"We have noticed iron and tin content in our standby equipment, specifically a diesel engine with 4,000 running hours and oil with 20 running hours. Surprisingly, the last three oil analysis reports have shown dramatic increases in iron and tin at values up to 205 and 80 parts per million. Do you have any thoughts as to what could be causing these increases? We are using ASTM D5185 for wear metal analysis."
Generally, there are two primary causes of high metal levels in oil analysis. The first is related to the sampling procedures. The second is associated with the actual wear metal levels.
When oil analysis is performed, it is important to ensure that the machine and oil conditions are reported properly. To achieve this, the correct sampling procedures will be critical.
An effective oil analysis program will have the appropriate sampling frequency and procedures clearly defined. The sampling frequency should be based on the machine's criticality, lubricant life cycle and environmental conditions. Typically, the frequency ranges from every week to every three or four months. More extended intervals should be avoided.
Oil samples should be collected from the same machine location consistently. For engines, the best method is to use a sample port installed between the pump and oil filter. Other sites such as the drain port may produce inconsistent results. If no sample port is available, consider utilizing a drop tube connected to a rod. This will enable you to take the sample from 1-2 inches above the bottom of the sump.
Lubricants should be sampled in their normal operating conditions, which means when it is safe to collect the sample while the engine is idling. If the machine has not been in operation for very long, allow it to reach its normal operating conditions before taking the oil sample. It is also a good idea to purge a small volume of oil prior to sampling. A sample bottle can be used to purge oil before the actual sample is obtained.
If you have followed these sampling practices and are certain the lab reports are accurate, there likely is a wear trend that must be corrected. Iron and tin content are characteristic of crankshaft bearings. A bearing may have severe wear because it has been displaced or incorrectly installed. In this situation, it is recommended to open the carter and inspect the crankshaft bearings.