4th Class Power Engineering lubrication and bearings practice questions (4B).

Five free practice questions on lubrication and bearings: unit B1 of paper B (4B) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · List the various classes and types of lubricants and describe their respective properties and application

   A conveyor chain carries parts through a heat-treating oven at about 400 °C at a metal-fabrication plant. The mineral chain oil now used smokes and leaves hard carbon on the links. Which class of lubricant suits this duty?

   1. A graphite lubricant in an evaporating carrier
   2. A heavier grade of the same mineral chain oil
   3. A lithium-soap grease packed into the chain links
   4. A water-mixed soluble-oil emulsion sprayed on the links
   Show the answer

   A. Correct. At this temperature an oil or grease oxidizes and cokes. A solid lubricant such as graphite keeps working: a carrier fluid takes it onto the links and then evaporates or burns off, leaving a dry graphite film whose layers slide easily over one another.

   Why not the others

   B. A thicker mineral oil still oxidizes and carbonizes at 400 °C; it would only leave more deposit. The temperature, not the viscosity, rules out a mineral oil here.

   C. A soap grease is oil held in a thickener, and the soap melts far below 400 °C; the oil then runs out and cokes just as the present chain oil does.

   D. The water in an emulsion boils off at once on a chain at 400 °C and leaves a little oil to burn; soluble-oil emulsions are cutting fluids and coolants, not high-temperature chain lubricants.

2. Recall · Describe the construction and operation of antifriction and thrust bearings

   The thrust bearing of a multistage boiler-feed pump has tilting pads arranged around a collar on the shaft. Each pad sits on a pivot so that it can

   1. roll along the collar and share the load like balls in a race
   2. tilt slightly and form its own wedge of oil against the collar
   3. move outward and keep the shaft centred in its casing bore
   4. swing clear of the collar so the rotor floats freely at start-up
   Show the answer

   B. Correct. The pivot lets each pad take up a small angle to the turning collar, so oil dragged in by the collar forms a converging, load-carrying film on every pad. The pads carry the rotor's axial (end) thrust on full fluid films and adjust themselves to changes in speed and load.

   Why not the others

   A. The pads do not roll; that describes a rolling-element thrust bearing. A tilting-pad bearing is a fluid-film bearing whose pads stay on their pivots and only tilt.

   C. Keeping the shaft centred radially is the job of the journal bearings. The thrust pads face the collar and resist movement along the shaft's axis.

   D. The pads stay against the collar, parted from it only by the oil film; the rotor must be held axially at start-up as well as at full load, or the impellers could rub on the casing.

3. Recall · List the properties of lubricating oils, the additives used, and their selection criteria

   Steam leaking past a turbine's glands condenses and gets into the lube-oil reservoir. Which property of the turbine oil lets this water separate out quickly, so it settles to the bottom of the reservoir and can be drained off?

   1. Viscosity index
   2. Demulsibility
   3. Pour point
   4. Oxidation stability
   Show the answer

   B. Correct. Demulsibility is an oil's ability to release water instead of holding it in a stable emulsion. Turbine oils are made to shed water readily because some steam or water leakage into the system is expected; the water settles and is drained or taken out by a purifier.

   Why not the others

   A. Viscosity index describes how much an oil's viscosity changes with temperature; a high index means a smaller change. It says nothing about how quickly water separates from the oil.

   C. Pour point is the lowest temperature at which the oil will still flow. It matters for cold starts and outdoor machines, not for separating water from oil in a warm reservoir.

   D. Oxidation stability is the oil's resistance to reacting with oxygen and forming sludge and acids. Water does hasten oil ageing, but the property that lets water drop out of the oil is demulsibility.

4. Procedural · Describe how to clean and replace roller and ball type bearings

   During a fan motor overhaul, a helper suggests putting the motor's sealed, factory-greased ball bearings through the solvent tank with the other parts. Why should these bearings not be washed?

