Fundamentals of Screws
Large and Small Screws
Screws are known by various names such as screws, bolts, rivets, screw rods, screws, small screws, screws with small nails, and others. They come in different 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 mobile phones. On the other hand, large screws are generally used in construction or bridge-building and can be as thick as 50 millimeters, or even as wide as 100 or 200 millimeters depending on the requirements.
Types of Screws
Screws come in various shapes, and there are countless new types being developed by manufacturers around the world.
They are generally classified as either female screws (which have nuts) or male screws (which include screws, small screws, fixed screws, turbine screws, wood screws, and others).
Small screws can further be divided according to the shape of their heads, which include round, butterfly-shaped, flat-round, pan-shaped, countersunk, and threaded screws with either plus or minus shaped heads.
Bolts, on the other hand, typically are six-sided, but four-sided, flat-headed, and round-headed bolts also exist.
Like screws, nut shapes also vary between four-sided or 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 instance, screws used in clocks, bicycles, construction, automobiles, machinery, and shipbuilding may differ in their materials and specifications.
In Japan, screw sizes are standardized based on ISO regulations, with uniform dimensions across the country. This standardization requirement was initially established for global uniformity.
However, these specifications are not implemented in other countries like Europe and the United States. During the repair of imported goods, it becomes necessary to follow overseas screw specifications.
Some commonly used screw specifications include Unified Thread Standard, SAE(Society of Automotive Engineers), Inches, and Bolts.
Japan used to primarily use British thread screws, but they are now transitioning to the ISO standard. British thread screws are currently mainly used in civil engineering and construction. The measurement of how deep screws go after they are turned is determined by the pitch, which is the distance traveled on 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 gentle movement during the twisting process.
Camera and mobile phone screws typically have small pitches, which must be preserved, even if the screws are short. Small pitch screws primarily aid 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 a head. According to the ISO standard, small screw heads can have flat, pan-shaped, countersunk, or adherent head shapes, whereas the JIS standard also includes flat-round, threaded, round, flat, and rounded-flat head shapes. Small screws serve as a way to tighten, and normally come with spiral slots or cross-holes to assist in turning them.
|Slotted round head small screw
|Cross recessed flat head small screw
(2) Positioning screw
It is used to fix the moving mechanical parts together through the tip of the screw. The tip of the screw can be flat, pointed, cylindrical, concave, round, etc. It is commonly used as a means of tightening, with slots, hexagonal socket, square head, etc.
|Slotted positioning screw
|Hexagonal socket positioning screw
(3) Machine screw
It refers to a screw that can tap its own threads. The head shape can be round, flat, countersunk, hexagonal, etc. It is commonly used as a tightening device, with slots, cross recesses, hexagonal sockets, etc.
|Cross recessed machine screw with a hole
|Cross recessed machine screw with 3 slots and a hole
(4) Wood screw
It is a screw with a pointed tip and threading used to screw into wood. The head shape can be round, flat, or oval. It is commonly used as a means of tightening and has slots, cross recesses, etc. as tightening methods.
|Cross recessed wood screw
|Threaded wood screw
It is a general term for bolts that are combined with nuts. There are various types depending on shape, performance, and usage.
|Nominal hex bolt (Part grade A)
|Effective diameter hex bolt (Part grade B)
It is a general term for female screw components.
|Hex nut type 1 (Part grade A)
|Hex nut with slot
It is a component used between the seat surface of small screws, bolts, nuts, and the tightening parts. There are many types depending on shape, performance, and usage.
|Flat washer with ground edges
It is a rod or cylinder-shaped component used for fixing joints, positions, fixing screw rotation, etc. into a hole. The pin may also have a head.
|Spring pin with slot
The following tools can be used to machine approximately M2 to M12.
It is used for drilling holes and machining internal threads on the inside of the hole. This process is called “tapping”.
It is used to machine external threads on the periphery of a cylindrical object.
The manufacturing process for screws is roughly carried out according to the above procedures. In addition to these, processes such as pickling, annealing, piercing, etc. are also required.
The methods for forming threads include cutting and rolling. Cutting forms the threads one by one, which is not suitable for mass production.
On the other hand, rolling uses steel dies arranged on a guide to apply pressure and roll the threads into the required shape.
