Outer Hexagon Bolt vs Inner Hexagon Bolt: How to Choose Between Them?

At present, hexagon bolts are widely used in the fastener industry. Compared with straight and cross heads, their assembly is not easy to slip and the torque is also high.

Compared with the dodecagonal and multi-tooth head, the utility model has simple structure and low cost.

Therefore, the comprehensive price of hexagon bolts is high, and it is also the type with the largest amount of bolts on automobiles.

Hexagon bolts can be divided into inner hexagon bolts and outer hexagon bolts. What are the differences between the two bolts? How to select when applying?

This article introduces the structure, cost, tightening tools, advantages and disadvantages and application scope.

1. Structure

Structurally, hexagon socket-head cap screws and outer hexagon bolts are easy to distinguish.

There is basically no difference in the threaded part. The outer hexagon is the hexagon of the head, and the head has no depression.

In order to increase the bearing area of the head, outer hexagon flange bolts can also be made, and this variant is also widely used.

The outer edge of the head of the hexagon socket bolt is round, and the middle is a concave hexagon.

The more common is the cylindrical head hexagon, and there are pan head hexagon socket, countersunk head hexagon socket, flat head hexagon socket.

Headless screws, stop screws, machine screws, etc. are called headless socket cap screw.

Of course, hexagon socket bolts can also be made into hexagon socket flange bolts in order to increase the contact area of the head.

In order to control the friction coefficient of the bolt head or improve the anti-loosening performance, the outer hexagon combination bolt and the inner hexagon combination bolt can also be made, as shown in the figure below.

2. Cost

The threads of socket head cap screws and hexagon socket bolts are generally formed by screwing. The screwing plates used are basically the same, and there is no significant difference in cost.

The difference is the head manufacturing process.

Because the head structure is completely different, different molds and processes need to be used.

The head manufacturing cost of the hexagon socket bolt is higher than the head manufacturing cost of the hexagon socket thread.

Based on my experience, the cost of the outer hexagon bolt is only half that of the inner hexagon bolt.

3. Tightening tool

It can be divided into assembly in life and production.

For assembly in life, the wrench shape for hexagon socket bolts is “L” type.

One side is long and the other side is short, and the other side is short to screw. Holding the long side can save effort and tighten screws better.

The outer hexagon bolt is a wrench with an equilateral hexagon head, such as adjustable wrench, ring wrench, open-end wrench, etc.

As shown in the figure below, you have done daily assembly or maintenance at home and should be familiar with the following two tools.

In order to meet the requirements of tightening quality and automation, the assembly in production will be carried out by fixed torque wrench and high-precision tightening gun.

Therefore, the corresponding tightening sleeve needs to be matched.

The sleeve of the outer hexagon bolt is concave hexagon, and the sleeve of the inner hexagon bolt is convex hexagon.

As shown in the figure below, there will be a series of sleeves matching the size according to the size of the bolt head.

4. Advantages and disadvantages

4.1 Advantages

Outer hexagon bolt/screw:

  • Good self-locking;
  • Large pre-tightening contact area and large pre-tightening force;
  • The length range of full thread is wider;
  • There can be reamed holes, which can fix the position of parts and bear the shear caused by transverse force;
  • The head is thinner than the hexagon socket, and in some places the hexagon socket cannot be replaced.

Hexagon socket head bolt/screw:

  • Easy to fasten;
  • Not easy to disassemble;
  • Non-slip angle;
  • Small space;
  • The load is large;
  • It can be countersunk and sunk into the interior of the workpiece, which is more exquisite and beautiful, and will not interfere with other parts.

4.2 Disadvantages

Outer hexagon bolt/screw:

It occupies a large space and is not suitable for more delicate occasions;

It cannot be used for countersunk head.

Hexagon socket head bolt/screw:

Small contact area and small preload;

There is no full thread beyond a certain length;

The fastening tool is not easy to match, easy to screw and replace;

Use a professional wrench when disassembling. It is not easy to disassemble at ordinary times.

5. Scope of application

It is the advantages and disadvantages of outer hexagon bolt and inner hexagon bolt, so how to choose them in the application?

If the axial force required for the tightening point is large, that is, the tightening torque is large, and the external tightening space is sufficient, then the outer hexagon bolt is used for tightening.

If there is space limitation in the tightening position, or there is a need to make the countersunk head beautiful, and the axial force required for the tightening point is small, that is, the tightening torque is small, then the hexagon can be made.

Take the car as an example. At the connection position between the subframe and the body, several bolts pass through the subframe through the bottom and are tightened to the body. Because the bottom is an invisible area, there is no aesthetic requirement, there is no interference in tightening, and the axial force and torque required for tightening are large (the bolts are tightened after yielding).

For this connection position, the outer hexagon bolts are suitable for tightening.

The connection of the interior trim is the visible area of the customer. Generally, there are aesthetic requirements.

The countersunk head structure needs to be made.

The top of the bolt head needs to be kept in the same plane with the structure of the connected part, or a layer of covering parts needs to be added outside.

The tightening axial force and torque of such positions are also small.

Therefore, it is very suitable to tighten with hexagon socket bolts or screws.

If you are not clear about how to choose, it depends on whether the connection position is exquisite, beautiful, and the assembly space is limited. If so, choose hexagonal socket bolts/screws.

If not, choose the outer hexagon bolt/screw. After all, the outer hexagon bolt is cheaper and other requirements can be basically met.

This is why the outer hexagon bolt/screw is more widely used than the inner hexagon bolt/screw.

6. Experience and Conclusion

Hexagon-head bolts/screws are widely used in various industries due to their simple structure, convenient use, moderate price, and easy-to-fail tightening.

Common hexagon-head bolts/screws have two types of structure: outer hexagon and inner hexagon.

In terms of structure, the head structure of the two can be clearly distinguished;

In cost, the outer hexagon is half of the price of the inner hexagon bolt;

The tightening tools are different in life and production;

According to the advantages and disadvantages of the two, in application, if the connection position has the requirements of delicacy, beauty and limited assembly space, the socket head bolt/screw shall be selected; if there is no such requirement, the outer hexagon head bolt/screw shall be selected.

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