A screw , or bolt , is a type of fastener characterized by a helical ridge, known as an external thread or just thread , wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread , often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as it is inserted. Their most common use is to hold objects together or to position objects.
Often screws have a head , which is a specially formed section on one end of the screw that allows it to be turned, or driven . Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface . There are exceptions; for instance, carriage bolts have a domed head that is not designed to be driven; set screws have a head smaller than the outer diameter of the screw; and J-bolts do not have a head and are not designed to be driven. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank ; it may be fully threaded or partially threaded.
The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread . Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to anticlockwise forces that work to undo a right-hand thread.
Differentiation between bolt and screw
A universally accepted distinction between a screw and a bolt does not exist. The Machinery's Handbook describes the distinction as follows:
A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut. A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head. An externally threaded fastener which is prevented from being turned during assembly and which can be tightened or released only by torquing a nut is a bolt. (Example: round head bolts, track bolts, plow bolts.) An externally threaded fastener that has thread form which prohibits assembly with a nut having a straight thread of multiple pitch length is a screw. (Example: wood screws, tapping screws.)
This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw and bolt .
The issue of what is a screw and what is a bolt is not completely resolved with Machinery's Handbook distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction and usage variations. Some of these issues are discussed below:
Types of screws and bolts
Threaded fasteners either have a tapered shank or a non-tapered shank. Fasteners with tapered shanks are designed to either be driven into a substrate directly or into a pilot hole in a substrate. Mating threads are formed in the substrate as these fasteners are driven in. Fasteners with a non-tapered shank are designed to mate with a nut or to be driven into a tapped hole.
Fasteners with a tapered shank (self-threading screws)
Fasteners with a non-tapered shank
Other threaded fasteners
Materials
Screws and bolts are made from 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 (steel screws can discolor oak and other woods), 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.
Joint analysis
Main article: Bolted jointScrews 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.
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 aluminum), 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 strength or the strength of the threads it goes into, or the compressive strength of the clamped layers (plates, washers, gaskets), whichever is least.
The thread on wood screws behave like a wedge.
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 classifications
The numbers stamped on the head of the bolt are referred to the grade of the bolt used in certain application with the strength of a bolt. High-strength steel bolts usually have a hexagonal head with an ISO strength rating (called property class ) stamped on the head. And the absence of marking/number indicates a lower grade bolt with low strength. 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. 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.
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.
SAE J429 defines the bolt grades for imperial sized bolts and screws. It defines them by grade , which ranges from 0 to 8, with 8 being the strongest. Higher grades do not exist within the specification.
The international standard for metric screws is defined by ISO 898, specifically ISO 898-1. SAE J1199 and ASTM F568M are two North American metric st
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