It's a busy day at the front counter. You're ordering safety supplies, training a customer on a jig saw, negotiating a deal with your sales rep, and trying to schedule a tight delivery to the job site. Never mind lunch—the soup is already cold. Now, imagine this phone call from one of your top customers, who is in need of your fastener expertise.

"You need us to tell you how to assemble the Fidget 2006 door flopper per the sketch you sent over? Well, let's start at the top," you say.

"First, is the joint going to be structural? That is, is it going to be a major load carrying joint? Is the load on this going to be in tensile or shear, in other words, is the load that it sees going to be at right angles to the joint or in a push-pull direction? Next, is the working load (how much force the parts sees when it's operating) going to be high, medium or low? This is a relative term, but is the load equal to more than twice the part weight, part weight or less than part weight?....Less than part weight! Then that means that it is a low-load joint.

"Now, the mating materials and their conditions. Is the mating material metal? Which? How thick? Can the joint be assembled from only one side or can you reach around to the back? Is the mating surface rough, have machine marks, bumps and grooves, etc.? How about the hole size? How big? Do we need extra security to prevent it shaking out, loosening? Does it get bounced around a lot?"

Exhausting? Sure. Fifteen questions later, this distributor suggested a tapping screw as the best fit for his customer's application.

While it's a fictional scene, the conditions under which it may happen are real in today's market. In their efforts to cut costs, many companies have moved the responsibility of fastener selection onto their suppliers. No longer will customers just be sending in orders; they will be asking distributors for application advice. To keep business from walking out the door, the distributor must know the advantages and disadvantages of various fasteners and be able to make suggestions about their use.

There are certain questions a distributor should ask a customer to make sure the right fastener is selected for the application (see table below). If the customer does not have the answers, then the distributor should seek more expert knowledge. He should also consult tables of torque values for recommended screws and thicknesses of metal, hole size tables for the various screws, and physical properties charts for bolt strength and hardness.

Fastener engineering is more of a black art than a science. The exact part chosen rests upon numerous decisions: what the customer wants, what type of part he is familiar with (stocking repair parts, installation tools), personal preferences, and company practice, for starters. Furthermore, just about any part can be designed to work. A rivet could be used in place of a bolt, a bolt in place of a clip, a weld nut in place of a pin. They may not work well, but they can be made to assemble.

Rather than throw up your hands at this nightmare of decisions, there are certain choices that a distributor can make which are best-choice options. In the end, you may decide that a bolt-and-nut joint is the safest and easiest, and be told, "Harry down the street holds his stuff with a pop rivet."

Most joints are safest with a bolt and nut joint. This combination is fairly foolproof. There are certain conditions and types of parts to consider but, overall, for loaded joints and most conditions, one of many bolt combinations will function well.

Bolted joints use bolts, nuts and sometimes washers. The other combinations are flanged parts (saves washers in many cases), or bolt and washer assemblies used with nut and washer assemblies, and finally, any mix of the above.

Bolts, like most modern headed product, are produced inexpensively on heading machines at great volume. When a smaller quantity is desired, screw machine parts are less expensive. The break point is in the range of about 50,000 pieces for standard configurations. Therefore, the use of a standard bolt (hex head size, threaded length, overall length, point) will be more economical, and procurable, than a special part.

The assembly of a washer onto the bolt or nut end of the joint on the assembly line is labor intensive and costly. If load spreading is desired, or slots and oversized holes are to be spanned, the use of a fastener and washer assembly is much less expensive overall. The part may be slightly more expensive than a loose washer and bolt but overall assembled joint cost is lower. Missing washers are also avoided.

Many joints require accurate torque to load the attachment with sufficient tension to achieve the design objectives. This is obtained through the use of the bearing face circle machine or formed on the underhead surface of the bolt (and face of the nut).

Washer assemblies have a number of problems with obtaining an accurate effective area, so bolt/nut and washer assemblies are usually not used in critical joints. They mostly are used to hold sheet metal (fenders, etc) and lower load joints. Flat washer parts prevent pull in and scratching of visible surfaces. Conical washer parts are used for higher loaded joints where load spreading is desired and other conditions exist. Flanged parts are used where the load needs to be spread out, where slightly oversized holes are present, and where the surfaces mated against are fairly even and smooth. Rough, uneven surfaces will loosen due to embedment of the part unless washers are used. Flanged parts will scruff surfaces, causing cosmetic damage.

Finally, bolts and flanged parts can be made to higher strength levels than washer assembly parts (the washer is heat treated at the same time as the bolt/nut, requiring a special heat treatment and material so that both harden correctly).

The second family of parts consists of the internal thread product, chiefly nuts. The use of a bolt and nut in joining rather than tapping into the base material allows the joint to be tightened to the limit of the fastener strength, rather than the strength of the base material. This is often a considerable difference. As mentioned before, standard product is much more procurable and comes in a larger variety of sizes, grades and sources. Screw machine quantities cost less in lot sizes of less than 50,000, headed product generally a larger amount. Standard hexagon nuts are designed to be stronger than the external threaded member in almost all cases. This is because a stripped nut may not be easily noticed on the assembly line but a broken bolt would be.

Internal thread product comprises hex nuts, hardened spring steel nuts, various shapes of weld nuts, clinch nuts, rivet-nuts, and other special styles. Some nut products require access to the back side to hold the part while the joint is being tightened. Weld nuts are welded and do not need holding but they must be attached to the backside first. Other non-backside accessible parts are clinched nuts and U-J Nuts which are positioned before the other member is installed. Some spring steel nuts (spin-on types) need the bolt member to be held or vice versa. Total assembled cost is a combination of part material, heat treatment (if any), the amount of labor required to set the part and the cost of the equipment used in the installation. For example, weld nuts are very economical but require several operations (cleaning, position, clamp, weld, etc,) to install with expensive welding machines. "In place" cost is very important in determining what fastener to choose.

The amount of strength required from the joint should be a consideration from the start. Bolts and nuts hold parts securely, all thickness, dissimilar materials, etc. U-Nuts are excellent for lower load joints but require the hole to either be near an edge or large opening or to have a cut out nearby where the part can be installed to position. Spring steel parts are harder than the mating part and are good where the nut side is not to be stripped during assembly. Weld nuts can only be welded to steel of similar composition. Medium to high hardness and alloy parts weld very poorly. Weld nuts are soft and will not sustain heavy loads. They are used where backside clearance and access is not available.

Clinched nuts come in a variety of styles. Some are clinched by having a portion of their body swaged over to lock the part onto the mating surface. Others have serrations that cut into and hold the part in a hole in a mating panel. They are used for smaller attachments with low loads. One type of clinched nut seen often is the rivetnut. Designed to operate like an internal threaded rivet, it grips the mating surfaces between the formed swaged collars of the head and formed "foot." It is easily installed but tends to turn out under low loads. This type of nut is useful where backside access is not available and a slightly stronger joint than that obtainable with a regular clinched nut is required.

To lower overall handling of parts, many stampings are made with an extruded section in them. This cone of metal is formed from multiple blows of the stamping process. It is often threaded, producing a nut, integral with the base material. One problem that occurs with extrusion nuts is that the walls may not extrude sifficiently to form enough thickness. When threaded, the remainder of the wall may shear, cut through or just be too weak to take much loading.