Gearbox Lubrication Calculations
Gearbox Lubrication Calculations
(OP)
I have very little experience/knowledge of fluid flow calculations and I would like some help answering some questions that were posed to me. These questions concern a gearbox with a gerotor oil pump used for gear mesh spray lubrication.
Here is what I know:
The sump from which the pump draws is constantly replenished. Oil enters the pump through a Ø.50" hole. The pump is running at 7000 rev/min. The pump's theoretical displacement is 0.325 in³/rev. Oil exits the pump through a Ø.50" hole. After exiting the pump, the oil enters a sprayer with two exit holes, each Ø.063" in diameter. The oil is SAE Grade 75W-90, viscosity index 106 cst @ 40°C and density 0.859 kg/l. Let's assume an oil temp. of 40°C.
From this information, can I calculate the oil pressure in the line between the sprayer and the pump? If not, what else do I need to know/define? If I can calculate the pressure, how is it done? Let's assume I can neglect piping losses, if that helps.
Thanks in advance for any help.
Here is what I know:
The sump from which the pump draws is constantly replenished. Oil enters the pump through a Ø.50" hole. The pump is running at 7000 rev/min. The pump's theoretical displacement is 0.325 in³/rev. Oil exits the pump through a Ø.50" hole. After exiting the pump, the oil enters a sprayer with two exit holes, each Ø.063" in diameter. The oil is SAE Grade 75W-90, viscosity index 106 cst @ 40°C and density 0.859 kg/l. Let's assume an oil temp. of 40°C.
From this information, can I calculate the oil pressure in the line between the sprayer and the pump? If not, what else do I need to know/define? If I can calculate the pressure, how is it done? Let's assume I can neglect piping losses, if that helps.
Thanks in advance for any help.





RE: Gearbox Lubrication Calculations
If you are attempting to design a system using a PD pump with no relief valve, I would not recommend it. Even if you could size everything perfectly so that you had the flow and pressure you want, bad things could happen if a nozzle plugged off or if the system was started up with ice cold oil. Any PD pump needs overpressure relief protection.
Johnny Pellin
RE: Gearbox Lubrication Calculations
RE: Gearbox Lubrication Calculations
RE: Gearbox Lubrication Calculations
1. Can I calculate the oil pressure in the line between the sprayer and the pump in the system described?
2. If not, what else do I need to know/define?
3. If I can calculate the pressure, how is it done?
4. Since no pressure relief valve is present, can the pressure still be calculated in the line between the pump and sprayer?
5. If so, may I be shown the necessary equations?
RE: Gearbox Lubrication Calculations
The Gerotor pump is an excellent choice due to the combination of the shaft speed and the viscosity. It is the only type of pump that I would seriously consider for the duty that you described. My preference would be to have the relief valve flow return to the sump well below the oil surface.
If the application includes very low ambient temperatures, you should consider a heater to warm the lube oil before a cold startup.
RE: Gearbox Lubrication Calculations
Can anybody address my specific questions?
1. Can anyone calculate the oil pressure in the line between the sprayer and the pump in the system described?
2. If not, what else does one need to know/define?
3. If a person can calculate the pressure, how is it done?
4. Since no pressure relief valve is present, can the pressure still be calculated in the line between the pump and sprayer?
5. If so, can you explain/demonstrate the necessary equations?
RE: Gearbox Lubrication Calculations
RE: Gearbox Lubrication Calculations
I am not going to post the orifice formulae here. They can be found in any fluid dynamics text book, Cameron Hydraulics, and probably on-line by searching for orifice flow calculations. And I am at home without access to my books. You need to know the configuration of the orifice (sharp edged, square edged, chamfered, etc.).
Most Sundyne gearboxes actually have a separate internal PSV that is screwed into a port in the center housing of the gearbox. The spring above the oil pump in the old configuration would provide a limit to oil pump pressure as well. But I believe that they are phasing out that design for a more robust pump. This upgrade also involves installing a larger PSV in some models (LMV-313, I believe)
Johnny Pellin
RE: Gearbox Lubrication Calculations
If this is an open system that cannot generate pressure hydraulically, there is no legal need for a relief valve.
However, if there is a point where you may have cold viscous oil, it would be advisable to limit the pressure in some way.
Delta P = v^2 x f x L x p
---------------
D2
Delta P in Pascals
V = velocity in mtrs/sec
f = friction factor
L = length of tube or pipe
p = fluid density
D = pipe diameter
Velocity(mtrs/sec) = Q x 21.22
-----
D^2
Q = Flow rate in ltrs/sec
As a simpler rule of thumb
Q
------- = X
Orifice area
The square root of X is the pressure drop...
Orifice area is in mm^2
Regards
Adrian
RE: Gearbox Lubrication Calculations
I would guess extremely high pressures determined by nonlinear flow conditions.
RE: Gearbox Lubrication Calculations
7000rpm x 0.325 inch3/rev = 2275 inch3/ minute.
= 1.32 ft3/min = 9.85 USgpm
2 nozzles = 4.9 USgpm/nozzle
nozzle velocity = 512ft/sec = 350mph
pressure at the nozzle approx 1770psi