Installation methods for small screws and bolts
There are various types of screws with different sizes, shapes of heads, threads, materials, lengths, and ease of removal, each with its own purpose. We use many screws in our daily work, mainly for fixation. Here, we will explain the general method of tightening screws.
When assembling machinery, there are two types of operations: “fixed” and “movable”. Therefore, it is important to understand the composition of the equipment and how to fix and move it.
Let’s think about it together.
Composition of screw fixation
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 fastened.
The blue and red parts are stretched by the rotation of the screw like a fishing net, which allows them to be tightly secured together when tightened.
What is the appropriate tightness?
The screws in mobile devices are live and may naturally loosen over time. During the tightening process, the screw extends forward, which cannot be seen but it is certainly happening. This is when the screw is working the hardest, providing the necessary fixing force.
If it is overtightened, it will relax and the fixing force will sharply decrease. 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 tightness,” it is difficult to reach a conclusion on the appropriate numerical value as it depends on the screw size, material, purpose, etc. I will explain the general distance of tightening later.
Screw tightening method 1:
First, both the screw and the material it will be fastened to should be clean. Foreign objects can 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. Similarly, check the nut, if using one, for scratches, burrs, and dust, and remove them if found.
When tightening, try to use a body posture that makes it easy to apply force, even when using a small handle. Operating with incorrect posture can easily twist the screw and reduce its lifespan with each subsequent tightening. Also, pay attention to the pressure, which should generally be “press: tighten = 7: 3”.
When tightening, start by hand to confirm that the screw can turn smoothly. Even if there are issues, it is easier to confirm them at this stage than when using a tool. Pay attention to the effective tightening size on the nut side as well; if the screw is longer than the effective length, the screw cannot move.
Special screws used in semiconductor equipment.
Use vacuum screws to fix components in the vacuum chamber. The purpose of the opening is to shorten the time for vacuuming the chamber and to 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 fix components, it is also important to pay attention to the sequence of tightening. The basic sequence should start from the shape shown as follows: tighten the one that is farthest first, and then proceed to the next one.
The basic order for temporary fixation, when there are tightening screws at both top and bottom, is to first tighten the top, then the bottom, and then the rest in a particular order. To reinforce and tighten the screws, follow the method described above. Once all the screws are temporarily fixed, they should be reinforced and fully tightened in order.
Moreover, when reinforcing and tightening the screws, it should be done gradually and with repetition, until the necessary torque is achieved. It is important to ensure that all the screws are placed in before doing temporary tightening, and putting screws in from the top 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 fixture must be confirmed. This is a professional practice in the industry.
How to Use a Tapper Tap?
Selecting and Using a Cross-Shaped Tap
Can a tap that does not match the “+” slot of a screw be used?
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 tap falls out or wobbles, it will cause wear on the tip of the tap and will not match the size of the tap. Of course, damages to the screw slot will also affect the tightening torque.
Should the tap be tilted when turning the screw?
From the wrist to the tap to the screw, it is important to maintain consistency. Twisting the tap too hard into the screw slot can cause abrasion on both the screw slot and the tap.
The expert approach
Select the appropriate screwdriver to match the screw. Place the screwdriver head onto 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 your wrist, tighten the screw with ease. Use the screwdriver to keep the screw level, without the use of magnets or adhesives.
The amateur approach
Using a high-quality screwdriver but with the wrong size. The tip of the screwdriver cannot fit into the screw and cannot be kept level, making it tempting to apply adhesive to the tip of the screwdriver.
As a result, the screw cannot be kept straight and is tilted. The screwdriver slot and the screw are unstable, causing damage to the screw slot. When turning, wrist movement causes friction in the slot and damages the screw hole.
Torque of hex key wrench (Allen wrench)
- ※Note 1: This refers to the torque of the wrench, which is different from the tightening torque.
- ※Note 2: The torque of a wrench with a ball end is lower.
The torque of a hex key wrench is roughly 1/2 the size of the wrench.
Instructions for using a hex key (Allen wrench)
There are two types of hex 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. The “B” side is for temporary fixation, while the “A” side is for reinforcement and tightening. Of course, using only the “A” side to tighten is also acceptable.
Do not use the “B” side for reinforcement and tightening, especially for models with a ball end, as the ball portion is weak and may break easily. Please be careful.
