Fastener Faults

Fastener Faults

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						There
						are
						various
						other
						methods
						
						
						of
						joining
						parts
						together
						
						
						so
						that
						they
						can
						
						
						be
						easily

						assembled
						
						
						and
						dismantled.
						Below
						are
						just
						a
						few
						types
						
						
						of
						other
						fasteners

Incorrect torque: Tightening a fastener too much or too little can cause it to fail.
Stripped threads: This can happen if a fastener is over- or under-tightened, or if the wrong length fastener is used.
Corrosion: Bolts can corrode when exposed to incompatible substances like oxygen, metal products, and chemicals.
Hydrogen embitterment: This is a major cause of fastener failure, and is thought to affect steels with Rockwell hardness above C30.
Fatigue and fracture: This can happen due to improper installation, and is a significant structural problem in fastener tension joints.
Excessive application of cyclic loading: This happens when the dynamic load exceeds the fastener's clamping load.
Impact shear: This can happen in bolts loaded in single shear, such as flywheel bolts and ring gear bolts.
Excess stress on bolts: This can cause bolts to break.
A stripped bolt head, or lug nut, is when the head has rounded off. Here are some ways to remove a rounded bolt

Incorrect torque

If the torque is not applied correctly and the tension on the bolt torque is too low, varying loads will act upon the bolt and fail. Proper torque control during the tightening of bolts ensures that they are securely fastened. Loose bolts can loosen over time from constant vibrations.

Choose the right torque wrench

Calibrate the torque wrench

Set the torque value

Align the torque wrench

Apply the torque force

Ensure right torque sequence with multiple bolts eg. Cylinder head

Reset the torque wrench

Striped Screw Threads

Striped threads occur when the strength of the threaded material is exceeded by the applied forces acting on the thread, causing a shear failure. Internal as well as external threads can strip.

Main reasons for stripping screw threads

Over tightening of nuts and bolts
Not having nut or bolt square when starting
No matching pitch and diameter sizes
Dirt in threads

Damaged internal thread can be repaired by enlarging hole and up the diameter of the bolt. If this is not possible then helicoill can be used tor repair internal thread

A Helicoil, also known as a screw thread insert or helical insert, is a stainless steel wire with a diamond-shaped cross-section that can repair stripped threads in hard metals like steel, or soft materials like wood or plastic. Helicoils are used in many industries, including mechanical engineering, electronics, automotive, and aerospace. They come in two versions: Free Running and Screw lock, and are available in green or red to make them easy to identify when installed.

Corrosion

It occurs when two different types of metal are joined, and electricity flows from one to the other, leaving behind corrosion. Uniform Corrosion: This occurs across the entire surface area of a fastener when it's not properly coated or plated.

To prevent corrosion the following can be done:

Use stainless steel fasteners
Use electroplated fasteners
Use galvanised fasteners
Cover with grease or copper slip

Hydrogen embrittlement

Hydrogen embrittlement (HE) is a dangerous reaction that occurs when hydrogen diffuses into a metal, causing it to become brittle. This can happen during welding, casting, pickling, or cathodic protection. The degree of embrittlement is determined by the amount of hydrogen absorbed and the microstructure of the material.

Fatigue is a common failure of fasteners during random vibration, especially after the loss of bolt pre-tension. Fatigue causes an estimated 90% of all mechanical service failures. Fatigue cracks usually initiate in the thread roots but can also initiate under the bolt head. Often failure in this location is the result of an inadequate under head radius (resulting in a high stress concentration) or the bolt being mounted on an inclined surface.

Stripped bolt head

Striped head bolts and nuts heads normally happens if you use a wrong size spanner or a shifting spanner. The use of shifting spanners should be limited.

Here are some ways to remove a stripped bolt head:

Bolt extractor kit: These kits have cone-like tips that dig into the metal to grip the bolt.
Locking pliers: Turn the adjustment screw so the teeth grip the bolt head, then lock the pliers and turn the bolt counter clockwise.
Drill: Drill directly into the head and shank of the bolt to create a new pilot hole.
Hair dryer: Apply heat to the bolt head for one to two minutes, then let it cool.
Blowtorch: Heat the bolt for a few minutes, then let it cool. Apply penetrating oil and try to wiggle the bolt free with pliers or a wrench.
Welding: The heat from welding the bolt onto a broken fastener can break corrosion bonds between the threads.
Torx wrench: Use a similarly sized Torx wrench to remove a stripped Allen-head bolt. The star shape can bite into the corners for more purchase.
Rubber band or steel wool: Place a rubber band or steel wool over the bolt head and press a tool into it and the bolt head to provide extra grip.
Make use of a nut spliier