Fault Analysis and Solutions of Rexroth Axial Piston Variable Motor A6VM in Rotary Drilling Rig

As the core equipment of modern infrastructure construction, the reliability of the hydraulic system of the rotary drilling rig is directly related to the construction efficiency and project quality. Rexroth's A6VM variable-speed axial piston motor has become a key power component of the main winch and travel system of the rotary drilling rig due to its advantages such as high pressure, high torque and wide speed range. However, in complex construction environments, the A6VM axial piston motor often faces typical faults such as overheating, leakage and speed failure. This article will deeply analyze the causes of these faults, provide a systematic diagnostic method, and give targeted maintenance and preventive measures to help equipment managers extend the service life of the motor and reduce maintenance costs.

The key role of A6VM axial piston motor in rotary drilling rigs

As an indispensable heavy equipment in modern infrastructure construction, the core functions of rotary drilling rigs, such as drilling rod lifting, power head rotation and whole machine travel, are highly dependent on the support of high-performance hydraulic systems. Among the many hydraulic components, Rexroth's A6VM series of inclined axis axial piston variable motors have become the preferred power unit for the main winch system and travel drive system of rotary drilling rigs due to its excellent power density, wide speed range and reliable load adaptability. This series of axial piston motors adopts an innovative inclined axis design, which realizes stepless displacement adjustment by changing the angle between the cylinder body and the drive shaft, and can accurately match the torque and speed requirements of rotary drilling rigs under different geological conditions.

However, the A6VM axial piston motor also faces many challenges in harsh construction environments and heavy-load conditions. Statistics show that about 35% of the hydraulic system failures of rotary drilling rigs are related to the travel and main winch motors. These failures may cause equipment downtime and delay the construction period, or may cause a chain reaction and damage other key components. Typical failure phenomena include abnormal heating of the motor housing, insufficient output torque, slow speed response, and hydraulic oil leakage. These problems are often closely related to the equipment operation mode, maintenance quality, and system matching design.

Based on the actual application cases and maintenance data of Rexroth axial piston motors, this article will systematically analyze the common failure modes of the A6VM series in rotary drilling rigs, deeply analyze the root causes of the failures, and provide operational diagnostic methods and solutions. At the same time, we will also explore how to extend the service life of axial piston motors through scientific preventive maintenance strategies, providing a comprehensive reference guide for equipment managers and maintenance technicians. By optimizing the operating status of the A6VM axial piston motor, not only can the overall working efficiency of the rotary drilling rig be improved, but also the maintenance cost of the equipment over its entire life cycle can be significantly reduced.

Structural features and working principle of A6VM axial piston motor

the bent-axis axial piston motor has a unique structural layout, which enables it to show excellent performance advantages in heavy-duty applications such as rotary drilling rigs. Unlike the traditional swash plate design, the A6VM motor's piston group is arranged at a certain angle to the drive shaft (usually 25° or 40°). This inclined axis structure can not only withstand higher radial loads, but also significantly improve the motor's displacement and torque output capacity by increasing the plunger stroke. The core moving pairs inside the motor include: plunger-cylinder pair, slipper-swash plate pair and cylinder-port plate pair. The fitting clearance of these three pairs of precision friction pairs is usually only 5-15 microns. They rely on hydrostatic oil film to achieve lubrication and sealing, and have extremely stringent requirements on the cleanliness of the hydraulic oil.

The variable mechanism of the A6VM axial piston motor is the key to distinguishing it from a fixed displacement motor. This mechanism adjusts the inclination angle of the swash plate in real time through a hydraulic servo control system, thereby changing the effective stroke of the plunger and achieving stepless changes in displacement. When the pilot pressure signal from the rotary drilling rig control system acts on the variable piston, the piston displacement is converted into a change in the swash plate angle through a mechanical connecting rod, thereby adjusting the motor displacement. In this process, the size of the damping hole on the control oil circuit directly affects the response speed of the variable. A damping hole that is too small will cause slow speed change, while a damping hole that is too large may cause system oscillation. It is worth noting that the A6VM motor is usually integrated with a high-pressure relief valve and an oil replenishment valve. The former limits the maximum pressure of the system to protect the safety of the components, and the latter provides the necessary cooling oil for the closed circuit to prevent the motor from being damaged due to overheating.

In the typical application of rotary drilling rigs, the A6VM axial piston motor mainly undertakes two key functions: one is to serve as the main winch drive motor , responsible for the lifting and lowering of the drill rod; the other is to serve as the travel drive motor , providing the traction required for the whole machine to move. In the main winch system, the motor needs to be started and stopped frequently and withstand huge impact loads. In particular, when the drill rod is suddenly stuck or released quickly, the hydraulic system may produce instantaneous pressure peaks, which poses a severe test to the motor bearings and valve plate16. In the travel system, the synchronization accuracy and speed response speed of the two A6VM motors directly determine the straight-line driving performance and steering flexibility of the drilling rig. Any slight internal leakage or variable mechanism jamming may cause the vehicle to deviate or lack of power.

