Why shouldn't fasteners be over-tightened?

Almost everyone has ever used fasteners such as screws and nuts, and may even damage them inadvertently. One of the most destructive methods overlooked by the research surface is over-tightening or over-twisting the fastener. This may cause the screw to peel off, the screw head to break, and the pre-tapped thread to be damaged.

Fastener installation

Installing fasteners is a simple task (usually speaking). To this end, torque is usually applied to the fastener, usually a nut or screw head, and then simply "screw in". When torque and pressure are applied to the driver, the fastener begins to rotate. Generally speaking, although there are exceptions, such as the left nut, turning it to the right will tighten, and turning it to the left will loosen ("right tighten, left loose"). When the fasteners are overtightened, problems start to appear.

An easy way to describe excessive twisting is to take a closer look at the deck screws. Most deck screws are flat head screws. This means that when properly installed, the screw head should be flush with the wood surface. As shown in the figure, if the fastener is too tight, the head will be pulled below the surface of the wood. The increased surface area of the pressed wood is sufficient to greatly increase the torque required to tighten or loosen the fastener, which may cause peeling, head breakage or thread damage, thereby affecting the functional integrity of the fastener.

When torque is applied to the fastener and the fastener has been fully tightened, most inexperienced people who use fasteners tend to think that more torque is needed to get more safety margin, so as to ensure that the fasteners are firm and believe that This will prevent the fasteners from loosening. However, this is usually not the case, and sometimes it will not only damage the fasteners but also have more chances of loosening. In order to prevent the fasteners from loosening over time due to vibration and other external factors, thread locking glue, lock washers, lock nuts or a combination of the three should be used. Click here to learn about some solutions to prevent loosening of threaded connections in the industrial field.

Things to consider when installing fasteners:

• The material of the fastener itself
•The material of the installed parts
•Thread type


Fastener material

When torque is applied to rotate the fastener, the driven part (driving groove or nut) of the fastener is subjected to huge stress. This is why it is important to use the right driver size and style on fasteners. Using the wrong size will exert uneven pressure on the groove, causing the groove to peel off or the nut to deform. Since fasteners can be made of different materials from soft metals to heat-treated hardened materials, the torque that can be applied to the fastener will depend on the material of the fastener. For example, aluminum bolts cannot withstand the same torque as grade 8 bolts.

Fastener drive head type

Fastener drive method is also very important. The following are the most common types of fastener drive, ranging from the best to the worst for torque absorption conversion capacity and slip resistance:

1. Star (Plum Blossom)
2. Hexagon socket
3. Inner Quartet
4. Pozi-Driv
5. Cross point
6. Slotting a hole

The material of the installed parts

The materials to be installed are varied, from plastic to steel, from wood to die-cast aluminum, and so on. This means that not only the fastener itself, but also the structure and strength of the material being installed are also important factors. For example, the critical value of screwing in plastic screws is much lower than that of screwing in steel screws.

In many installations, damage to the mounting hole may eventually damage the entire component. If you twist it excessively in a softer material, the threads in the hole may be damaged or fall off completely. This is very common when using plastic holes. It is usually easy to over-twist and damage the thread. To solve this problem, it is necessary to re-tap the entire thread or install a new thread using a threaded insert, and it is likely that the screw diameter needs to be increased.

Thread type

The thread type also differs when twisting the fastener. There are two basic types of threads:

•Coarse teeth

•Fine or very fine teeth

Coarse thread is a deeper but more scattered thread. This makes coarse-threaded fasteners more durable because slight damage on the threads will not prevent the threads from rotating.
A fine thread is a shallow thread, but there are more threads per unit length (for example, per inch). Their tighter, shallower structure makes them less likely to loosen, but it also means that there are more threads to hold the fastener in place. Due to these additional threads, the fastener can withstand greater torque and distribute it better evenly on the mounting material.

If the torque is too large, both types of fasteners will cause the threads to twist slightly, making it difficult to remove the fasteners when repairs are needed in the future. This distortion will also weaken and change the retaining force of the fastener.

In order to avoid this situation, a torque wrench is the best way to avoid excessive torque. A torque wrench is a wrench that sets and senses torque digitally. Once the optimal torque is reached, the clutch in the wrench will slip, preventing further tightening of the fastener. Many professional industries follow these twisting guidelines and use these tools to prevent over-tightening.