Monitoring Bearing and Gear Failures in
How do aircraft engines work? What you need to know to monitor wear of engine components
David J. Aslin, Eaton Corp.
Residence Time. This is the volume of oil in the reservoir (which is measured in gallons) divided by the lube pump flow out of the tank’s oil outlet port (gallons per second). The residence time should be sufficient to allow the scavenge mixture’s air to migrate via buoyancy to the tank’s vapor space. Depending on the internal geometry of the tank and the method used to introduce the oil into the lube tank, re-entrainment of air is possible.
The use of residence time alone to allow the air in the scavenge mixture to escape increases the oil volume requirements of a lube oil tank. The buoyancy of air bubbles in the air-oil mixture is related to the mixture’s temperature. At high temperatures, the air becomes more buoyant because of the expansion of the air bubbles, the natural convection forces, and a decrease in the oil’s viscosity. Oil viscosity creates drag that retards the speed of an air bubble migrating to the vapor surface of the mixture.
In-Tank Horizontal Baffles. These devices are used to slow the incoming fluid velocity to allow time for the air to separate from the oil. These baffles are usually 10°–15° off horizontal. The incoming fluid is funneled over the baffle, where buoyancy causes the air to separate from the fluid. The end of the baffle is submerged in the reservoir’s fluid. This prevents splashing and subsequent re-entrainment of air.
Vertical Tank Baffles. Hydraulic tanks also use vertical tank baffles. The incoming mixture is forced to travel the length of the tank and back to the hydraulic pump’s suction strainer. This allows the air to escape via buoyancy and prevents short-circuiting of the incoming fluid with the hydraulic pump’s suction strainer.
You can integrate inline, three-phase separators (see Photos 1 and 2) into the scavenge lube system’s oil reservoir. Inline and in-tank separators operate in the same manner. The scavenge air-oil mixture tangentially enters the constant diameter chamber, which causes the
As wear debris moves into the separator’s chamber, it’s forced to the outer chamber wall by its own high density (the heaviest phase of matter contained in the scavenge lube oil mixture). A debris exit in the chamber wall lets particles escape in a small slipstream of lube oil. The slipstream with concentrated wear debris is directed to either a debris sensor pocket or into a flow-through-type debris sensor.
Because cyclonic separators operate at g (acceleration) levels that exceed the force of gravity, the mounting attitude of the separator has no effect on its ability to separate air or debris from oil. Therefore, these devices function efficiently in negative and micro-gravity (zero g) environments. Cyclonic separators also make ideal pulsation dampers.
Experiments have been conducted on air-oil separators to measure the effect of various two-phase flow regimes (e.g., slug flow and annular flow) on the separator’s performance. Static inline mixers were placed at the separator’s inlet, and air-oil separation efficiencies were compared with separation efficiencies recorded prior to the mixers’ installation. The results were identical. This proves that irrespective of the two-phase flow regime, the air separates from the oil immediately upon entering the separator’s chamber.
The amount of acceleration the mixture experiences upon entering the separator is calculated as follows (the equations in Part 2 are numbered consecutively from those in Part 1):
Fluid acceleration is normally expressed in multiples of gravity (32.2 ft/s2) and calculated as follows:
Normal fluid acceleration required in a cyclonic phase separator to remove air from oil is between 50 and 100 g. The g level is directly related to pressure drop. The higher the fluid acceleration in the separator’s chamber, the higher the associated friction loss. The friction loss results in a higher separator inlet pressure. The higher the inlet pressure, the smaller the actual air-to-oil ratio of the scavenge lube oil mixture at the separator’s inlet. New low-pressure drop, three-phase cyclonic separators have been developed that let you remove wear debris from the scavenge lube oil mixture with a minimal fluid acceleration increase.
The frictional loss through a cyclonic separator includes inlet and exit losses, and the friction loss associated with the fluid-to-chamber wall interface. The higher the fluid velocity at the fluid-to-wall interface, the higher the corresponding friction loss. These devices are efficient at separating air and wear debris from oil, providing the oil viscosity is <30 cp. Above this point, a liquid vortex is destroyed.
Typical air and oil separation efficiencies over the operating envelope of an aircraft gas turbine engine are 95% by volume. Wear debris capture and/or indication efficiency depends on the fluid acceleration level; the transport fluid viscosity; and the size, material density, and shape of the wear particle. Typical capture efficiencies are:
Air separation efficiency is calculated as follows:
Oil separation efficiency expressed as a decimal is calculated as follows:
Concentrating wear particles into a small slipstream in an oil reservoir cyclonic separator has advantages:
Next month, Part 3 of this series discusses the various types of metallic wear debris collectors, detectors, sensors, and systems predominately used in the aerospace industry for monitoring metallic bearing and gear wear.
David J. Aslin, Ph.D., is Group Leader, Engineered Sensors, Systems Engineering Group, Eaton Corp., 7370 College Pkwy., Ft. Myers, FL 33907; 941-849-5762, fax 941-939-0152, email@example.com.