Screw Basics
Big and Small Screws
Screws are known by various names, such as screws, bolts, rivets, screw rods, screws, small screws, small nail screws, among others. They come in a variety of sizes, and with modern technology, screws can be made in sizes of 1 millimeter or even smaller.
Small screws with sizes of 0.5 millimeters or less are commonly used in wristwatches, computers and cell phones. On the other hand, large screws are generally used in construction or bridge construction and can have a thickness of up to 50 millimeters, or even a width of 100 or 200 millimeters, depending on the requirements.
Types of screws
Screws come in many shapes and there are countless new types being developed by manufacturers around the world.
They are generally classified as female screws (which have nuts) or male screws (which include screws, small screws, set screws, turbine screws, wood screws, and others).
Small screws can be further divided according to the shape of their heads, which include round, butterfly-shaped, flat round, pan-shaped, countersunk and threaded screws with positive or negative shaped heads.
Screws, on the other hand, are typically six-sided, but there are also four-sided, flat-head, and round-head screws.
Like screws, nut shapes also vary between four-sided and round shapes, depending on the specific use of the screw.
Screws are also classified based on the specific industry in which they are used. For example, screws used in watches, bicycles, construction, automobiles, machinery and shipbuilding may differ in their materials and specifications.
Screw Specifications
In Japan, screw sizes are standardized based on ISO regulations, with uniform dimensions throughout the country. This standardization requirement was initially established for global uniformity.
However, these specifications are not implemented in other countries such as Europe and the United States. During the repair of imported goods, it becomes necessary to follow the overseas screw specifications.
Some commonly used screw specifications include Unified Thread Standard, SAE (Society of Automotive Engineers), Inches and Bolts.
Japan used to mainly use British threaded screws, but now they are transitioning to the ISO standard. British threaded screws are currently mainly used in civil engineering and construction. Measuring the depth of screws after they are turned is determined by pitch, which is the distance traveled during rotation.
The standard pitch is 1.0, but for automotive parts and other heavily stressed components, smaller pitches such as 0.8 are the norm. Screws with small pitches and shallow angles require smooth movements during the twisting process.
Camera and cell phone screws typically have small pitches, which must be preserved even if the screws are short. Small pitch screws mainly help in the miniaturization of products as they have smaller thread pitches and screw threads, making them smaller overall.
Types of screws
(1) Small Screws
Small screws are screws with small diameters and come with heads. According to the ISO standard, small screw heads can have flat, pan, countersunk, or grip head shapes, while the JIS standard also includes round, threaded, round, flat, and rounded flat head shapes. Small screws serve as a means of tightening and typically come with spiral grooves or cross holes to aid in turning.
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| Small round head slotted screw | Small flat head screw with cross recess |
(2) Positioning screw
It is used to fix the moving mechanical parts through the screw tip. The tip of the screw can be flat, pointed, cylindrical, concave, round, etc. It is commonly used as a clamping medium, with grooves, hexagonal socket, square head, etc.
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| Slotted positioning screw | Hex Socket Positioning Screw |
(3) Machine screw
Refers to a screw that can drill its own threads. The shape of the head can be round, flat, countersunk, hexagonal, etc. It is commonly used as a clamping device, with grooves, cross recesses, hexagonal sockets, etc.
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| Cross Recessed Machine Screw with Hole | Cross Recess Machine Screw with 3 Slots and One Hole |
(4) Wood Screw
It is a screw with a pointed tip and thread used to screw wood. The shape of the head can be round, flat or oval. It is commonly used as a clamping means and has grooves, transverse recesses, etc. as tightening methods.
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| Cross Inlaid Wood Screw | Threaded wood screw |
(5) Screw
It is a general term for screws combined with nuts. There are several types depending on shape, performance and use.
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| Nominal Hex Bolt (Part Grade A) | Effective Diameter Hex Bolt (Part Grade B) |
(6) Walnut
It is a general term for female screw components.