The shaft seal system of the A6VM axial piston motor also deserves special attention. The motor output shaft usually adopts a double seal design: the inside is a high-pressure rotary seal to prevent the pressure oil in the working chamber from leaking out; the outside is a dustproof seal to block the invasion of external pollutants. When the internal leakage of the motor increases abnormally, the pressure in the oil drain chamber may rise sharply, which will not only accelerate the wear of the shaft seal, but in severe cases, it may even directly flush the oil seal out, causing a large amount of hydraulic oil leakage. In addition, the oil drain port on the motor housing must be kept unobstructed. If the oil drain line is bent or blocked, the housing pressure will increase, which may cause damage to accessories such as sensors (such as the burning of the speed sensor mentioned in the case), or even cause serious consequences such as the housing bursting.

Table: Typical technical parameters of A6VM axial piston motor in rotary drilling rig

Understanding the structural characteristics and working principles of the A6VM axial piston motor is the basis for accurate diagnosis of on-site faults. In the actual maintenance process, many seemingly complex fault phenomena often originate from problems at the basic principle level. Only by grasping the core mechanism can we avoid being confused by surface phenomena and make correct judgments and disposal.

Common failure modes and causes analysis

axial piston motors will exhibit a variety of typical failure modes, and each failure often hides a specific formation mechanism. A deep understanding of the characteristic manifestations and root causes of these failures is a prerequisite for implementing precise maintenance. Based on the actual maintenance cases and data statistics of Rexroth A6VM series motors, we can classify these failures into several main categories, each of which has its own unique symptoms and diagnostic points.

Motor overheating and abnormal temperature rise

Abnormal rise in housing temperature is one of the most common failure phenomena of A6VM axial piston motors, and is also the initial cause of many chain failures. Under normal operating conditions, the motor housing temperature should be 10-20℃ lower than the hydraulic oil temperature. If the motor housing feels hot to the touch (usually over 80℃), it indicates abnormal heating. Overheating problems mainly come from two sources: one is mechanical friction heat generation. When the bearing clearance is too large or the sliding surface of the swash plate is poorly lubricated, direct contact friction between metals will generate a lot of heat; the other is hydraulic energy loss. High-pressure oil leaks into the low-pressure cavity through the worn distribution plate or plunger gap, and the energy is converted into heat energy. A construction site once reported an extreme case in which the plastic housing of the speed sensor melted after the A6VM200 motor had been running for less than 50 hours. After disassembly and inspection, it was found that the motor cylinder and the distribution plate had adhered due to high-temperature sintering. The root cause was that the oil drain line was blocked, causing the housing heat to be unable to dissipate in time.

Specific factors that lead to motor overheating include: insufficient axial preload of the bearing causing abnormal friction between the raceway and the roller; hydraulic oil contamination causing scratches on the surface of the distribution plate, increasing internal leakage; insufficient oil replenishment pressure causing the static pressure support of the friction pair to fail; or the system flushing flow is too small to effectively cool the inside of the motor. It is worth noting that when the rotary drilling rig is continuously piling, the main winch motor is often in a low-speed and high-torque condition. At this time, it is difficult to form an oil film and is more prone to local overheating. Operators should avoid maintaining this working state for a long time.

Insufficient output torque and reduced speed

When the rotary drilling rig is unable to lift the drill or the travel speed drops significantly, it often indicates that the performance of the A6VM axial piston motor has declined. This type of failure can be divided into two situations: one is that the motor housing temperature is normal but the output torque is insufficient. The problem usually lies in the oil supply of the hydraulic system, such as insufficient main pump flow, low control pressure or reversing valve stagnation; the other is the torque drop accompanied by severe heating of the housing, which is mostly caused by increased internal leakage caused by internal wear of the motor.

The internal leakage paths are mainly concentrated in three key friction pairs: the increase in the gap between the plunger and the cylinder bore causes the high-pressure chamber to leak oil into the housing; the wear of the joint surface between the distribution plate and the cylinder body causes the high and low pressure chambers to communicate; the failure of the variable mechanism control piston seal causes the pilot pressure to leak. During detection, the degree of internal leakage can be quantified by measuring the flow difference between the motor inlet and return oil ports. Under normal circumstances, the volumetric efficiency should not be less than 90%. The A6VM motor at a construction site had a speed fluctuation problem. After disassembly, it was found that the variable mechanism control piston was scratched by metal chips, forming grooves that caused the pilot pressure to leak, making the swash plate unable to stabilize at the set position, and ultimately manifested as irregular fluctuations in the output speed.

