Analysis of Failure Causes for Main Components of Hydraulic Rock Drills

A hydraulic rock drill is a type of equipment used for drilling holes in rock, equipped with multiple functions such as impact, rotation, propulsion, and flushing—essential for creating blast holes or anchor bolt holes during construction. Due to its relatively complex structure, the causes of component failure in hydraulic rock drills are also intricate. This article analyzes the primary reasons behind the failure of key components in hydraulic rock drills.

1. Accumulator End Cap Cracks

The cracks generated in the accumulator end cap are shown in Figure 1. There are three main reasons for these cracks:

First, the method used to check the accumulator pressure was inappropriate. The accumulator pressure should not be checked too frequently, as this can easily lead to nitrogen leakage from the accumulator bladder during inspection. Operating the accumulator at low inflation pressure will result in excessive impact forces generated by the hydraulic oil. These forces, when applied to the accumulator end cap, may cause cracks to develop in the cap.

Second, the nitrogen charging pressure valve has not been replaced. During the accumulator overhaul, the nitrogen charging pressure valve should be replaced. This is because the sealing surfaces of the old nitrogen charging pressure valve suffer wear after prolonged and frequent opening and closing. Once the sealing surfaces of the nitrogen charging valve are worn, it can lead to nitrogen leakage from the accumulator bladder, ultimately causing cracks in the accumulator end cap.

Third, the tightening force on the accumulator end cap is too high. Failing to tighten the accumulator end cap according to the specified torque can lead to additional internal stress, resulting in early cracking.

2. Rinse head cracked

The flushing head is made from high-strength, corrosion-resistant steel and serves to keep the water seal in the correct position while supporting the stop ring. The fractured area of the flushing head is shown in Figure 2, and the causes of the fracture can be attributed to the following three aspects:

First is operator error. After analyzing multiple cases of flushing head failures, it was concluded that the primary cause of these failures is the improper operation of hydraulic rock drills when performing impact actions without forward thrust—particularly during high-impact or reverse-propulsion (back-drilling) scenarios, which significantly increases the risk of flushing head rupture.

Second, the flushing head becomes corroded. Since the material used to manufacture the flushing head cannot simultaneously offer both high strength and excellent corrosion resistance, if the flushing water contains acidic or alkaline corrosive substances, the flushing head will inevitably be eroded. This corrosive action can lead to cracks forming in the flushing head.

Third, the front end has been corroded. An irrigation head is installed inside the drill's front end; if the front end becomes corroded, the irrigation head will shift forward. Once the irrigation head moves forward, the recoil force generated by drilling is transmitted through the stop ring to the connection plate, leading to stress concentrating at the hole in the front-end connection plate.

Around the mouth area. Since the connecting plate is the component that links the flushing head, stress concentration can easily lead to cracking of the flushing head.

3. Anterior Cracks

The front end of the hydraulic rock drill is internally equipped with a pilot sleeve and a flushing head, bearing all the loads transmitted from the drill bit shank. The cracked area at the front end is shown in Figure 3, and its failure can be attributed to the following three factors:

First, operator error occurs when the rock drill is operated for extended periods under low, no, or even reverse (back-breaking) thrust conditions. In these situations, the impact force from the piston is transmitted through the stop ring and flushing head to the front end, potentially causing cracks to develop at the front.

Second, internal corrosion within the tool bit occurs. If corrosion develops inside the tool bit, stress concentrations will form at the corroded areas during hydraulic rock drilling operations, leading to cracks in the tool bit. Over time, these cracks will continue to propagate until the tool bit eventually breaks completely.

Third is corrosion from flushing water. If the hydraulic rock drill uses corrosive flushing water, it can cause corrosion at the front end, leading to stress concentration in the corroded areas and ultimately resulting in cracks forming at the front tip.

