Terminology commonly used for mechanical properties of fasteners
Time：2019.05.20 Browse：315
Torque is a force that generates rotation. Here are some of the most common examples of torque:
1. Chord the watch 2. Turn the cap 3. Rotate the door handle 4. Screw in the screw.
Torque needs to be considered in most applications.
There are slight differences in the following four kinds of torques.
1. Driving Torque
2. Seating Torque
3. Break away Torque
4. Prevailing Torque
All these torques will be encountered in practical applications, but their importance varies with different operating conditions.
1. Driving Torque - The Rotating Force that must be applied to assemble the parts.
2. Locking Torque - The force required to achieve a predetermined degree of tightness in the assembly.
3. Loosening Torque - The rotating force required to separate the assembled parts.
4. Preset Torque - A characteristic set on the fastener that makes the fastener resist loosening by friction force when it is locked into the workpiece with thread. The driving rotating moment needed to overcome the friction moment is called preset torque.
Driving Torque: Driving Torque is the main consideration in the application of screw cutting, screw rolling and self-locking parts. As the maximum force required for rotating parts, the requirement is necessary. Excessive driving torque will cause rotation failure and rotation failure, all of which will increase the cost of fasteners, so it is necessary to reduce the driving torque as much as possible. This demand leads to another one. Drive-pull ratio is the relationship between the torque needed to drive fasteners and the torque needed to resist breakage or breakage of internal threads. The wider the range of this value, the more conducive to reducing poor assembly, repeated assembly and corresponding costs, the more applicable fasteners will be.
For self-tapping threaded screw, the required drive-to-Strip Ratio is 1:3, that is to say, there is one unit of driving torque, which requires three units of pull-off strength to match the thread strength.
Locking Torque: Locking Torque is the force that rotates the fastener to the required degree of tightness or torsion-tension. It is expressed as a maximum value. That is to say, it is important in any case that the action of less than the specified maximum torque should be given a specified degree of tightness or clamping force. Because fasteners should be able to lock properly to ensure normal assembly, but this should not be done. Excessive torque requirement will cause the failure of driving system, increase labor intensity and failure of matching, all of which will result in the increase of fastener cost.
Loose Torque: Loose Torque refers to the rotating force when the fastener is loosened from the locking state. This has the most practical significance under the condition that the fastener is easy to loosen. Loose Torque is generally related to the self-locking fastener (not only the self-locking fastener, but also the anti-loosening fastener). It is expressed as a minimum value, that is, the fastener can not be separated from the assembly by a moment less than the loosening torque.
Preset Torque: Technically speaking, preset Torque is a kind of counter-moment to rotational moment. It usually refers to the moment when the fastener which is fastened but not yet locked is turned out. Like the loosening torque, the preset torque is used in the situation of easy loosening, and its prescribed value is the minimum. The fastener must be fastened (but not all) under the action of less than the prescribed torque. Fixed locking) can not be rotated, each fastener requires a certain amount of torque to drive, which is the "driving torque" of the preset torque. Each preset torque fastener requires a larger torque to lock to ensure the appropriate locking state, which is the "locking torque". Each fastener needs a certain amount of torque to release it from the locking state, which is the "loosening torque". Each fastener requires a certain amount of torque to remove it from the unlocked but still locked state. This is the preset torque. Torque is measured in pounds or foot pounds. A pound-inch is the force exerted by a pound at an inch from the center of rotation. A pound-inch is the force exerted by two pounds at an inch perpendicular to the center of rotation. The force produced by a force of one pound acting on two inches. From this, it can be concluded that
Torque = Axis to Torque Vertical Distance x Force
A pound-inch divided by 12 gives a pound-foot. On the contrary, a pound-inch multiplied by 12 is a pound-foot. The unit of choice depends on the size of the value. For example, we usually use 100 pounds-feet instead of 1200 pounds-inches.
Note: Points-inches and pounds-feet are the correct units of technology. But we often write in terms of inches-pounds and feet-pounds. Torque is the force necessary to produce rotation. It is the force acting on the end of the arm to produce rotation.
Answer the questions (don't look at the previous ones):
1. What is torque?
2. What are the four different torques?
3. What is the unit of torque?
Check your answers by looking at the aforementioned materials. Write down the correct answers if there are any omissions.
