For turbocharger bearings to perform without any fault, it needs to be adequately lubricated.
Property of Turbocharger Bearings
- Must handle high loads: The turbocharger bearings need to carry heavy rotating parts, and the load will increase when the engine is in operation. The bearings should have a high load-carrying capacity.
- Withstand challenging and extreme conditions: The turbocharger fitted on the ship has to operate in conditions different from that on land. The bearing should run smoothly even in rolling/ pitching/ yawing conditions or in extreme weather.
- Enhanced operational reliability: The bearing should be able to operate even during long-running hours of the oil or when the oil is contaminated.
- Reduced temperature peaks: The turbocharger speed can be relatively high during the starting up of the engine or when the engine is suddenly stopped during manoeuvring, leading to an increase in bearing temperature. The bearing should be able to cope with this temperature peak occurring for a short period of time.
- Does not react: The bearing material should not react with the lubricating oil provided in the turbocharger or with other parts of the turbocharger made from different materials.
- Compensate high thrust loading: High boost pressure acting on the compressor wheel can create significant thrust loads. The thrust is even higher in VGT turbochargers with variable geometry turbines, as they are designed to provide higher boost pressure at low loads. The bearing should be able to withstand and run smoothly in high-thrust load situations.
- Oil supply delay-tolerant: Some turbochargers do not have their oil sump and oil supply pump for the bearing. The lubrication oil is provided from the main engine system through long-running lube oil pipes which will lead to a delay in oil reaching the bearing after startup. The bearing should work correctly even in oil-starved conditions.
- There are mainly two types of turbocharger bearings popularly used in marine turbochargers. They can be defined by the locations of their fitting in the turbocharger shaft: I) Outboard bearing type II) Inboard bearing type
Lubrication System for outboard bearing
- Ball and Roller bearing uses an integral lube oil supply system for lubrication, i.e., it is fitted with a self-contained gear type pump (centrifugal pump in some models) operated by the rotating shaft of the turbocharger provided on both blower & turbine side
- An independent oil sump is provided on both sides of the casing of the turbocharger, and the pump draws oil from the sump. A gear pump is provided on both the blower & turbine side
- A sight glass is provided in the casing with a maximum/ minimum oil level mark (oil should be renewed at stated intervals)
- The outboard bearing may be lubricated by an external system (M/E LO System) through a fine filter or by the independent system (Pump, filter, cooler, oil, sump, alarm, etc.)
- All lubrication systems must contain adequate lubrication when tilted to an angle of 15° in any direction or a temporary tilt of 22½
- If a separate system is used, the following criteria must be fulfilled – a 6 m head of the tank; pressure >1.6 bar; supply time of 10 minutes
Lubrication System for inboard bearing
- A sleeve-type white metal bearing is fitted inboard to a radial-type turbocharger. In this case, the shaft is centrally supported, and the thrust bearing is located adjacent to the sleeve bearing.
- There is no separate sump for inboard bearings. The lube oil is supplied from the main lube oil system through a gravity tank. Lubrication oil is supplied through the pipeline connected to the bearing housing via a non-return valve.
- Lube oil is supplied to the gravity tank by a main engine lube oil system through an orifice and one line from the tank goes back to the sump tank, thus maintaining the oil level in the gravity tank at all times.
- The oil tank should have the capacity to supply oil for 10- 15 min after a low-level alarm for the oil level in the tank is sounded.
- The long service life of the bearing is claimed because of a stable rotor on a larger supporting surface with a low rate of bearings wear.