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Lubrication for DTH Hammer
Aug 02, 2018

Due to the poor working conditions of the DTH Hammer, it has a large dust content and a high humidity, and the reciprocating motion of the piston has a high frequency and a large impact speed, so the lubrication of the impactor is very important.


The purpose of lubrication of DTH Hammer. 

The purpose of impactor lubrication is as follows:

1. Reduce the friction loss of moving parts. Excessive friction due to insufficient lubrication can cause fine cracks on the contact surface, such as crack propagation, which can cause damage to the part. In addition, overheating of the surface caused by excessive friction may cause local softening or regional plastic deformation of the metal, eventually leading to destruction of the cylinder and the piston.


2. Anti-corrosion effect. The impactor parts are susceptible to corrosion by gas and chemical substances in the water during the drilling process. Corrosion and the stress in the parts will cause the strength of the parts to drop seriously. Corrosion will further enlarge the cracks or cause corrosion cracks. The rust zone directly causes damage to the parts. Therefore, corrosion protection is one of the important functions of lubrication.


3. Sealing effect. Appropriate lubrication ensures the seal between the moving parts of the impactor and prevents the frequency of the impactor from being lowered due to poor sealing.


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Requirements for the quality of lubricating oil. The Gardner-Denver Compressed Air Institute in the United States has proposed the following requirements for the lubrication of wind motives:

1. Must have high liquid film strength;

2. It is not easy to erupt or interfere with the movement of the valve;

3. No smoke or poisonous gas can be discharged;

4. No corrosion under any conditions;

5. Can quickly lubricate all parts to be lubricated;

6. Fully lubricated under high speed, high temperature and low temperature;

7. No sticky residue is formed in cold gas or hot gas;

8. Has a higher oil quality.


To sum up the above requirements, it is summarized that the lubricating oil should have a suitable viscosity, a good emulsified quality for the compressed gas flow, and a high liquid film strength.


Therefore, the lubricating oil is required to have a suitable viscosity. This is because the impactor works under the condition of large temperature difference. In order to ensure that the lubricating oil can lubricate the parts well at any temperature, the viscosity of the lubricating oil should not be too high at low temperature, so as not to affect the flexibility of the valve plate. , thereby slowing down the drilling speed; at high temperatures, the viscosity should not be too low, otherwise it will not protect.


The emulsified mass of the compressed gas flow means that the lubricating oil must have the ability to resist washing and adhere to the metal surface in the case of encountering water, that is, when drilling, the water vapor generated in the compressed air is under the action of the emulsifier. It can quickly form a continuous lubricating liquid film to protect the surface of the parts, and also protect against the rust and corrosion caused by acidic water. The so-called emulsifier is a kind of combination of lubricating oil and saponified raw materials. Short-term emulsion (water-oil mixture).


The term "liquid film strength" refers to the strength of a liquid film formed by lubricating oil that is squeezed between two pressed metal surfaces without breaking. In order to increase the strength of the liquid film, it is necessary to add an additive to the lubricating oil, which is an increase in the load supported by the lubricating oil to prevent scratches and damage when the metal surface of the sliding member contacts.


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The lubrication method of DTH Hammer  and the consumption of lubricating oil. The lubrication of the impactor is mostly carried out by means of an injection method, that is, an oil feeder is connected at an appropriate position of the air supply line of the down-the-hole drilling machine, and the pressurized gas carries the mist-like lubricating oil to lubricate the various components of the impactor.


According to the size of the impactor and the amount of gas consumption, the oil feeder should have sufficient volume to ensure the supply of lubricating oil; and an adjustment device is arranged outside the oil feeder to properly adjust the oil supply speed. In order to prevent the impactor from continuing to work without lubricating oil, some oil feeders are equipped with an automatic shut-off device for the gas supply system when there is no oil.


The consumption of lubricating oil can be calculated as follows:


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In the middle

G0——the consumption of lubricating oil per hour, g/h;

P0——rated working pressure of the impactor, MPa;

P——the actual working pressure of the impactor, MPa;

V0——the standard air consumption of the impactor when the compression pressure is P0, m3/min;

a——The oil concentration of the lubricating oil in the compressed air, g/m3.

The oil concentration of lubricating oil in the gas pressure is different from that of national manufacturers. For example, scholars in the Soviet Union proposed (0.17~0.23) g/m3; Japan's Furukawa Mining has a regulation of 0.87~0.88g/m3.


case analysis

According to the general working condition of DHD 340 high-pressure down-the-hole impactor, the simulated impactor is subjected to the stress of 17mm aperture granite under the pressure of 17kg, and the equivalent stress distribution of the piston is obtained. During a drilling cycle, the maximum equivalent stress of the piston appears on the strike surface of the piston and the drill bit. The equivalent stress is about 350 MPa. The minimum stress occurs at the tail of the piston and the cylinder. The equivalent stress is about 3 megabytes. Pa. (As shown below)

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The equivalent stress exceeds the fatigue strength of the material and even the yield strength, which is the main cause of the early failure of the piston. According to this force analysis, for other reasons, piston failure is most likely to occur in and around the plane in contact with the drill bit; it is not likely to occur in the tail of the piston with less stress.


If the piston is not in place or lubricated under these conditions, the piston will wear and break. Because the piston is quenched, the surface hardness reaches HRc58~60 degrees, which is extremely sensitive to microcracks. In a work environment lacking lubricating oil, the surface temperature of the piston rises sharply, and many fine microcracks are formed on the surface. The microcracks expand further under the action of high-frequency stress, causing the piston body to break along the metal lattice, causing the piston to fail. The failure of the piston in this case has obvious trace characteristics:

The surface of the piston will have a tensile mark caused by friction at high temperature;

Piston fracture will produce a smoother fracture at the pull mark;

The piston fracture will extend to the body in a short time until the remaining part of the material is unable to resist the fracture, causing the remaining part to split.



Below we will use examples to illustrate

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Figure 1 shows the impact surface of the DHD340 DTH Hammer piston. According to the stress analysis, the early failure of the piston is most likely to occur at this position. However, by observing the striking end face on the photograph, except for the partial stress eclipse, the piston striking end face is basically flat, indicating that the load generated by the stress is not enough to cause the piston to fail under this condition; likewise, according to the stress analysis, the stress at other positions of the piston Below this position, it should also be normal.

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Figure II

However, in the opposite direction, the position where the piston tail fits with the cylinder is broken, and there is a marked high temperature burn mark at the fracture.

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Figure III

The figure above shows the plane of the fracture after the piston has broken. It can be seen from the figure that the fracture surface is centered at the most obvious position of the burn and spreads to the periphery. When the diffusion range reaches a certain extent, the remaining part exhibits obvious splitting phenomenon.


The phenomena shown in these pictures are a good example of our analysis. Therefore, the lack of lubrication of the piston is a major killer that causes the piston to fail. Usually, this is not caused by product design, materials and processes, but by product failure caused by improper use. The domestic DTH drilling tool manufacturers are not paid, which increases the user's use cost and delay. Unnecessary losses such as construction period.


Here, we must solemnly remind everyone that no matter what type of manufacturer and type of down-the-hole drilling tools are used, the lubrication and correct use of the products are the fundamental guarantee for protecting their own investment and interests, as well as product design, material selection and process. The ultimate guarantee.

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