Fastener Selection: Guidelines

By Fred C. Jensen

Here a fabricated nut plate is shown with threaded inserts installed.

A fastener facilitates assembly as well functionality,  making it is a significant part of any design. In fact, it can make or break the machine or product. Fasteners selected should not be an afterthought — an after-the-fact fix can be complicated and impracticably expensive. Therefore,  we offer the following guidelines to help you in the selection of fasteners.

  Grade 8 Socket Head Cap Screws (SHCS) are the screws of choice,  in my opinion. The head design of the SHCS allows the use of ball-nose hex key tooling. This is a much more forgiving system with regards to the tool angle and thrust pressure that must be applied by the assembler. Avoid using screwdriver-driven or hex-head bolts. Screwdriver slots are too easily stripped. It is best to apply the wrench to the nut side only, not to both the nut and bolt sides at once.

  Try to use only the fit 2A for external threads or fit 2B for internal threads. Fit 2A and 2B is the middle of the tolerance range for screw systems. Fit 1A and 1B are for loose, open tolerances whereas fit 3A and 3B are for tight tolerance systems.

  When your design allows, use only coarse thread fasteners. Designing with only coarse threads reduces bolt inventories. Fine threads require very clean assembly conditions for reliable torque readings. The practical use of fine threads is in adjustment mechanisms. Also, since a fine thread is slightly stronger, it could be used where size or weight limitations will not allow the next size of coarse thread. These are marginal uses for the added cost of also stocking fine sizes.

  Stainless steel bolts and nuts are preferred whenever possible. Cadmium and zinc plating is toxic in manned environments. The extra cost of stainless steel fasteners is largely offset by the cost of plating and the environmental restrictions on the plating vendors.

  Decide on which system to use, inch or metric, and only use that system. This reduces the inventory of stocking a complete set of both systems.

  Design to use bolts in 0.125-in. (3mm) length increments.

  If locking fasteners are required, use a chemical locking method such as a liquid loctite thread sealer and locking fluid in lieu of a mechanical locking device. Bolts that rely on deformed metal threads are chip producers and can only be used once. They are also difficult to torque to precise low torque values. Bolts with nylon locking inserts can only be used once and are temperature sensitive. Avoid lock washers of any kind since they are also chip generators.

  Avoid self-tapping screws. These are also chip generators and remove some of the thread material each time they are used. Chip generation is death to an electronic assembly and even a pure mechanical system may be installed near an electronic system.

  Use a commercial threaded stainless steel insert in all aluminum assemblies, no exceptions. An aluminum thread galls too easily to risk use. If a part is too thin to use an insert, then use a nut.

  Never use a jam nut, only use full height nuts. One cannot be sure that the jam nut was not over-tightened by someone in the past. Why take the chance on a fastener that is really an undersized, half-sized component.

  Use half-dog internal hex-socket set screws. Cup-point set screws will mark the mating shaft but have very high holding. A prehardened shaft will be marked less with half-dog set screws.

  Try to design using only a few preferred bolt sizes. The following inch sizes are preferred: Number 0, 2, 4, 6, 8, 10, 0.25, 0.312,  0.375, 0.437, 0.5, 0.625 and 0.75.

  We prefer the Metric Coarse pitch M Profile in even sizes. Nominal Coarse Metric sizes include: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24 and 30 millimeters.

  Both from a redundant reliability stand point and to distribute the stress pattern, it is much better to have bolt patterns with a greater numbers of smaller sized bolts than bolt patterns with fewer larger sized bolts. Look at the engine compartment of the newer cars, they understand this.

  A quarter-inch diameter bolt (6mm) is the most misused screw size. It is almost big enough to get the job done. Many people over-torque this size. In most cases, it is better to use a larger size.

  Use only regular size washers, not the narrow or wide sizes.

  If you must use a wide washer under a fastener, it is far better to use a single fabricated nut plate that matches the entire bolt pattern at once. As seen in the opening figure, Fabricated Nut Plate Shown with Threaded Inserts Installed, we show a circular bolt pattern nut plate with commercial inserts installed in lieu of tapping the holes in the plate. Of course, the simplest nut plate has only tapped holes. If required, the nut plate can also have a dowel pin installed to key the pattern.

  Limit your designs to only four sizes of dowels unless you are building very large structures. Dowels should only be used to locate a part during assembly, not during use. If you need to locate a part during use,  design an assembly with one or two keys at right angles. A key has much higher holding power than a dowel. We have never seen an elongated key slot. However,  everyone has seen an elongated dowel hole. Our preferred dowel sizes include 0.062 dia. x .37 long, 0.125 dia. x .50 long, 0.25 dia. x .62 long, and 0.375 dia. x .75 long.

  There is a wide variety of commercial nuts and inserts available for use with sheet metal, plastics, rubber, ceramics, and wood. These are well engineered and very useful. We suggest using the same SHCS for the bolts but recommend taking advantage of what is available in these special purpose nuts and inserts. Some examples include:

  Pem fastener nuts for sheet metal are very useful but only in the smaller sizes such as #4, 6, 8, and possibly 10. The self-clinching floating nut adjusts for mating hole misalignment. Look for Penn Engineering & Manufacturing Corp. (Danboro,PA; pemnet.com).

  Yardley Products Corp. (Yardley, PA;  yardleyproducts.com), makes an entire line of threaded inserts for plastics,  rubber, ceramics, and wood.
Microdot Aerospace Fastening Systems Group –Alcoa Fastening System (Fullerton, CA; alcoa.com) makes a line of thin wall inserts.
Dodge inserts from Helicoil (New Delhi, India;  helicoil.in) makes expansion inserts for soft materials.

  Helicoil inserts are the old standby, for inserting into aluminum, cast materials and many other materials.

  Aluminum and stainless steel pop rivets in 0.125 and 0.187 inch diameter sizes are useful. This is a very user-friendly fastening method that is widely used but for only non-critical applications. It is far better to drill and tap an assembly with a stainless steel fastener for critical applications.

  Careful consideration must be given to the use of all fastener systems for their galvanic potential with the parent material being fastened. In most cases wet fastening with a general-purpose low-adhesion sealant such as those specified in Mil-S-8784B is very desirable to prevent corrosion.

  Carbon fiber composites are very cathodic with metal fasteners. As such they must all be wet assembled with a corrosion-inhibiting sealant. Examples include the Polysulfides as per Mil-S-81733C.  Because of galvanic compatibility issues, the preferred fasteners for assembling carbon fiber are titanium, Inconel 600,  austenitic stainless steels and superalloys such as A286 may be used with carbon fiber (1). Low alloy steels and martensitic stainless steel fasteners are not acceptable when fastenening carbon fiber components. Special conical stainless steel finishing washers are useful for spreading the loads from the fastener into the carbon fiber material.

  Fred C. Jensen is Director of Engineering at Patriot Engineering Co. Send your feedback to [email protected]

References Used:
“Composites”, Volume 1, Engineered Materials Handbook, by ASM International, 1987

Information:
Patriot Engineering Co.
Chagrin Falls,OH
patriotengineeringco.com