Tensile Strength
Tensile strength refers to the material's ability to withstand breaking under external tension. Have you ever experienced breaking rubber strips? If so, you are testing the tensile strength of rubber strips. Tensile strength of fasteners is the same. It is the tensile value that fasteners can withstand without breaking them.
Tensile strength is one of the most common physical properties of fasteners. It is the ultimate strength (Ultimate Strength) of fasteners. It is also the basic index of the Load Bearing Ability (Load Bearing Ability) considered by fasteners in application.
Tensile strength is expressed by pound/inch 2 (PSI). It refers to the forces that can be applied to fasteners by means of an average distribution of the minimum cross-sectional area of minor diameters. For example, the breaking tension of fasteners is 100,000 pounds, and the minimum cross-sectional area of fasteners is 1 square inch, so the tensile strength of fasteners is 100,000 PSI.
Tensile strength = force / area = 100,000 pounds / 1 square inch = 100,000 PSI
Tensile strength is the ability of fasteners to resist axial tension (Axial Tensile). It shows the ability of fasteners to withstand axial tension loads. Tensile strength usually refers to Ultimate Tensile Strength (UTS). Because Yield Strength and Proof Load of fasteners are related to their tensile strength, we will discuss them later.
Answer the questions (don't look at the previous ones):
1. What is tensile strength?
2. What is the unit of tensile strength?
Check your answers by looking at the aforementioned materials. Write down the correct answers if there are any omissions.
Yield Strength
In theory, each axial tension will cause the fastener to elongate to varying degrees. Because the existing metal has its elastic modulus (Degree of Elasticity, or Young's modulus Moulde), the fastener will usually be restored to its original length after the load is removed. When the fastener cannot restore its original length, the load value is its Yield Point. It is the point where plastic deformation begins to occur when fasteners are subjected to axial loads.
In the fastener industry, we really hope that it can be used within its elastic limit to ensure the safety of the connection, rather than stretching it to the drop point. This will reduce the effectiveness of the fastener. When the fastener is stretched to the drop point, it will not be able to shrink back to the original length. This shrinkage provides an effective locking force for fastener connections. See how fasteners lock and work. Imagine a fastener as a coiled spring. Imagine a door tightened by a spring. When the spring does not exceed its drop point, it can effectively close the door. But when the spring is over-stretched and cannot return to its original length, the spring will fail and cannot close the door. But when the spring is over-stretched, it will not close the door. When stretching, it will reach its lowering point, and the spring will fail to tighten the door. The spring will lose its original tension. The same is true of fasteners. Once stretched excessively, it will lose its original tension.
Generally speaking, the yield strength is equal to 25% of the ultimate tensile strength. The drop point of a fastener is the point at which it generates permanent elongation under axial load.
Answer the questions (don't look at the previous ones):
1. What is the yield strength of fasteners?
2. What is the relationship between the tensile strength and the yield strength of fasteners?
Check your answers by looking at the aforementioned materials. Write down the correct answers if there are any omissions.
Note: The relationship between yield strength and tensile strength is not unchanged 25%. Generally speaking, the higher the tensile strength of the same material (whether caused by work hardening or heat treatment), the higher the ratio of yield strength to tensile strength, and the lower the ductility. For example, bolts of grade 4.6 have to be annealed because the ductility after cold forging can not reach 30% requirement. Tensile strength decreases, the drop strength and tensile strength are about 45-50%. After cold forging, the 5.6 grade bolts are not treated. The drop strength and tensile strength are about 35-40%. For 8.8 grade bolts, after cold forging, the drop strength and tensile strength are about 20-25%. For 10.9 and 12.9 grade bolts, after cold forging, the drop strength and tensile strength are about 10-20%.
Guarantee Load
To ensure that the load is the maximum axial tension that the fastener can bear without permanent elongation, let's take the spring as an example, assuming that the fastener is a spring, we can imagine pulling the spring to the maximum length without permanent elongation, that is to say, the fastener can be restored to its original length after removing the load.
