The concept of class and grade bolts
Depending on the type of use, the screws are usually made of low carbon iron grade 4.6 and 5.6, high carbon dry iron grade 8.8 and above, steel and titanium. Each of these raw materials has its advantages and disadvantages that give the screw a unique feature for a special use. In general, steel and galvanized screws are suitable for wet areas or in industry and acidic and corrosive environmental conditions due to their high resistance to rust and corrosion. Bolt grade is the most important factor in determining the amount of stiffness, tensile stress, shear stress, torsion, and in fact the physical and inherent properties of bolts, and in fact what is stated in engineering handbooks for bolt selection is usually more In relation to the dimensional characteristics of nuts and bolts and this class or grade. The bolt class is specified by two numbers and is in the international standard system. The first number is multiplied by 10 tensile loads in kilograms, which can withstand one square millimeter of the screw cross section. The second number multiplied by 100 is the percentage of the first time that the screw can withstand without changing shape in the tensile state.
Class 5.6 bolts and nuts
They are known as iron screws and are classified as medium to low end nuts. They are not widely used in heavy industries and heavy loads and only have a connecting role. Each square millimeter of them can withstand a load of only 50 kg
Class 8.8 bolts and nuts
They are known as dry bolts and because they have a moderate level of hardness and durability, they are widely used in various industries and are in fact the most widely used bolt class. They can withstand a load of 80 kg in tensile per square millimeter
Class 10.9 bolts and nuts
They are one of the most widely used bolts and nuts in the construction industry. Silos and rigs are more brittle than the lower class and each square millimeter of them can withstand a load of 100 kg and can withstand up to 90% of this load without changing shape and elongation.
Class 12.9 bolts and nuts
Class 12.9 nuts are the most widely used types of connections in the heavy industry of telecommunication towers and bridge and dam construction industries. Based on this, each square millimeter of the cross-sectional area of the screw can withstand a tensile load of 120 kg and can withstand up to 90% of this load without deformation.
Grade bolts and nuts 2
Grade 2 bolts in the American system are according to ASTM A307 standard and are usually supplied with zinc coating and its consumption is more sufficient in cases where medium hardness is sufficient. They are usually produced from low carbon steel.
Grade bolts 5
Grade 5 bolts and nuts in the American machine are very similar to the A325 standard in the international system and have more carbon grade 2 and are widely used in the valve and pump machinery industry.
Grade bolts 8
The hardest and most carbon at the same time the driest type of nut in the American system. These bolts are compared to class 12.9 in the international system and although they have slightly different heat and combination operations, they have very similar efficiency.
Galvanized bolts and dachromat
The plating of bolts and nuts has long been a process for making them resistant to humid, acidic and ammonia environmental conditions. Plating can vary depending on the sensitivity, type of use and concentration of corrosive substances
For example, the bolts used in the oil and gas industry require more resistant plating than the water and wastewater industries, and the former uses cadmium plating and the latter cold and hot galvanizing.
The most common types of plating are hot-dip galvanized and hot-dip galvanized bolts. The cold galvanizing process only puts a thin layer of zinc on the outer surface and provides any small scratches in those conditions for rusting and is therefore only suitable for cases where there is a possibility of rising humidity and in conditions with humidity. The above is not very efficient
The use of hot-dip galvanizing to protect the surface of bolts in hot, humid and somewhat acidic conditions has long been common. However, this process has many weaknesses compared to dachromat plating. One of the most important is the hardness reduction due to the high process temperature of nearly 460 degrees. Another disadvantage compared to dachromate coating is the increase in surface thickness and the problem in easy connection of hot-dip galvanized bolts and nuts, so that usually after hot-dip galvanizing, the nuts are oversized for easy connection, which in turn increases the corrosion of the nuts.
Metric and inch bolt code reading
Metric and inch bolt code reading Codes are used to introduce bolts. These codes are introduced differently in the American metric system and the standard system. What is fixed in all codes in all systems is the type of screw, the dimensions of the screw including the diameter and length in the metric system in millimeters and in the American system in inches and the grade or grade of the screw.
Bolts in general
In general, for all types of screws, what is written in the screw description is included
Screw type + wrench type + screw flower + screw material + screw dimensions
for example
Machine screw Phillips pan Stainless steel 18-8 1/4 – 20 x 2 ”
Four-way machine screw, lens head, stainless steel grade 18-8, diameter 1/4 inch, 20 gears per inch and length 2 inches
Allen screw code reading in metric system
Hex Bolt M22X3.0X80 DIN933 8.8 Hot deep Galv
Hexagon screw diameter 22 mm length 80 mm step length 3 mm DIN933 all-gear production standard, class 8.8 with hot-dip galvanized coating
Allen Bolt M16X2.0X60 DIN912 10.9 Dacromet
Allen screw diameter 16 mm Step length 2 mm Length 60 mm Production standard 912 Class 10.9 Dachromat plating
Therefore, in the metric system, the elements that make up the bolt and nut code are:
HEX Bolt and Socket Bolt / Allen Bolt
Screw dimensions M22X3.0X80 Screw diameter-step length-screw length
Production standard DIN933, DIN931, DIN9614, DIN320, DIN490,….
