About threaded fasteners

What are bolts and screws?

hex head cap screw

A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally tightened or released by means of a nut. This definition would include threaded fasteners which are prevented from being turned during assembly and which can be tightened or released only by means of a nut (Example: round head bolts, track bolts, plow bolts). A screw, on the other hand, is an externally threaded fastener capable of being inserted into holes having preformed internal threads or unthreaded holes by which the screw forms the threads during insertion. Screws are tightened or released by turning the head because there is no nut.

One must realize that these definitions are broad and are not strictly adhered to in industry. Threads for screws and bolts are most often standardized, so a screw can be used as a bolt and a bolt as a screw, for example. The American Society of Mechanical Engineers (ASME) standards specify a variety of machine screws in diameters ranging up to 0.75 in (19.05 mm). These fasteners are often used with nuts as well as driven into tapped holes. They may be defined as a screw or a bolt based on accepted industry definitions. Typically though, machine screws tend to be available in smaller sizes and are referred to as screws or machine screws.

Often, fasteners having hex shaped heads are referred to as bolts while others with different configurations may be referred to as screws regardless of the exactness of the definition. The critical aspects of these fasteners certainly do not include the correctness of their definition, but rather the design characteristics of a given product such as tensile strength, hardness and dimensions. These characteristics fall into ranges of industry standards that make fastener selection easier.

Usage

It is well understood that the purpose of bolts and screws is to join two or more components together. For a screw , one component (to be joined to another) would have a through hole large enough that the screw can slide through. The component being attached would therefore have a threaded hole for the screw to bind to. In a bolted connection, both (or more) components would have through holes for the bolt to slide through and joining would be accomplished by a nut. In either case, flat washers can be used to provide a bearing surface for the nut and or screw head. Furthermore, split ring spring washers, star washers or other types of washers may be used to facilitate a locking condition of a tightened screw head or nut.

Theory

In consideration of a bolted joint, the joint must be engineered to withstand its intended purpose. Tensile forces, bending forces, shear forces, vibration, corrosion and thermal changes all play a role in determining the needed design of a bolted joint. I have no intention of going through the engineering analysis of a bolted joint in this article, but will briefly discuss the fundamental concept of what should happen in the bolt (or screw) itself.

At the most basic level, a bolt must be tightened sufficient enough to securely hold a joint together. "How tight" would be determined by engineer. Of course, the resulting criteria as determined by an engineer are not usually of concern to an end user who may be an assembler or repair person. Tightness of the bolt is important and is the fundamental being discussed here.

The tightness is typically specified as torque. Torque is a property of rotation and is defined as the product of the length of a lever fixed to a central axis of rotation and an applied force perpendicular to the opposing lever end. That is: force x lever arm. The most common units of measure today are either foot-pounds (ft-lbf), inch-pounds (in-lbf), Newton-meters (N-m) or deciNewton-meters (dN-m).

A bolt or screw, when properly tightened, should act as a retaining spring so to speak. When tightened to the specified torque value, a bolt (or screw) should have a resulting tensile force. The resulting tensile force (force in tension), ideally would stress the bolt material enough that it would be in its elastic range. This means that if the bolt tension is released, the bolt would spring back to its normal relaxed state. If the tightness of a bolt exceeds the elastic characteristics, the material is then stressed so much that it yields. The stress region where yielding occurs is called the plastic region. It is in the plastic region that, when released, the bolt cannot return to its normal geometry. Stripping of the threads could occur before this permanent stretch , but in either case, this would indicate a failed fastener. To summarize a bit, it is the spring action of tightened bolt that holds a joint together properly. The amount of tension required is by design which is why torque values are specified by manufacturers.

Types of bolts and screws

There are many types of threaded fasteners today. So many that it may seem overwhelming. Of course, the scope here is limited to machine type screws and bolts. Obviously, there are more such as wood screws, sheet metal screws etc, which are not relevant here. Additionally, the theory of tightening as mentioned above may not apply to all types of threaded fasteners. There are exceptions, but the above theory is indeed the norm. Some of the many types in industry are as follows:

  • Hex head cap screws
    hex head cap screw

    Likely one of the most common types, a hex head cap screw is one that employs a hexagonally shaped head. This type is tightened by means of a 6 or 12 point socket tool, open end wrench or box end wrench having 6 or 12 points.

  • Twelve point cap screws
    twelve point cap screw

    Similar to hex head screws, 12 point screw heads have 12 points that require the use of twelve point sockets or box end wrenches having twelve point design. Twelve point screw heads often are flanged.