4. Impact Piston Guide Bushing Area Damaged

The most common issue with impact pistons is damage to the guide bearing area, leading to the piston becoming stuck in the guide sleeve. As shown in Figure 4, the damage to the impact piston's guide bearing area stems from the following three factors:

First, contaminants are present. Damage caused by contaminants in the shock piston guide bearing area includes: contamination of the hydraulic oil, leading to poor contact between the shock piston and the guide sleeve; and the presence of contaminants between the sealing chamber and either the rear end cap or the shock piston guide sleeve, resulting in misalignment of the shock piston. These issues cause a sharp rise in localized surface temperatures on the piston, triggering minute thermal cracks on its surface. Over time, these cracks progressively propagate inward toward the core of the shock piston, ultimately leading to its fracture.

Second, the tightening forces of the bolts are uneven. If the bolts on both sides of the rock drill are damaged or if the tightening forces are imbalanced, or if the rear cover bolts are not tightened evenly—especially when the bolts haven’t been re-tightened within the prescribed maintenance period—it can lead to a reduction in the coaxial alignment of the rock drill’s components, resulting in impact piston malfunction.

Force does not transmit in a straight line, ultimately leading to the piston guide bearing area becoming jammed or damaged against the guide sleeve.

Third is corrosion of the mating surfaces. Corrosion at the interface between the guide sleeve and the piston leads to increased friction during impact operations, resulting in premature wear and damage to both the impact piston guide bearing area and the guide sleeve itself.

5. Impact Piston Cavitation

The impact piston of the hydraulic rock drill experiences pulsed forces, causing cavitation to frequently occur on the surfaces of the front and rear drive areas of the piston, as well as in the piston sealing region. The cavitation phenomenon occurring in the impact piston is illustrated in Figure 5; the causes of cavitation can be attributed to the following two aspects:

First, excessively low thrust pressure can lead to cavitation in the impact piston. When a hydraulic rock drill operates continuously under low thrust pressure, it forces the buffer piston to move forward, shifting the impact position of the impact piston further ahead. This results in an extended stroke for the impact piston and a reduced impact frequency. Meanwhile, since the reversing valve’s switching time remains unchanged, the timing of the impact piston’s reversal no longer aligns with that of the valve, creating sudden high-pressure spikes that may ultimately cause cavitation in the impact piston.

Second, frequent reverse thrust (backfiring) in hydraulic rock drills, as well as excessively low or high nitrogen pressure in the accumulator bladder, or even accumulator failure, can all accelerate cavitation wear on the impact piston. For instance, when the accumulator's nitrogen pressure is too low, the buffering capacity of the buffer piston diminishes, leaving the impact piston’s pulses unbuffered and causing a sharp increase in hydraulic oil pressure—potentially leading to cavitation damage on the impact piston.

6. Impact piston sealing surface damaged

The damage to the impact piston sealing surface is shown in Figure 6. Typically, this type of damage occurs when the piston sealing area becomes seized against the cylinder body. Such seizure between the steel components in this critical area can cause the impact piston to get stuck, prompting operators to stop the operation before the piston completely breaks. There are two primary reasons for this piston seizure: First, contaminants may be present in the hydraulic oil, or impurities could have entered from outside the rock drill, wedging themselves between the impact piston sealing surface and the cylinder body. Second, improper tightening torque on the side bolts might lead to poor alignment, or wear in the impact piston guide sleeve could also contribute to the issue.

7. Impact piston impact face damage

The damage to the impact piston's striking face is shown in Figure 7. Typically, this type of damage to the impact piston's striking face is caused by the following reasons:

(1) Salt Corrosion

If a hydraulic rock drill is placed in a saline environment for even a short period, the metal will still suffer from salt-induced corrosion—even when the machine is not in operation. As a result, its fatigue strength will drop by as much as two-thirds compared to normal conditions, and the piston will fail after just a few hours of normal impact. The impact piston cannot withstand saltwater erosion, so the only way to extend its lifespan is to prevent saltwater from entering the rock drill altogether.

(2) Corrosion Damage

If corrosive liquids enter the piston surface and drill bit shank area while the rock drill is operating, corrosive grooves will form on the impact face of the piston. These grooves can initiate fatigue cracks, ultimately leading to piston failure. If the piston damage isn’t severe, it can be reconditioned by grinding.

8. The rotating bearing is severely worn.

The wear condition of the rotary bearing in the rock drill is shown in Figure 8.

