Research Progress on Monitoring and Separating Suspension Particles for Lubricating Oil

Table 1

Main monitoring methods for the suspended particles in lubricants.

Project	Monitoring principle	Advantages	Disadvantages
Particle counting method [40]	When the light path illuminates the sample, the light path is blocked, and the photoelectric receiver receives the change of the photoelectric intensity, which is converted into a voltage pulse signal. The number of particles of different sizes is recorded through different voltage valves.	The particle counter is simple to operate and has a fast counting speed. It is suitable for on-site monitoring.	Particles smaller than 100 μm in diameter cannot be monitored, and the particles cannot be qualitatively analyzed.
Atomic spectroscopy [41]	The content of this element is calculated by detecting the number of photons consistent with the characteristic frequency.	Easy to operate and no need for on-site treatment of lubricants.	Cannot detect the suspended particles larger than 10 μm.
Infrared spectroscopy method [42]	Based on the characteristic absorption peaks, numbers, and relative intensities of different substances, the presence of particles in the oil sample was deduced and its molecular structure was determined.	It can analyze oil degradation and pollution status quickly and efficiently.	Insensitive to metal particles, unable to analyze wear particle size and morphology, and no qualitative analysis.
Ferrography method [43]	A high-gradient magnetic field device was designed to separate the metal from the oil sample and to deposit it on a transparent substrate in order of size and then qualitatively and quantitatively analyze the abrasive particles.	Abrasive particle detection range size is from 1 to 1000 μm. It can perform the quantitative and qualitative analysis of abrasive particles at the same time.	The analysis is slow, and the analysis results strongly depend on the experience of the engineer.
Magnetic plug and magnetic detection methods [44, 45]	The metal’s magnetic principle is used to analyze the morphology, number, and size of the captured metal abrasive particles.	Can detect larger abrasive particles (100–1000 μm).	Cannot detect aerosols below 100 μm.