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| Type 1 Hex Nut (Part Grade A) | Hex nut with slot |
(7) Washer
It is a component used between the seat surface of small screws, nuts, bolts and tightening parts. There are many types depending on shape, performance and use.
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| Flat washer with ground edges | Elastic washer |
(8) Pin
It is a rod or cylinder shaped component used to fix joints, positions, fix screw rotation, etc. The pin may also have a head.
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| Conical pin | Spring pin with slot |
Screw machining
The following tools can be used to machine approximately M2 to M12.
1. Tap
It is used to drill holes and machine internal threads on the inside of the hole. This process is called “tapping”.
2. Die
It is used to machine external threads on the periphery of a cylindrical object.
Screw Formation
The screw manufacturing process is carried out approximately according to the above procedures. In addition to these, processes such as pickling, annealing, drilling, etc. are also required.
Methods for forming yarn include cutting and rolling. Cutting forms strands one by one, which is not suitable for mass production.
On the other hand, rolling uses steel dies arranged in a guide to apply pressure and curl the wires into the desired shape.
Installation methods for small screws and nuts
There are several types of screws with different sizes, head shapes, threads, materials, lengths and ease of removal, each with its own purpose. We use a lot of screws in our daily work, mainly for fixing. Here we will explain the general method of tightening the screws.
When assembling machines, there are two types of operations: “fixed” and “mobile”. Therefore, it is important to understand the composition of the equipment and how to repair and move it.
Let's think about this together.
Screw fixing composition
Generally, people think that they should tighten the screw as much as possible, but they may not know the condition of the connection between the screw neck and the material being fixed.
The blue and red parts are stretched by the rotation of the screw like a fishing net, which allows them to be firmly fixed to each other when tightened.
What is the appropriate tightness?
Screws on mobile devices are live and may loosen naturally over time. During the tightening process, the screw extends forward, which cannot be seen but is certainly happening. This is when the screw works hardest, providing the necessary fixing force.
If it is tightened too much, it will relax and the clamping force will decrease drastically. If the screw is tightened too much beyond this point, it will break and the task, which was almost completed, will fail.
Regarding the question of “what is the appropriate tightening”, it is difficult to reach a conclusion about the appropriate numerical value, as it depends on the screw size, material, purpose, etc. I will explain general clamping distance later.
Screw tightening method 1:
Firstly, both the screw and the material to which it will be fixed must be clean. Foreign objects may prevent the screw from turning smoothly and being firmly secured.
Inspect the screw for scratches, burrs and dust and confirm that the tightening tool is being used correctly. Likewise, check the nut for scratches, burrs, and dust if you are using one, and remove them if you find them.
When tightening, try to use a body posture that makes it easier to apply force, even when using a small handle. Operating with incorrect posture can easily twist the screw and reduce its life with each subsequent tightening. Also, pay attention to the pressure, which should generally be “press:press = 7:3”.
When tightening, start manually to confirm that the screw turns smoothly. Even if there are issues, it's easier to confirm them at this stage than using a tool. Also pay attention to the effective tightening size on the nut side; if the screw is longer than the effective length, the screw cannot move.
Special screws used in semiconductor equipment.
Use vacuum screws to secure components in the vacuum chamber. The purpose of the opening is to shorten the chamber's vacuuming time and reduce the effect of external air. The model with opening is weaker than the one without, so pay attention to the tightening torque!
Screw tightening method 2:
When using multiple screws to secure components, it is also important to pay attention to the tightening sequence. The basic sequence should start as shown below: press the one that is furthest away first and then proceed to the next one.
The basic order for temporary fastening, when there are tightening screws at the top and bottom, is to tighten the top first, then the bottom, and then the rest in a specific order. To reinforce and tighten the screws, follow the method described above. After all screws are temporarily fixed, they must be reinforced and fully tightened in order.
Furthermore, reinforcing and tightening the screws must be done gradually and repeatedly, until the necessary torque is reached. It is important to ensure that all screws are placed before performing temporary tightening, and placing screws from above is done to ensure safety.