The preload of the rotating bearing should be appropriate; if the preload is too low, the bearing balls will drift off their raceways, leading to bearing damage. Rock drills equipped with a simple buffer piston (such as models COP 1032/1238/1440) are particularly sensitive to insufficient preload in the rotating bearing. In these rock drills, the impact of the buffer piston against the rotating sleeve generates vibrations that cause the bearing balls to stray from their raceways, resulting in deformation of the bearing housing and ultimately leading to the failure—specifically, the rupture—of the rotating bearing.

Excessive preload results in excessive friction acting on the bearing, which can lead to premature wear. When assembling a rotating bearing, it is essential to perform a preload test.

9. Rotating bushing damaged

The rotating bushing transmits the reaction force of the impact from the drill bit shank to the buffer piston, and its damage is shown in Figure 9. Typically, there are two main reasons for the failure of the rotating bushing:

(1) Insufficient lubrication

Sufficient lubrication is a critical requirement for maintaining the optimal performance of rotating bushings. High thrust forces and large-diameter drilling operations necessitate enhanced lubrication. Discoloration around the end faces of the rotating bushings typically indicates inadequate lubrication; in cases of severe lubrication deficiency, it can lead to cracking of the bushing and even damage other components of the rock drill.

(2) Fatigue Damage

The rotating bushing is a wear part that typically needs to be replaced after 400 hours of operation under impact conditions, to prevent fatigue-related damage from compromising other components.

10. Side bolt damaged

The side bolts are used to assemble the various components of the rock drill, and their damage is illustrated in Figure 10. These bolts endure impact forces generated during the drilling process due to severe vibrations. To prevent fatigue failure, the side bolts must be tightened according to the specified manufacturing guidelines. There are two primary reasons for side bolt damage: first, the tightening torque of the bolts is not checked within the recommended intervals; second, contaminants between the nut and bolt, or thread corrosion, can cause the bolts to seize up.

In cases of thread corrosion, even tightening the side bolts according to the specified torque will not generate sufficient clamping force. Indentations and corrosion spots on the threads can lead to cracking, which may ultimately result in fatigue failure of the side bolts.

Side bolts that are severely rusted or cracked should be replaced. During each major overhaul, the side bolts, nuts, and washers should be replaced to prevent secondary damage. Mixing new and old bolts is strictly prohibited.

11. Retaining ring damaged

The most common causes of rock drill damage are severe reverse blows, low thrust force, or complete lack of thrust. When the piston drives the drill bit into its forward position, it retains some residual impact energy, which is then transferred to the stop ring. Such reverse blows, combined with low or no thrust force, accelerate wear on the stop ring. The damage to the stop ring is illustrated in Figure 11.

When replacing the drill bit tail, the retaining ring must be inspected. If it is damaged or worn beyond 1 mm, it should be replaced immediately. As the retaining ring is a wear-prone component, to prevent damage to related parts (such as the buffer piston and drill bit tail) caused by fatigue failure of the retaining ring, it is generally recommended to replace the ring every 400 hours of operation.

12. Drive sleeve wear

The most common failure phenomenon of the drive sleeve (triangular sleeve) is premature wear, as shown in Figure 12. Typically, drive sleeve wear is caused by the following four reasons: first, insufficient or no lubrication; second, using the wrong type of lubricant; third, the diameter of the bore being larger than the size specified for the rock drill; and fourth, overheating of the drive sleeve.

Overheating of the drive sleeve can lead to cracking in the spline area of the pin end. Additionally, damage to the internal rack within the drive sleeve and severe wear of the front guide sleeve can both cause the drive sleeve to crack. Moreover, excessive clearance between the drive sleeve and the rotating bushing may also result in cracking of the drive sleeve.

13. Buffer Piston Cavitation

Sometimes, the buffer piston experiences severe cavitation after only a short period of operation, as shown in Figure 13.

Cavitation in the buffer piston is typically caused by a ruptured accumulator diaphragm or incorrect nitrogen charging pressure. The resulting flow fluctuations can lead to cavitation on the buffer piston.