Nuts and Bolts

2010 September 14
by Joe Johnston
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It is time to start thinking about the physical components that will make up the roaster. A key class of components is the mechanical fasteners used to hold all of the other parts together. I have a personal fondness for mechanical fasteners, as they come in an endless variety of types, sizes, and finishes. Choosing the correct one, mechanically, for each application is important to the reliable functioning of the whole. However, the style and appearance of the fastener also figures into whether a machine is a great design or simply a functional design.

For example, as I type this post on my Apple PowerBook, I note the use of polished stainless steel, flat head, allen head machine screws holding it together. This fastener speaks volumes about the design of my computer. It is a very clean, simple, elegant fastener made of a great looking, durable material. The use of it implies simplicity, style, and durability, all of which are characteristics of the Mac as a whole. I intend to put as much attention into the selection of each and every faster as Apple does.

Metric or SAE?

The first decision is whether to use all metric fasteners or all SAE fasteners. SAE stands for Society of Automotive Engineers and is the system of sizes for fasteners in the automotive and aerospace industries that are based upon the English Standard System (inches). The decision does not greatly affect the variety and quality of fasteners available. It is more of a philosophical choice. I choose SAE, because the roaster is American-made and is a modern classic. I grew up using 1/2″ wrenches to work on tractors and go-carts, rather than 13mm (the metric version) and so I have a special fondness for SAE.

Material, Grades, and Finishes

Mechanical fasteners come in a wide variety of materials, grades, and finishes. For the roaster project application the two materials that will be used almost exclusively are steel and stainless steel. Both materials are quite strong and durable. In a few applications, it is possible that brass or nylon fasteners will be used, but these are much weaker materials.

Steel bolts are available in several grades related to strength. Strength is the resistance to breaking by being pulled (tensile strength). Grade 2 has a tensile strength of 74,000 psi (pounds per square inch), grade 5  is 120,000 psi, and grade 8 is 150,000 psi. Stainless steel has a tensile strength of 100,000 psi. It is important to match the strength (grade) of the bolt to the anticipated load.

Steel bolts are also available in a variety of finishes, primarily designed to inhibit rust and corrosion and to reduce the friction on the threads. My favorite is bright chrome plating. Grade 8 bolts are often finished with yellow zinc, which results in a yellow color with a hint of green. Another common finish is black phosphate or black zinc, resulting an off-black finish. A fairly standard finish for bolts is clear zinc.  Stainless is normally used in its natural finish.

Bolts (L to R) Bright Chrome Grade 5, Stainless, Grade 8 Yellow Zinc, Grade 5 Zinc, Grade 2

I will use mainly grade 5 and grade 8 bolts. My current thinking is that I will use grade 8 yellow zinc or black phosphate bolts for most high load applications that are not visible. I am also thinking of using grade 5 bright chrome bolts for visible applications.

Bolts

Bolts are the mainstay of mechanical fasteners. They come in a wide range of sizes, quoted as head size and thread length. For example, a 1/2″ x 1″ bolt has a head width (face-to-face) of 1/2″ and a thread length of 1″. Larger bolts are used for larger loads. The length of the bolt is normally determined by total width it must span to connect parts together and have room left for washers and a nut.

Bolts are made with a variety of head styles. The most common is the hex head (six sided). A variation on the hex head is the flanged hex head, which helps distribute the load and may have serrations to help prevent the bolt from loosening. Several heads using an allen wrench are also available. The most common are the cap socket head, button head, and flat head.

Black Phosphate Bolts (L to R): Flat Head, Button Head, Socket Cap, Flange

I like to use hex heads (flanged or not) for internal spaces. For finished areas, I prefer a flat head or a button head. It makes for a much “cleaner” look.

Bolt Threads: Standard (L), Fine (R)

One last variable in choosing a bolt is the thread. Many bolts are available in standard (coarse) thread and fine thread. It is best to choose a coarse thread for rapid assembly and in non-ferrous materials such as aluminum. Fine threads are about 10% stronger and resist loosening due to vibration. I plan on using coarse threads for most applications with some fine threads for higher strength/high vibration areas.

Nuts and Washers

Paired with a bolt is  a nut, unless the component receiving the bolt is threaded. The nut is matched to the bolt in terms of size, material, grade, finish, and thread. Virtually all nuts are hex nuts, even if the head of the bolt uses an allen wrench. The standard nut is the basic hex nut. To cover the protruding bolt end, the choices are a cap nut or an acorn nut (taller), which give a very finished look. Some nuts are designed to resist loosening due to vibration. A nylon insert lock nut has a ring of nylon embedded in the top. This ring grabs the threads, reducing the chance of loosening. A serrated flange nut has grooves under the flange designed to dig into the metal under the nut. Similarly, a kep nut has an attached star lock washer that also digs into both the nut and the metal under the nut.

Nuts (L to R): Standard, Nylon Insert Lock, Serrated Flange, Kep, Cap

Choosing a nut for a particular application depends mainly on the initial choice of bolt. After that, it is more a matter of style.

Washers (L to R): Split Lock, External Tooth Star Lock, Wave, Fine Flat, Coarse Flat

Washers are used for two main purposes: to spread the load and to prevent loosening. Flat washers are used to spread the load over the surface of the components being joined. They also prevent marring of the surface of the component due to the rotation of the nut during tightening. Flat washers are matched to the diameter of the bolt for their inside diameter. The outside diameter varies depending upon how thin the metal of the joined component is. The thinner the metal, the wider the washer needed to smoothly spread the load to the component.

Lock washers come in a variety of forms, but the most common are the split lock washer, wave washer, and star lock washers (external tooth and internal tooth styles). In all cases, the goal is to prevent the nut from loosening. I make generous use of lock washers and augment with Loctite® threadlocker (an adhesive).

Machine Screws

For less strenuous fastening jobs, machine screws are used. Similar to bolts, the main difference is size and the head styles commonly offered. Normally, machine screws have a head that is engaged by a flat blade screwdriver, a phillips head screwdriver, or an allen head wrench.  There are other unusual heads that accept torx or are tamper-proof (for public restrooms, for example). Otherwise, the head shapes are very similar to bolts.

Machine Screws (L to R): Pan Head (Phillips), Button Head (Phillips), Socket Cap, Button Head (Allen), Flat Head (Allen)

Machine screws are available in various lengths, diameters, thread pitches, materials, and finishes. They also have matching nuts and washers. Since machine screws are some of the most visible fasteners, it is important to think of overall design and style when selecting one.

Conclusions

I love mechanical fasteners. They tend to be under-appreciated as simply a means of holding components together. The variety of available fasteners is truly astounding and many are gorgeous to look at. Choosing the right one for the job is something I look forward to. Great design is in the details and I intend to pay attention to each and every fastener and its contribution to the overall roaster design.

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2 Responses
  1. Victor permalink
    September 15, 2010

    I guess you are aware of this already, but there are probably some standards or regulations with regard to choosing the right material in the food industry especially when it comes in contact with the food.
    Of course, roasting coffee beans is not the same as processing raw meat, but it’s better to be on the safe side.
    Regards,
    Victor

  2. November 3, 2010

    Big fan of allen socket caps right here.

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