Answer EKM Question 28

Q28. With reference to turbocharger bearings:

A. Discuss the relative advantages and disadvantages of white metal sleeve and ball race bearings for turbocharger rotor support.

B. State with reasons how axial location of the rotor is achieved.

C. Explain how the bearing are kept cool in service.

D. Indicate how the bearings are sealed from the atmosphere and exhaust gas.

Answer: White metal sleeve bearing
- Usually known as Inboard Bearing, in this type, the location of the bearings are inside the rotating turbine and compressor blade on the shaft.
- On the sleeve type white metal bearing, hydrodynamic fluid film lubrication prevails. Still, the friction losses and heat generated are better compensated than ball race bearing.
- The hydrodynamic film is used for lubrication of the bearing and it is dependent on the maintenance of the correct oil viscosity, high purity, etc.
- Misalignment is not very significant as an anti-friction bearing Hence failure is not sudden but progressive with warning signs.
- A thrust bearing arrangement is necessary to be fitted into the main bearing assembly of the turbocharger to compensate for the axial thrust which is taking place during the startup, shut down & low loads operation of the turbocharger.

Advantages:-
- Compact and takes very less space
- Low grade of lube oil can be used as compared to ball race bearing. Also High operating temperature is possible as they have a better cooling arrangement
- Not subjected to brinelling.
- Longer Time Between Overhaul, hence more bearing life
- Less significance for any misalignment
- Due to external lubrication, any sudden failure of engine or a black out the bearings continues to be cooled. Inbuilt pump is not necessary.

Disadvantages: -
- High friction losses at low loads hence low efficiency
- No easy access to the bearings, the rotor needs to be removed.
- Takes longer time to complete overhaul compared to ball race bearing hence longer port stay is required.
- Poor response to a change of load
- Chance of oil contamination is higher as it used system oil of the main engine.

White metal ball rare bearing
- The ball bearings consist of either a single cartridge design with a row of tapered ball bearing on either end, or two separate ball bearing to support either end of the turbocharger's main shaft. These bearing are also lubricated by an oil feed but the actual rotating area between the main shaft and the bearing is greatly reduced.
- This means that less lubrication is required in the ball bearing design. In addition, the use of a tapered ball bearing eliminates the need for thrust, bearings as the taper design carries the thrust load on the main shaft.
- Ball race and Roller bearing uses integral lube oil supply system for lubrication, i.e., it is fitted with a self-contained pump operated by the rotating shaft of the turbocharger provided on both blower & turbine sides. An independent oil sump is provided on both sides of the casing of the turbocharger, and the pump draws oil from the sump.

Advantages:- 
- Low friction losses at low loads hence High efficiency
- Easy access to bearing for any inspection or repair, not necessary to remove rotor for bearing access
- Highly suitable for axial flow type turbocharger
- Can handle very high temperatures.
- No header tank or additional LO piping is essential hence No additional fittings such as cooler, filters are required

Disadvantages:-
- Require high-grade lubricating oil with higher viscosity and stability as the operating temperature of these bearings without cooling system is very high.
- Ball race Bearing subjected to brinelling.
- Requires resilient mountings and greater alignment. Hence, overhaul is preferably done by shore side technicians by most of the ship operators.
- High operational and maintenance cost due to low running life. Shorter Time Between Overhaul
- Suitable mainly for a large size of turbocharger system as compared to an inboard sleeve bearing type.

B. State with reasons how axial location of the rotor is achieved.
- A means for axially securing the shaft and the turbine wheel connected thereto is provided between the compressor wheel and the turbine wheel, wherein this means for axial securing limits an axial movement of the shaft and of the turbine wheel connected thereto in the direction of the turbine in the event of the compressor wheel bursting.
- The means for axially securing the shaft and the turbine wheel, connected thereto comprises a component which is connected to the shaft or mounted on the shaft and is fixed In the axial direction in the housing.
- As the gas forces acting on the compressor and turbine wheels in axial direction are of differing strengths, the shaft and turbine wheel assembly is displaced in an axial direction.
- The axial bearing, a sliding surface bearing with tapered lands, absorbs these forces. Two small discs fixed on the shaft serve as contact surfaces. The axial bearing is fixed in the centre housing.

C. Explain how the bearing are kept cool in service
- Turbocharger bearings are cooled by the lubrication system, there are. 2 different type of lubrication used which is, inbuilt lubrication system and gravity type. Inbuilt system has its small sump inside and a pump, which is run by the shaft it self.
- Gravity system uses a overhead tank which is filled up constantly by the main engine lube oil system and the oil drained due to gravity, lubes the bearings and drains down to the engine. 
D. Indicate how the bearings are sealed from the atmosphere and exhaust gas.
Labyrinth Seals
- The bearings are separated from the blower and turbine by labyrinth seals.
- These seals are sealed by air supply from the discharge of blower to prevent oil entering the blower and to prevent contamination of the oil by the exhaust gas.
- In case of defective labyrinth seals on blower side, oils leaked into the air system the cooling surfaces are covered with oil which are Insulators and the cooling efficiency will be reduced. Also there will be deposits on the blower and the blower efficiency will be reduced. - In case of defective labyrinth seals on turbine side, the oil is leaked into the exhaust side and there will be carbon deposits on the nozzle and turbine blades. Reduce blower efficiency and fire in the exhaust piping. It Will cause turbocharger surging. Oil will contaminate with exhaust gas.