Documentation:SBME-Teaching-Labs/Fasteners

Fasteners and Adhesives

While fasteners and adhesives seem relatively simple at first glance, differences arise in the materials they can be used with, the types of physical stresses they are meant to resist, and their applications. These differences are not insignificant and can lead to a failure if not the right fastener is not properly chosen. Regardless of the fastener or adhesive chosen, selecting the proper size, length or amount to use is essential (you do not want a half inch of nail protruding from your prototype). As such, please see below for a quick breakdown of some common options.

Fasteners

Fasteners are generally defined by a physical object used to provide support to hold objects in place as intended. These can include bolts, screws, rivets, pins and more. Each type has its own use cases and benefits and draw backs.

Screw	Machine Screw	Bolt	Lock Nut	Castle Nut
Tee Nut	Nail	Rivet	Cotter Pin	Retaining Ring

Screws

Screws are one of the most versatile fasteners and are used for wood, plastics, and metals. They can be either self-tapping or not. Self tapping screws will cut the threads into the hole as you drill, though a pilot hole is still beneficial as to not split the material. Non self tapping screws require a pre-threaded hole to engage. Screws are used to join two objects together by engaging both sides to create a clamping force and may noy may not have a shank (an unthreaded section near the head of the screw). While full thread screws will have a stronger holding force, they may cause an offset between the two joined surfaces if the threading in each object does not align based on the object's desired rotational position. Screws are not generally intended for high shear stress, and specialty screws may be needed in such cases such as lag screws. However, screws do have a high resistance to pull-out (a high clamping force).

Machine Screws

Machine screws are somewhat of a cross between a bolt and a screw. They are intended to thread into pre tapped holes like a screw but share a more similar structure with a bolt. They are generally used for holding two metal pieces together and come in smaller sizes with full length threads.

Bolts

Bolts are used to connect two pieces of material through an unthreaded hole, secured by a nut on the opposite side. They have a high shear strength and are therefore less likely to snap under force. A washer is generally used on either side of the materials being joined to help distribute the clamping force over a wider area. Bolts are usually used for one of two main reasons; disassembly may be needed or a secure yet adjustable joint is required (possibly to change the clamping force or alignment). Due to their higher strength, they are also used in heavy load applications more commonly. Bolts may or may not have a shank (generally larger bolts do have one).

Lock Nut

A nut for a bolt which features some method to prevent the nut from backing off (loosening). This is generally done in the form of nylon insert to prevent backward motion.

Castle / Slotted / Retainer Nut

These types of nuts are used in conjunction with a drilled shank (a bolt or rod with a hole drill through it laterally) and a retaining or cotter pin. The addition of the pin keeps the nut and bolt in the same relative position, preventing any change from vibration or disassembly and reassembly.

Tee Nut

A tee nut is a specialized nut meant to be driven into wood to create a threaded hole. The "tee" are a set of sharp teeth which bite into the wood preventing rotation and pull-out and therefore allowing high-quality threads to be used with a fastener on a wood piece.

Nails

Nails are simplistic lengths of metal with a sharp point and a flat head which can be hammered. They are forced directly into the material without pre tapping or making a pilot hole. Generally, they are reserved for softer materials like wood, plastic, drywall, etc. The friction of the nail and internal elasticity of the material provides the holding force. Nails are generally fairly good in both shear and clamping force but are mainly use for their inexpensiveness and speed.

Rivets

Rivets are a specialized type of fastener generally reserved for sheet metal or fabrics / leather. They are permanent fixtures that can only be removed by destroying them. A hole is drilled through the pieces and aligned. The rivet is inserted and a rivet gun is used to compress the rivet, creating a mushroomed end on the either side of the 2 joined objects, therefore creating a strong holding force. Rivets are good when used in high-vibration applications as they are not threaded, and have a large width relative to their length compared to bolts, minimizing protrusion from the joint while retaining strength.

Retaining Pins

Retaining pins are used to connect a shaft and bore. A hole is drilled through both the shaft and overlapping bore so that when they are aligned, the pin is pushed through both to maintain their alignment. Retaining pins are generally are used more so for repeatable assembly in the correct orientation where decent strength is also needed or when the use of bolts is not feasible..

Snap or Retaining Rings or Clips

Retaining rings uses are similar to retaining pins except that the sit on the inside of outside of a shaft or bore to keep the shaft and bore, and any other integral components correctly aligned by creating a physical barrier, or shoulder.

Adhesives

Wood Glue	Metal Epoxy	2-Part Epoxy
Silicone Adhesive	Acrylic Glue	Super Glue

Wood Glue

Wood glue is specifically meant to bond pieces of wood together. It will work with some other porous materials but is not intended for use with plastics, metals or other non-porous materials. It has a high bond strength and moisture resistance, but has low temperature resistance and can be very rigid when cured, making it unsuitable for applications requiring flexibility.

Metal Epoxies and Glues (ex. JB Weld)

These 2 part epoxies are used mainly for metal-metal bonding but can be used for other materials in some cases. Epoxies such as JB Weld have high strength, temperature resistance, and water and corrosion resistance. Metal epoxies like JB Weld can be sanded, tapped, drilled and more after curing. While an excellent option for bonding metal and some other surfaces, it is generally not suitable for high impact or vibration applications.

Epoxy

General epoxies are a good option for bonding most materials, but can be outdone by some materials specific bonding agents. It is generally used for plastics, woods, and some metal applications and provides a strong, reliable bond that is resistant to many chemicals. Some epoxies are also UV sensitive, becoming brittle or cracking after long-term exposure.

Silicone Adhesive

Silicone adhesives benefit from flexibility and temperature and environmental resistance. These adhesives may not be suitable for heavy load applications due to limited adhesion strength and may not be suitable for all materials (such as plastics). The silicone can also be used to provide electrical insulation.

Acrylic Adhesives

Acrylic adhesives are versatile and strong bonding solutions that have a wide variety of applications. These adhesives have a fast cure time but may cure to become brittle. The curing process also produces strong odors and an exothermic reaction, which may affect the materials being bonded.

General Use Adhesives (Super Glue)

'Super glues' are great general use options for bonding objects. They provide good shear and tensile strength but may create brittle joints. They also have limited gap curing abilities (super glue does not expand to fill holes or gaps in bonding surfaces) and have low chemical resistance. For most general, quick prototyping options, super glue is a good option.

A Final Note

While it is always important to consider how the fastener will resist the expected forced and interact with your material, it is also important to think about what it leaves free. For example, using a single unthreaded pin will allow the two objects to rotate relative to one another while preventing them from translating out of place. Moreover, many of these fasteners can be used in conjunction to get the ideal result of freedom and restriction of movement. Just as you think about your overall design, think about and plan your connection points and type of fasteners you will need. Lastly, do not overcomplicate and add to your work. While many specialized fasteners and adhesives exist, if a more common option can be used in its place without an unacceptable negative effect, use the more common option. This will both simplify your prototyping process and likely reduce cost, but will also provide a simpler manufacturing and replacement pathway in the future, especially if you are designing something to be manufactured at scale.