Lubricants Questions and Answers

Answer: A

Correct answer: cool the tool and lubricate the chip interface

A cutting fluid does two things at once, and both are needed: it removes the heat generated at the cutting zone, which preserves the tool’s hardness, and it lubricates the sliding contact between the chip and the tool face, which reduces the heat generated in the first place. It also flushes the chips away.

Answer: B

Correct answer: combine water's cooling with oil's lubricity

Water is much the better coolant and oil much the better lubricant, and an emulsion delivers both from one fluid. The oil is supplied as a concentrate that must emulsify readily and stably when the operator adds water, since a poorly formed emulsion separates in the sump and delivers neither property reliably.

Answer: C

Correct answer: 200 cSt

Kinematic viscosity is dynamic viscosity divided by density: \[ \nu = \frac{0.18}{900} = 2 \times 10^{-4}\ \mathrm{m^2/s}. \] One centistokes is \( 10^{-6}\ \mathrm{m^2/s} \), so this is 200 cSt. Multiplying rather than dividing gives 162, which is one of the distractors.

Answer: D

Correct answer: 80

The viscosity index compares the oil against two reference oils of the same viscosity at 100 degrees: \[ VI = \frac{L - U}{L - H} \times 100 = \frac{100 - 60}{100 - 50} \times 100 = 80. \] L is the poor reference and H the good one, so an oil matching H would score 100 and one matching L would score zero.

Answer: A

Correct answer: 60

\[ VI = \frac{100 - 70}{100 - 50} \times 100 = \frac{30}{50} \times 100 = 60. \] Note the direction: raising U moves the oil towards the poor reference L, so the index falls. U is the low-temperature viscosity, and a lower value there means the oil has thinned less on cooling, which is what a high index describes.

Answer: B

Correct answer: 5.6

The saponification value is defined as milligrams of potassium hydroxide per gram of oil, so the measured figure is the value itself: 5.6. Such a low number indicates an essentially mineral oil with a small ester addition; a vegetable oil would give something near 190.

Answer: C

Correct answer: 70 cSt

On a simple linear blend by volume, \[ \frac{60 \times 100 + 40 \times 25}{100} = \frac{6000 + 1000}{100} = 70\ \mathrm{cSt}. \] Real blending indices are logarithmic rather than linear, so an actual blend would come out below this - the linear figure is a first estimate rather than a prediction.

Answer: D

Correct answer: unsuitable, the flash point being below the working temperature

An oil must not be run above its flash point: at 220 degrees it is producing enough vapour to flash, which is both a fire hazard and a sign that the oil is volatilising away. A margin is required, not merely a value above some round number, and the fire point being higher does not rescue it.

Answer: A

Correct answer: soften and run out of the bearing

The drop point is the temperature at which the thickener structure collapses and the grease flows, so a housing at 110 degrees will take a 95 degree grease past that point. The grease will run out and the bearing will be left with no lubricant at all. Service temperature must be well below the drop point, not merely near it.

Answer: B

Correct answer: 1-Q, 2-R, 3-P

Three properties governing three quite different things. The viscosity index describes how much the viscosity changes with temperature. The pour point is the lowest temperature at which the oil will still flow, and so governs whether it can be pumped on a cold start. And the flash point is a safety limit, setting the temperature above which the oil must not be run.