The influence of lubrication on the shank broken of down-the-hole drill bits

In June 2025, customer A reported that four of our 115mm down-the-hole drill bits had broken during late-stage use. After an on-site inspection, our technicians discovered that the drilling rig was using waste engine oil with a viscosity of 150. This high viscosity resulted in a low amount of lubricant introduced by the compressed air during operation. Furthermore, the rig lacked a dry dust collector, and during drilling, large amounts of water were injected into the compressed air, washing away the oil inside the hammer and at the interface between the caliper sleeve and the drill bit, preventing the formation of an effective lubricating film.

 

 

The crack source was detected at the fracture surface by metallographic examination. From the metallographic photos, we can see that there are obvious burns on the surface of the product, and tiny cracks extend inward at the burns. This is the main reason for the cracks.

 

 

We recommend that customers lift the hammer after drilling, close the water injection connection, and then open compressed air for three minutes to flush out any water stains inside the hammer. This will allow a sufficient oil film to form between the hammer's internal components and between the drill bit shank and the drill sleeve , preventing rust on the drill bit shank during downtime. We also recommend using a lubricant specifically designed for pneumatic rock drilling tools and ensuring proper lubrication of the drill bit during operation, achieving a minimum lubrication consumption of 2.3 liters per hour (16 liters per shift).

 

The impactor lubricating oil injection standards are as follows:

 

For dry drilling (water injection less than 2 gpm (7.6 lpm)), the recommended rate of lubricant injection is 1/3 pint (0.16 liters) per hour for every 100 scfm (2.8 m³/mm) of air. For example, if a 900 scfm (25.5 m³/mm) compressor's airflow were to be directed entirely to the hammer, 900 ÷ 100 × 1/3 = 3 pints (1.6 liters) of lubricant would be required per hour. For wet drilling (water injection greater than 2 gpm (7.6 lpm)), the recommended rate of lubricant injection is 2/3 pint (0.32 liters) per hour for every 100 scfm (2.8 m³/mm) of air. Additional lubricant is required to compensate for leaks and oil loss caused by water. Additional lubricant injection is also required if foam is present around the drill hole. (See the table below.)

 

 

After the improvement, the life of our drill bit reached 3,500 meters, the drill bit spline wore out and failed, and the shank no longer broke. Our products have been recognized by customers and are now in stable supply in batches.

 

 

About Us

 

As a global supplier of Rock Drilling Tools, Coal Mining Tools, Construction Tools, Wear And Crushing Tools and Rotary Drilling Tools, Luoyang Golden Egret Geotools co., Ltd. continues to invest in research and development and is committed to providing customers with high-reliability, low-energy consumption products.