Hexagon Bolts and Nuts

Threaded fastener A screw used as a threaded fastener consists of a shaft, which may be cylindrical or conical, and a head. The shaft has a helical ridge or thread formed on it. The thread is essentially an inclined plane wrapped around a shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix (Taps and dies), or the screw may create it when first driven in (a self-tapping screw). The head is specially shaped to allow a screwdriver or wrench (UK: spanner) to grip the screw when driving it in. It also stops the screw from passing right through the material being fastened and provides compression. Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse. A screw that is tightened by turning it clockwise is said to have a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Examples include rotating items such as the left hand grinding wheel on a bench grinder or the left hand pedal on a bicycle (both looking towards the equipment). Threaded fasteners are traditionally made by a cutting action such as taps and dies provide, however recent advances in tooling allows them to be made by rolling the blank (a section of rod) between two specially machined dies. The thread form and shape of the fastener are squeezed onto the blank. This method work hardens the threads and saves material. A rolled thread is obvious after manufacture because the outside diameter of the thread is greater than the diameter of the blank material. Bicycle spokes, which are just very long thin bolts, always use rolled threads for strength. Bolt A bolt is a cylindrical (as opposed to conical) threaded fastener that passes through the work piece and is held in place by a nut or a threaded hole on the other side. This is a very common way of holding together temporary and permanent constructions. An unthreaded hole is known as a clear hole. The thread on a bolt sometimes occupies only part of the shaft, the remainder of the shaft being clear; if the thread continues up to the head it is known as a 'set'. Other fastening methods When screws and bolts cannot be used, nailing, riveting, roll pins, pinned shafts, welding, soldering, brazing, gluing, and duct tape (taping) are some alternatives. Materials and strength Screws and bolts are made in a wide range of materials, with steel being perhaps the most common, in many varieties. Where great resistance to weather or corrosion is required, stainless steel, titanium, brass or bronze may be used, or a coating such as brass, zinc applied. Electrolytic action from dissimilar metals can be prevented with aluminum screws for double-glazing tracks, for example. Some types of plastic, such as nylon or Teflon, can be threaded and used for fastening requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. Even porcelain and glass can have molded screw threads that are used successfully in applications such as electrical line insulators and jamjars. The same type of screw or bolt can be made in many different grades of material. For critical high-tensile-strength applications, low-grade bolts may fail, resulting in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is impressed on the heads to allow inspection and validation of the strength of the bolt. However, low-cost counterfeit fasteners may be found with actual strength far less than indicated by the markings. Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar critical applications. Mechanical analysis Rotating screw and fixed trough A screw is a specialized application of the wedge or inclined plane. It contains a wedge, wound around a cylinder or shaft, that either fits into a corresponding inclined plane in a nut, or forms a corresponding inclined plane in the wood or metal as it is inserted. The technical analysis (see also statics, dynamics) to determine the pitch, thread shape or cross section, coefficient of friction (static and dynamic), and holding power of the screw is very similar to that performed to predict wedge behavior. Wedges are discussed in the article on simple machines. Critical applications of screws and bolts will specify a torque that must be applied when tightening. The main concept is to stretch the bolt, and compress the parts being held together, creating a spring-like assembly. The stretch introduced to the bolt is called a preload. When external forces try to separate the parts, the bolt sees no strain unless the preload force is exceeded. As long as the preload is never exceeded, the bolt or nut will never come loose (assuming the full strength of the bolt is used). If the full strength of the bolt is not used (e.g., a steel bolt threaded into aluminum threads), then a thread-locking adhesive may be used. If the preload is exceeded during normal use, the joint will eventually fail. The preload is calculated as a percentage of the bolt's yield tensile strength, or the strength of the threads it goes into, or the compressive strength of the clamped layers (plates, washers, gaskets), whichever is least. Tensile strength Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. For example, when two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough so that the friction between the two bars can overcome the longitudinal force. If the bars slip, then the bolt may be sheared in half, or friction between the bolt and slipping bars may erode and weaken the bolt (called fretting). For this type of application, high-strength steel bolts are used and these should be tightened with a torque wrench. Rusty hexagonal bolt heads High-strength bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head. The property classes most often used are 8.8 and 10.9. The number before the point is the tensile ultimate strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa. Tensile yield strength is M10, property class 8.8 bolt can very safely hold a static tensile load of about 15 kN.