Many precision machined parts are useless until they are assembled into mechanical components. These assemblies require the use of many different types of fasteners. In this unit you will be introduced to different types of fasteners and their proper usage. One of the most fundamental tasks of the machinist is the use of fasteners. Threaded fasteners take on many different shapes and forms, but they all have one thing in common, the use of a thread. Although threads are used for adjustment purposes, measuring tool applications, and the transmission of power, the main use of a thread is as a fastening device.
Angle of Thread- The angle of the thread is the included angle between the sides of the thread (Figure 1). For example, the thread angle for Unified Screw Thread forms is 60 degrees.
Major Diameter-Commonly known as the outside diameter (Figure 2). On a straight screw thread, the major diameter is the largest diameter of the thread on the screw or nut.
Minor Diameter-Commonly known as the root diameter (Figure 2). On a straight screw thread, the minor diameter is the smallest diameter of the thread on the screw or nut.
Number of Threads-Refers to the number of threads per inch of length.
Figure 2 Major parts of a thread
Lead-The distance a screw thread advances in one revolution. The lead and the pitch of a single lead thread are the same. On double lead threads, the lead is twice the pitch. A double lead thread has two start points.
There are a great number of thread forms. In the later units of this course, you will examine these in more detail and you will have the opportunity to cut some threads. As far as fasteners are concerned, we will concentrate on the unified screw thread form. The unified thread form is an attempt to standardize thread forms in the United States, Canada, and Great Britain. Unified threads are divided into the following series:
Unified coarse and unified fine refer to the number of threads per inch on fasteners. A specific diameter of bolt or nut will have a specific number of threads per inch of length. For example, a ¼-inch diameter unified national coarse bolt will have 20 threads per inch of length. This bolt will be identified by the following specifications:
The ¼ is the major diameter and 20 refers to the number of threads per inch. A ¼ inch diameter bolt with a fine thread would be identified by the following specifications:
The ¼ is the major diameter and 28 refers to the number of threads per
This series, UNC, is the one most commonly used in the mass production of bolts, screws, nuts and other general fastening applications. It is also used for threading into lower tensile strength materials (bronze, brass, aluminum, and plastics) to obtain the best resistance to stripping of the internal thread. It is also used on quick assembly or disassembly, or if corrosion or slight damage is possible.
This series, UNF, when used on external threads has a greater tensile stress area than coarse threads of the same size. The fine series will resist stripping out better than coarse threads in areas where the external and mating internal threads are subjected to loads equal to or greater than the capacity of the screw or bolt. Fine threads are also used where the length of engagement is limited or where wall thickness demands a fine pitch.
Some thread applications can tolerate loose threads, while others require a tighter fit. An example of this would be the head of an engine. The head of your car or truck engine is held down by a threaded fastener called a stud. A stud is threaded on both ends. One end is threaded into the engine block. The other end uses a nut to tighten down the cylinder head. When the head is removed, you want the stud to remain in the engine block. This end requires a tighter fit than the end of the stud accepting the nut. If the fit on the nut is too tight, the stud will unscrew as the nut is removed.
Unified thread fits are classified as lA, 2A, 3A ... or 1B, 2B, 3B .... The A indicates an external thread. The B indicates an internal thread. The numbers indicate the class of fit. The lower the number the looser the fit and vice-versa. Class 2 fits are used on the largest percentage of threaded fasteners. The tighter the fit the closer the tolerance of the sizes of the thread and hence the more expensive to purchase. A typical notation of a unified thread form with fit tolerance would be:
In this particular case the class of fit would be a 2. The symbol A indicates an external thread.
With the importation and exportation of goods, especially in the automotive industry, metric threads have become the prevalent thread type on many kinds of equipment. The metric thread form is similar to the unified thread form in that it is based on a 60-degree thread angle. Metric thread series take the following form:
Where M is the major diameter in millimeters and the 1.5 is the pitch (distance from one thread to the next thread) in 1.5 millimeters, the 6 is the class of fit and the "g" symbolizes the external thread. This external thread would have a major diameter of 10 millimeters, a pitch of 1.5 millimeters, and a "medium" thread fit.
A general definition of a bolt is "an externally threaded fastener that is inserted through holes in an assembly." A bolt is tightened with a nut (figure 4). A screw is an externally threaded fastener that is inserted into a threaded hole and is tightened by turning the head (Figure 4).
