On-Site Debris Monitors Add Value to PdM Programs

Tags: wear debris analysis, oil analysis

Machinery maintenance programs based on lube oil condition-monitoring techniques offer rapid payback on investment by reducing equipment downtime and maintenance costs. Wear progression in many types of operating machinery may now be monitored on-site in seconds by untrained staff.

Swansea Tribology Centre introduced the first PQ™ Ferrous Debris Monitor in 1985. PQ™ technology has progressed to become an essential component of oil condition-monitoring programs in almost 400 laboratories worldwide. PQ™ data is interchanged routinely between laboratories, equipment manufacturers and users alike, all use this numerical index as a measure of wear condition. One oil analysis laboratory has established a number of satellite labs at customer sites. These on-site laboratories use PQ™ Ferrous Debris Monitors to determine if a sample needs expert assessment at the central lab. Similarly, a lubricating oil manufacturer offers a mobile condition-monitoring service visiting mining customers at regular intervals. Six vehicle-based minilabs use PQ™ Ferrous Debris Monitors to select samples that should be sent to a central lab for more detailed examination.

What is PQ™?
The PQ™ is a sensitive magnetometer that measures the mass of ferrous wear debris in a sample and displays this as a PQ™ index. This unitless number is quantitative and can be trended with acceptable linearity over a wide range of ferrous debris concentrations and particle sizes. The PQ™ measurement may be made directly without removing the fluid from the sample bottle.

Identify Problems On-Site
Statistics from several laboratories show more than 80 percent of the oil samples they receive show no abnormality. Many types of oil lubricated compartments such as gearboxes and drive trains (differentials, final drives, etc.) could be adequately monitored by a PQ™ measurement on-site. No extra effort is involved because the PQ™ uses the same sample that would be taken for laboratory examination. However, engineers have traditionally shown reluctance to implement this approach, despite the obvious advantages of cost and the immediate availability of a wear index at the precise location where this information is required. One reason for their reluctance is that many oil suppliers provide a specific number of free analysis samples as part of the supply contract. Another reason is a perceived difficulty in interpreting the significance of the PQ™ index measurement.

What is Done With the Results?
Machines rarely operate under identical conditions from company to company; therefore it is inadvisable to define PQ™ index limits for normal and abnormal wear conditions. Nevertheless, two leading earthmoving equipment manufacturers have independently reported PQ™ index limits, which are remarkably similar (Table 1).

Trends are Better Than Limits
For on-site applications it is better to examine the trend in the results rather than limits. Three measurements made at weekly intervals will identify what is a normal reading. For later measurements, any change in PQ™ index greater than 10 percent will indicate that something is changing and the sample should be sent to the laboratory for more expert assessment. Most models in the PQ™ range permit measurement of the sample directly in the sample bottle so the laboratory receives that actual sample for more detailed examination.

Sampling Frequency
Some laboratory oil analysis programs typically sample every three months. This probably derives from the OEM’s recommended servicing interval, and for many types of equipment, this is insufficient. At the other extreme, locomotive gearboxes are monitored daily, and a prominent European national railway is considering the installation of PQ™ Ferrous Debris Monitors at intermediate locations on a single journey. PQ™ Ferrous Debris Monitors offer flexibility to the on-site sampling schedule. If a gearbox shows little change in PQ™ index when measured at four weekly intervals, that frequency is probably sufficient. When the trend suddenly changes, the device can be monitored every day.

Case History #1 - Pulverizing Mill Gearbox at a UK Cement Works
Traditionally, as part of a free analysis package offered by the lube oil supplier, oil samples were sent to a commercial oil analysis laboratory four times per year. To monitor the pulverizing mill gearbox, samples were taken via the dipstick hole of the oil reservoir serving the main bearing, bevel gear and pinion. To increase the frequency of monitoring the more critical machinery, on the recommendation of the commercial oil analysis laboratory, the company purchased a PQ™.

When an oil sample from the mill was measured on the PQ™, an index value of around 50 was expected and an actual index of 285 prompted further investigation. The first sample was sent to the commercial oil analysis laboratory four weeks ahead of schedule, and a second sample taken the following day, gave a PQ™ index of 375. Visual inspection of the mechanism by endoscopy failed to identify any obvious problem. When spectrometric oil analysis (SOA) by the commercial oil laboratory confirmed a high result on the first sample, and a sample on the fifth day gave PQ™ index of 497, the gearbox was taken out of service.

Examination of the oil sump revealed the presence of metal stitching originating from an old repair to the gear mechanism cover and shaft support plate. This stitching had fallen from the cover plate’s ceiling and had been pulverized in the gear and rolling mechanisms. The gearbox may have been within hours of catastrophic failure (Figure 1).

Case History #2 - Drive Train Problem
The ability of the PQ™ to detect large wear particles (above 10 microns) permits a more accurate diagnosis of a problem. In many machines, a single oil lubrication system may supply several compartments, such as a differential and two final drives. When a problem occurs, much of the large debris will remain resident in the compartment where this debris is generated. If several compartments are sampled individually, the PQ™ will identify the specific compartment where the problem is located.

Figures 2, 3 and 4 show PQ™ index measurements from samples pulled over a period of five months from a Caterpillar 777B drive train; a common oil system supplies all three compartments within the drive train. While both the spectrometric and the PQ™ data flagged a problem in the drive train, only the PQ™ correctly identified the location of the faulty component. Once the repair and oil change were completed, both the spectrometric and PQ™ data reduced to normal levels.

On-site measurement of oil and grease samples by PQ™ Ferrous Debris Monitors offers significant cost-savings, flexibility and quick turnaround times. By identifying trends over an appropriate period, the user can establish PQ™ index values for normal and abnormal wear conditions. By identifying the abnormal samples, the number of samples sent for expert laboratory examination will be considerably reduced. Alternatively, where the oil supplier provides some oil analyses free of charge as part of its supply contract, that facility can be used to preselect only the samples that need expert assessment.

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