Classification, Principles, Tightening Methods and Usage Precautions of Bolts
(Last Updated On: November 8, 2023)
Bolts are a very common thing in daily life. It also plays a huge role in industrial production. So in this article, TorcStark has collected and compiled some basic information about bolts, hoping to help everyone better understand bolts.
Classification of bolts
Bolts are mainly classified in the following ways
Head shape: hexagonal head, round head, square head, countersunk head, etc. Thread length: full thread and half thread. Thread profile: triangle, trapezoid, tube shape, etc. Thread direction: right-hand and left-hand.
The thread size of the bolt determines the size of the wrench used
Dimensions across sides(s/mm)
13 / 14
16 / 17
18 / 19
Bolt performance grade
The performance grade number of the bolt will be printed on its head, generally represented by X.Y.
X*100=nominal tensile strength value of this bolt material, unit: MPa
X*100*(Y/10)=Yield strength value of this bolt, unit: MPa
The performance levels are divided into more than 10 levels such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9.
① High-strength bolts: refer to bolts with a performance level of 8.8 and above. They are made of low-carbon alloy steel or medium-carbon steel and have been heat treated (quenched and tempered).
② Ordinary bolts: The rest are generally called ordinary bolts, which can be divided into refined bolts (Grade A and B: 5.6 or 8.8) and rough bolts (Grade C). The performance level is generally 4.6 or 4.8.
Bolt connection principle
Bolts and nuts are used to fasten two components with through holes and are detachable connections.
Bolt connection types can be divided into: according to the degree of fit between the screw and the through hole:
Ordinary bolt connection: There is a gap between the hole and the rod after assembly. The structure is simple, easy to assemble and disassemble, can be assembled and disassembled multiple times, and is widely used.
Reamed hole bolt connection: There is no gap after assembly. It mainly bears lateral loads and can also be used for positioning.
How bolted connections work
According to Huco’s law: after a solid material is stressed, there is a linear relationship between stress and strain (unit deformation) in the material.
Stress analysis of bolted connections (1)
Bolt shear connection: mainly relies on the mutual extrusion of the bolt rod and the hole wall to transmit force.
Bolt tensile connection: mainly relies on the pretightening force after bolt tightening.
When the stiffness of the connected components is large and the bolts are arranged symmetrically, each bolt will bear the tensile force acting on the connection equally. When the stiffness of the connected components is small, the flange of the connection will bend and deform, resulting in leverage force.
Stress analysis of bolted connections (2)
In addition to shear force and tension force, it will also be subject to other forces such as vibration, temperature, lateral stress (wind force, flow, pressure, etc.), load changes, etc.
Bolt tightening method
There are three bolt tightening methods:
①Torque tightening method: The principle is that there is a certain relationship between the torque and the axial pretightening force. This fastening method is simple and intuitive to operate and is currently widely used.
② Angle tightening method: The rotation angle is roughly proportional to the sum of the bolt elongation and the looseness of the tightened parts. Therefore, the method of achieving the predetermined tightening force according to the specified rotation angle can be adopted.
③Yield point tightening method: The theoretical goal is to tighten the bolt to just past the yield limit point.
Torque tightening method
Pretension is applied to the fastener by rotating the nut portion of the fastener, and the applied torque complies with the 5-4-1 rule.
Principle of bolt tightening sequence: tighten in the middle first, then both sides, diagonally, clockwise, and in stages:
Generally tighten in two stages: the first step is to tighten about 50% of the torque; the second step is to tighten to 100% of the torque.
The end of the bolt should be exposed 1 to 3 pitches outside the nut.
Precautions for using bolts
Preparing for bolt assembly
Bolts and nuts should be checked to see if they are clean and rusty, and whether there are burrs or bumps;
Check whether the plane where the connected parts contact the bolts and nuts is perpendicular to the bolt holes;
You should also check the tightness of the bolt and nut fit.
When assembling a nut and a flat washer, both the nut and the washer are facing away from the connected parts.
The marked side of the nut is the front side, and the rounded side of the washer is the front side.
