I. Analysis of Friction and Wear

Friction and wear in hydraulic components can cause structural deviations beyond tolerance ranges, leading to a gradual decline in hydraulic system performance. Here is a brief overview of the mechanisms of friction and wear in hydraulic components, as well as the causal relationships of related failures.

Classification of Friction and Wear

Relative motion between hydraulic components generates friction, which leads to various types of wear, mainly adhesive wear, fatigue wear, and abrasive wear.

(1) Adhesive Wear

Tribological studies indicate that despite being ground and polished, the sliding surfaces of friction pairs exhibit microscopic irregularities. Under load, the contact peaks experience extremely high pressure, causing plastic deformation at the contact points. When the two surfaces slide at high speed relative to each other, the contact points adhere; under the action of sliding shear forces, the adhesion is then torn off. This cycle of adhesion, tearing, re-adhesion, and re-tearing constitutes adhesive wear.

(2) Fatigue Wear

Fatigue wear refers to fatigue spalling caused by alternating loads acting on metal surfaces. Typical examples in hydraulic components include: fatigue spalling of bearings in various pumps and motors; fatigue spalling on rollers (or steel balls) and guide rails in internal-curve low-speed high-torque hydraulic motors; and surface fatigue spalling between valve cores and seats in hydraulic valves due to the impact of alternating loads from frequent opening and closing.

Fatigue wear occurs under repeated alternating loads, manifesting as pitting and patchy spalling on metal surfaces. It does not develop linearly over time; instead, it is less noticeable initially and gradually progresses over a period.

(3) Abrasive Wear

Abrasive wear refers to phenomena such as component wear, seizure of moving pairs, and blockage of damping holes caused by particulate contaminants entering the hydraulic system.

Abrasive wear is related to the nature of contaminants. Studies have shown that harder contaminants with higher crushing strength cause greater wear damage.

II. Analysis of Fatigue Fracture

Fatigue fracture failure is also a common failure mode in hydraulic components. Fracture failure generally occurs in pressure-containing chambers of hydraulic components (e.g., cylinder blocks of axial piston pumps, valve housings, and cover parts), certain rods and plates subjected to tension, compression, or bending moments, and rotating shafts of pumps or motors. The characteristic of fracture failure is that under repeated alternating loads (related to changes in pressure or force acting on the chambers or parts), the material’s strength decreases due to fatigue. Cracks initiate in areas where stress exceeds the fatigue strength, and further propagation of these cracks leads to fracture.

III. Vibration Analysis

Vibration analysis is an important method for hydraulic fault diagnosis. The basic process involves using sensors to measure and record pressure and flow pulsations in the hydraulic system or vibration signals (displacement, velocity, and acceleration) of hydraulic component housings. These signals are then processed through spectrum analyzers for frequency spectrum transformation. By comparing these spectra with those of various standard states (normal operation and typical fault states), the closest matching state can be identified. Spectrum analysis primarily judges faults based on changes in amplitude and the frequency at which vibration peaks occur.

IV. Ferrographic Analysis

Ferrographic analysis is a promising fault analysis method, also known as ferrography. Its basic process involves passing lubricating oil containing wear debris through a high-intensity, high-gradient magnetic field. The magnetic force separates ferromagnetic wear debris from the lubricating oil, which then precipitates on a substrate in order of particle size to form a ferrograph for observation and analysis.

V. Thermographic Analysis

In hydraulic equipment, most faults in hydraulic components are accompanied by energy loss. For example, blockages and leaks are associated with the loss of pressure energy in the fluid medium, and abnormal wear is accompanied by the loss of mechanical energy in moving parts. Most of this lost energy is released in the form of heat. Additionally, temperature monitoring is convenient, as it can directly target component housings and the outer walls of pipelines. Temperature signals are relatively stable, and monitoring sensors are relatively inexpensive.