When looking for ways to reduce waste, cost-conscious and environmentally aware manufacturers and consumers may consider reusing metric bolts and screws. But not all bolts and screws can be safely and effectively reinstalled after being removed from an assembly or component. In fact, an unequivocal “yes” is never the answer to the question of whether a fastener can be reused. Depending on the situation, the answer is always either “no” or “it depends.”
High-strength bolts that have been pre-tensioned into their proof load range – the point at which they permanently deform and do not “snap back” when relieved from their loads – should not be reused. This is the case in many applications, as structural and other bolts used in the manufacturing, construction, and automotive industries are commonly tightened to a torque value that creates a sufficient clamp force to prevent joint failure.
Tightening a nut onto a bolt causes the bolt to stretch slightly. Much like a spring, the bolt resists this stretching, and its tendency to return to its natural state creates clamping action between, say a cylinder head and a manifold, or two pieces of sheet-metal housing.
It is critical to the component’s operation that the amount of tension created holds the parts together strongly enough to prevent their separation by outside forces such as the machine’s vibration, the load stress generated during operation, gasket creep, temperature fluctuations, and more. Too much torque, however, can stretch the fastener too much, to the point where it chips, breaks, or yields. Bolts and screws are rated by their “proof load” – how much tension they can withstand before they fail. As a rule of thumb, torque a “clamp load” (also known as “preload”) 75 percent to 90 percent of the proof load is optimal.
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Metric fasteners seem simple enough until it’s time for you to actually select the exact kind that you need. There are actually a wide variety of types, styles and grades to choose from. This is not only true of metric bolts or metric screws – it also applies when you’re looking for the right metric washer for the job. Flat washers, curved washers, split washers, internal and external washers – what’s the difference?
Let’s take a look at the various categories offered by Mr. Metric. Our best seller is the good old flat washer, a thin, flat metal sphere with a large hole cut in its center. This versatile washer provides plenty of tension between the head of a screw or nut and the substrate for general-purpose applications. Curved washers and wave washers exert a lighter thrust load than flat washers and are used in rotating mechanisms such as bearings, motors and seals.