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Table of Contents

1.The category and the use of fasteners

The meaning of grade 8.8 and 8.0 for bolts and nuts.

For unified imperial threads, there are three thread grades for external threads: 1A, 2A and 3A, and three grades for internal threads: 1B, 2B and 3B, all of which are clearance fits. The higher the grade number, the tighter the fit.

1.1A and 1B, the very loose tolerance grades, are used for the tolerance fits of internal and external threads.

2.2A and 2B, the most common thread tolerance grade specified for imperial series mechanical fasteners.

Grades 3.3A and 3B, the tightest fits, are used for fasteners with tight tolerances and for safety-critical designs.

For metric threads, there are three thread grades for male threads: 4h, 6h and 6g, and three thread grades for female threads: 5H, 6 H and 7H. The thread fit is best combined as H/g, H/h or G/h. For bolts, nuts and other fine fastener threads, the standard recommendation is to use a 6H/6g fit.

Carbon steel: strength grade mark code consists of two parts of numbers separated by “-“. Marking code “-” before the meaning of the digital part of the nominal tensile strength, such as 4.8 grade “4” said nominal tensile strength of 400N/MM2 1/100. “-” and the meaning of the digital part after the point indicates the yield strength ratio, that is, the nominal yield point or nominal yield strength and nominal tensile strength ratio. Such as 4.8 grade products yield point of 320 N/mm2.

Stainless steel product strength level mark by “-” separated by two parts. Sign code in the “-” before the symbol indicates the material. Such as: A2, A4, etc. sign “-” after the sign indicates the strength, such as: A2-70

Carbon steel: metric bolt mechanical properties grade can be divided into: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9 a total of 10 performance levels.

Stainless steel is divided into 60, 70, 80 (austenitic); 50, 70, 80, 110 (martensitic); 45, 60 (ferrite) three categories

Bolts and nuts can not be the same performance level, bolts choose A2-70, nuts choose A1-50. A2-50

In general, according to the principle of equal strength, 8.8 grade bolts choose 8 grade nuts, 10.9 grade bolts choose 10 grade nuts.

2. Stainless steel fasteners locking causes and countermeasures

Stainless steel bolts, nuts with thread locking cause analysis:

  1. the nature of the material reasons: locking or bite, often occurs in stainless steel, aluminum and titanium alloy fastener threads, these types of metal alloys have good ductility, and carbon steel has the essence of the difference; itself has the characteristics of corrosion prevention, will be damaged on the surface, the metal surface to produce a thin layer of oxide (in the case of austenitic stainless steel, is chromium oxide) to prevent further more deeper rusting. When a stainless steel fastener is locked, the pressure and heat generated between the threads will destroy and erase the chromium oxide layer between them, causing the metal threads to block/shear directly, and the phenomenon of adhesion will occur. When the adhesion continues (usually no more than one full thread turn), the stainless steel fastener will lock up completely and can no longer be removed or locked. Usually this series of blocking, shearing, adhesion, locking of a series of actions occur in a short moment, therefore, understand the characteristics of stainless steel products and follow the correct method of operation is the first step to prevent stainless steel fasteners locking.
  2. the internal causes of the locking phenomenon.

① The same furnace number material production of bolts and nuts assembled with the probability of thread locking will increase.

② The influence of thread accuracy, the external thread grade 6h with the internal thread 6H will increase the friction, (over fit) our production of external threads using 6g, (gap fit), thread size in the lower limit of the tolerance range.

The higher the Cu content in the material composition, the higher the chance of locking of the thread fit, but some fasteners do not use materials with higher Cu content due to cold heading deformation. The materials used by our company will cause the probability of locking with the thread from large to small: 302HQ ≥ 304HC ≥ 304

fastener's types
  1. the external reasons for locking phenomenon.

① not the correct choice of products, before use should confirm whether the mechanical properties of the product can meet the use of demand (such as the tensile strength of the screw and the safety load of the nut). In addition, the length of the screw should be selected appropriately, to tighten the nut to reveal 1-3 tooth pitch is appropriate.

② rough or foreign material sticky teeth, such as welding joints and other metal chips caught in between the teeth, often lead to locking.

③ too much force or locking speed is too fast as far as possible, choose torque wrench or socket wrench, avoid using movable wrench or electric wrench, because electric wrench often leads to locking speed too fast, the temperature rises rapidly and locking.

④ the angle of the force applied is not reasonable, the nut must be screwed perpendicular to the axis of the screw, do not tilt.

⑤ not use the washer, the use of the washer can effectively prevent the problem of locking too tight.

Prevent stainless steel fasteners in the use of the locking method.

Due to the good ductility of stainless steel, and carbon steel has the essence of the difference, improper use will lead to the bolt and nut with the inability to spin, that is, “lock up”, commonly known as “bite”, to prevent lock up mainly in the following areas.

① Correct choice of products.