This shows that there is a close relationship between the lowering point of fasteners and the assurance of load. In theory, as there are two adjacent points in a range, one is a little smaller than the other, the smaller value is to ensure the load, and the larger one is the lowering point. Because the two points are too close, they are regarded as the same in practical application. For example, the tensile strength of carbon steel fasteners is 100,000 PSI, so its guaranteed load is its lowering point, which is 75,000 PSI.
The knowledge and significance of load assurance is very important for business personnel, because sometimes they will be required to provide the tension and load that fasteners can bear in practical application. Remember that the general principle is that the best effect can be achieved by applying 75% of the force guaranteeing the load. This is a general principle of the tension of fasteners when they are in use. For example, the tension strength of fasteners mentioned just now is 1. 00,000 PSI, because the load is guaranteed to be 75% of the tensile strength, so the guaranteed load is 75,000 PSI. If the customer asks you how much tension this bolt can bear, you should answer "75% or (25,000*75%) 56, 250 PSI. Guarantee that the load is the maximum force that fasteners can bear without plastic deformation. Remember the following three important principles:
1. The guaranteed load of carbon steel is 75% of its tensile strength.
2. Locking the fastener to 75% of its guaranteed load will give full play to its maximum effect.
3. The fastener must be locked up to 50-60% of its tensile load.
Answer the questions (don't look at the previous ones):
1. What is the guarantee load?
2. What is the relationship between guaranteed load and tensile strength?
3. Fasteners should be locked up to what percentage of the guaranteed load?
4. A fastener with a strength of 100,000 PSI, how much tension would you lock it to (write out the calculation formula)?
Check your answers by looking at the aforementioned materials. Write down the correct answers if there are any omissions.
Torque-Tension
The relationship between torque and elongation: The relationship between torque and elongation refers to elongation and resistance when applied to fasteners. The relationship between torque and resistance is very important in application. As mentioned earlier, business people usually advise customers to ensure that 75% of the load is locked tightly. Customers will then ask "How much torque is needed to achieve this tension?" Before you ask the question, you must know why some customers put forward this question is reasonable. When using fasteners, we first consider applying appropriate tension. In this case, why do customers ask about the torque? Because fasteners are locked by applying torque when they are in use, it is more convenient to measure the torque than to measure the actual tension value.
Now you know why this is a meaningful question? You can think about the answer. First of all, there are different relationships between torque and resistance. Here are some situations that affect their mutual relations:
1. Surface condition of fasteners (natural color or plating)
2. Surface condition of thread fit
3. Undertaking Status
4. Thread Grade
5. Thread Type
6. Strength of fasteners
7. Strength of Material Matched with it
8. Lubrication status
All these differences will affect the relationship between torque and resistance in practical use.
Businessmen should be careful not to infer the relationship between torque and resistance for customers. Youhui has purchased devices specially designed to determine the relationship between torque and resistance. Obviously, it is difficult for business people to solve such a complex problem. It is the duty of business people to understand the actual usage of fasteners. What's more, and feedback back so that we can best solve the customer's problems. If you encounter problems related to torque, you should know:
1. How to use fasteners.
2. What material are fasteners made of?
3. What type of fasteners will you use?
4. How much tension (clamping force) do you need?
5. What kind of surface is covered?
6. What lubrication do you plan to use?
7. Other usage situations that you think are more important.
If possible, try to obtain the assembly samples of fasteners used. It is important to have a general understanding of the torque pull. Torque pull refers to the pull that occurs when the torque is applied to the fastener. Another related concept of torque pull is clamping force. This will be discussed later.
Answer the questions (don't look at the previous ones):
1. What is the relationship between torque and tension?
2. What is the relationship between the customer's torque and the pull force in a particular service condition?
Check your answers by looking at the aforementioned materials. Write down the correct answers if there are any omissions.
Clamp Force
The locking force is the force acting on the locked surface (i.e. the bearing surface) when the fastener fastens the object.
In application, the force acting on the locked surface is exactly the same as the tension on the fastener; in fact, the relationship between the torque locking force in the fittings and the ratio of the torque stress relationship of the fastener are exactly the same.
Torque is very important in most applications because proper tension in fasteners ensures correct assembly; remember that it is not the responsibility of salesmen to draw up a torque-stress diagram. But it is not that salesmen can avoid these problems, but that salesmen should collect necessary information to the relevant departments for providing it. The best possible answer.