Before tightening all the screws, it is necessary to confirm the combination of the female and male screws that are inserted. At this point, the position of the device must be confirmed. This is a professional practice in the industry.
How to use a Tapper Tap?
Selecting and using a cross-shaped touch
Can a tap be used that does not match the “+” slot on a screw?
Insert the tap into the “+” slot of the screw and observe it horizontally. For example, for screws with a length of 1 cm or less and a diameter of less than φ4 mm, the tip of the tap should fit smoothly. If the faucet falls or wobbles, it will cause wear on the faucet tip and it will not match the size of the faucet. Of course, damage to the screw groove will also affect the tightening torque.
Should the faucet be tilted when turning the screw?
From the wrist to the tap to the screw, it's important to maintain consistency. Twisting the tap too tightly into the screw groove can cause abrasion to both the screw groove and the tap.
The specialized approach
Select the appropriate screwdriver to match the screw. Place the screwdriver head on the screw and make sure it fits snugly into the slot horizontally, or even if it has to be placed downwards, make sure it can be inserted into the screw hole.
Then, keeping the screwdriver in a straight line from the screw to the wrist, tighten the screw easily. Use the screwdriver to keep the screw level, without using magnets or adhesives.
The amateur approach
Using a high quality but wrong size screwdriver. The screwdriver tip does not fit the screw and cannot be held level, making it tempting to apply adhesive to the screwdriver tip.
As a result, the screw cannot be kept straight and becomes skewed. The screwdriver slot and screw are unstable, causing damage to the screw slot. When turning, the movement of the wrist causes friction in the groove and damages the screw hole.
Hex key torque (Allen wrench)
| Day. | N·m (kgf·cm) |
| 0.7 | 0.08（0.82） |
| 0.9 | 0.18（1.84） |
| 1.3 | 0.53（5.4） |
| 1.5 | 0.82（8.36） |
| two | 1.9（19.4） |
| 2.5 | 3.8（38.7） |
| 3 | 6.6（67.3） |
| 4 | 16（163） |
| 5 | 30（306） |
| 6 | 52（530） |
| 8 | 120（1,224） |
| 10 | 220（2,244） |
| 12 | 370（3,775） |
- ※Note 1: Refers to the wrench torque, which is different from the tightening torque.
- ※Note 2: The torque of a ball-end wrench is lower.
The torque of a hex wrench is approximately half the size of the wrench.
Instructions for using a hex key (Allen wrench)
There are two types of hexadecimal keys. Regardless of the model, the shorter end is referred to as the “A” side and the longer end is referred to as the “B” side. Side “B” is for temporary fixation, while side “A” is for reinforcement and tightening. Of course, using only the “A” side for tightening is also acceptable.
Do not use side “B” for reinforcement and tightening, especially for models with a spherical tip, as the spherical part is weak and can break easily. Please be careful.
So why have a ball ending? Due to the consideration of work efficiency, the ball end allows easy rotation.
Compared to models without a ball tip, it is easy to understand how to use it. One-handed tightening is not easy on models that do not have a ball end.
Direction of rotation to tighten a screw
In general, the direction of rotation for tightening a screw is clockwise (CW). Of course, there is also a counterclockwise rotation (CCW) to unscrew the screw, which is used for specific purposes. Therefore, not all screw tightening directions are clockwise.
Counterclockwise rotation (CCW) is mainly used to prevent loosening and adjust the length. Have you ever used a screw that requires counterclockwise rotation? Like on a bike?
The screws installed for left or right use are not the same. The right side pedal cannot be installed on the left side and vice versa.
Some tips and knowledge about using screws
“Screws” are metal components that we often use around us. If used incorrectly, screws may become loose or fall out. Tightening screws also requires a certain level of technique and related knowledge.
Here are some tips on tightening screws:
1. When a screw is not tight enough, it can loosen and come apart.
When a screw is turned (referred to as tightening torque), it generates a clamping force on the part being tightened. Proper tightening produces the best clamping force. When a screw comes loose, it may be due to insufficient tightening.