Then why have a ball end? Due to consideration of work efficiency, the ball-end allows for easy rotation.
Compared to models without a ball-end, it is easy to understand how to use it. Single-handed tightening is not easy with models that do not have a ball-end.
Rotational direction to tighten a screw
In general, the rotation direction for tightening a screw is clockwise (CW). Of course, there is also a counterclockwise (CCW) rotation for unscrewing the screw, which is used for specific purposes. Therefore, not all screw tightening directions are clockwise.
The counterclockwise (CCW) rotation is mainly used to prevent loosening and adjust length. Have you ever used a screw that requires a counterclockwise rotation? Like on a bicycle?
Screws that are 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 on screw usage
“Screws” are metal components that we often use around us. If used improperly, screws may loosen or fall off. Screw tightening also requires a certain level of technique and related knowledge.
Here are some tips on screw tightening:
1. When a screw is not tightened enough, it may loosen and become undone.
When a screw is rotated (referred to as tightening torque), it generates a tightening force on the part being tightened. Proper tightening produces the best tightening force. When a screw becomes loose, it may be due to insufficient tightening.
2. Screws can also loosen if they are overtightened.
Overtightening not only makes screws loose, but when tightening a soft material, it may cause screws to become loose. In this case, please retighten.
3. Over-tightening screws can sometimes cause damage.
Generally, if bolts are tightened too tightly, the bolts themselves will be damaged, and the tips of the cross-hole screws of the bolts will also be damaged.
When normal nuts (such as soft steel and brass) are used as nut components for objects with high-strength bolts (such as hexagonal socket bolts), the nuts may be damaged when the bolts are tightened too tightly.
At this time, the operator may not notice the damage to the nuts. Be careful not to over-tighten and damage the screws. If other loads are applied after tightening, the screw and surrounding parts may be deformed and the screws may become loose.
4. Please tighten properly.
As mentioned earlier, if screws are not tightened, they may become loose. In addition, screws that are overtightened may also become loose. Proper tightening is necessary, and proper tightening guidelines are the 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 tightening force of the tightening operation is also specified in the JIS standards.
5. Recommended tightening torque is not absolute.
There are various methods for tightening management, including torque method, angle of rotation method, and bolt inclination method.
Among these, the torque method is the most widely used. Therefore, the recommended tightening torque in various data and sample books is only suitable for use under certain limited conditions and is not absolute.
It can be roughly used without any problem, but it should not be over-relied upon.
6. Calculation method of target tightening torque:
Tfa = 0.001kdFf / (1 + 0.01m)
- Tfa: target tightening torque (N・m) When using the “torque method” for tightening, the target tightening torque can be obtained approximately by calculating according to the above format.
- k: Torque coefficient (minimum value obtained from the tightening test in JIS B 1084)
- d: Nominal diameter of the screw (mm)
- Ff: The endurance (or yield point) of the common screw component x 85% of the As (N / mm2)
- As: Effective cross-sectional area of the screw (mm2)
- m: Torque accuracy of the tightening tool (%)
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 various factors:
a. Rapid changes in temperature. When screws are combined with parts made of materials with different coefficients of thermal expansion and the temperature changes rapidly, the screws may become loose. It is necessary to use rivets for connection or replace the screws with the same material as the parts.
b. Severe vibrations. Screws may become loose due to vibration at the fixed screw positions.
c. Forces around the shaft and perpendicular to the axis. The force of rotating screws and the force applied perpendicular to the screw can easily cause the screws to become loose and damaged. In this case, it is important to carefully consider the direction of screw installation and the force applied.
d. When the joint surface being tightened has paint. If the surface of the part being tightened has paint, screws may become loose very quickly.
e. When the rigidity of the part being tightened is low. The part tightened using fastening screws is called the part being tightened. If this part is deformed, not only plastic deformation but also elastic deformation can cause screws to become loose.
f. When light alloys and plastics are tightened. Even if the temperature changes only slightly, screws may become loose if the part being tightened is made of aluminum alloy and plastic.
g. Other factors:
- When several parts are tightened together with one screw.
- When washers are used to tighten the screws.
- When using screws with shorter nominal lengths.
There are various methods to prevent screws from becoming loose, 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 tightening force may decrease. After tightening, if screws are retightened again after some time, it can significantly prevent screws from becoming loose.
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