Torque
Torque is a force that produces rotation Here are some examples of the most common torques:
1. Chord the watch 2 Unscrew the cap 3 Rotate the door handle 4 Screw in screws
Torque is an important consideration in most applications
The following four torques are slightly different
1. Driving Torque
2. Locking torque
3. Break away Torque
4. Prevailing Torque
All of these torques will be encountered in practical applications, but their importance varies according to different service conditions
1. Driving torque - the rotating force that must be applied to combine the parts
2. Locking torque - the force required to make the assembly of parts reach the preset tightness
3. Loosening torque - the rotational force required to separate the combined parts
4. Preset torque - a feature set on the fastener, which makes the fastener produce resistance due to friction when it is locked into the thread workpiece to prevent loosening. The driving rotation torque required to overcome the friction torque 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 necessary for rotating parts, requirements are 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 driving torque as much as possible This requirement leads to another engineering requirement "drive pull off ratio" It is the relationship between the torque value required to drive the fastener and the torque value required to match the internal thread to resist detachment or damage. The larger the range of this value, the more conducive to reducing poor assembly, repeated assembly and reducing the corresponding cost, the more suitable the fastener will be
For thread rolling self tapping screws, the required drive to strip ratio is 1:3, that is, with one unit of driving torque, three units of tensile strength are required to match the thread strength
Locking torque: The locking torque is the force to turn the fastener to the required tightness or torque tension value It is expressed as a maximum This means that the specified tightness or clamping force should be obtained when the torque is less than the specified maximum torque, which is very important in any case The fastener should be able to lock properly to ensure normal assembly, but this should not increase the torque excessively Excessive torque requirements will lead to failure of the drive system, increased labor intensity, and failure of coordination, all of which will lead to increased fastener costs
Loosening torque: Loosening torque refers to the rotating force when loosening the fastener from the locked state This is most practical when fasteners are easy to loosen Loosening torque is generally related to self-locking fasteners (not only self-locking fasteners, but also fasteners for locking). It is expressed as a minimum value, that is, fasteners cannot break away from assemblies with a torque less than the loosening torque
Preset torque: technically, preset torque is a resistance to rotation It usually refers to the torque when screwing out the fasteners that are locked but not locked Like the loosening torque, the preset torque is used in the situation where it is easy to loosen, and its specified value is also the minimum value Fasteners must be locked (but not necessarily locked) without rotation under the effect of less than the specified torque. Each fastener requires a certain torque to drive, which is the "driving torque" of the preset torque. Each fastener with a preset torque requires a greater torque to lock, so as to ensure an appropriate locking state. This is the "locking torque" Each fastener needs a certain torque to release it from the locked state, which is called "loosening torque" Each fastener needs a certain torque to take it out when it is not locked but still locked. This is the preset torque Torque is measured in pounds inches or foot pounds One pound inch is the force produced by one pound force at a distance of one inch perpendicular to the center of rotation 2 lb in is the force generated by a 2 lb force acting at a distance of 1 in. perpendicular to the axis of rotation or a 1 lb force acting at 2 in From this, it can be concluded that
Moment=vertical distance from axis to torsion x force
Divide the pound inch by 12 to get the pound foot. Conversely, multiply the pound inch by 12 to get the pound foot. The unit should be selected according to the size of the value For example, we usually use 100 lb ft instead of 1200 lb in
Note: Both pound inch and pound foot are technically correct units But we often write in inch pounds and foot pounds Torque is the force necessary to produce rotation It is the force acting on the end of the arm to make the object rotate
Answer the question (don't read the previous content):
1. What is torque?
2. What are the four different torques?
3. What is the unit of torque?
Check your answers by checking the above materials and write down the correct answers if there are any omissions
Tensile Strength
Tensile strength refers to the ability of materials to resist breaking under external tension Have you ever had the experience of breaking the rubber strip? If so, you are testing the tensile strength of the rubber strip The tensile strength of fasteners is the same. It is the tensile value that fasteners can bear to be applied on them without breaking them
Tensile strength is the most common physical property of fasteners It is the ultimate strength of fasteners It is also a basic indicator of the load bearing capacity of fasteners in application
The tensile strength is expressed in pounds per inch 2 (PSI) It refers to the force that can be applied to the fastener evenly distributed on the minimum cross-sectional area of the fastener (Cross Sectional of Minor Diameter) For example, if the breaking force of a fastener is 100000 pounds and its minimum cross-sectional area is 1 square inch, then the tensile strength of the fastener is 100,00PSI It means:
Tensile strength=force/area=100000 pounds/1 square inch=100000 PSI
Tensile strength is the ability of fasteners to resist axial tension It shows the ability of fasteners to withstand axial tensile load The tensile strength usually refers to the Ultimate Tensile Strength (UTS) Since the Yield strength and Proof Load of fasteners are related to their tensile Strength, we will discuss them later
Answer the question (don't read the previous content):
