Lubricants Questions and Answers

Answer: C

Correct answer: a high drop point with water resistance

Lithium soap gives a drop point around 190 degrees, so it serves at temperatures a calcium grease could not, and it resists water where a sodium grease would wash out. That combination, together with good mechanical stability, is why lithium greases displaced the older types as the general purpose choice.

Answer: D

Correct answer: bentonite clay or fine silica

A non-soap grease uses an inorganic thickener - modified bentonite or finely divided silica - which has no melting point, so the grease has effectively no drop point and is limited instead by the oxidation of the base oil. That makes such greases the choice for high temperature service.

Answer: A

Correct answer: layers that slide easily over one another

Graphite is built of sheets of covalently bonded carbon held to one another only by weak forces, so the sheets shear apart under a small stress while each remains intact. That anisotropy is the whole of its lubricating action, and it is why the same element in the tetrahedral diamond structure is the hardest substance known.

Answer: B

Correct answer: needs no adsorbed moisture to lubricate

Graphite depends on adsorbed water vapour to keep its layers apart and passivate the sheet edges, so in a vacuum or a very dry gas it ceases to lubricate and can behave abrasively. Molybdenum disulphide has the same layer structure but shears without adsorbed species, which is why it is used in space mechanisms.

Answer: C

Nothing adheres to the fluorine sheath and the chains slide past one another readily, giving one of the lowest friction coefficients of any solid. Its poor thermal conductivity and low load capacity limit where it can be used unsupported.

Answer: D

The viscosity changes remarkably little between low and high temperature, which no mineral oil approaches. Their weakness is poor boundary performance with steel on steel, so they suit light loads and wide temperature ranges.

Answer: A

Correct answer: keep deposits suspended and neutralise acids

The detergent keeps surfaces clean and carries a reserve of alkali that neutralises the acids formed by combustion and oxidation; the dispersant keeps the resulting soot and sludge finely suspended so that it is carried to the filter rather than settling. Together they are what allow long drain intervals.

Answer: B

Hindered phenols and zinc dithiophosphates intercept the radicals and peroxides that would otherwise propagate the oxidation, so sludge and acid formation are delayed. Once the additive is exhausted the oil degrades rapidly.

Answer: C

Correct answer: cannot maintain a load-bearing film

Foam is a mixture of oil and air, and air will not carry load, so a foaming oil loses its film and the surfaces come into contact. Foam also impairs heat transfer and can cause a pump to lose suction. Silicone anti-foam additives break the bubbles as they form, at very low concentration.

Answer: D

Sulphur, chlorine or phosphorus compounds decompose only at the hot spots of a heavily loaded contact and build a sacrificial inorganic film there. Below that temperature they are inactive, which is why they are reserved for severe duty.