Hydraulic Pump Journal Bearing considerations

The pressure across a gear pump can be thought of as acting on the projected cross-sectional area of the two gears * the gear depth. This substantial load needs to be supported by bearings which control shaft displacement such that the gears are constrained from translating by less than their radial running clearance. Also, the gear shafts create a potential leakage path. For these reasons, journal bearings are the bearing of choice for gear pumps.

Journal bearings can be the most critical aspect of the design. The bearings are designed to operate in hydrodynamic lubrication (HDL) where there is no metal-to-metal contact. If HDL is lost, then performance penalties and severe damage will likely result. Conversely if HDL is maintained, gear pumps can last a surprisingly long time with minimal wear.

Journal bearings are designed to operate in the region between the dotted lines:

Chart from Shigley & Mischke

Note that "P" is not pump pressure! It is applied load / projected total bearing area (and you have 4 such bearings). Applied load is pressure * gears cross-section.

This chart provides the minimum film thickness as a function of original radial clearance. As can be seen from the chart, for low bearing characteristic (Sommerfeld) numbers, the film thickness can be very small, considering that the radial clearance can easily be only a few tenths of a thousandths of an inch.

In theory, the friction variable is near-linear, converging on zero as the bearing is loaded right to contact:

Chart from Shigley & Mischke


Loss of Hydrodynamic Lubrication in Practice

In practice, this never happens.

This assumes perfect:

• Cylindricity
• Runout
• Perpendicularity, and
• Alignment

...between half-shafts and housings, as well as perfect surface finish and no bending moment applied to the shafts.

Instead, the above chart behaves like this (the Stribeck curve):

In practice, the transition point where HDL is lost will depend on practical tolerances and surface finish where metal-to-metal contact starts.

The Effect of Heating on Viscosity

There is one important exception to this: note that viscosity is a term in the Sommerfeld number. Even with the low friction coefficients in the order of .01, there is a power loss due to applied side force * friction coefficient * moment arm * rpm, which is converted to heat. Keep in mind that this heat goes into a film of oil the thickness of a sheet of paper. While there is a flushing of oil due to both pump leakage and also the journal bearing's natural (and considerable) pumping action, the heating of the oil can be considerable.

When this happens, the viscosity decreases. But note that viscosity decreases exponentially with increasing temperature: a 50 degree C increase in temperature can decrease viscosity by a factor of 10x or more. This process massively decreases the Sommerfeld number and can result in inexplicable journal bearing failures in design that appear sound on paper.

Be careful also of using an increase in viscosity to solve journal bearing problems: increased viscosity will increase viscous friction work and thus heating of the fluid, which decreases viscosity. Happily, the temperature increase and thus viscosity decrease due to HDL can be calculated easily and the design checked for this issue, before the drawings are released.

Temperature Control on Your Test Loop

Be very careful about this effect when testing: a typical hydraulic test loop consists of a small oil reservoir and a gear pump throttled back to produce ~ 1 kW of power into ~ 1 gallon of fluid, which is dissipated into the oil and raises its temperature.

This can easily result in a 50 degree C temperature rise in the fluid, which will render your test results useless!

Temperature must be recorded and controlled during testing.

Housing Material

One unexpected source to this effect is the choice of housing material: one of the reasons aluminum is so widely used is because its thermal conductivity is ~ 5x higher than that of steel or cast iron.

Shaft Aspect Ratio

In support of these practical considerations, it is recommended to keep the length / diameter aspect ratio of the half shafts (the shaft on either side of the gear) to between 0.8 and 1.2. Less than 0.8 results in poor sealing and limited carrying capacity of the bearing, and greater than 1.2 runs the risk of early metal-to-metal contact between shaft and housing.

This point bears repeating: particularly when adapting an existing design to operate at higher pressures, it is very temping to "save" the existing shaft diameter by increasing its length, thus preserving the existing production tooling and gauging for both shaft and housing bores. Don't do it. Exceeding an aspect ratio of 1.2 will lead to journal bearing crashes at high pressures.

The way to detect a journal bearing crash is to look at the pump curve generated in your testing: this should be a straight line. Particularly when driving the pump with a PMDC motor, which slows down under load, which further lowers the Sommerfeld number, will hasten the early onset of metal-to-metal contact. In the test data, this shows up as the straight-line pump curve taking a nose dive.

Shaft diameter vs. Gear diameter

Be careful about something else: increasing the journal diameter will increase the carrying capacity of the bearing, but the resulting friction coefficient will act on a larger moment arm, which increases the power consumed. This also eats into the gear face, which decreases the available sealing area there.

Be careful to allow for a grinding relief at the intersection of the shaft and gear face, since both the shaft and gear face are typically ground to achieve the required surface finish. These reliefs decrease the sealing area. The relief does not count toward the shaft aspect ratio.