   1. Solvent would get past the seals and thin grease that cannot be renewed.
   2. Solvent would leave a film on the races that stops grease from sticking.
   3. Only hot light oil is safe on them, because it will not thin the grease.
   4. Washing would remove the preload set between the inner and outer rings.
   Show the answer

   A. Correct. A sealed bearing is greased for life at the factory and cannot be repacked. Solvent that seeps past the seals thins or washes out that grease and leaves dirty solvent inside. Wipe the outside clean only, and replace the bearing if it is doubtful.

   Why not the others

   B. The objection is not a film on the outside; it is solvent getting inside, where it thins the sealed-in grease, which cannot be replaced or topped up.

   C. Hot oil thins and flushes out grease just as solvent does; it is a way of cleaning open bearings, not sealed ones. A sealed bearing is not washed in anything: wipe the outside clean and replace the bearing if it is doubtful.

   D. A deep-groove ball bearing's internal clearance is fixed when it is made and is not changed by washing. The reason not to wash it is the sealed-in grease, which solvent would thin and which cannot be renewed.

5. Recall · Discuss the concept of lubrication and list the purposes of a lubricant

   In the lubricated cylinder of a reciprocating air compressor, the thin oil film between the piston rings and the cylinder wall does more than reduce friction and wear. It also helps to

   1. cool the air as it is being compressed in the cylinder
   2. seal the rings against air blowing past them
   3. absorb the moisture drawn in with the intake air
   4. keep carbon from building up on the discharge valves
   Show the answer

   B. Correct. Sealing is one of the purposes of a lubricant: the oil film fills the tiny gaps between ring, groove and cylinder wall, so less air leaks past the rings during compression and the cylinder keeps its capacity and efficiency.

   Why not the others

   A. A lubricated reciprocating cylinder is fed only a small, metered amount of oil, far too little to absorb much of the heat of compression (unlike an oil-flooded screw compressor, whose large oil flow does cool the air). The air is cooled by the cylinder jacket or fins and by the intercooler and aftercooler; the film's extra job here is sealing the rings.

   C. Water in lubricating oil is a contaminant that causes rust and weakens the film. Moisture is taken out of compressed air by aftercoolers, separators, drain traps and dryers, not by the cylinder oil.

   D. Oil carried onto hot discharge valves is a cause of carbon deposits, not a cure; over-lubricating a compressor cylinder makes valve fouling worse. The film's extra job is sealing between the rings and the cylinder wall.

02Answer key

03What unit B1 covers

Why plant machinery is lubricated and how its bearings are cared for: the purposes of a lubricant, the classes of lubricants and the properties and additives of lubricating oils, boundary and full-film lubrication, sleeve, rolling and thrust bearings, cleaning and replacing ball and roller bearings, and reading the causes of a bearing failure.

Part B of the SOPEEC 4th Class syllabus names this unit “B1 Lubrication” and lists two objectives under it (objectives 1 to 2 of 52). SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Oil does more than cut friction

A lubricant also carries heat away, flushes out dirt and wear particles, seals and protects against rust. A circulating oil system uses those jobs: the cooler takes back the heat the oil picks up, and the filter and reservoir catch what it flushes out of the bearings.

Turbine oil must shed water and resist oxidation

Demulsibility is an oil's ability to release water so it settles and can be drained, which matters where gland steam leaks into the reservoir. Oxidation inhibitors slow the sludge, varnish and acids warm oil forms in contact with air, and they are used up slowly in service.

Hotter oil means a thinner film

A hydrodynamic film's thickness depends on the oil's viscosity, which falls steeply as the oil heats. Lose the oil cooler and the journal runs closer to the bearing. In boundary lubrication, at starting, stopping or overload, the film is too thin to keep the high spots apart.

A failed bearing shows its cause

Milky grease and rust-stained etch marks at roller spacing point to water getting in. A polished outer ring and an oversize bore mean the ring was turning in a loose housing. Sealed, greased-for-life bearings are wiped clean, never washed in solvent.

05The other units

Paper 4B has twelve units, B1 to B12. Each has its own page of free questions:

The 4th Class Power Engineering practice exam (4B) page describes the full question bank; paper A has its own 4A practice exam. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.