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One of the buzzwords used in regard to condition monitoring is oil pressure mapping. This article explains oil pressure mapping, why this diagnostic technique was developed and how it is used to measure engine bearing wear.
Testing for Bearing Wear
Traditionally, testing for abnormal wear or damage to the main or big end bearings was carried out by measuring engine oil pressure at idle and maximum engine speeds. The reasoning was that excessive bearing clearance would cause excessive oil leakage and a resulting drop in oil pressure.
This test is no longer reliable as modern diesel engines are fitted with high-capacity oil pumps, which are needed to deliver oil to the spray jets used to cool the pistons. As a result, the pumps can cope with higher oil leakage rates, so there is little noticeable drop-off in oil pressure. This is why oil pressure mapping is used.
Before discussing oil mapping in more detail, it’s helpful to take an in-depth look at bearings, their importance in maintaining healthy equipment, and why oil pressure affects bearing wear.
Bearings and Oil
A bearing is often an inexpensive part in a machine, but the failure of a bearing results in a considerable amount of consequential damage to other components. It is for this reason that maintenance personnel are concerned about the health of bearings.
Oil analysis is an important tool used to assess the soundness of a bearing. Once a problem is detected through oil analysis, it must be investigated to establish the cause and extent of the problem.
The following are a few of the common causes of bearing faults:
overloading or shock loading
contamination of lubricating oil
overheating of lubricating oil
overheating of the bearing
misalignment or incorrect assembly of the bearing
insufficient pressure and/or volume of the lubricating oil
The secret to long bearing life, after installation and operational problems have been corrected, is to ensure that the bearing is supplied with the correct grade of oil in sufficient quantities, and that the oil is clean and runs at the correct temperature.
Plain bearings, in particular, are sensitive to oil volume and pressure. Insufficient pressure will normally result in insufficient oil volume being delivered to the bearing. The decreased oil volume causes the bearing to wear out faster, due to increased operating temperatures and contact between the journal and bearing.
Antifriction (roller or ball-type) bearings can normally run on relatively small volumes of oil. However, a drop-off in oil volume will cause wear on the cage due to the increased sliding contact in this area. The wear then allows the rollers or balls to move out of position and accelerate wear of the bearing races.
An oil pump, like most other pumps, produces a rapid increase in output as speed increases until a critical point is reached, after which the output drops off again.
To prevent erosion damage to the bearing from the excessively high oil pressures possible with the high-capacity oil pumps fitted to modern diesel engines, a pressure-relief valve is fitted to the engine oil pump. This vents excess oil back to the sump when the maximum allowable pressure is reached.
Oil pressure mapping is simply a method of verifying that the oil pump pressure follows this curve. When the shape of the curve changes, it indicates a fault with the oil pump or excessive oil leakage in the engine, possibly due to bearing wear.
How to Map Oil Pressure
To map the oil pressure, you need an accurate oil pressure gauge which can accommodate a range of pressures. Dashboard-mounted gauges are generally not accurate enough for mapping purposes. An accurate tachometer is needed to measure engine speed while performing the test. Graph paper to plot the resultant pressure curve is also useful.
Make sure the engine has reached full operating temperature. Water temperature is not a reliable indicator of oil temperature. The water temperature stabilizes quickly due to the action of the thermostat, but the oil temperature lags behind during warm-up. If possible, record the oil temperature when the pressure readings are taken.
The oil temperature is important because the oil thins rapidly as the oil temperature increases, then thickens again when it cools down. The thickness of the oil (viscosity) affects the pressures obtained and may give inaccurately high readings if measured at too low of a temperature.
Connect the oil pressure gauge to the main oil gallery before it enters the bearings; the oil pressure sender unit is generally a good point. Connect a tachometer if required.
Oil pressure should then be measured at 10 or more speed intervals equally spaced between idle and maximum engine speed. It is helpful to draw a straight line from the first reading taken at idle to the last reading taken at maximum engine speed. All the intermediate points on the curve should lie above this line.
If the pressure falls below the line or follows the line closely, there is likely a problem with the oil pump or the pressure-relief valve, or excessive leakage is occurring within the engine itself. Even if the oil pressure is close to normal at full speed, it may be too low at peak torque where the bearings are subjected to maximum loading.
There are many different designs of engines and pumps, so when a problem is suspected, it helps to have a reference graph against which to compare the results. It is, therefore, advisable to map an engine shortly after it has been run in, and then use this baseline for comparison with later graphs.
Any change or drop-off in the graphs should be investigated. First, check the pump and pressure-relief valve. If no fault is found with the pump, the engine itself should be inspected for excessive leakage.
Engines Fitted with Scavenge Pumps
Engines on some earthmoving equipment are designed for operating on slopes and are fitted with scavenge pumps. It is generally not possible to check the function of the scavenge pump. It tends to wear out faster than the main pump due to oil aeration in the scavenge pump when the engine is running level. An undetected scavenge pump failure will soon result in bearing failure. Engines fitted with this type of pump must have the pump stripped and checked for damage at planned intervals of at least every 10,000 hours of operation.