10 exam-style questions with answers and explanations, straight from our 1,050-question bank. Tap an answer to check yourself. When you're ready, take the scored version in the free practice test.
The CLS exam has 150 questions and runs 3 hours.
These 10 free CLS questions are organized by exam domain, so you can see how each part of the Certified Lubrication Specialist (CLS) blueprint is tested. Reveal the answer and explanation under each question.
A base oil is tested and found to contain 91% saturates, 0.02% sulfur, and has a viscosity index of 118. Under the API base oil classification, this oil is:
Correct answer: B - Group II, since the viscosity index falls below the Group III threshold
A used gearbox oil returns a viscosity 11% BELOW the new-oil specification. Fuel dilution is not possible in this application and the flash point is unchanged. The MOST likely cause is:
Correct answer: C - Permanent shear of the viscosity index improver in the charge
An operation runs slowly under heavy load, and the lubricant film has collapsed to the point where surface asperities are in contact. Which additive is doing the work of protecting the surfaces?
Correct answer: A - The antiwear additive, which forms a sacrificial film on the surface
A technician is told a grease is 'NLGI 3' and concludes it will provide a thicker lubricating film than an NLGI 1 product. This conclusion is:
Correct answer: C - Incorrect, because NLGI grade describes consistency, not film thickness
A hydraulic system is converted from mineral oil to a phosphate ester fire-resistant fluid. The nitrile (NBR) seals are left in place. What will happen?
Correct answer: D - The seals will fail, as NBR is attacked by phosphate ester
A hydraulic pump develops a loud, high-pitched whine. The reservoir level is correct, no suction leaks can be found, and the fluid is the specified grade. The MOST likely cause is:
Correct answer: B - A restricted inlet causing the fluid to vaporize
A plant improves its hydraulic fluid cleanliness from ISO 21/19/16 to ISO 18/16/13. At each measured particle size, the particle count has been reduced by a factor of:
Correct answer: C - 8
Six consecutive oil samples from a gearbox show iron steady at approximately 20 ppm by ICP spectroscopy, while the PQ index has climbed from 9 to 45. The BEST interpretation is:
Correct answer: C - Large ferrous particles are forming that ICP cannot detect
A filter is rated Beta(10) = 200. Its efficiency at 10 micrometers is:
Correct answer: C - 99.5%
A gearbox that has run reliably at 60 degrees C for years is relocated and now runs at 80 degrees C. All other conditions are unchanged. The approximate effect on the oil's oxidative life is:
Correct answer: A - It falls to roughly one quarter
The CLS exam also covers these domains. Drill them in the full free practice test:
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