The Experts: Why use wedge-locking instead of friction based?

Q: What is the difference between friction-based locking and wedge locking and when is lubrication needed?

A: The principle of friction locking is based on increased friction in the thread or under the bolt head/nut. The main disadvantage of this method is increased torsion stress in the joint. High torsion may cause the fastener to yield at a lower preload than expected. Since friction conditions are uneven you may not reach the necessary preload. Insufficient preload is the most common cause of the bolt’s failure, often due to fatigue. Lubricants are often used to minimise friction and to obtain uniform clamp load. However lubrication will significantly reduce or even discharge locking ability of any friction-based method. Friction-based locking results in little or no control over achieved clamp load.

The wedge locking method is based on tension instead of friction. The most common example of the wedge locking system is a pair of washers which have cams with a rise greater than the thread pitch of the bolt. The washer pair is installed cam face to cam face. When the bolt/nut is tightened teeth grip and lock the mating surfaces, allowing movement only across the cam faces. Any rotation of the bolt/nut is blocked by the wedge effect of the cams. The wedge locking ability is not affected by lubrication; furthermore by using it you can utilise each bolt to its full capacity.

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