Speed shifting malfunction and slow response

a variable motor , the speed change performance of A6VM is crucial to the operational sensitivity of the rotary drilling rig. When speed change failure or response delay occurs, the control oil circuit should be checked first: whether the control pressure reaches the set value (usually 20-40bar); whether the damping hole is blocked; whether the servo valve core is stuck. There was a case where the motor displacement switching took more than 5 seconds (normally less than 1 second). Inspection found that the control oil filter was blocked, resulting in obstruction of the control oil flow. The fault was eliminated after cleaning the filter.

Mechanical stagnation can also cause speed change problems, such as mechanical interference caused by wear of the variable head and variable body, or rust of the inclined plate trunnion due to poor lubrication. In low temperature environments, the increased viscosity of the hydraulic oil may cause the variable mechanism to move slowly, which reminds us to use low-condensation hydraulic oil and fully preheat the system before winter construction. In addition, electrical signal failures such as proportional solenoid coil open circuit or abnormal control module output will also manifest as speed change function failure. At this time, it is necessary to use an ammeter to measure the solenoid resistance and input current for judgment.

Abnormal noise and vibration

A healthy A6VM axial piston motor should make a uniform "buzzing" sound when running. Any metal knocking sound or intermittent abnormal noise indicates potential problems. Bearing damage is a common source of noise. When pitting occurs on the raceway or the cage is broken, a high-frequency "crackling" sound will be emitted, and it will intensify with the increase of speed. Another type of noise comes from cavitation. When the resistance of the oil inlet pipeline is too large or the gas content of the oil is too high, vacuum bubbles may be generated in the plunger cavity during the oil suction stage. These bubbles will collapse instantly in the high-pressure area, causing a crisp popping sound. Long-term cavitation will also corrode the surface of the cylinder body and the distributor.

Vibration problems are often related to unbalanced rotating parts or loose fits. In one case, an A6VM motor vibrated violently in a specific speed range. After disassembly and inspection, it was found that the coupling cushion was damaged, causing the motor and reducer to be out of center. After replacing the elastic coupling, the vibration disappeared. Vibration will accelerate the aging of seals and the loosening of bolts, forming a vicious cycle. Therefore, once abnormal vibration is found, the machine should be stopped immediately for inspection to avoid secondary damage.

Hydraulic oil leakage

Leakage failures can be divided into two categories: internal leakage and external leakage. Internal leakage has been discussed in the previous article, while external leakage is more intuitive, usually manifested as oil seepage at the shaft seal, pipe joint or housing joint surface. Spindle oil seal failure is a common cause of external leakage. When wear grooves appear on the shaft surface or the oil seal lip ages, high-pressure oil will leak out along the shaft neck. It is worth noting that excessive internal leakage will increase the pressure in the oil leakage chamber, indirectly leading to increased leakage at the shaft seal. Therefore, simply replacing the oil seal often cannot completely solve the leakage problem, and the root cause of the internal leakage must be solved at the same time.

Another special type of leakage occurs at the casting defects of the motor housing, such as sand holes or micro cracks. In a maintenance case, the A6VM motor housing temperature sensor interface continued to leak oil, and repair welding still could not solve the problem. It was finally discovered that there were casting pores inside the housing, and the pressure oil leaked along the pore channel. The only option was to replace the entire housing assembly. This reminds us that when purchasing hydraulic components, we should choose original products from regular channels to avoid early failures due to casting quality defects.

Table: Correspondence between A6VM axial piston motor fault symptoms and possible causes

By systematically sorting out these failure modes and their internal mechanisms, maintenance personnel can establish a structured diagnostic approach and avoid detours in the troubleshooting process. It is worth noting that many failures do not occur independently, but are interrelated and cause and effect. Therefore, while dealing with the dominant failure, the potential inducing factors should also be checked to truly achieve a thorough cure for the failure.

Fault diagnosis methods and steps

Accurate diagnosis is the key prerequisite for solving A6VM axial piston motor failures. The lack of a systematic diagnostic process often leads to misdiagnosis and repeated repairs. In view of the characteristics of axial piston motors used in rotary drilling rigs, we have developed a set of clearly defined fault diagnosis methods, from simple appearance inspection to complex internal disassembly, to gradually locate the root cause of the fault. This method has been proven effective on multiple construction sites and can significantly improve maintenance efficiency and accuracy.

Initial examination and symptom analysis

Sensory diagnosis constitutes the first line of defense for troubleshooting. Experienced maintenance technicians can find many potential problems by "looking, listening, touching, and smelling". Checking the appearance of the motor for oil stains can determine the location of the leak; listening to the uniformity of the running sound can identify bearing or plunger abnormalities; touching the housing temperature to feel the cooling effect; smelling the oil odor can find signs of overheating and burning. For example, when fresh oil stains appear near the oil drain port of the A6VM motor, it is likely that the shaft seal has begun to fail; if the motor is running with intermittent "clicking" sounds, it may indicate that the swash plate support bearing is damaged.

