Bolts, screws, studs, etc. are also graded according to:

1. Strength
2. Composition
3. Finish

These are defined in the ASTM (American Society for Testing and Materials) and SAE (Society of Automotive Engineers) tables and charts….a simplified version of both is in your text book. The simple version is….the higher the grade of fastener, the stronger it is and the more torque that can be applied to increase the holding power of the fastener. The composition of the fastener is determined by the strength desired and the environment in which it is being used. As an example, inside a building a simple steel fastener could be used……in a corrosive environment such as on a ship or continual exposure to moisture, a stainless fastener would be used. In most cases, the manufacturer of the of the device, component, fixture, etc. has pre-determined the correct fastener for the job at hand.

Bolts (and some screws) are designed to be used in combination with nuts for securing parts, devices, pieces, etc. Nuts are governed by the same agencies, SAE and ATSM. They are configured in as many different ways as bolts and screws are….dependent upon their intended usage. They also are of different strengths, composition and finish, just like bolts and screws.

Bolts, screws and nuts are relatively small in size in relation to the items being fastened, so the surface area that is actually exposed to the clamping or holding forces is also small. This is where flat washers are used….one job of the flat washer is to increase the surface area over which the clamping force is applied. They also protect the surface of the item being clamped together from being marred by the rotational force of the fastener. There are three basic types of washers

….the fender washer, which is larger in diameter and spreads the clamping force over a larger area

…..the standard washer which provides a smaller surface area, but still larger then the fastener head or nut face

….and the lock washer…..which is used to help "lock" the fastener and/or nut in the proper position and prevent loss of torque by the fastener rotating in relation to the surface or nut being used (unscrewing).

There are numerous methods of locking the fasteners in position…..lock nuts, lock washers, safety wire, etc. They all accomplish the same task…..prevent the fasteners from rotating and loosing torque….or "holding force". The most common are lock washers…..they’re used throughout industry and consist of a washer that has some provisions for a single (or multiple) sharp metal edge biting into the surface of the flat washer and the head or nut on the fastener. These come in numerous configurations and are used according to the configuration needed to accomplish the task. These are normally reusable for a few times, provided they are not damaged or over compressed. The next most common locking method is the "jam nut"…..this method uses a smaller nut screwed down on top of the main nut used to supply the primary holding power of the fastener. This prevents the main nut from rotating because the nuts are "jammed" together. This method has been overshadowed by the "lock nut". The locking nut has a fibrous or plastic insert inside the nut to "jam" it in place….some "lock nuts" have simply had the threads deformed in a few places to pinch or clamp the nut on the threads. The most secure method is the use of safety wire or a cotter pin……the fastener has a hole through the head or shank and the nut has slots that align with the hole….safety wire or a cotter pin is passed through the slot/hole combination and provides the most secure way to prevent the fastener from turning and releasing torque on the surfaces. This method provides the most security but also requires the removal and installation of the safety wire or cotter pin every time the fastener is used, which is labor intensive.

Stud bolts have no head and are use in combination with a nut and washers. These are primarily used where access is limited to one side of the piece, part, device, etc. being fastened.

In all cases, to get proper utilization of the fastener, proper torque should be applied according to the size and tensile strength of the fastener. This torque should be measured by use of a "torque wrench" on only unlubricated fasteners. If the fastener is lubricated, values for the torque being applied are not valid and a "torque value chart" for lubricated fasteners must be used (these are hard to find). Care must be taken when applying "torque" to covers with multiple fasteners or multi-fastener devices to prevent distortion or warpage of the parts or covers. When ever possible, use a cross pattern and tighten/torque the fasteners in steps or increasing values. As an example, torque all the fasteners in the cover in a cross pattern to 50% of rated torque value….retighten in the same cross pattern to 75% of rated torque value…..then to 90%...etc. The "cross pattern" consists of torquing fasteners at 180 degrees from each other on a cover or device.