  1. Reasonable choice of screw length and specifications, to tighten the exposed nut 1-3 tooth pitch is appropriate.
  2. Before use to confirm whether the mechanical properties of the product can meet the requirements of use.

② Reduce the coefficient of friction.

  1. The thread surface is clean (e.g. no dust sand, debris).
  2. It is recommended to apply water wax or add lubricant (such as butter, 40# oil) on the surface.

③ Correct operation method.

  1. The thread must be screwed perpendicular to the axis of the thread, do not tilt it.
  2. In the process of tightening, the force must be applied evenly and the tightening torque must not exceed the safety torque (see the safety torque table).
  3. Use torque wrenches or socket wrenches as much as possible, avoid using movable wrenches or electric wrenches.
  4. Avoid using them at high temperature and do not rotate them quickly, as the rapid rise in temperature will make them lock up more easily.

④Material interchangeable fit: (such as 304 material bolt and 316 material nut assembly fit)

Safety torque table (the following data is for reference only) unit: N.m

Specification M4*0. 7 M5*0. 8 M6*1.0 M8*1.25 M10*1. 5 M12*1.75

Safety torque 2.5 5 9 20 40 55

Specification M14*2.0 M16*2.0 M18*2. 5 M20*2.5 M22*2. 5 M24*3

Safety torque 100 150 180 220 400 510

3.The locking phenomenon of fasteners

Fastener users often reflect: why stainless steel fasteners sometimes have the problem of locking, while in the use of carbon steel fasteners does not often occur similar phenomena, is not the stainless steel fasteners material is soft, carbon steel fasteners are relatively hard reason? That’s right! Stainless steel and carbon steel has the essential difference. Stainless steel has better ductility, but its hardness and carbon steel has a certain gap. Austenitic stainless steel grade 316 head label A4-80 hardness can actually only reach the equivalent of carbon steel hardness grade 0.88. However, this statement can only be said to be half right.

hook and loop fastener

Locking (or bite) often occurs in stainless steel, aluminum and titanium alloy fasteners, these types of metal alloys themselves have anti-rust characteristics, will be damaged on the surface, the metal surface to produce a thin layer of oxide (in the case of austenitic stainless steel, is chromium oxide) to prevent further deeper corrosion, when the stainless steel fasteners are locked, the pressure and heat generated between the tooth pattern will When the stainless steel fastener is locked, the pressure and heat generated between the threads will destroy and erase the chromium oxide layer between them, making the metal threads directly block/shear, and then the phenomenon of adhesion occurs. When the adhesion phenomenon continues to occur (usually no more than a full circle of tooth diameter), will make the stainless steel fasteners completely locked, can no longer be removed or locked. Usually this series of blocking → shearing → adhesion → locking a series of actions occur in just a few seconds, therefore, understanding the characteristics of stainless steel products and follow the correct operating procedures are the first step to prevent stainless steel fasteners locking.

External causes of the locking phenomenon:

(1) not the correct choice of products before use should first confirm whether the mechanical properties of the product to meet the needs of use (such as the tensile strength of the screw and the safety load of the nut). In addition, the length of the screw should be selected appropriately, to tighten the nut to reveal one or two tooth pitch shall prevail.

(2) rough teeth or foreign objects sticky such as welding joints and other metal chips caught in between the teeth, often lead to locking.

(3) The force is too much or the locking speed is too fast. Use torque wrenches or socket wrenches as much as possible and avoid using movable wrenches or electric wrenches. Because electric wrenches often lead to too fast locking speed, the temperature rises rapidly and locks up.

(4) Wrong angle of force application direction, the nut must be screwed perpendicular to the axis of the screw. Do not tilt.

(5) No washer is used. The use of washers / retaining ring can effectively prevent the problem of locking too tight.

Prevent locking of stainless steel fasteners to prevent flicking measures.

  1. When using stainless steel fasteners, is it always a fixed speed to lock the nut?

If it is the first time or not familiar with the stainless steel product process users, you should ask your supplier about the relevant characteristics of stainless steel. Generally speaking, slowing down the locking speed can significantly reduce (or even avoid altogether) the chance of locking. Because heat often occurs when locking, when heat increases, the chances of locking will also increase. In use, the speed of locking on stainless steel fasteners should be lower than the speed of locking on carbon steel.

  1. Is the screw or nut lubricated before locking?

If the answer is “no”, it is recommended to use grease, molybdenum disulfide, graphite, mica or talc to lubricate the inner and outer threads to reduce the occurrence of locking. In fact, it is an additional layer of lubrication film between the nut and the screw.

  1. whether to use the same material grade of screws and nuts?

If the answer is yes, we can recommend the use of different grades of screws and nuts, such as 304 with 316, etc.. But we should pay attention to the choice of stainless steel and other needs to meet their own rust and corrosion resistance. Improve the locking.

