A screw is a shaft with a helical groove or thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.
Screws and bolts
A screw used as a threaded fastener consists of a cylindrical shaft, which in many cases tapers to a point at one end, and with a helical ridge or thread formed on it, and a head at the other end which can be rotated by some means. The thread is essentially an inclined plane wrapped around the shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix using a tap, 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 (British English: spanner) to rotate the screw, driving it in or releasing it. The head is of larger diameter than the body of the screw and has no thread so that the screw can not be driven deeper than the length of the shaft, and to provide compression.
Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse.
The vast majority of screws are tightened by clockwise rotation; we speak of 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. Left-hand screws are used on 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) or hub nuts on the left side of some automobiles.
Threaded fasteners were made by a cutting action such as dies provide, but recent advances in tooling allow them to be made by rolling an unthreaded rod (the blank) between two specially machined dies which squeeze the blank into the shape of the required fastener, including the thread. This method has the advantages of work hardening the thread and saving material. A rolled thread can be distinguished from a thread formed by a die as the outside diameter of the thread is greater than the diameter of the unthreaded portion of the shaft. Bicycle spokes, which are just bolts with long thin unthreaded portions, always use rolled threads for strength.
Differentiation between bolt and screw
Carriage bolt with square nut.
Structural bolt DIN 6914 with DIN 6916 washer and UNI 5587 nut.
A universally accepted distinction between a screw and a bolt does not exist.
In common usage the term screw refers to smaller (less than 1/4 inch) threaded fasteners, especially threaded fasteners with tapered shafts used in un-threaded substrates and the term bolt refers to larger threaded fasteners that are designed to be used with nuts or in tapped holes. The term machine screw is commonly used to refer to smaller threaded fasteners that are used with nuts or in tapped holes. The term lag bolt (also known less commonly as a lag screw) is used to refer to larger threaded fasteners with tapered shafts.
Various methods of distinguishing bolts and screws exist or have existed. These methods conflict at times and can be confusing. Old SAE and USS standards made a distinction between a bolt and a cap screw based on whether a portion of the shaft was un-threaded or not. Cap screws had shafts that were threaded up to the head and bolts had partially threaded shafts. Today a bolt that has a completely threaded shaft might be referred to as a tap bolt.
ASME B18.2.1 defines a bolt as "an externally threaded fastener designed for insertion through the holes in assembled parts, and is normally intended to be tightened or released by torquing a nut." Using this definition to determine whether a particular threaded fastener is a screw or a bolt requires that an assumption be made about the intended purpose of the threaded fastener, and as a practical matter does not seem to be followed by most threaded fastener manufacturers. It also conflicts with common usage such as the term "head bolt", which is a threaded fastener that mates with a tapped hole in an engine block and is not intended to mate with a nut.
It is possible to find other distinctions than those described above, but regardless of the particular distinction favored by an individual or standards body the use of the term screw or bolt varies. More specific terms for threaded fastener types that include the word screw or bolt (such as machine screw or carriage bolt) have more consistent usage and are the common way to specify a particular kind of fastener.
The US government made an effort to formalize the difference between a bolt and a screw, because different tariffs apply to each. The document seems to have no significant effect on common usage and does not eliminate the ambiguous nature of the distinction for some fasteners. The definition is available online.
Other fastening methods
Alternative fasteners to screws and bolts are nails, rivets, roll pins, pinned shafts, welding, soldering, brazing, and gluing (including taping).
Another option is the threaded insert. Examples include Helical Inserts [2] and Keensert .
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, bronze, monel or silicon bronze may be used, or a coating such as brass, zinc or chromium applied. Electrolytic action from dissimilar metals can be prevented with aluminium 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 canning jars.
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. Gradings are indicated as markings, while grade 0 is the lowest, grade 10 is the highest. Here is the sequence of bolt strength and markings, from least to most. Grade 0, 1 and 2 bolts have no markings, grade 3 has 2 radial lines, grade 5 has 3, grade 6 has 4, grade 7 has 5, grade 8 has 6, grade 9 has 7, grade 10 has 8.
In some applications joints are designed so that the screw or bolt will intentionally fail before more expensive components. In this case replacing an existing fastener with a higher strength fastener can result in equipment damage. Thus it is generally good practice to replace fasteners with the same grade originally installed.
Mechanical analysis
Rotating screw and fixed trough
A screw or bolt is a specialized application of the inclined plane. The inclined plane, called its thread, is helically disposed around a cylinder or shaft. That thread usually either fits into a corresponding (negative or female) helical thread in a nut, or forms a corresponding helical cut in surrounding softer material as it is inserted. A simple screw, such as for fastening, is typically pointed, and thereby is commonly distinguished (in informal terminology) from a bolt or machine screw. Common screws, and usually bolts, have a head which may be mechanically driven or rotated, which usually serves as a stop, and may have an unthreaded shoulder portion beneath the head.
The technical analysis (see also statics, dynamics) to determine the pitch, thread profile, coefficient of friction (static and dynamic), and holding power of a screw or bolt 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 driving it. The main concept is to tension the bolt, and compress parts being held together, creating a spring-like assembly. The stress thus introduced to the bolt is called a preload. When external forces try to separate the parts, the bolt experiences 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 (for example, a steel bolt threaded into aluminium, then a thread-locking adhesive or insert 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
Rusty hexagonal bolt heads
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 should be tightened to a specified torque.
High-strength steel 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 5.8, 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 ultimate strength is the stress at which the bolt fails (breaks in half). Tensile yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2 % offset strain. When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.
Mild steel bolts have property class 4.6. High-strength steel bolts have property class 8.8 or above. An M10, property class 8.8 bolt can very safely hold a static tensile load of about 15 kN.
There is no simple method to measure the tension of a bolt already in place other than to tighten it and identify at which point the bolt starts moving. This is known as 're-torqueing'. An electronic torque wrench is used on the bolt under test, and the torque applied is constantly measured. When the bolt starts moving (tightening) the torque briefly drops sharply - this drop-off point is considered the measure of tension.
Recent developments enable bolt tensions to be estimated by using ultrasonic testing. Another way to ensure correct bolt tension (mainly in steel erecting) involves the use of crush-washers. These are washers that have been drilled and filled with orange RTV. When the orange rubber strands appear, the tension is correct.
Large volume users such as auto makers frequently use computer controlled nut drivers. With such machines the computer in effect plots a graph of the torque exerted. Once the torque ceases to rise (the point where the bolt begins to deform) the machine stops. Such machines are often used to fit wheelnuts and will normally tighten all the wheel nuts simultaneously.
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