Proper Use
The movable jaw should be snugly adjusted to the nut or bolt head in order to prevent damage to the fastener's head, or rounding. In addition, it is important when applying significant force to ensure that the fixed jaw "leads" the rotation (it follows its tip) and the movable jaw "trails" the rotation, leaving its tip behind, so to speak. The area of contact for the fixed jaw should be farther from the body of the tool. That means the movable jaw's area of contact is relatively close to the body of the tool, which means less bending stress. The fixed jaw can withstand bending stress far better than can the movable jaw, because the latter is supported only by the flat surfaces on either side of the guide slot, not the full thickness of the tool.
This type of spanner should not be used on a rounded off nut, as this can overload the movable jaw. Nor should such a spanner be used "end on" in cramped quarters (except perhaps when the nut is barely more than finger-tight), where a ratchet is more appropriate.
Some cheaper brands' jaws move when twisting on tight nuts.
Read more about this topic: Adjustable Spanner
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