Figure 4 A screw is used in a threaded hole, while a bolt is used with a nut.
From these general definitions a bolt can become a screw or the reverse can be true. This depends on how they are used. Bolts and screws are the most common of threaded fasteners.
The strength of an assembly in large part depends on the diameter of the bolt or the thread engagement of the screw. Thread engagement is the distance a screw extends into the threaded hole. The minimum thread engagement should be a distance equal to the diameter used; preferably you would like to have 1-1/2 times the screw diameter, for it is easier to remove a broken stud than it is to drill and tap for a larger screw.
Machine bolts are made with hexagonal or square heads.
The body diameter, the diameter of the unthreaded portion of the bolt below the head, is typically slightly larger than the nominal or standard size of the bolt. A hole that is to accept a bolt must be drilled slightly larger than the body diameter.
The machine screw is used for general assembly work. It is manufactured in both fine and coarse thread series and fitted with either a slotted or recessed head.
Machine screw sizes vary from No. 0(0.060) to ½ in(0.500) in diameter, and come in many different lengths (Figure 5).
Set screws have several different points (Figure 9). The flat head set screws are used where minimal indentation to the part is needed and where frequent adjustments are made. They are also used to provide a jam screw effect when a second set screw is added to prevent vibrating loose. A dog point set screw is used to hold a collar to a shaft. Alignment is always maintained with a dog point set screw because the shaft is drilled with a hole of the same diameter as the dog point. A cup pointed set screw will give a very good slip resistant connection.
In some instances bolts need to be fastened with just the right amount of pressure. In these instances the manufacturer of certain products will recommend a certain clamping force be applied to a particular fastener. Insufficient torque will usually result in parts working loose and causing a malfunction due to misalignment. Over tightening, on the other hand, can cause stress or warpage, which also might disturb alignment on assemblies. The "armstrong" method of tightening fasteners can also cause broken castings, broken bolts, or stretching of the fastener.
Steel has excellent elasticity; the ability, like a spring to stretch and then snap back to its original shape. Any fastener must reach its limits of stretching in order to exert clamping force. But also like a spring, an over stretched fastener takes on set, loses its elasticity, and cannot snap back to its original shape. Proper torquing will prevent this condition.
A popping or snapping sound is sometimes heard during the final tightening of a fastener. This popping sound indicates that the fastener is undergoing set. When a new fastener is being used and the popping occurs, the remedy is to back it off and retighten to the proper torguing specifications. When an old fastener is being used, and you hear this popping, take the fastener out, and clean the bolt and the internal threads out completely. The safer, more economical thing to do is to replace the old fastener with a new one.
Nuts utilize a hexagonal or square head and are used with bolts with the same shaped head. They are available in different degrees of finish.
REGULAR is un-machined (except for the threads).
REGULAR SEMIFINISHED is machined on the bearing face to provide a straight , flat surface for the washer.
HEAVY SEMIFINISHED is the same in finish as the semi-finished; however, the body is thicker for greater strength.
CASTELLATED or SLOTTED NUTS have a milled slot across the flats so they can be locked in place using a cotter pin or wire.
ACORN NUTS are used when appearance is of the most importance or where projecting threads must be protected.
Washers are used to distribute the clamping pressure over a larger area and to prevent marring. They can also be used to provide a larger bearing surface for bolt heads and nuts.
Non-threaded fasteners make up a large group of fastening devices.
Retaining rings are stamped rings, both internal and external, and are used to keep parts from slipping or sliding apart. While most retaining rings need a groove to seal them in position, some types are self locking and do not require the use of a recess.
A key is a small piece of metal imbedded partially in the shaft and partially in the hub to prevent rotation of a gear or pulley on a shaft. Here are some different types of keys.
SQUARE KEYS-The width is usually one-fourth of the shaft diameter. One half of the key is fitted into the shaft and one half is fitted into the hub (Figure 16).
GIB HEAD KEY-Except for the gib head, this key is identical to the square key. The gib head provides easy removal(Figure 16).
PRATT & WHITNEY KEY-The ends are rounded and this key is fitted into a slot on the shaft of the same shape (Figure 16).
WOODRUFF KEY-A woodruff key is semi-circular in shape and fits into a keyseat of the same shape. The top of the key fits into a keyway in the mating part (Figure 16).
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