Operation test is another important preliminary inspection. By actually operating the main winch and travel system of the rotary drilling rig, observe the response characteristics of the motor under different working conditions: whether it is stable and without creeping at low speed; whether there is impact during speed change; whether it can maintain stable torque under maximum pressure, etc. In one case, the right side of the drilling rig was obviously weak when it was moving, but the pressure gauge showed that the system pressure on both sides was the same. It was finally found that the variable mechanism of the A6VM motor on the right side was stuck in the small displacement position and could not provide sufficient torque.

Instrument measurement and parameter analysis

When sensory inspection cannot determine the root cause of the fault, instrumental measurement is required to obtain quantitative data. The most basic testing tools include hydraulic pressure gauges, flow meters and thermometers. By measuring the motor inlet and outlet pressures, flow rates and temperatures, the actual efficiency can be calculated and compared with the standard values. For example, if the motor inlet pressure is measured to be 350 bar and the return oil pressure is 30 bar, the theoretical output torque should be:

Torque (Nm) = (350-30) × 10⁵ × displacement (cm³/rev) / (20π)

If the measured torque is significantly lower than the calculated value, it indicates serious internal leakage.

Control oil circuit detection is particularly important for variable motors. A pressure gauge should be installed at the servo control port to check whether the control pressure reaches the set value (usually 10-20% of the system pressure) and whether the response time is within a reasonable range (usually <0.5 seconds). A construction site reported that the A6VM motor was slow to change speed. Measurements found that the control pressure was slow to build up. It was eventually found that the damping hole on the control oil circuit was partially blocked by colloid, which returned to normal after cleaning.

For electrically controlled variable motors, a multimeter is also required to check the resistance and supply voltage of the proportional solenoid to ensure that the coil is not broken and the control signal meets the requirements. Complex faults may require the use of an oscilloscope to observe the control current waveform, or connect Rexroth's dedicated diagnostic software to read the motor's internal parameters and fault codes.

Oil testing and contamination analysis

The condition of the hydraulic oil directly reflects the internal health of the axial piston motor. Taking oil samples for particle counting and spectral analysis can determine the degree of wear and the source of contamination. For example, a sudden increase in the copper content in the oil may indicate wear of the bearing cage; excessive silicon content indicates external dust intrusion; and a large number of 10-20μm steel particles indicate wear of the valve plate or plunger. Rexroth recommends that the oil cleanliness of the A6VM motor should be maintained within ISO 4406 18/16/13 level. Exceeding this range will significantly shorten the motor life.

Moisture detection should not be ignored either. Moisture will destroy the strength of the oil film, increase the wear of the friction pair, and promote the oxidation and deterioration of the oil. A simple test can be done by dropping oil on a hot plate. If there is a "crackling" sound, it means that the water content is too high; accurate measurement requires the use of a special moisture meter. The A6VM motor at a coastal construction site frequently experienced cavitation of the distribution plate. Testing found that the moisture content in the oil reached 0.15%, far exceeding the limit of 0.05%. The problem was solved after replacing the oil and repairing the breather.

Disassembly inspection and wear assessment

When all external tests still cannot determine the cause of the fault, motor disassembly becomes the final diagnostic method. The disassembly process should follow the standard steps in the Rexroth maintenance manual, paying special attention to recording the relative positions of each component and the number of adjustment shims. Key inspection areas include: whether there is ablation and scratches on the surface of the valve plate; the clearance between the plunger ball head and the sliding shoe; the sealing condition of the variable mechanism piston; and signs of fatigue on the bearing raceway.

Wear assessment requires the support of experience and technical data. For example, the flatness deviation between the cylinder block and the valve plate of the A6VM motor should not exceed 0.005mm. If it exceeds this value, it needs to be ground or replaced; the standard clearance between the plunger and the cylinder hole is 0.015-0.025mm. If it exceeds 0.04mm, the component must be replaced. In a maintenance case, it was found that the swash plate trunnion was slightly rusted during disassembly, resulting in limited variable angle. After polishing with fine sandpaper and applying special grease, the normal variable range was restored.

System Interaction Impact Analysis

Many times, the real root cause of motor failure is not the motor itself, but the system matching problem. For example, the flow pulsation of the main pump may cause motor pressure oscillation; unreasonable oil tank design may cause cavitation; or insufficient cooler capacity may cause excessive oil temperature. When diagnosing, the hydraulic system should be considered as a whole and the working status of all related components should be checked.

Particularly noteworthy is the flushing circuit of the closed system . In closed applications (such as travel drives), the A6VM motor relies on a continuous flushing flow to remove heat and pollutants. If the flushing valve is not set properly or the filter is clogged, the motor will overheat quickly. It is recommended to regularly check the flushing flow, which should not be less than 10% of the main pump flow, and the flushing oil temperature should not exceed 70°C.

