Hex bolts are currently widely used in the fastener industry.
Compared with slotted and Phillips head types, they are less prone to stripped threads during assembly and have a higher torque. Compared with twelve-point and multi-tooth head types, they have a simpler structure and lower cost.
Therefore, hex bolts have a relatively high overall cost performance and are currently the most commonly used type of bolt in automobiles. Hex bolts can be divided into socket hex bolts and regular hex bolts. What are the differences between these two types of bolts and how should they be selected for application?
01 Structure
Structurally, it is easy to distinguish between an Allen bolt and a hex bolt. The threaded part has almost no difference. The hex bolt has a hexagonal edge on the head and no indentation. To increase the bearing area of the head, it can also be made into a hex flange bolt, which is also widely used.
The head of an Allen bolt is round on the outside and has a hexagonal recess in the middle. The most common type is the cylindrical head Allen bolt, but there are also countersunk head Allen bolts, flat head Allen bolts, and socket head Allen bolts. Headless screws, set screws, and machine screws are also referred to as headless Allen bolts.
Of course, to increase the contact area of the head, Allen bolts can also be made into Allen bolts with a flange. To control the friction coefficient of the bolt head or improve the anti-loosening performance, they can also be made into hexagon socket combination bolts and Allen socket combination bolts.
02 Cost
The threads of both hex socket and hex head bolts are generally formed by threading, and the threading dies used are basically the same, with no significant cost difference. The difference lies in the manufacturing process of the head. Due to the completely different head structures, different molds and processes need to be used. The manufacturing cost of the head of hex socket bolts is higher than that of hex head bolts.
03 Tightening Tools
Here it can be divided into assembly in daily life and in production.
In daily life, the wrenches used for hex socket bolts are L-shaped, with one side longer than the other. The shorter side is used to tighten the screws, and holding the longer side can save effort and better secure the screws.
The wrenches for hex bolts have heads in the shape of an equilateral hexagon, such as adjustable wrenches, box wrenches, and open-end wrenches. if you have done any assembly or maintenance at home, you should be familiar with these two types of tools.
In the assembly process of production, to ensure the quality of tightening and meet the requirements of automation, assembly is carried out using fixed torque wrenches and high-precision tightening guns. Therefore, corresponding tightening sockets are required.
The socket for the external hexagon bolt is concave hexagonal, while the socket for the internal hexagon bolt is convex hexagonal, as shown in the following figure. There is a series of sockets of different sizes that match the size of the bolt head for both types.
04 Advantages and Disadvantages
4.1 Advantages
- Hexagon head cap screws/bolts: Good self-locking property; Large pre-tightening contact area, providing greater pre-tightening force; Broader range of full thread length; It can have reamed holes, which is capable of fixing the position of parts and withstanding shear forces caused by lateral forces; Thinner head than hexagon socket head cap screws/bolts, in some cases, hexagon socket head cap screws/bolts cannot replace them.
- Hexagon socket head cap screws/bolts: Easy to tighten; Difficult to remove; Less prone to slippage; Occupies less space; Can withstand greater loads; Can be countersunk, allowing them to be recessed into the workpiece, making them more delicate and aesthetically pleasing, and not obstructing other components.
4.2 Disadvantages
- Hexagon socket bolts/screws: They occupy a relatively large space and are not suitable for more delicate applications; they cannot be used for countersunk heads.
- Hexagon socket bolts/screws: They have a small contact area and low preload; they do not have full threads beyond a certain length; the tightening tools are not easy to match, they are prone to stripping when tightened, and replacement is not convenient; a professional wrench is required for disassembly, and they are not easy to disassemble in daily use.
05 Application Scope
Given the advantages and disadvantages of hexagon socket head cap screws and hexagon bolts, how should one choose when applying them?
If the axial force required to tighten is large, that is, the tightening torque is large, and there is sufficient external tightening space, then an external hexagon bolt should be used for tightening. If there are space limitations at the tightening position or there is a requirement for a countersunk head for aesthetic purposes, and the axial force required for tightening is not large, that is, the tightening torque is not large, then an internal hexagon can be used.
Take automobiles as an example. At the connection point between the subframe and the vehicle body, several bolts pass through the bottom of the subframe and are tightened to the vehicle body. Since the bottom is an invisible area with no aesthetic requirements, there is no interference during tightening, and the required axial force and torque for tightening are relatively large (the bolts are tightened beyond the yield point). For such a connection position, it is suitable to use hexagon socket bolts for tightening.
The connection of interior parts is within the customer’s visible area and usually has aesthetic requirements. It needs to be recessed, with the top of the bolt head being on the same plane as the structure of the connected part, or an additional cover part is needed on the outside. The tightening axial force and torque at such positions are also relatively small. Therefore, it is very suitable to use hex socket bolts or screws for tightening.
If you are still not quite clear about how to choose, then look at whether there are requirements for the connection position to be delicate, aesthetically pleasing, and have limited assembly space. If there are, then choose the socket head cap screw/bolt. If not, then select the hexagon head screw/bolt.
After all, hexagon head screws are cheaper and can basically meet other requirements. This is also why hexagon head screws/bolts are more widely used than socket head cap screws/bolts.
06 Experience and Conclusions
Hexagon head bolts/screws are widely used in various industries at present due to their advantages such as simple structure, convenient use, moderate price, and less prone to failure when tightened. Common hexagon head bolts/screws include two structures: external hexagon and internal hexagon.
Structurally, the two can be clearly distinguished by their head structures. In terms of cost, the price of an external hexagon bolt is half that of an internal hexagon bolt. Whether in daily life or in production, the tightening tools for the two are different.
According to their advantages and disadvantages, in application, if the connection position has requirements for delicacy, aesthetics, and limited assembly space, an internal hexagon head bolt/screw should be chosen. If there are no such requirements, an external hexagon head bolt/screw should be selected.
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