Essentials of force application (for reference only):
Only apply wrist force
Apply wrist force and elbow force
Apply full body arm strength
Apply upper body force
Exert full body force
Apply full body weight
Apply full body weight
The key to preventing bolted connections from loosening is to prevent the relative rotation of the thread pair. There are three common ways to loosen bolts:
① Friction to prevent loosening: spring washer to prevent loosening, double nut to prevent loosening, self-locking nut to prevent loosening
② Mechanical anti-loosening: cotter pin anti-loosening, stop washer anti-loosening
③Permanent anti-loosening: edge punching method to prevent loosening, bonding to prevent loosening
Frequently Asked Questions about Bolt Removal
① When loosening the flange bolts of pipes and valves:
You should first loosen half of the bolts away from the body so that the remaining oil, steam, and water can be discharged from the opposite side to prevent being sprayed and burned.
② If the bolt cannot be unscrewed due to glue:
Most glues are organic solutions. You can soak them in banana water or alcohol for half an hour, or cover the bolts with alcohol cotton. After a longer time, the glue can be dissolved.
③If the bolts are rusty and cannot be unscrewed:
Spray WD-40 and wait half an hour. The spray will penetrate into the middle of the bolt and dissolve the rust.
Other methods: vibration, lubrication, fire baking, electric drill.
④If the bolt sliding teeth cannot be unscrewed:
Corrosive solutions can be used to slowly erode the bolts, such as dilute sulfuric acid and hydrochloric acid.
Use acids for metals and bases for plastics
Common misunderstandings about bolt use
① Replace fine with coarse:
For some important connecting parts, such as transmission shafts, flying shafts, etc., the bolts mostly use fine threads.
Fine thread bolts have smaller pitch and outer angle, have the advantages of high strength, good self-locking performance, and strong ability to withstand impact, vibration and exchange loads.
After using coarse-threaded bolts instead, they will easily loosen, come off, or break, causing accidents.
Bolts that bear lateral loads and shear forces (such as drive shaft bolts and flywheel bolts) must have a transitional fit with the bolt holes. The assembly should be strong and reliable, and can withstand lateral forces.
If you do not pay attention to inspection during assembly and install the bolts even when there is a large gap between the bolts and the bolt holes, the bolts may become loose or cut off.
Misconception: Thickening the nut can increase the number of working turns of the thread, thereby improving the reliability of the connection.
In fact, the thicker the nut, the more uneven the load distribution among the threads, and the easier it is to cause the connection to loosen.
④One female with multiple pads: (≥2 pieces)
Sometimes the bolt after assembly is too long, so someone installs many spring washers on one bolt.
During the tightening process, the spring washers are unevenly stressed, and some may break, reducing the pre-tightening force of the bolts.
Or produce eccentric load, which reduces the connection reliability of the bolt.
⑤The washer is too large: (GB/T 97.1-2002 M5~M36 in Table 1: the inner diameter of the washer is 0.3mm~1mm larger than the major diameter of the thread)
Wrong approach: Due to lack of suitable gaskets, replace them with ones with a larger inner diameter.
In this way, the contact area between the bottom of the bolt head and the washer is small, and the bearing pressure or locking force of the washer is reduced. Under the action of vibration and impact load, the bolt is easy to loosen.
⑥Torque does not match:
Wrong view: It is believed that bolts should be “rather tight than loose”, so they consciously increase the tightening torque of bolts.
Consequences: causing the bolt to slip or even break.
Wrong approach: For important bolts that need to be tightened with torque, use an adjustable wrench to save trouble.
Consequences: loosening due to insufficient torque, causing failure.
⑦ Improper locking: important bolts should be locked with anti-loosening devices after assembly
When locking with a cotter pin, a common mistake is to lock with a cotter lock that is too thin or a half-piece cotter lock.
When using spring washers for locking, a common mistake is that the opening distance of the washers is too small and the elasticity is lost.
When locking with a locking plate, a common mistake is to lock the locking plate at the edge of the nut.
When using a double nut lock, a common mistake is to install the thinner nut on the outside and not tighten it tightly.
There are rust stains, scale, iron filings, sand, burrs, etc. in the threads of bolts, nuts or holes.
Clean them before assembly. When tightening the bolts, due to the blocking effect of impurities, the torque value appears to meet the requirements, but in fact, the connector is not really tightened.
In the end
The above is an introduction to the classification, principles, tightening methods and usage precautions of bolts. TorcStark, as a service provider that provides bolt tightening solutions, has been studying bolts carefully. In this way, we can better provide you with perfect solutions based on bolts and working conditions. If you are currently facing bolt tightening problems, please contact us for solutions.