Through this well-organized diagnostic process, maintenance personnel can gradually identify the root cause of the A6VM axial piston motor failure from the phenomenon to the essence. Practice has proved that following a systematic diagnostic method is more efficient and reliable than guessing based on experience, and can effectively avoid unnecessary parts replacement and repeated repairs. In the next section, we will discuss specific maintenance solutions and preventive measures based on the diagnostic results.

Repair solutions and replacement standards

Scientific maintenance is the key to restore the performance of the A6VM axial piston motor. Improper maintenance methods not only fail to solve the problem, but may also introduce new potential faults. For different types of faults and wear levels, we need to adopt differentiated maintenance strategies, from simple on-site adjustments to professional factory renovations, to form a complete solution system. This section will elaborate on the specific maintenance methods for various typical faults and provide clear parts replacement standards to help maintenance personnel make reasonable decisions.

Repair technology of friction pair wear

Repairing the valve plate is one of the most common processes in A6VM motor maintenance. When there are slight scratches on the surface of the valve plate (depth <0.01mm), grinding repair can be used: use a grinding plate with a particle size of 800# or above, use kerosene as the medium, and manually grind in an "8" shape until the scratches disappear and the flatness reaches within 0.005mm. After grinding, it needs to be thoroughly cleaned to avoid abrasive residue. For valve plates with severe ablation or detached coating, new parts should be replaced because the damage of the surface hardened layer will accelerate wear.

the plunger assembly requires careful evaluation. The standard clearance between the plunger ball head and the slide shoe is 0.02-0.05mm. If it exceeds 0.1mm, the slide shoe or the entire plunger assembly must be replaced. It is worth noting that the plungers and slide shoes of the A6VM motor should be replaced in groups. Mixing parts with different degrees of wear will cause uneven force. In a repair case, only 3 of the 7 plungers were replaced. As a result, the new plungers bore most of the load and soon showed abnormal wear.

Cylinder repair is usually limited to minor wear. When the cylinder bore roundness error is <0.01mm, honing can be used to restore the surface quality; if the wear is severe or there are signs of cylinder pulling, it is recommended to replace the entire cylinder assembly. When assembling after repair, special attention should be paid to the running-in of the cylinder and the valve plate: the initial start should be run at low pressure (50-100bar) for 30 minutes to gradually build up an oil film to avoid secondary damage caused by direct high-load operation.

Troubleshooting methods for variable mechanism failures

the servo valve jam . When disassembling the servo valve, make a mark to avoid reverse installation; the clearance between the valve core and the valve hole should be less than 0.005mm. If there is burrs or rust, use a fine oilstone to slightly trim it, and then polish it with suede. All parts must be fully lubricated with hydraulic oil before assembly, and the valve core should be able to slowly slide through the valve hole by its own weight. If the valve core is severely worn and cannot be repaired, the entire servo valve assembly must be replaced to avoid variable instability due to internal leakage.

of the variable piston seal will result in the inability to establish control pressure. When replacing the seal, pay attention to the material and specifications of the original seal. Ordinary nitrile rubber seals age quickly in high temperature environments, and high-performance seals made of fluororubber or polyurethane should be used. Check the surface finish of the piston before installation. Any scratches may cut the new seal. If necessary, use fine sandpaper (above 1000#) to gently polish it along the axial direction.

Wear of the swash plate trunnion will limit the variable angle. The clearance between the trunnion and the bearing should be <0.02mm. If it is loose due to wear, the shaft diameter can be repaired by brush plating, or the swash plate assembly can be replaced. When adjusting the variable mechanism, Rexroth special tooling is required to ensure the center position accuracy to avoid excessive zero flow due to mechanical deviation.

Replacement criteria for bearings and rotating parts

Bearing life is the main factor that determines the overhaul cycle of the A6VM motor. According to official data from Rexroth, the average service life of bearings under normal conditions is about 10,000 hours, but the actual service life may be greatly shortened due to contamination, overload or misalignment. Disassembled bearings should be replaced even if they appear intact, because increased clearance (>0.05mm) cannot be determined by visual inspection. When replacing bearings, the original model must be used. Different brands of bearings may have differences in preload and load capacity.

The spindle repair needs to be particularly careful. The journal surface roughness should be less than Ra0.2μm. If there are wear grooves (depth>0.01mm), laser cladding or brush plating can be used for repair, but the bonding strength between the repair layer and the substrate must be ensured. The wear of the shaft seal contact area will directly affect the sealing effect. Minor wear can be polished with fine sandpaper. Severe wear requires the replacement of the spindle or the use of a sleeve repair process.

