The Professor: Oil Pumps and Net Oil Pressure
Understanding the Differences is Key for Successful Servicing
Often, compressors in the refrigeration and air conditioning fields have forced oiling systems. These compressors are usually over 5 horsepower (hp). They contain an oil pump located at the end of the compressor’s crankshaft (Figure 1).
The oil pump is actually keyed into the compressor’s crankshaft, which supplies power to turn the oil pump (Figure 2). Oil pumps can be of the gear (Figure 3) or eccentric type. The star gear in this type of pump actually squeezes the oil to a higher pressure. The added pressure that the star gear puts into the oil can be referred to as the net oil pressure. (Figure 4 shows the star gear disassembled from the oil pump and out of its seat within the oil pump.)
One has to remember that the oil pump picks up the oil from the compressor’s crankcase (under crankcase pressure), adds pressure to the oil, and then delivers the oil into drilled holes in the crankshaft. The oil is then delivered to the compressor’s bearings and connecting rods through these drilled holes or oil galleys. The oil now drops to the crankcase to be picked up again by the oil pump. Notice that the oil was first picked up by the oil pump from the compressor’s crankcase, and after lubricating essential compressor parts, ends up back in the crankcase.
An eccentric-type oil pump (Figure 5) uses an eccentric-type of mechanism to also squeeze the oil to a smaller and smaller volume, which adds pressure to the oil. Again, this squeezing action by the eccentric adds pressure and is referred to as net
oil pressure.
Measuring Net Oil Pressure
When dealing with compressors that employ an oil pump, many service technicians confuse net oil pressure with oil pump discharge pressure. However, it is of utmost importance that technicians understand the difference between these two pressures when servicing compressors with oil pumps.
As mentioned earlier, the oil pump’s rotating gear, or eccentric, adds a certain pressure to the oil pumped through the crankshaft. This pressure is considered net oil pressure. Net oil pressure is not the pressure that can be measured at the discharge of the oil pump. The oil pump picks up oil (at crankcase pressure) from the compressor’s crankcase through a screen or filter.
The oil pump discharge port’s pressure included both crankcase pressure and oil pump gear pressure it adds to the oil. This is why net oil pressure cannot be measured directly with a gauge. A gauge at the oil pump’s discharge port would register a combination of crankcase pressure and oil pump gear pressure. The technician must realize this
and subtract the crankcase pressure from the oil pump discharge port’s pressure to get the net oil pressure, which will be shown in
the following
equation.
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Equation
First, note in Figure 4 where a technician would measure oil pump discharge pressure. This would include both crankcase pressure and oil pump gear or eccentric pressure.
The equation is this:
Oil pump discharge pressure — Crankcase pressure = Net oil pressure
Note: Net oil pressure is simply the pressure put behind the oil from the oil pump’s star gear or eccentric.
Let’s take the example of an oil pump discharge pressure of 65 psig. The crankcase pressure is 15 psig. What would be the net oil pressure?
Simply subtract the crankcase pressure from the oil pump discharge pressure to get net oil pressure. (65 psi) – (15 psi) = 50 psi net oil pressure.
This means the oil pump’s star gear or eccentric is actually putting 50 psi of pressure into the oil when delivering it into the crankshaft’s drilled passages. This is why oil safety controllers are called differential-type controllers. They sense the difference between oil pump discharge pressure and crankcase pressure. This is why these controllers have a capillary tube or pressure transducer connected to the discharge of the oil pump and the crankcase to sense a difference of pressures or net oil pressure.
Net oil pressures vary from compressor to compressor. Net oil pressures usually range from 20-40 psi. Most oil pressure safety controllers will shut the compressor down if the net oil pressure falls below 10 psi. Variables which affect the net oil pressure are:
• Compressor size;
• Viscosity of the oil;
• Temperature of the oil; and
• Bearing clearance.
Larger compressors need more net oil pressure because they have more surface area to lubricate. The oil pumps must also pump and carry the oil greater distances within the larger compressor. Also, as the oil gets hotter and its viscosity drops, the net oil pressure will also usually drop. As a compressor wears, its tolerances will become greater and easier for the oil to escape through its clearances.
Publication date: 6/30/2014
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