  • Torx head cap screws
    torx head cap screw

    Like a twelve point screw, a Torx will likely have a flange. At first glance, a Torx may appear like that of a twelve point screw, but they are certainly different. A Torx, having the appearance of a star shape with six points, requires a Torx style tool for tightening.

  • Flange head cap screws
    flange head cap screw

    As the name implies, these screw heads have a flange at their base. Flange head cap screws can employ any of the many different shapes of heads and sockets. The implementation of a flange incorporates the concept of a rigid flat washer at its base for distributing head load across a larger area thus reducing stress on the mating surface.

  • Vibration resistant cap screws
    vibration resistant cap screw

    Often, vibration resistant screws (and nuts) will be flanged. The underside of the flange is formed with tapered ridges. The slope of these ridges is such that the edges resist rotation in the direction of loosening. There are many design types of vibration resistant fasteners in addition to ridged flanges. Other vibration resistant fastener designs may have locking insersts in the threaded portion or elastomeric bands under the fastening head.

  • Socket head cap screws
    socket head cap screw

    Socket head cap screws have heads of varying heights but cylindrical in shape. The name "socket" implies the forming of the driving shape which is formed as a socket within the cylindrically shaped screw head. The most common type of socket head cap screw is the hex socket. It is common for machine parts to be counter-bored so that these screw heads can reside below the material surface.

  • Button head cap screws
    button head cap screw

    This type of screw utilizes a head in the shape of a button. Sockets are formed into the heads for driver insertion. A disadvantage of this type of screw is that the sockets are generally shallow which increases the likelihood of socket damage with repeated removal and reinstall. Common sockets are slots, crosses and hex.

  • Flat head cap screws
    flat head cap screw

    Flat head screws have a flat top surface. The base of the screw head is an inverted cone shape that tapers from the flat top diameter to the shoulder diameter of the screw. The purpose of this design is to have screw installation where the screw head is flush with the surface of the material the screw passes through. A method of drilling called countersinking is required for these screw types.

  • Set screws
    set screw

    Set screws are usually socket head in nature, but do not have heads. Set screws are basically threaded rods having a socket at one end and a specially formed tip at the other end for "setting". Setting tips can be pointed, flat, round, concaved or may have inserts such as bronze plugs. These screws are used to lock threaded rings to shafts to prevent them from moving or to set sheaves and gears to keyed or "D" bore shafts. Of course, their uses are not limited to these.

    set screw tips

  • Shoulder screws
    shoulder screw

    Shoulder screws are made with a smooth shoulder between the head and the threads. The shoulder is larger in diameter than the threads and smaller in diameter than the head. A shoulder screw functions as a pivot pin where mechanisms must be fastened together while allowing rotational movement, or where movement up and down along the shoulder is necessary.

Socket styles

  • Hex socket
    hex socket

    The hex socket is probably the most prevalent type used in socket head cap screws. Hex keys (often referred to as Allen wrenches) are used to rotate screws such as these.

  • Torx socket
    torx socket

    This socket has a star shaped pattern with 6 points and requires the use of Torx keys to rotate them properly. Torx is advantageous in that the "socket to tool" connection is very secure and resists slipping of the tool very well yet is quite strong.

  • Square socket
    square socket

    This type of socket isn't very prevalent, although some manufacturers do use them. The socket is fairly rugged. The disadvantage is that wrench insertion is limited to angles of 90 degrees apart.

  • Slotted head
    slotted socket

    This simple design is very common to machine screws having flat and button heads. The design is simply a slot cut straight across the diameter of the screw head. Slotted screws have the disadvantage that the screw driver easily slips out of the slot, requiring greater attention on the part of the person installing the screw.

  • Phillips head
    phillips socket

    This is the traditional cross shape formed into the heads of many button and flat head screws. With this type of socket, it is more difficult to produce torque as high as one could with slotted screws. This design makes it much easier to install screws because much less effort is needed to keep the driver located in the screw head.

Standards

  • Sizes

    Bolts and screws manufactured are made to accepted industry standards with regard to size. Though lengths may be standardized by a manufacturer, standards for bolts and screw sizes typically refer to diameter and thread pitch. Standards exist for metric and inch sizes.

    Metric sizes

    Note: The following table may not include every possible size of metric fastener.