Principles of overall replacement of sealing system

Hydraulic seals are the first line of defense against leaks. When repairing the A6VM motor, all dynamic and static seals should be replaced, including shaft seals, O-rings, and combination gaskets. When selecting seals, pay attention to material compatibility: standard nitrile rubber (NBR) is suitable for mineral oil; when using water-glycol or phosphate ester hydraulic oil, ethylene-propylene (EPDM) or fluororubber (FKM) seals should be selected.

the oil drain system is often overlooked. After maintenance, check whether the oil drain line is unobstructed. The pipe diameter should not be less than the size of the motor oil drain port, and the pipeline should avoid "bag-shaped" air accumulation sections. The oil drain back pressure must be controlled within 0.5 bar. Too high will cause premature failure of the shaft seal. In a maintenance case, the newly installed A6VM motor had a shaft seal leak shortly after operation. It was eventually discovered that the oil drain line was too long (more than 5 meters) and had multiple bends, causing the back pressure to be too high.

Performance test process after maintenance

No-load test is the first step of maintenance acceptance. The motor should start smoothly under no-load condition, and the positions of various variables should be switched flexibly without abnormal noise. During the test, the speed should be gradually increased to the maximum value, and the vibration and temperature rise should be observed. The housing temperature should not exceed the ambient temperature by 30°C.

The load test verifies the actual working performance. The hydraulic test bench is gradually loaded to the rated pressure to check whether the output torque and speed at different displacements meet the standards. Special attention is paid to the stability of the variable transition zone. There should be no torque mutation or speed fluctuation. The test time should last at least 30 minutes to ensure that each friction pair is fully run and reaches a thermal equilibrium state.

The sealing test should not be neglected. Maintain the pressure at the maximum working pressure for 5 minutes and check whether there is leakage at each static seal and shaft seal. For variable motors, the sealing of the control oil circuit should also be tested to ensure that there is no internal leakage of the servo piston.

Table: Replacement standards and maintenance methods for key components of A6VM axial piston motor

By strictly implementing these maintenance standards and process flows, the A6VM axial piston motor can be restored to a performance state close to that of a new one. It is worth noting that for motors with severely worn core components such as cylinders and valve plates, sometimes overall replacement is more economical and reliable than repeated repairs, especially for key construction equipment, where reliability is often more important than repair costs. In the next section, we will explore how to reduce failures and extend the life of the motor through scientific preventive maintenance.

Preventive maintenance and optimization suggestions

prevention is better than repair is particularly evident in the maintenance of A6VM axial piston motors. As a high-value construction equipment, the downtime loss of rotary drilling rigs far exceeds the regular maintenance costs. By establishing a scientific preventive maintenance system, the failure rate of A6VM motors can be significantly reduced and the service life can be extended. This section will systematically explain the daily maintenance points, oil management strategies and system optimization suggestions of axial piston motors to help users reduce the occurrence of failures from the source.

Hydraulic Fluid Management and Contamination Control

Oil cleanliness is the most critical factor affecting the life of the A6VM axial piston motor. Studies have shown that more than 70% of hydraulic failures are related to oil contamination, and solid particles will accelerate the wear of precision friction pairs such as the valve plate and plunger. Rexroth recommends that the system oil cleanliness of the A6VM motor should be maintained at ISO 4406 18/16/13 or higher standards, which requires the use of a high-efficiency filter with β₅≥200 and regular monitoring of contamination. In actual applications, an online particle counter can be installed at the motor return oil port to monitor the oil status in real time, and the filter element can be replaced in advance when the contamination is close to the critical value.

The choice of oil is also crucial. The A6VM motor should use anti-wear hydraulic oil that meets the DIN 51524 standard. The viscosity grade should be selected according to the ambient temperature: ISO VG 46 is recommended for normal temperature environments (15-40°C); ISO VG 68 is used for high temperature environments (>40°C); ISO VG 32 is used for cold areas (<15°C). Special attention should be paid to the fact that hydraulic oils of different brands and models cannot be mixed. Even if the viscosity is the same, the difference in additive formula may cause chemical reactions, precipitation or corrosion of components. A construction site mixed two brands of VG 46 hydraulic oil, causing the oil to produce flocs that blocked the filter and caused insufficient oil supply to the motor.

Regular oil changes are the basis for maintaining oil performance. It is usually recommended to change the hydraulic oil every 2000 working hours or once a year, but it should be shortened to 1000 hours in harsh environments (dusty, high temperature, high humidity). When changing the oil, all filters must be replaced at the same time, and the oil tank must be thoroughly cleaned to prevent old oil residue from contaminating the new oil. It is worth emphasizing that oil changes alone cannot solve the problem of system contamination. The source of contamination must be found, such as failed shaft seals, worn components, or water ingress into the breather.

Daily inspection and regular maintenance

Daily inspection is an effective means of discovering early faults. Operators should check the following items every shift: motor housing temperature (it should not feel hot to the touch); whether there is oil leakage at the shaft seal and pipe joints; whether the operating sound is normal; and whether there are abnormal fluctuations in system pressure. Simple temperature patches can be attached to the motor housing, and they will change color and alarm when the set temperature (such as 80°C) is exceeded. Although these inspections are simple, they can detect potential problems in time and prevent minor faults from developing into major repairs.