Classes 5.6, 8.8, 10.9 and 12.9
Plating
Coding Allen screws and hexagons in the American system
In the American system, all numbers are expressed in inches, and unlike the metric standard, which specifies the step length of the screw, in the American system, the number of gears is expressed in inches.
Hex Bolt 1-1 / 4 – 20U.C x 3 ″ G5
1-1 / 4 inch diameter hex screw with 20 gears per inch, coarse gear, 3 inch screw length and 5 screw grade
The UC standard United Coarse refers to a large tooth that stands in front of the UF United Fine fine gear.
Master bolts are also produced and read in the American system and are usually inches
Stud Blot 1-3 / 4X2-1 / 2 A193 B7
Master bolt with a diameter of 1.3 x 2 inches and a length of 2.1 inches according to ASTM B7 standard
Common standard bolts are B7, B8, Stainless steel, B16
Step length and gear depth of metric bolts
Step Length and Gear Depth of Metric Bolt We have already talked about how to measure step length, screw diameter, step diameter and gear depth, and described the phase differences of the opposite gears in the bolts. Here we will discuss the dimensions and its importance in strength and yield stress
Metric standard bolts and nuts, which are now produced all over the world and accepted as standard, are produced with two types of standard coarse thread or coarse gear, fine thread or fine gear. Each of these two types of gears has advantages and disadvantages that we will discuss in the following
Material of screws, nuts and washers
stainless steel
Stainless steel is a combination of low carbon steel with chromium to increase corrosion and rust resistance and is a common material in the production of bolts. Stainless steel has a very high resistance to corrosion and since the strength properties are in the main structure of the material, the surface scratch does not reduce their resistance.
Of course, it is a misconception that stainless steel screws have higher strength, stiffness or tensile strength than steel screws. Rather, the structure of stainless steel is such that it has higher flexibility or malleability than steel, but it has less tensile strength and stiffness than steel bolts. The only resistance to corrosive agents is stainless steel bolts and nuts, which are preferable to steel. In addition, stainless steel material is not capable of thermal hardening due to low carbon. Stainless steel screws, unlike steel, do not have magnetic absorption properties of magnets or some of their grades have a very small amount
8-18 stainless steel, which has 18% chromium and 8% nickel in its structure and is the most widely used type of stainless steel in the production of tools, bolts and nuts.
Stainless steel 316, known as non-absorbent, which is not absorbed by magnets and is highly resistant to corrosive agents.
Stainless steel 410 which is harder than 8-18 but less corrosion and rust resistance
Steel bolts
Steel is the most widely used material in the production of fittings. Steel bolts are supplied both black and with various placements of zinc, hot and cold galvanized, chrome and cadmium plating. Steel screws are usually made in four grades 2, 5, 8 and alloy steel in the American standard machine and in four classes 5.6, 8.8, 10.9 and 12.9 in the international system. Grades 2, 5, and 8 are usually coated with a light blue or light yellow layer of zinc or galvanized to withstand rust.
Bronze silicone
Silicone bronze screws, also known as bronze screws, are made of copper and silicon alloy. Bronze screws are commonly used in marine structures and in the construction of boats, stainless steel is preferred because it has a higher corrosion resistance than stainless steel and has a higher hardness due to the copper inside. Bronze is very similar in color to copper, but the reason for less use of bronze screws is the high price of this metal.
aluminium
Aluminum is a soft, lightweight and corrosion-resistant metal. Like stainless steel, aluminum has an inherent resistance to corrosion, so surface scratching does not reduce its corrosion resistance.
Bolts are made from different alloys of aluminum. Compounds such as iron, manganese, copper, zinc and silicon are found in different percentages in the structure of these alloys, which give aluminum better tensile and thermal strength.
Advantages and disadvantages of hot-dip galvanized bolts
Advantages and disadvantages of hot-dip galvanizing bolts Hot-dip galvanizing process is one of the most widely used types of bolts and nuts along with cold galvanizing and dachromat plating. However, in the engineering world today, this plating has been replaced by dachromat plating due to its problems. Below we will discuss these advantages and disadvantages more
Advantages of hot dip galvanized bolts
In the hot-dip galvanizing process, zinc penetrates into the surface of the steel and creates a very durable surface coating so that scratches on the surface of the bolts and nuts do not expose them to rust. This coating of zinc or zinc varies from 35 to 85 microns depending on the diameter of the bolts.
Disadvantages of hot-dip galvanized bolts
The hot-dip galvanizing process takes place at a very high temperature of 445-465 ° C, which causes the hard bolts to drop a few rockets in most cases, so that the grade 10.9 bolts after the hot-dip galvanizing process are sometimes used in tensile tests. And the impact is as hard as 9.8. In this regard, the dachromat plating process is preferable to hot-dip galvanizing
During the hot galvanizing process, a layer of zinc is placed on the surface of the bolts and nuts, causing their nominal diameter to increase by a few microns on the surface and the entire circumference of a circle, which makes it difficult to fasten the nuts on the bolts. To solve this problem, the beads are sized before the hot-dip galvanizing process, which has its own problems, and again, after the galvanizing process, the connection problems are not well solved. The definitive solution to the problem of over-sizing in the hot-dip galvanizing process is to over-size the beads after galvanizing, which is actually removing the protective layer of zinc from the inside of the nut and exposing them to corrosion again.