Design and analysis of a passive filtration system for splash lubricated gearboxes
Creators: name: YANG, Chi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400103
subject: Aircraft performance and flight control systems
classificationCode: 400199
subject: Aerospace engineering not elsewhere classified
classificationCode: 401201
subject: Aerodynamics (excl. hypersonic aerodynamics)

description: Lubricants are used in many applications to reduce frictions between contacting surfaces. Machines with clean lubricants will be able to achieve a longer life cycle and better reliability. It is unavoidable that wear particles and environmental ingress will be generated continuously during machine operating time. Wear is a primary cause of failure in most machines. Wear is defined as damage to a solid surface, generally involving progressive loss of material, due to relative motion between the surfaces and contacting substances.  Wear debris and environmental ingress will suspend in lubricant, enter contacting surfaces and cause damages to those surfaces. In order to maintain lubricant clean during the operating of machines, different types of circulating oil systems have been introduced. However, due to extra weight and space required for installing conventional filtration systems, many lubricated systems such as splash lubricated gearboxes used in helicopters and mining machines have not been fitted with a filtration system in their oil sump.

The maintenance of lubrication in most unfiltered splash lubricated machines relies on periodic onsite inspection and lubricant change. The predominant issues caused by unfiltered gearbox constitute not only the routine maintenance costs, but also substantial reduction of bearings and gears' life due to fatigue crack initiation sites or accelerated wear caused by environmental ingress and wear particles. In the literature, there are a few researchers engaged in exploring the possibility of introducing force-feed filtration system to splash lubricated gearboxes. However, force-feed filtration method greatly increases the weight of lubricated system due to the introduction of pipes, oil pumps and oil filters, as well as degradation of its controllability and fuel efficiency. Apart from filtration system using pressure, little attention has been paid to gravity filtration with less required components and added mass. In this research, a novel light weight passive filtration system that utilizes gravitational sedimentation and functional materials is presented for unfiltered splash lubricated gearboxes. Helicopter's tail rotor gearbox is used as a sample to be analyzed in this report.
Typically, when splash lubricated systems are in a stationary condition, particles suspended in lubricant starts to settle to bottom of the oil sump due to gravity. Shape memory effects of shape memory alloy Nitinol is utilized in the design of passive filtration system to enable particles to pass through the filtration frame to settle and prevent them from re-entering the lubricant before the lubricated components start to operate. The shape memory effect of shape memory alloy allows it to be plastically deformed in the low-temperature martensitic condition. When the external stresses are removed and shape memory alloy is heated, it will regain its original memorized shape as a result of the martensitic phase transformation during the heating. A predictive model is also developed to predict theoretical particle concentrations of flake-like particles suspended in the lubricant during settling process, as well as the estimated filtration effectiveness of the passive filtration system. The experimental investigation undertaken in this research clearly confirms that the passive filtration system has the capability of removing particles suspended in the lubricant in the simplified test rig. It also validates the predictive model's ability to provide reasonable estimation of the change of particle concentrations during settling process.