When doing some screw and bolts tightening, have you ever wonder if you are using the correct size of screws and bolts? Are the screws and bolts you are using can withstand the stress?
Screws and Bolts are marked with specific load in tensile strength capacity in either numbers or markings on the head.
1.Grade 2 (//) screw and bolts. Made up to light duty, low carbon steel and usually between 25-30k psi (pounds per square inch) tensile strength load capacity.
2.Grade 4 (////)screw and bolts. Made up of medium duty, moderate carbon steel and has a load capacity between 42-50k psi tensile strength. It is either made using a cold rolled or forged approach of metal hardening.
3.Grade 5 (/////) screw and bolts. Made up of light heavy duty alloy carbon steel and has a load capacity of 58-70k Psi tensile strength. Its is usually made using a hot rolled and heat treated metal hardening process.
4.Grade 6-8 (////////) screw and bolts. Made up of heavy duty alloy steel (Cr-Moly)and has a load capacity of 72–180k Psi tensile strength. It is usually made using hot rolled and tempered SUS 440C / 316 for food grade for the heaviest working load capacity.
Separations of term from SCREW to BOLT start from defined ASME standard as follows: a 0 – 10 wire gauge sizes is screw, bigger than this size from 1/4” up is a bolt. For Metric, 1-5mm is screw while 6mm and up is a bolt.
Types of bolts
1.The Hexagonal head bolt. Hex-Heads are generally used for assembly where ease of tool access is sufficient such as socket, open end wrenches, or adjustable wrenches.
2.The Button head carriage bolt. Carriage bolt are used for wood and crate application. Note that head shank is made square to serve as anti-torsion lock when it is embedded in the material.
3.The Flat head slotted counter sink screws are designed for flushed head application so that surface is maintained flat. Classification can be from wood to metal sheet where location is significantly important as the counter-sink head normally locates the part in place. The counter-sink angle comes from 90° to 100° inclusive angle.
4.The Button head slotted stove bolts are good for aesthetics where smoothness of the head plays an impact on the assembly. It also use wider diameter of the head to gain more surface area.
5.The Pointed wood or metal screw (if slotted point, it is self tapping metal screw). Typical wood and metal screws are made from low carbon steel. They are driven in a pre-drilled. Holes as the size of the minor screw diameter. Self tapping or threads forming metal screws are normally with end slit to cut threads in metal.
6.The Pan Head slotted stove bolt and the Philips head stove bolt. These types of screws come in large commercial quantities as they are commonly used in general fastening applications as in toys, appliances, electrical connectors and terminals. They are normally made from low carbon steel, coated with zirconium, zinc, brass or copper where electrical conductivity is required. Some special electrical applications are made from whole material as brass, copper, beryllium or aluminum alloy.
7.The Winged nut bolt and screw are often used in applications where hand locking or tensioning is required.
8.The Stud bolt is threaded on both ends are intentionally designed to fully hold tight at one end having class “A” fit while the other end is with regular “B” or “C” fit to where the nut is fastened. The “A-fit” end is normally retained.
9.The Hex machine bolt. The significant difference of a machine screw or bolt is that the shank is smaller than the thread major diameter. These are made from medium to high tensile strength material and are used for assembly of motors, pumps and similar applications.
10.The Retainer screw. There are several designs of retainer screws, from square shank, rectangular or diamond dependent of application. The intention is to hold the specific part in place, or they rotate or move with the part.
11.The Socket head cap screw (Allen head cap screw). Socket head cap screws are commonly found in industrial machineries. The major consideration when using this type of screws includes; commonality, generally high tensile strength, miniaturization as it can serve in confine spaces for assembly and maintenance. These screws and bolts comes in heat treated, tempered, alloyed or SUS materials for food grade application.
12.The “U” Bolt. Typically designed for fastening and gripping on cylindrical parts as pipes and posts. For automotive industry, high tensile application “U” bolts are used to fasten suspension springs and engine mounts.
Rules in thread engagement on screws and bolts.
1.Minimum thread engagement must be 100% of the fastener major diameter. At best, 2x the bolt or screw diameter are essential considering softer materials / parts used in the assembly.
2.Nut engagement shall be at minimum of 1 major diameter of the fastener.
3.Where blind threaded hole is applied, a minimum of 1 major diameter clearance will be considered. This will allow room for foreign matters and trapped air to prevent rupture of the part.
4.When thread lock adhesive is applied, caution must be taken to apply only sufficient amount on the threads and not on the head. Sealant, gasket must be avoided to contaminate the fasteners, as they can go inside the threaded hole that will cause rupture.
5.Never attempt to use suspected worn threads as this will potentially create irreversible damage.
6.Use only true and certified replacement fasteners. Design engineers have carefully considered the forces and tensile strength of fasteners. Bad replacement is a mitigating safety risk.
7.Tighten the fasteners to its prescribed torque, from the designer specs and the fastener manufacturer recommendations.
Knowing your screw and bolts load capacities as well as its different type may determine the safety of your work not only to you but for others.
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