Regular maintenance plans should be made based on the number of working hours. Check the motor mounting bolt torque and coupling alignment every 500 hours; replace the return oil filter and sample the oil contamination every 1000 hours; check the variable mechanism responsiveness and oil drain back pressure every 2000 hours. Maintenance records should be archived in detail, including measurement data, replaced parts and abnormal phenomena. These historical data are extremely valuable for analyzing failure modes and predicting remaining life.

The maintenance of the oil drain system is often overlooked but is crucial. Check the oil drain line every month to see if it is unobstructed. The pipe diameter must not be smaller than the size of the motor oil drain port, and the pipe route should avoid U-bends that cause air blockage. The oil drain back pressure should be measured regularly. If it exceeds 0.5 bar, the cause must be investigated. It may be a pipe blockage or filter saturation. The case shows that an A6VM motor had an oil drain filter clogged, resulting in increased housing pressure, which eventually caused the speed sensor seal to melt and leak oil.

Operational specifications and working condition optimization

Correct startup procedures can significantly reduce cold start wear. In low temperature environments, the viscosity of hydraulic oil increases and it is difficult to flow. The A6VM motor should be run at no load for 5-10 minutes before starting, and then gradually loaded after the oil temperature rises to above 30°C. An oil preheating device can be installed in extremely cold areas to avoid poor lubrication due to oil solidification. During winter construction at a northern construction site, the operator ran the motor at high load without preheating, causing the motor valve plate to be severely scratched due to insufficient lubrication.

Load management is also critical to extending motor life. Try to avoid operating the A6VM motor under extreme pressure (>90% rated pressure) for a long time. This condition not only accelerates wear, but also causes the oil temperature to rise sharply. When the rotary drilling rig encounters hard rock formations, it should adopt "intermittent impact" rather than continuous pressurization drilling to allow time for the hydraulic system to dissipate heat. Operation training should emphasize smooth operation and avoid sudden acceleration or emergency stops. These impact loads will significantly shorten the fatigue life of bearings and gears.

System matching optimization can improve overall reliability. The displacement ratio of the A6VM motor to the main pump should be reasonably designed, usually recommended to be in the range of 1:1 to 1:1.5. Too large or too small will affect efficiency and control performance. The flushing flow in the closed system should not be less than 10% of the main pump flow to ensure sufficient heat exchange capacity. After a drilling rig was modified, the motor frequently overheated. Later, it was found that the flushing valve set flow was only 5%. After adjusting to 12%, the temperature returned to normal.

Condition Monitoring and Predictive Maintenance

Vibration analysis can detect bearing and gear defects early. Install a vibration sensor on the A6VM motor housing to monitor the changing trend of acceleration and speed values. When high-frequency components (>1kHz) appear, it often indicates early damage to the rolling bearing. Perform spectrum analysis regularly to establish baseline vibration characteristics, and early warning can be issued when abnormal peaks are found in subsequent tests.

Temperature monitoring is a direct means of preventing overheating failures. Install temperature sensors on the motor housing and the oil inlet and return ports to monitor temperature differences in real time. Under normal circumstances, the temperature difference between the oil inlet and the housing should be <20°C. If the temperature difference suddenly increases, it may indicate that internal leakage has intensified or cooling efficiency has decreased. Internet of Things technology makes remote monitoring possible, wirelessly transmitting temperature data to the cloud to achieve centralized management of multiple devices and abnormal alarms.

Oil analysis technology has developed into a powerful predictive tool. Regular sampling of the oil for particle count, moisture content, element spectrum, and viscosity changes can assess internal wear and remaining life. For example, a sustained increase in iron content indicates increased wear of steel components; an increase in silicon indicates seal failure or air filter penetration; and an increase in acidity reflects oil oxidation and deterioration. Based on this data, a scientific overhaul plan can be formulated to avoid sudden failures.

Table: A6VM axial piston motor preventive maintenance cycle and content

By implementing these preventive maintenance measures, the mean time between failures (MTBF) of the A6VM axial piston motor can be extended by 30-50%, and the overall maintenance cost can be reduced by more than 20%. More importantly, preventive maintenance ensures the construction continuity and reliability of the rotary drilling rig, avoiding construction delays and economic losses caused by sudden failures. For large construction companies with multiple equipment, establishing standardized hydraulic system maintenance procedures and equipping them with necessary testing equipment and training personnel will yield a considerable return on investment.

Conclusion and Outlook

The axial piston motor directly affects the construction efficiency and economic benefits of the entire equipment. Through an in-depth analysis of the Rexroth A6VM series of inclined axis axial piston variable motors, we can clearly see that most failures do not occur by chance, but are closely related to factors such as design selection, operation and maintenance, and system matching. The failure modes, diagnostic methods, and maintenance strategies systematically sorted out in this article provide a practical reference framework for on-site technicians, which helps to improve the standardization and effectiveness of fault handling.

