Pins have great importance in the movement of excavators and other construction machines and in achieving their production purposes. Pins and bushings in construction machines provide the mobility that as same as joints provide to the human body. Considering the operating conditions and weights of the construction machines, the durability of these parts becomes more important. This situation increases the importance of pin manufacturing and production. However, costs also play an active role in determining this durability.
For example; If we suddenly apply force to the arm of the work machine perpendicular to the pin axis. This force is also transmitted to the pin, bushing and bushing slots on the arm. Various scenarios arise here. The main goal at the end of these scenarios should be to return the arm to its former state with the least cost and labor.
Bushings fit tightly into their sloths. For this reason, the slohts are slightly deformed with each bushings change. As a result, bushings produced in standard sizes will not fit perfectly. This will require either revising the bushing sloth in the arm and using non-standard bushings, or replacing the entire arm. This situation leaves the machine inactive for a long time and is costly. In this case, breaking the pin would be the most logical option. That’s why the bushings are completely hardened and their hardness values are higher than the surface hardness value of the pin.
The surfaces of the pins are hardened by induction or nitriding, but they are not completely hardened. Because fully hardened steels become more fragile. If the pin is too flexible, it will be difficult to control the construction machine. These flexibilities appear as mechanical values as a result of the chemical structure of steel.
Raw Material Selection in Pin Production
Some types of steel used in pin production are listed below.
34CrNiMo6 / +QT:
This type of steel is generally used in concrete pumps, but can also be used in other work machines. This type of steel is used in concrete pumps since they are generally exposed to wind and their arm lengths are thin and long. It has a more flexible structure than other tempering steels. It costs more. This cost should be ignored since there will be people underneath it while it is working and it will endanger human life if the pins break. By quenching, tempering and stress relieving, the internal structure of the material becomes more durable and homogeneous. While this process increases the cost, it also increases safety to a higher level.
42CrMo4 / +QT:
This type of steel is used in construction machinery. Torsion and impact resistance is high. Since it is against occupational safety, people are not allowed to be around construction machines while they are operating. Thus, if the pin breaks, human life will not be in danger. It is a price performance product. By quenching, tempering and stress relieving, the internal structure of the material becomes more durable and homogeneous. Big construction machinery manufacturers use raw materials in this way.
C45/1040/1050:
These steel types are manufacturing steels. Although it is not suitable for use in construction machinery, its material cost is cheaper than other types of steel. They cannot reach the sufficient hardness value when surface hardening by induction. Although it varies depending on the manufacturer, there is a possibility of central cracks appearing in these steels.
Raw material selection is of great importance in pin production and manufacturing. You can consult us on this issue, our engineers will inform you in detail both in terms of sector and cost.
Lathe Process in Pin Production
We cut the raw material to allow machining allowance in both diameter and length. We process the raw material, make angles and radii, drill holes and make threads. If the pin needs to be induction hardened, we leave an allowance of approximately 0.5 mm for the diameter of the pin. This allowance is in case the material warps while the surface is hardened by induction. If the surface is to be hardened by nitriding, less than 0.5 mm can be left.
We must be careful that there are no sharp corners on the pins. Cracks may occur at induction due to heat accumulation in sharp corners.
Surface Hardening in Pin Production
We generally harden pin surfaces using two methods. The first of these is induction and the other is nitration. Temperature values here can reach up to 400-500 Celsius. After this heating process, we cool it rapidly with water to ensure the irregular bonding of atoms on the material surface.
With induction, we create an electromagnetic field around the material and cause the atoms in the material to move. The kinetic energy generated here turns into heat energy because the material boundaries are rigid. Temperature values here can reach up to 400-500 Celsius. After this heating process, we quickly cool it with water to ensure irregular bonding (distortion) of the molecules on the material surface. In this way, we obtain a hardened surface. The depth of hardness varies depending on the type of raw material, the value of the electric current applied and the time elapsed between heating and cooling. Unless there is a special request, we set the hardness value as 50-55 HRc and the hardness depth as 3-4 mm.
After heat treatment, we make hardness measurements and linearity checks of the pins. Then we bring it to size by grinding.
Surface hardening by nitriding is not a heat treatment. For this reason, we machine the pins on the lathe, then grind them according to their size and perform the necessary surface treatments. In nitration baths, they send nitrogen atoms onto the material. These nitrogen atoms fill the space between other atoms on the surface, causing the surface to harden.
Grinding
Machine manufacturers produce pins to certain tolerances because they fit into bushings and are part of a moving mechanism. These tolerances are very difficult to achieve on lathes. Additionally, machining the pin to exact size on a lathe is very risky due to distortions that may occur during heat treatment.
After the heat treatment, we grind the pins in grinding machines, we grind the pins to their pre-plating dimensions or final precise measurement.
Since precision here is important in passing the pin into the bushing, grinding has a very important place in pin manufacturing and production.
When grinding pins on grinding machines, gently compress the material. This causes the surface hardness value to increase. The hardness value seen as 52 HRC after induction may have increased to 55 HRC after grinding.
Coating
Machine manufacturers determine the type of coating they will apply according to the area where the machines will be used and the environmental factors to which the parts on the machine will be exposed. Although there are various coating types, the most commonly used coating types are hard chrome plating and phosphate coating. Although titanium coating technology has developed recently, it has not yet found a place in the spare parts industry due to factors such as installation costs. However, considering both the coating quality and its less negative impact on humans and the environment compared to other coating types, we think that titanium coating will find a place in this sector as well.
The coating creates a smooth surface on the pin, ensuring good lubrication and smooth movement. It reduces the friction force. Good coating quality has a positive impact on the maintenance time of the machine. Therefore, coating has a very important place in pin manufacturing and production.
Welding in Pin Production
Construction machinery manufacturers produce some of the pins with caps and some with flanges. In capped pins, they leave a section at the end of the pin slightly larger than the pin diameter to prevent further movement of the pin. Flanged pins have the same purpose of use, but they prevent the pin flange from rotating by fixing it.
We produce flanged pins by welding a sheet shaped with a round pin. Everything that is valid in welding technology is also valid in this welding process. Determining the welding wire types and determining the preheating temperature according to material have great importance for the health of the weld. The manufacturer creates Welding Process Specifications (WPS) based on the size of the welds. It is important to ensure standardization at the source.
The weld must hold both the sheet metal and the pin, penetrating both. This depends on the preparation of a plan by the welding engineer, which includes the electric current, rotation speed of the pin and the number of welding rows. We perform dimensional control of the weld. If there is no problem with the size control, we cut the welded area and check the penetration with the help of the UT device. This way we confirm the welding. And we use the WPS form on other weldings as well.
Breaking the flange weld of a pin renders the pin unusable and the construction machine inactive. Therefore, welding has a very important place in pin manufacturing and production.
Paint in Pin Production
We do not apply coating to the flanges of flanged pins. Because in the operation of preparing the flanges for welding, we remove the oil layer on them. When they heat up during welding, the oil inside them decreases even more. For this reason, flanges oxidize more quickly. We paint the flanges in order to prevent the harmful effects of oxidation and ensure the appearance integrity of the machine. We paint the flanges with primer and top coat paints in accordance with the color codes and thicknesses specified by the machine manufacturers.
