenience. If the joint isn’t fast enough tightened, the bolt might start to leak gas or fluid. The bolt could break, or the equipment could be damaged, or even catastrophic accidents could occur.
There are at minimum five possible causes for loose bolts that can happen in combination.
In essence, an untightened bolt is loose, and the joint doesn’t contain enough strength to secure each section together.
This can cause an unintentional slippage between sections and put unnecessary shear tension on the bolt, which could cause it to break eventually.
Tests on joints with bolts under vibration reveal that a variety of tiny “transverse” movements cause the two joint sections to move in a parallel manner with one another and both the bolt’s head and the nut.
These repeated movements of cutting threaded rod threaten tension between threads of the bolt and the joint that hold the joint. Then, vibrations result in the bolt “unwinding” from the mating threads, and the joint will lose its clamping force.
The engineers responsible for determining the tension of bolts permit a break-in phase that occurs when the bolt’s tightness decreases to a certain extent.
This is due to the micro-embedding of the bolt’s head and nut on the joint’s surface. It is possible with soft materials, like composites and polished, hard metals.
If the joint hasn’t been properly designed or if the required tension was not met at the beginning, the embedding of the joint may result in a loss of the clamping force.
Many joints with bolts include an extremely thin and flexible gasket between the head of the bolt and the outside of the joint. This gasket is used to protect the joint from leaks of liquid or gas.
The gasket acts as springs, pushing back against the force from the bolt and its surface. As time passes, particularly close to high temperatures or chemical corrosive substances, the gasket could “creep,” which means it loses its springiness which can cause a loss of clamp force.
Differential Thermal Expansion.
Suppose the materials of the joint and the bolt differ. In that case, huge variations in temperature caused by extreme environmental changes or industrial processes could cause the bolt’s material to contract or expand quickly, resulting in the bolt becoming loose.
In the event of alternating or dynamic loads, such as those generated by generators, machines, wind turbines, generators, and wind turbines, can trigger mechanical shock.
It is an abrupt force applied to the joint or the bolt which causes the threads of the bolt to move concerning the joint’s threads. Like vibrations, the slippage could eventually result in the loosening of bolts.
Steps to Prevent Loose Bolts
Here are five fundamental kinds of prevention techniques:
Washers tend to be larger than the bolt head and the extra surface creates friction in the joint, ensuring the force of clamping.
Conical, also known as bolt with washers, are cup-shaped washers that are less effective than spring washers in vibration tests.
This could cause permanent harm to the joint’s finish or the surface, which could be unacceptable for critical aerospace situations where surface imperfections could result in fatigue stress. It could also prevent the tightening of the joint to the correct tension.
Wedge-locking washers operate in two sets, with each washer featuring opposite-facing wedges that interact with each other and the surfaces of the joint and the nut to stop the self-turning of the bolt.
Locking fastener systems come with an elongated flat retainer that is like a washer, along with a clip inserted into the groove of the bolt’s head.
Tab washers come with two tabs that are opposite, that fold inwards to hold the bolt head in place or nut once installed, and they may also have teeth that can penetrate the joint’s surface to keep it in place.
Although these small nuts and bolts keep the bolt from falling off, the bolt usually does not assist the joint in keeping the specified clamping force.
Torque Nuts With A Predetermined Torque.
Inserts of metal or nylon inside the nuts (sometimes called lock nuts) can create additional friction and prevent loosening.
Another option is to put an internal spring within the nut. Nylon inserts aren’t suitable in high heat or harsh chemical applications.
Since the inserts on many lock nuts cover only some of the threads inside, an intense transverse motion or shock may result in the bolt self-loosening.
Modern applications use two nuts with different thread sizes that advance at different speeds on a double-threaded bolt. So transverse motions which could result in one nut moving do not affect the other one.
Liquid adhesives and heated thermoplastic coatings, or set screws with adhesive, have been successfully used to ensure that bolts in specific applications don’t break. The issue is that they make it more difficult to take the joint apart later.
Maintaining Proper Tension Ensures Bolts Stay Tight
A combination of well-designed hex set screws with a well-designed clamp force development and the right bolt retention tools can effectively protect a bolted joint from all the problems mentioned in this article.