1. What is tensile strength?
2. What is the unit of tensile strength?
Check your answers by checking the above materials and write down the correct answers if there are any omissions
Yield Strength
Theoretically, each axial tension will make fasteners stretch to different degrees Because all existing metals have their elastic modulus (Degree of Elasticity, or Young's Module), usually the load is removed and the fastener will return to its original length When the fastener cannot recover its original load value for a long time, it is the Yield Point The yielding point is the point at which the fastener begins to produce plastic deformation under axial load
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 yield point for use Because this will reduce the effectiveness of fasteners When the fastener is stretched to its yield point, it cannot shrink back to its original length This shrinkage provides an effective locking force for fastener connection We can clearly see how fasteners are locked and how they work Imagine that a fastener is like a spring that is wound tightly Imagine a door tightened with a spring. When the spring does not exceed its falling point, it can effectively close the door But when the spring is overstretched and cannot be restored to its original length, the spring will fail and cannot close the door tightly However, when the spring is overstretched, it will reach its falling point. At this time, the spring will become invalid and cannot tighten the door The spring loses its original tension The same is true of fasteners. Once they are overstretched, they will lose their original tension
In general, the reduced strength is equal to 25% of the ultimate tensile strength The yield point of a fastener refers to the point at which it bears axial load and produces permanent elongation
Answer the question (don't read the previous content):
1. What is the yield strength of fasteners?
2. What is the relationship between tensile strength and yield strength of fasteners?
Check your answers by checking the above materials and write down the correct answers if there are any omissions
Note: The relationship between yield strength and tensile strength is not an invariable 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 will be, and the lower the ductility. For example, for bolts of Grade 4.6, because the ductility cannot meet the 30% requirement after cold forging, they must be annealed. At this time, the yield strength and tensile strength will be reduced, The yield strength and tensile strength are about 45-50%. After cold forging, Grade 5.6 bolts will not be treated after cold forging. The yield strength and tensile strength are about 35-40%. As for Grade 8.8 bolts, after cold forging, they will be quenched and tempered. The yield strength and tensile strength are about 20-25%. Grade 10.9 and 12.9 bolts will be quenched and tempered after cold forging. The yield strength and tensile strength are about 10-20%
Proof Load
To ensure that the load is the maximum axial tension that the fastener can withstand 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, after removing the load, the fastener can return to its original length
This shows that the yield point of fasteners is closely related to the guaranteed load Theoretically, if there are two adjacent points in a range, one is a little smaller than the other, then the smaller value is the guarantee load, and the larger value is the surrender point Because two points are too close together, we regard them as equal in practical application The guaranteed load of carbon steel fasteners is 75% of their maximum tensile strength For example, if the tensile strength of carbon steel fasteners is 100000PSI, its guaranteed load is its drop point, which is 75000PSI
It is important for business personnel to ensure the knowledge and significance of load, because sometimes they will be required to provide the tension and load that fasteners can withstand in practical application Remember the general principle that the best effect can be obtained by applying 75% of the force of the guaranteed load This is the general principle for the tension of fasteners in use For example, the tensile strength of the fastener we just mentioned is 100, 000 PSI. Because the guaranteed load is 75% of the tensile strength, the guaranteed load is 75, 000 PSI. If the customer asks you "how much tension can this bolt withstand", you should answer "75% of the guaranteed load or (25, 000 * 75%) 56, 250 PSI Ensure that the load is the maximum force that the fastener 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 function
3. Generally, the fastener must be locked to 50% - 60% of its tensile load to ensure its effectiveness
Note: The load shall be guaranteed to be a fixed value according to the nominal size and product grade according to the provisions. When the fastener bears the load to this specified value, it shall not produce any deformation that may cause assembly failure Similarly, the relationship between the guaranteed load and the tensile strength is not an unchanging 25%, which is the same as the annotation in the previous paragraph
Answer the question (don't read the previous content):
1. What is the guaranteed load?
2. What is the relationship between the guaranteed load and the tensile strength?
3. Fasteners shall be locked to what percentage of the guaranteed load?
4. For a fastener with a strength of 100,000PSI, how much tension will you lock it to (write the calculation formula)?
Check your answers by checking the above materials and write down the correct answers if there are any omissions
Torque Extension
Relationship between torque and elongation: The relationship between torque and elongation refers to the elongation and resistance when torque is applied to fasteners The relationship between torque and resistance is very important in application. As mentioned above, business personnel usually advise customers to lock fasteners with tension at 75% of the load The customer then asks, "How much torque does it take to reach this tension?", Before answering the question, you must be clear about why some customers are asking reasonable questions When using fasteners, the first consideration is to apply appropriate tension In this case, why do customers still ask about torque? Because the fastener is locked by applying torque during use, it is convenient to measure the torque than the actual tension value
Now do you know why this is a meaningful question? You can consider the answer. First, torque and resistance have different relations The following are some conditions that will affect their relationship:
1. Surface condition of fasteners (natural color or electroplating)
2. Surface condition of thread fitting
3. Bearing surface condition
4. Thread grade
5. Thread type
6. Strength of fasteners
7. Matched materials