Stainless steel and carbon steel has the essence of the difference, stainless steel has a better ductility, improper use will lead to screws and nuts with the inability to spin off. That is commonly known as “lock-up” or “bite”. Improve locking can be considered from the following aspects:

  1. The correct choice of products.

(1) Before use to confirm whether the mechanical properties of the product can meet the requirements of use (such as the tensile strength of the bolt and the safety load of the nut).

(2) The length of the bolt should be selected appropriately, in order to tighten the nut to reveal 1-2 tooth pitch shall prevail.

  1. Reduce the coefficient of friction.

(1) The threads must be kept clean.

(2) Properly add lubricant before use (such as: 40# oil, grease, preferably using bolt anti-seize).

  1. Correct operation method.

(1) the nut must be screwed perpendicular to the axis of the screw, do not tilt;

(2) in the process of tightening, the force must be applied evenly, and the force must not exceed the safe torque value (now this is a bit difficult).

(3) Use torque wrenches or socket wrenches as much as possible and avoid using movable wrenches or electric wrenches.

(4) They must be cooled when used at high temperature and not rotated rapidly when used to avoid rapid rise in temperature leading to locking (such as electric wrenches, etc.).

The screw bite includes nut and bolt bite, planting wire and bottom buckle bite, which is the most common problem.

Cause analysis.

  1. The threads were not cleaned up during the last reassembly, and there are burrs and rough surface on the screw buckle.
  2. The threads were damaged before reassembly or the threads were blindly screwed on without high temperature anti-burning agent, lubricant or molybdenum disulfide during assembly, resulting in thread bite.
  3. The heating process is incorrect when disassembling, so that the temperature of the screw part of the buckle is too high and it rises to death.
  4. When the temperature of the device is high, the bolt is disassembled.
  5. When the bolt works at high temperature for a long time, the oxide skin on the surface is thicker and a hard oxide film is formed by the large squeezing pressure between the bolt and the buckle. When the oxide film is broken, the burr is pulled out on the thread surface, causing the thread to bite.
  6. The bolt processing quality is not high enough, the finish is too poor, and the clearance between the threads is small.

Solution

  1. Each bolt, each nut and bottom buckle should be carefully tested and inspected before reassembly so that no burr is left.
  2. Before assembling the high temperature bolts, make sure to clean the original grease and lubricate them well before assembling.
  3. Strictly implement the maintenance procedures and manufacturer’s standards, control the temperature, and disassemble the bolts when the temperature of the device is reduced to the allowable temperature.
  4. About 4 h before disassembly, pour kerosene or loosening agent on the threads to lubricate the oxidation between the threads to facilitate disassembly. During the maintenance phase after disassembly, clean the oxidized skin of the bolt thoroughly.
  5. Check the new bolt carefully, and if you find that the screw buckle is not good or the finish is not good enough, replace the bolt or reprocess it and use it again after it is qualified.
  6. The original method is to ask a skilled gas welder to cut the nut with a gas cut to protect the screw, but the threads of the screw are often injured by mistake when cutting.

In the mechanical repair work, often encounter the nut or screw can not be disassembled, it is very difficult to think:

The main reason for this phenomenon is that the thread is often loose in use, some users of this riveting, welding and other fixed work; or due to collision or incorrect disassembly, resulting in the head of the thread upsetting or make the threads buckle; perhaps because the wrench is not suitable, the opening is too wide, the nut screwed round. In these cases, only the application of mechanical processing methods, such as chiseling, filing, will be fixed, upsetting, the phenomenon of disorderly buckle eliminated; the edge of the nut filed square, and then wrench to screw, in order to disassemble.

In addition, in addition to the above-mentioned cases, there are also due to the threaded connection, under long-term pressure, the phenomenon of adsorption, engagement, or because of oxidation and rust and make the two bonded together, so that the screw can not be. This can be disassembled by the following methods.

First, apply gasoline or kerosene to the threads to penetrate the rust layer of the threaded connection, and then unscrew it after 30 minutes.

The second is to use suitable disassembly tools. For example, use the movable plate hand as little as possible, and use the suitable fixed plate hand or socket wrench to unscrew it, and do not add the connecting rod at will.

snap fastener

Third, repeatedly tighten and loosen. Xu Xu screwed into 1/4 turn, and then out, repeatedly tighten and loosen. In this way, the flattening and shearing can be carried out at the same time, and the rust layer can be subjected to repeated tensile and compressive stresses to separate the rust layer from the base metal, so that it is easy to disassemble.

Four is to use the hand pound to knock around the nut, so that the threaded connection is shaken so as to separate, and then screw out.

Fifth, use a blowtorch to heat up the nut, so that the nut is heated and expands, while the screw is heated less and the expansion is smaller, resulting in a gap, when it is quickly unscrewed.

When using the above method, we should pay attention to the direction of the threads of the positive and negative buckle, otherwise, screwing the opposite, will be tighter and tighter, so as to break the screw. If you can’t screw it down, you should soak it with kerosene overnight.

Today’s message is about fasteners’ types and uses, if you have any question, please contact us.

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