2. Screws can also come loose if overtightened.
Not only does over-tightening cause the screws to loosen, but over-tightening a soft material can cause the screws to loosen. In this case, retighten.
3. Over-tightening screws can sometimes cause damage.
Generally, if the screws are tightened too tightly, the screws themselves will be damaged, and the cross-hole screw tips of the screws will also be damaged.
When normal nuts (such as mild steel and brass) are used as nut components for objects with high-strength bolts (such as hex bolts), the nuts may be damaged when the bolts are tightened too tightly.
At this time, the operator may not notice damage to the nuts. Be careful not to overtighten and damage the screws. If other loads are applied after tightening, the bolt and adjacent parts may become deformed and the bolts may loosen.
4. Tighten correctly.
As mentioned previously, if the screws are not tightened, they may come loose. Additionally, screws that are overtightened can also come loose. Proper tightening is necessary and proper tightening guidelines are most important.
The target tightening force is determined by the type of screw, the strength of the screw, the friction between the screw and the base surface, and the tightening method. The tightening method is specified by JIS standards.
The formula for the target clamping force of the clamping operation is also specified in the JIS standards.
5. The recommended tightening torque is not absolute.
There are several methods for tightening management, including torque method, rotation angle method and screw tilt method.
Among these, the torque method is the most used. Therefore, the tightening torque recommended in various data books and samples is only suitable for use under certain limited conditions and is not absolute.
It can be used approximately without any problems, but it should not be overly reliable.
6. Target tightening torque calculation method:
T fa = 0.001kdF f / (1 + 0.01 m)
- T fa : target tightening torque (N・m) When using the “torque method” for tightening, the target tightening torque can be approximately obtained by calculating according to the above format.
- k: Torque coefficient (minimum value obtained in the tightening test in JIS B 1084)
- d: Nominal screw diameter (mm)
- F f : The strength (or yield strength) of the common bolt component x 85% of As (N/ mm2 )
- How: Effective cross-sectional area of the screw (mm 2 )
- m: Tightening tool torque accuracy (%)
For example, when the bolt strength grade is 8.8 and the bolt size is M10, and assuming k = 0.195 and m = ±5%, the target tightening torque (Tfa) is calculated using the following formula: Tfa = 0.001 × 0.195 × 10 × 0.9 × 640 × 58 / (1 + 0.05) = 63(N・m)
7. Screws can become loose due to several factors:
The. Rapid temperature changes. When screws are combined with parts made of materials with different coefficients of thermal expansion and the temperature changes rapidly, the screws may loosen. It is necessary to use rivets for connection or replace the screws with the same material as the parts.
B. Severe vibrations. Screws may loosen due to vibration at fixed screw positions.
w. Forces around the axis and perpendicular to the axis. The force of rotating screws and the force applied perpendicular to the screw can easily cause screws to loosen and become damaged. In this case, it is important to carefully consider the screw installation direction and the applied force.
d. When the surface of the joint to be tightened contains paint. If the surface of the part to be fastened is painted, the screws may come loose very quickly.
It is. When the stiffness of the part to be tightened is low. The part tightened with fastening screws is called the part to be tightened. If this part is deformed, not only plastic deformation but also elastic deformation can cause the screws to loosen.
f. When light alloys and plastics are tightened. Even if the temperature changes only slightly, the screws may become loose if the part to be tightened is made of aluminum alloy and plastic.
g. Other factors:
- When several parts are tightened with one screw.
- When washers are used to tighten screws.
- When using screws with shorter nominal lengths.
There are several methods to prevent screws from loosening, but “reinforcement tightening” is the most effective method to prevent loosening. Regardless of the type of screw, they will have initial looseness. After a period of time, the degree of looseness and clamping force may decrease. After tightening, if the screws are retightened again after some time, this can significantly prevent the screws from loosening.
