The Nuts and Bolts of Threaded Fasteners, Part 1

Tom Benford Jul 8, 2008 0 Comment(s)

What would you call a bolt or screw without a washer? Probably loose, for starters! Most fasteners are installed where vibration and temperature changes occur. Mechanical motion, over time, causes fasteners to back off and loosen, hence the need for something to help prevent that effect: the washer. The primary function of a washer is to provide a surface against which the head of the fastener or surface of a nut can bear. Flat washers do this very well and spread loads, but normally don't help to keep the fastener tight. Lock washers are designed to keep fasteners from loosening. They come in many forms, the two most common being the split (helical) ring washer and the toothed washer. Split washers function in much the same way that helical springs do; when the bolt is tightened sufficiently, the ends of the washer come together under compression, resisting movements of the bolt by creating a certain degree of friction against which the bolt would have to overcome. Toothed washers work very well because their many teeth bite into the surface against which the head or nut bears, creating large amounts of friction. They come in external, internal, and internal/external tooth forms. The softer the surface under the teeth, the better the locking washer works.

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Gauge It : These plastic hole and thread gauges-available in metric (yellow) and fractional (white) from extremely handy tools to have in your garage. You can also download paper gauges for free at the company's Web site,>>>

Essentially, a bolt can't do its job if it doesn't have a nut holding it on. Therefore, any threaded fastener that doesn't screw into a mating surface needs a nut to hold it in place. There are as many different nuts as there are washers and other hardware items because they are designed to accommodate different methods of tightening, strengths to suit varied applications, and some also have provisions for locking. Most nuts are hexagonal in shape to accommodate wrenches, but there are also some square nuts out there as well. If there's one cardinal rule when it comes to nuts, it's this: The nut used must be the same grade of metal as the bolt or failure will surely and eventually occur.

Here are some of the more common nuts you'll probably be dealing with in your garage, aside from the human ones.

Machine screw nuts are fine threaded and frequently used to hold brackets and other assemblies.

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AWL That :Have a screwdriver with a broken or worn tip? regardless of whether it's a flat-blade or a Philips, you can use a bench grinder to grind the tip off and taper it for use as an awl, seat bolt-hole locator (particularly handy if you're replacing the carpeting in your Corvette), or as a drift-pin. It's also a good screw-starter for projects around the house.>>>

Track bolt nuts are usually square and frequently found holding fixtures in place. These are often held "captive" in some sort of housing that allows some degree of movement and adjustment, such as door latch mechanisms.

Jam nuts are typically thin and used for applications where space is critical. You'll frequently find them used for automotive dash trim and radio assemblies.

Thick nuts are typically used for coarse-threaded bolts and in applications where great holding strength is necessary -for instance, securing bumpers to their mounting brackets.

Slotted nuts are cut in several places to accommodate the use of cotter pins or other devices that act as safety systems to prevent loosening. Axle nuts are typically slotted.

Castle nuts are also known as castellated nuts. They are also slotted, taller nuts used for high-hold situations. Castle nuts are frequently available with plastic or metal inserts that exert high friction while turning as added insurance to keep them tight.

Flange nuts are made with a wide bottom flange to spread loads over a large surface. They are frequently found securing fenders to car and truck bodies.

Cap nuts are typically nuts with solid tops that are acorn-shaped. Cap nuts are used in situations where they will be seen in the finished product and are often chrome-plated. These nuts require the use of a specific-length bolt.

As I said at the beginning, there's a lot to know about threaded fasteners, so we'll continue this discussion next month in Part 2 where we will cover course and fine threads, metric and fractional standards, hardness, torque considerations, and head markings. Don't miss it!