Jeremy,
A couple of comments to assist you in diagnosing the situation.
I find it easiest to solve problems by coming back to basic principles.
Firstly, pressure is created by resistance to flow. The pump does not create the pressure you are measuring; the pump only produces oil flow. It is the resistance in the oil flow path, that create the pressure you are measuring.
Think of a garden hose. When turned on and pouring onto the lawn, there is maximum water flow out but no velocity (the stream falls at your feet). But, as you progressively block the end with your finger, you can feel the pressure (back resistance) grow, and a diminishing stream of water comes out (at an increasing velocity).
I recently measured both sections of the A10 pump, and calculated that the displacements are:
Crankshaft supply section: Theoretical flow of 4.14 l/min @ 6,000 rpm
However, because of (necessary) internal clearances between the gear teeth and the mazac housing AND between the sides of each gear and the end plate (and the mazac housing on the other side) the ACTUAL flow will be less than theoretical. This is expressed as volumectric efficiency, and I expect the A10 pump in good condition to produce only 85% of its theoretical flow (at best). I.e the pump has a volumetric efficiency of 85%. This means that the actual flow of the pump section is 3.52 litres / min @ 6,000 rpm, which is 85% of the theoretical flow.
Note that the flow is directly related to shaft speed. Therefore at 3,000 rpm (a typical riding speed), the crankshaft oil supply section of the pump will produce 1.75 litres / min AT BEST.
The following factors will all act to reduce the volumectric efficiency of a pump (resulting in less ACTUAL pump flow), and often more than one are having an effect:
- Oil Viscosity (the thickness of the oil). Viscosity is a measure of an oil's propensity to flow. The thicker the oil, the slower it flows (the higher the internal friction within the oil molecules). A typical engine oil with an ISO rating of 100, will have a viscosity of 1,200 centistokes (cSt) at 5 degrees Celsius (a cold start condition), but will have fallen to 30 cSt at 70 C (a running engine). you can see that the viscosity is almost a factor of 10 times lighter (it flows almost 10 times easier) This means that the thinner oil will escape more easily through clearances such as across the pump gear sets, big end bearings, timing bush (or bearing),and the OUTSIDE (fixed) diameter of the timing bush.
- Wear across rotating components. This includes the big end bearings, and their journals, the timing bush (or bearing) and its journal, the oil pump end plate, and inner face the gears run against, the gear teeth and the mazak housing. As these clearances increase, the oil finds it easier to leak away Remember: Flowing fluids take the path of least resistance. Like us; we all take the easiest path!
- Temperature. Internal clearances become greater as the engine heats up. This includes the big end bearings, and their journals, the timing bush (or bearing) and its journal, the oil pump end plate, and inner face the gears run against, the gear teeth and the mazak housing.
- operating pressure; The greater the pressure, the greater the leakage rate. However not relevant in your instance, as the pressure is falling because:
The engine warms up / eventually reaches operating temperature. The temperature increase causes the oil to thin dramatically, which increases the internal leakage rate from every potential location. It also causes the pump volumetic efficiency to fall further as well (pump flow reduces). The raised engine temp also causes the dynamic (operating) clearances to increase at the same time, which further increases the rate of internal leakage(s)
How do you notice the rise in internal leakage (and probable reduction in pump output flow)? - the oil pressure falls!
If you increase the revs, the pressure will increas a little, as you have increased the pump output flow.
Th relief valves in our bikes are set to approximately 50 psi. You only see this pressure on the gauge when the rate of pump output flow exceeds the rate of internal leakage. When the engine is hot and the oil is many times thinner (slips away easily) and the clearances are greater (larger leakage paths), the oil pressure falls as the internal leakage rates increase. When (if) you have no pressure, then the leakage rate exceeds the pump output flow rate. 10 psi seems to be an acceptable pressure when the engine is hot; 0 psi would worry me!
I would check the integrity of the oil seal delivering oil to the quill as well. If the wrong type of seal has been fitted, then it could fail at apressure less than the 50 psi you will get when cold. Many lip seals are not rated beyond 15 - 20 psi; the lip will tear or turn inside out. Further, the (static) OD of this seal is as critical as the lip. Oil could be leaking past if the crush is not sufficient, and a sealing compound has not been used.
Note that the scavenge side of the A10 pump has a theoretical fow of 5.8 l/min @ 6,000 rpm. With volumetric efficiency of 85%, the flow will be 4.9 l/min. This is approximately 30% more flow than the crankshaft delivery section.
Tthe pressure filter installation has the advantage of controlling the oil cleanliness to the bearings. However, as the filter blocks, it will reduce pressure to the bearings. Hopefully the element has a bypass valve. This means that when the pressure on the dirty (upstream) side of the filter reaches the setting of the bypass valve (say 15 psi), the bypass valve opens and unfiltered oil flows through, to the bearings. However, oil pressure downstream of the filter (but upstream of the crankshaft bearings) will always be 15 psi less, than upstream. As the engine wears, and leakage increases, this could result in no pressure at the bearings.
Don't use a non bypass element, as it could block to the degree that there is no pressure at the crankshaft, even at cold start.
I fit a filter to the return line.
Hope this helps,
Richard
