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We at Salina Nut & Bolt would like to get down to the "nuts and bolts" of your fastener needs. All too often a project or repair is done with inappropriate fasteners because of a lack of engineering knowledge of what the appropriate fastener is, how it is chosen and how it is installed. Any improperly chosen or misinstalled fastener can lead to several unexpected problems. Some of those surprises can include unnecessary expense, less bonding strength than assumed, corrosion, thread stripping or premature assembly failure.
You must also keep in mind that any assembly failure has the potential for property damage or personal injury that can be much more costly than the cost of providing the proper fastener in the first place. So, for the long run and your sanity, avoid those problems that will arise from using just any ol' rusty bolt from that old coffee can in the back room.
Consumers and manufacturers recognized early the need for standardization of minimum and maximum performance capabilities, dimensional conformity and uniform coding systems. Although each group has adapted standards particular to their user industries, most groups follow a similar system of standards which is basic to the entire industry.
The American National Standards Institute (ANSI) in New York and the Society of Automotive Engineers (SAE) located in Pennsylvania are among the most commonly seen specifications data. Other institutes issuing standards include the American Society of Testing and Materials (ASTM) and the Industrial Fasteners Institute (IFI). Also, the U.S. Government as well as particular industry associations have specialized requirements for fasteners with applications in military, scientific and other unusual applications.
Complete SAE and ASTM marking systems are much more extensive than can be illustrated here but the three most common grades of fasteners and their familiar markings are shown in the chart above. A comparison of two common marking systems (SAE & ASTM) used extensively in industry shows the similarity in the systems, but also some important differences in strength specifications.
Selecting the Right Fastener Grade:
Cap Screws, commonly referred to as "bolts," should be used for their firm clamping action along the length of the bolt and not as a pivot point for rotation of the connected parts around the bolt shaft. The residual tension that is set up between the cap screw and the nut when tightened is the purpose of the fastener. The degree of that tension is, in part, what determines the proper grade of fastener to use. The three most common grades in the SAE specifications are grades 2, 5 and 8 and they (or their equivalents in other systems) can meet the needs of most applications.
A Grade 2 fastener is made from a low or medium carbon steel. A Grade 5 fastener is also made from a medium carbon steel but is Quenched and tempered for additional strength. Grade 8 fasteners are made from a medium carbon steel which is alloyed for increased strength and are also Quenched and tempered in finishing for superior strength. Each grade is, of course, progressively more expensive. Hex Head Cap Screw:
The workhorse of the various grades of fasteners is the Grade 5 cap screw. Its use is common because of its cost-to-strength ratio. It is not, however, an answer to all your clamping needs. Knowing what clamping strength you need helps determine the proper grade of fastener to use. Use of fewer bolts at a higher grade could cost less than its lower grade counterpart of equal combined strength. Fewer fasteners also speeds assembly time in manufacturing but select the fasteners to save you time without sacrificing Quality or safety. Also, using the same number of fasteners of a higher grade allows you to use a smaller diameter bolt to achieve the same assembled strength.
Socket Head Screws:
Whenever space is limited around the fastener head, the use of an internally wrenched fastener, perhaps in a counterbored hole, may solve the problem. But remember that the use of alloy steel in socket head fasteners is just as much for wrenching strength as it is for proof load requirements. In fact, trying to 'wrench to proof load may strip the bearing surface of the socket before proof load is achieved. The smaller the wrench size, the more likely this is to occur. Even the wrench itself may fail from excessive torque before a proper full bolt load is achieved.
The engineering that goes into producing a Quality hex nut belies its simple appearance. When torque is applied to tighten a nut/bolt combination, two forces are created: longitudinal and radial. Longitudinal force lies parallel to the bolt shaft and radial force is generated by the inclined plane of the threads and radiates perpendicular to the longitudinal axis. These two combined forces sustain the load that is placed on the nut/bolt combination. Torsion is applied around the bolt shaft while tightening and disappears as soon as the twisting action of wrenching ceases.
The nut must be able to transfer all this load (stress) through its physical structure onto a bearing surface without producing failure in the form of thread shear, dilation or crushing. Thread shear can be caused from the nuts material makeup or not enough nut height (i.e.: not enough threads) to distribute the load evenly on sufficient threads. Dilation is caused when the torque applied in wrenching combined with the inclined plane of the thread causes the nut to expand out away from the bolt shaft. When this happens, the contact between the threads of the hex nut and bolt lessens and is concentrated on the weaker tips of the threads.
The doughnut-shaped bearing surface of a hex nut must be large enough to sustain the load generated when tightened. If it is too small, crushing of this bearing surface will cause the bolt load tension to relax and loosen its bond. After a nut/bolt combination is tightened and the residual bolt-load is in place, vibration and fatigue can still cause any of these stresses to occur.
Decendants Of The Hex Nut:
Going beyond the familiar Hex Nut, other derived forms of the Hex Nut include the" Heavy" Hex Nut, Thick Nuts, Jam Nuts, and various forms of the Lock Nut.
Heavy Hex Nuts are 1/8" larger in wrench size thus having thicker walls, resist dilation in heavier loads. Thick Hex Nuts are larger in the height direction. The added height provides more threads to distribute the load. For high temperature applications, use a 2H Nut. Fine threaded High Nuts are used on "U" bolts, shackles and tractor pad bolts.
The simplest of the locking nuts is the Jam Nut. Thinner than the standard Hex Nut, it is used against a Hex Nut to lock its position forming an economical and secure bond. The Jam Nut is placed between the Hex Nut and the bonded surface to allow the stronger Hex Nut to carry the load placed on it.
To counteract the effects of vibration in an assembly, nuts are manufactured incorporating a locking device of special design which provides added friction or resistance against loosening. Numerous manufacturers produce many familiar and not-so-familiar forms of the Lock Nut for just about any possible environment of application.
Lock Nuts are manufactured and graded A, B or C and correspond in load requirements to SAE Grades 2, 5 and 8 respectively, An improper application would be using a Grade A or B Lock Nut on a Grade 8 screw failure is inevitable. Conversely, using a Grade B or C Lock Nut on a Grade 2 screw is a waste of money. In either case, improper stressing of the nut (the first case) or the screw (the other case) will occur if the pair is tightened to full load capacity of the stronger component.