Shear Strength
Shear strength is the ability of fasteners to prevent deformation when loaded perpendicular to the axial direction.
You may have broken the head of a golf ball or at least seen others do so. You may have noticed that when the club hits the ball, when the ball flies out, it also lands on the ground. People say: the ball head is cut off by the club. In fact, you accidentally tested the shear strength of the ball head, and you can see whether the ball head can withstand the club. The force perpendicular to the axis of the golf ball head. Therefore, the shear strength of the golf ball head should be small enough to not affect the hitting.
When fasteners are applied to loads perpendicular to the axis, the shear strength must be taken into account. Unlike golf heads, fasteners must be able to withstand these loads to complete their work.
The shear strength is usually estimated to be 2/3 of the tensile strength, so if the tensile strength of the fastener is 180,000 PSI, the shear strength is 120,000 PSI; in other words, the fastener should be able to withstand 120,000 PSI loads perpendicular to its axis.
Usually, the shear force occurs when the fastener is pinned vertically on two overlapping parallel materials while the direction of the two materials is opposite.
Only when the shear strength of the fastener is greater than this force will the fastener not fail.
Fatigue Strength
Fatigue strength is the ability of fasteners to withstand fatigue failure under cyclic vibration stress. In other words, it is the ability of fasteners to withstand variable loads for some reason.
Generally speaking, fatigue strength is much lower than its maximum tensile strength.
There are several kinds of fatigue, but vibration is the most common type.
A typical example of "vibration" fatigue is all "Head-bolt" used in diesel engines. Head-bolt is not loaded when the piston goes down along the piston cylinder, but when the piston rises again, the pressure in the piston cylinder gradually increases until it goes down again. The change and period of load during the whole process are related to the speed of the motor.
In order to overcome and ensure the fatigue failure of vibration variable stress, fasteners should be able to withstand more than the maximum stress in the cycle that may be encountered.
Fatigue failure is a factor to be considered for fasteners under shear or tension.
Ductility
Ductility is the ability of a material to deform permanently without cracking.
Ductility is mentioned in many different parts of the fastener industry, but the concepts are the same. Firstly, for the materials to be used in fastener forming, the better the material ductility is, the better the deformation ability is without cracking. We use clay as an example to illustrate ductility, although it is not a fastener material.
You can knead a spherical clay into a flat shape and its surface will never crack or break; that is, ductility; on the contrary, even slight deformation will lead to cracking and brittle fracture in the same test with stone. The two extreme cases mentioned above have the same properties. Some materials, such as aluminium, have good ductility and can deform very large without cracking. Material, while other materials, such as harder steel, will crack if slightly deformed; material ductility is the decisive factor of product forming process.
The ductility of materials used to form threads also needs to be considered, because threading is formed by deformation of materials around fasteners rather than cutting off redundant materials; and then, if the threads are rolled from clay and stone materials; first, a hole of the same size is punched on each material and then inserted into the screw, it can be seen that the materials around clay will move or deform without cutting effect; But when you insert the same screw into the stone, the stone material will crack. This is the brittleness of the material. Of course, no one will screw into clay or stone, but the material used to make the screw should have similar ductility. That is, the material can deform without cracking or fracture.
In some cases, fasteners will be impacted or hammered. Generally speaking, these fasteners should be deformed before fracture, which is very important. If fasteners are easy to break, they will crack under impact; if they have good ductility, they will only deform and not break.
Think again about clay and stone. If you have two cylinders of similar size and shape, one is clay and the other is stone; if you apply the same impact perpendicular to its axis, the clay will bend and the stone will crack. Fasteners are made of similar materials. Their ductility is different.
Only by setting all the requirements of fastener application can Friendship and customers work together to develop fasteners that are most suitable for each special occasion.
Ductility is the ability of a material to deform permanently without cracking.
Three elements of ductility should be considered in fasteners:
1. Material ductility of fasteners themselves.
2. Material ductility at the moulding site.
3. Ductility requirements of fasteners for special occasions.
Hardness
Hardness is an index of material resistance to friction, depression and bending.