Main research findings and practical value

Failure mechanism research shows that the typical failures of the A6VM axial piston motor show obvious regularity. Data show that bearing wear, valve plate damage and variable mechanism jamming account for more than 75% of the total failures, and these failures are often directly related to hydraulic oil contamination, overheating and improper operation. Understanding these inherent connections can help maintenance personnel quickly locate the root cause from the symptoms and avoid one-sided maintenance of "treating the symptoms instead of the root cause". For example, when the temperature of the motor housing is found to be abnormally high, not only the cooling system problem should be considered, but also potential factors such as bearing preload, oil back pressure and internal leakage should be checked.

Maintenance economic analysis reveals the important value of preventive maintenance. Comparative data shows that the overhaul period of the A6VM motor of equipment that implements systematic preventive maintenance can be extended to 12,000-15,000 hours, which is more than 50% higher than the "repair only when it breaks down" model. Although regular replacement of filters, oil and analysis and testing increase direct costs, they avoid greater losses caused by unplanned downtime and major damage. The practice of a large infrastructure engineering company shows that after the introduction of condition monitoring and predictive maintenance, the failure rate of the hydraulic system has dropped by 40% and the annual maintenance cost has been reduced by 25%.

Technological innovation is changing the traditional maintenance model. With the development of the Internet of Things and big data technology, intelligent monitoring of A6VM axial piston motors has become possible. By installing vibration, temperature and pressure sensors at key locations, real-time collection of operating data and uploading to the cloud for analysis, early fault warning and remaining life prediction can be achieved. Rexroth's latest generation of A6VM motors have begun to integrate smart chips to record operating parameters and load spectra, providing data support for precise maintenance. These technological advances will gradually promote the transformation of maintenance strategies from "regular maintenance" to "on-demand maintenance", further improving the scientificity and economy of equipment management.

Future Development Trends and Technology Outlook

Advances in materials and manufacturing technology will enhance the inherent reliability of the A6VM motor. New surface treatment technologies such as diamond-like carbon (DLC) coating can greatly improve the wear resistance of the distribution plate and plunger; high-strength composite materials are used in variable mechanisms to reduce weight and inertia; 3D printing technology realizes one-piece molding of complex flow channels and optimizes the flow characteristics of internal hydraulic oil. These innovations are expected to extend the life of next-generation axial piston motors by 30-50% while improving energy efficiency and power density.

Intelligence and integration are the clear directions for the development of hydraulic components. Future A6VM motors may integrate pressure, temperature and flow sensors, and built-in controllers to achieve adaptive adjustment, automatically optimizing displacement and pressure settings according to load changes. Through coordinated control with the main pump and valve group, an "intelligent hydraulic system" is constructed to achieve optimal efficiency and fault self-diagnosis. This intelligent upgrade will significantly reduce reliance on operator experience and make equipment maintenance more standardized and convenient.

Green environmental protection requirements drive hydraulic technology innovation. With increasingly stringent environmental regulations, the A6VM motor faces multiple challenges of reducing noise, reducing leakage and improving energy efficiency. The new shaft seal design achieves almost zero leakage; optimized flow channels reduce flow noise; and efficient variable control reduces energy loss. At the same time, the promotion and application of biodegradable hydraulic oils also puts forward new requirements for motor material compatibility, prompting continuous innovation in sealing and coating technologies.

Recommendations for industry practice

practical recommendations to rotary drilling rig users and maintenance service providers :

1. Establish standardized maintenance process: formulate detailed A6VM motor inspection, maintenance and overhaul specifications to ensure that each operation has rules to follow and each repair has documentation. Special emphasis is placed on oil management and pollution control, which are the most economical and effective measures to extend the life of the motor.

2. Invest in status monitoring capabilities: Gradually equip basic diagnostic tools such as oil analyzers, vibration detectors, and infrared thermal imagers. Enterprises with conditions can consider the Internet of Things remote monitoring system to achieve a shift from passive maintenance to active prevention.

3. Strengthen technical training for personnel: Regularly organize special training on hydraulic system maintenance to enhance technical personnel's understanding of the working principle and failure mechanism of axial piston motors, and avoid misdiagnosis and incorrect repairs due to insufficient knowledge.

4. Optimize spare parts management strategy: Stock up on key consumable parts of A6VM motors such as seal kits, bearings and valve plates, but avoid overstocking.

5. Participate in data sharing platforms: Join the industry equipment health management network, share fault data and maintenance experience, use collective wisdom to solve difficult problems, and provide feedback for new product improvements.

By adopting these suggestions, users of rotary drilling rigs can effectively improve the operating reliability of the A6VM axial piston motor and maximize the return on investment. With the advancement of technology and the innovation of maintenance concepts, we have reason to believe that the failure rate and maintenance cost of the hydraulic system will be further reduced, providing a stronger and more reliable power guarantee for basic engineering construction.