    Thread diameter (mm) Thread pitch (mm)question mark icon
    1.6 0.35
    2 0.4
    2.5 0.45
    3 0.5
    3.5 0.6
    4 0.7
    5 0.8
    6 1
    8 1.25
    Thread diameter (mm) Thread pitch (mm)
    10 1.5
    12 1.75
    14 2
    16 2
    20 2.5
    24 3
    30 3.5
    36 4

    Inch sizes

    Note: The following table may not include every possible size of inch fastener.

    Thread diameter (inches) Thread pitch (tpi)question mark icon
    #0 80
    #1 64
    #1 72
    #2 56
    #2 64
    #3 48
    #3 56
    #4 40
    #4 48
    #5 40
    #5 44
    #6 32
    #6 40
    #8 32
    #8 36
    #10 24
    #10 32
    #12 24
    #12 28
    1/4 20
    1/4 28
    5/16 18
    5/16 24
    3/8 16
    3/8 24
    Thread diameter (inches) Thread pitch (tpi)
    7/16 14
    7/16 20
    1/2 13
    1/2 20
    9/16 12
    9/16 18
    5/8 11
    5/8 18
    3/4 10
    3/4 16
    7/8 9
    7/8 14
    1 8
    1 12
    1-1/8 7
    1-1/8 12
    1-1/4 7
    1-1/4 12
    1-1/2 6
    1-1/2 12
    1-3/4 5
    1-3/4 12
    2 4-1/2
    2 12
    2-1/4 4-1/2
    2-1/2 4
    3 4
  • Grades

    Several grades have been defined for bolts and screws. Different grades exist for metric fasteners than for inch fasteners. These grades define the mechanical properties for a given fastener size, mainly tensile strength and recommended torque.

    Inch Grades

    The most common inch grades are grade 2, grade 5 and grade 8, where grade 8 has the highest tensile strength of the three. Other grades are manufactured as well, with grades exceeding grade 8. These grades classify screws in a category of psi (pounds per square inch) rating according to SAE standards (Society of Automotive Engineers). For standard steel bolts, the psi ratings for tensile strength are the following.

    • Grade 2: 60,000 psi to 74,000 psi
    • Grade 5: 105,000psi to 120,000 psi
    • Grade 5.2: 120,000 psi
    • Grade 7: 133,000 psi
    • Grade 8: 150,000 psi

    Many cap screws are marked as to the grade. Hex head cap screws are typically marked with slashes around the top surface of the hex. Grade 2 typically has no slashes, grade 5 has three slashes and grade 8 has 6 slashes. It is important to realize that not all screws are marked, so identification of a grade may not be so easy. Some may be marked differently, according to a manufacturer's specific standards, or to agency standards other than SAE (such as ASTM), so it is important to know what you are dealing with before choosing a specific screw.

    Metric grades

    Understanding metric grades is no different than inch grades. Only units of measure differ, and of course the grade categories do not fall in line with the inch grades. Regardless, tensile strength is the primary determinate. For standard steel metric bolts, the tensile strength is given in Mega Pascals ([newtons per square meter]x10^6).

    • Grade 4.6: 400 MPa
    • Grade 4.8: 420 MPa
    • Grade 5.8: 520 MPa
    • Grade 8.8: 830 MPa
    • Grade 9.8: 900 MPa
    • Grade 10.9: 1040 MPa
    • Grade 12.9: 1220 MPa

    1 Pa = 1 N/m2 = 1.4504x10-4 lb/in2 (psi)

Materials

The most common material used in bolts is low to medium carbon steel that is heat treated, quenched and tempered. Fastener materials today are not limited to steel. To generalize a bit, it really depends on the application (the engineering requirements). For example, bolts that would be submerged in salt water would have different material requirements than bolts that reside in oil. The bolts in oil would likely never suffer from corrosion whereas the bolts in salt water would corrode if made of carbon steel or low grade stainless steel. Therefore a high grade stainless steel may be needed.

Again, material selection for bolt manufacture depends on intended use. Materials that are used are low carbon steel, medium carbon steel, several different grades of stainless steel, aluminum, titanium and even plastics. Coatings such as zinc may be used for corrosion resistance. For carbon steel bolts, "bluing" or "blackening" can be done to improve corrosion resistance. Certainly more can be said, but the intent here is to provide a basic understanding as to the scope of materials.

Conclusion

This could be a very extensive article, but my attempt here is to provide a basic understanding of machine related threaded fasteners. For repair work and assembly processes, the people involved need not be concerned with the engineering side of screws. Touching on the matter as I have done here, though, should enlighten the reader as to the importance of using the correct fastener as may be specified by a manufacturer for a product. Size, material, grade and thread pitch are important and should not be underestimated.

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