The most important meaning of fastener hardness is to resist friction during assembly and/or its role in mechanical applications. For thread forming and thread cutting self tapping screw, because it uses internal threads from the matching hole extruded or tapped out, it is obvious that its hardness must be higher than that of the matching assembly. If not, then self tapping threads should be used in assembly. Two different hardness materials can be clearly separated, such as clay and steel. If you want to screw the steel threaded self-tapping screw into clay, because clay is soft and easy to deform, of course, it can be easily screwed in. Next, if you want to screw the clay threaded self-tapping screw into the steel, how can xxxga? Therefore, the self-tapping screw for thread forming must be harder than the matching workpiece.
Fasteners can also be used in some working situations. Especially for some shoulder-mounted screws, hardness is only important in several important bearing areas. These bearing surfaces must have sufficient hardness to bear the excess friction that increases rapidly and accumulates during assembly to ensure the function of the assembly and reduce maintenance. At one time, we take clay and steel as our work. For example, because steel is harder than clay, it will last longer as a mobile fastener than clay.
Another important factor is that the hardness of the material is directly related to the tensile strength, shear strength and ductility of the material.
1. The tensile strength increases.
2. The shear strength increases.
3. Reduced ductility.
Businessmen should understand how customers determine the associated hardness values for each strength level and ductility requirement application. Ask these questions carefully and refer them to the technical department so that Friendship can recommend what fasteners customers use.
There are two basic methods for hardening fasteners:
1. Cold working or work hardening.
2. Heat treatment.
When fasteners deform at room temperature, we call it intercooling. intercooling stresses are applied on deformed materials to deform them and residual stresses will harden them. This phenomenon occurs in both cold heading and extrusion.
The hardness of materials increases after several different heat treatments. These methods will be dealt with in the heat treatment chapter.
There are two basic methods for hardening fasteners:
1. Cold working or work hardening.
2. Heat treatment.
When fasteners deform at room temperature, we call it intercooling. intercooling stresses are applied on deformed materials to deform them and residual stresses will harden them. This phenomenon occurs in both cold heading and extrusion.
The hardness of materials increases after several different heat treatments. These methods will be dealt with in the heat treatment chapter.
The hardening of fasteners can be divided into three different ways according to their application situations:
1. Through Hardened.
2. Case Hardened.
3. Selectively Hardened.
All parts of fully hardened fasteners are hardened. Both the core and the surface are hardened. Surface hardened fasteners are hardened at shallow parts of the material surface. On fasteners, the surface will be much harder than the core. Selective hardening is done only at selected parts of the fastener. Treatment is usually done at the tail. Selective hardening is also much harder than other non-hardened parts.
The strength series of fasteners can be increased after full hardening treatment, and fasteners such as bolts can resist more tension without breaking after full hardening treatment.
The surface hardening treatment of fasteners can mainly increase their wear resistance during assembly. Surface hardening treatment is used in most thread forming and thread cutting self-tapping screw systems because the self-tapping screw must tap the matching internal threads on the matching workpiece. If these screw are fully hardened to the surface hardening level, the fastener will break, so in most fields. Closing is not practical.
When full hardening or surface hardening of fasteners is dangerous, selective hardening treatment can be used. Selective hardening treatment is mainly used for self-tapping threads in thread forming. Some self-tapping threads in thread forming hope to preserve the strength grade and ductility of full hardening because of their special use occasions. At the same time, sufficient tail and thread hardness must be used to form internal threads. Selective hardening treatment is used. Firstly, the fastener is hardened to the desired strength level, and then the tail and the end of the fastener are hardened to 4-5 threads to maintain the self-tapping function.
The cost of selective hardening treatment is much higher than that of full hardening treatment or surface hardening treatment. However, it is very valuable if such treatment is necessary in use.
As for the implementation of heat treatment, reference can be made to the heat treatment chapter of this manual.
Hardness is an index of material resistance to friction, depression and bending.
Hardness affects the friction of fasteners during assembly.
Hardness testing should be carried out on a machine that can press a material under a specific load and measure its depth. The most commonly used Rockwell hardness tester and Brinell hardness tester (or Vickers hardness tester) have a variety of scales to measure the hardness under different conditions.
Hardness is related to strength and ductility and is obtained by work hardening or heat treatment hardening.
