A Review of Reverse Threaded Bolts

If you are one of the many that enjoy doing work for yourself rather than hiring a professional, you have probably happened onto a reverse threaded bolt or two in your maintenance adventures. Reverse threaded bolts are exactly what the name implies. The threads are cut in such a way that the bolt tightens to the left in a counter-clockwise fashion, completely opposite of normally threaded bolts. This article will review common questions about reverse-threaded bolts.

Overview

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You may be wondering why anyone would bother to create a reverse-threaded bolt. If you happen onto one unexpectedly, it will probably take you a lot of time and effort trying to loosen it before finally trying to unscrew it in a clockwise manner. Bolts tend to rattle loose under vibration. Reverse-threaded bolts are used in situations where a normally threaded bolt may loosen on its own. If you find that no matter how you try to loosen the bolt in a counter-clockwise manner it seems impossible, consider turning the bolt the other way. Even if it is a normally threaded bolt, often a slight turn clockwise can dislodge the bolt enough to allow removal. If it turns easily to the right and continues turning as you apply force, it is a reverse-threaded bolt.

Review

Screw s and bolts in general are designed to deter rotational movement from linear force. In other words, pressure can be applied parallel to a screw or bolt’s length without causing rotation. This works because adequate friction exists between the surface area of the bolt and the material into which it is screwed. The reverse threaded bolt applies the same holding power, but is used in areas where rotational force may be present. Lawnmower blades are a good example of an application for reverse-threaded bolts. Because of the counter-clockwise spin of the blades, reverse-threaded, or left-handed thread, bolts are used. This has the effect of the angular momentum applied to the bolt complementing its ability to deter rotational movement and therefore, keeping your lawnmower blades from flying off.

pros

Reverse-threaded bolts can be used to deter theft. The thought behind this is simple; the time a thief spends trying to loosen the bolts often deters the person from stealing the item secured. Subways often use reverse-threaded bolts to hinder people in their attempts to remove light fixtures. In cases where counter-clockwise rotational force is present, reverse-threaded bolts will not loosen whereas, normally threaded bolts may.

cons

It is common for reverse bolts to be twisted into two pieces. If a person wrongly assumes that a bolt is normally threaded he or she may break it into pieces bolts will only take so much torque before breaking. This applies to both threading schemes. Once a bolt is broken, it is very difficult to get out. Normally, a hole must be bored to the remaining piece of the bolt and an easy out is normally applied.

Reverse threaded bolts have their purpose. When used in situations where rotational force is present, a thread that tightens in the direction of the existing force is much less likely to come loose. It is common for people to unwittingly snap the heads off reverse-threaded bolts by over tightening them when they are trying to loosen the bolt incorrectly. Luckily, easy-outs allow the broken bolt to be removed.