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Why You Shouldn’t Over-Torque Fasteners

Why You Shouldn’t Over-Torque Fasteners

Everyone who has ever worked with fasteners has accidentally messed one up at some point. One of the most damaging ways to do that is by over-tightening, or over torquing the fastener. This can result in stripping screws, snapping screw heads and damaging pre-tapped threading.

Fastener Torquing

Installing fasteners is an easy task (usually). To do so, you normally apply torque to the fastener, usually a nut or a screw head and simply “screw it in”. When torque and pressure is added to the driver, the fastener begins to spin. In general, although there are exceptions such as the left-hand nut, spinning to the right tightens and spinning to the left loosens (“righty-tighty, lefty-loosey”). The problems start when fasteners are driven too far, or over tightened.

Proper torquing of a flat head deck screw and an over-torqued flat head deck screw

An easy way to picture over-torquing is to take a look at deck screws. Most deck screws have a flat head style. This means when installed correctly, the screw head is supposed to be flush with the surface of the wood. As you can see in the picture to the right, if the fastener is over-tightened, the head is pulled beneath the surface of the wood. The increased surface area pushing against the wood is enough to greatly increase the required torque to tighten or loosen the fastener, which can result in stripping, snapped heads or thread damage which compromises the integrity of the fastener.

When torque is applied to a fastener and it is tightened, it will take an increased amount of torque to further tighten. Most inexperienced people working with fasteners tend to severely over-tighten fasteners thinking it will prevent them from loosening, however, this is not normally the case, and will cause damage to the fastener. To keep a fastener from loosening over time due to vibration and other external factors, a threadlocker solution, locking washer, locking nut or a combination of the three should be used.

While this seems simple enough, when torque is applied to a fastener and it is tightened, it will take an increased amount of torque to further tighten. Most inexperienced people working with fasteners tend to severely over-tighten fasteners thinking it will prevent them from loosening, this is not normally the case. To keep a fastener from loosening over time due to vibration and other external factors, a threadlocker solution, locking washer, locking nut or a combination of the three should be used.

Things to Consider When Torquing Fasteners:

  • Fastener Materials
  • Installation Materials
  • Thread Type

Fastener Materials

When torquing a fastener, the driven portion of the fastener – drive recess or nut – is put under a tremendous amount of stress. This is why it is crucial to use the proper drive size and style on the fastener. Using the wrong size will place an uneven pressure on the recess resulting in a stripped recess or a rounded nut. Since fasteners can be made from different materials ranging from soft metals to heat-treated hardened ones, the torque that can be applied to the fastener will depend on the material the fastener is made out of. For example, an aluminum bolt will not be able to take nearly as much torque as a Grade 8 bolt.

Fastener Drives

The fastener drive style will also matter. Below are the most common fastener drive styles listed from best to worst in terms of torque-taking ability and resistance to stripping:
Drive Styles: Slotted | Phillips | Square | Hex | Star

  1. Star (Torx)
  2. Internal Hex
  3. Robertson (Square)
  4. Pozi-Driv
  5. Phillips
  6. Slotted

Installation Materials

Installation materials can range from plastic all the way to steel which means not only does the torque the fastener can handle matter, the torque the material threading can handle also matters. Torquing a screw in plastic will have a much lower threshold then torquing a screw in steel.

In many installations, ruining the installation hole can end up ruining an entire build. By over-torquing in a softer material, the tapped threading in the hole can be damaged or stripped entirely. This is very common when working with plastic holes. It is generally very easy to over-torque and destroy the threading. To fix this, new threads need to be installed either by re-tapping the whole, or using a threaded insert and more than likely, the diameter of the screw will also need to be increased.

Thread Type

Thread type can also make a difference when it comes to torquing fasteners. There are two basic types of threading:

  • Coarse
  • Fine

Coarse Threading is a deeper but more spread out threading. This makes coarse threaded fasteners more durable because light marring on the threading won’t prevent the threads from spinning.

Fine Threading is a shallower threading but with many more threads per inch. Their tighter and shallower structure makes them less likely to be vibrated loose, but it also means there are more threads holding the fastener in place. Due to these extra threads, the fastener can withstand more torque and distribute it better on the installation material’s threading.

Both types, if torqued too much can cause the threading to slightly warp making it very difficult to remove the fastener later. That warping also weakens and changes the holding power of the fastener.

Torque WrenchThe Best Way to Avoid Over-Torquing

For most DIY projects the best way to avoid over-torquing is just to practice. With practice, knowing when to stop torquing will become second nature.

A torque wrench is a wrench that digitally sets and senses the torque. Once the optimal torque is reached, the clutch inside the wrench will slip preventing the fastener from being tightened further.  Many professional industries follow these torquing guidelines and use these tools to prevent over-tightening.

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