CPT CPT Fluid Systems & Hydraulics 2 — Questions and Answers
Question 1: In a hydraulic system, what does a directional control valve control?
- Fluid temperature
- The path fluid takes through the circuit (Correct answer)
- System operating pressure
- Pump displacement
Correct answer: The path fluid takes through the circuit
A directional control valve routes hydraulic fluid to different actuator ports, controlling the direction of cylinder or motor movement.
Question 2: Which hydraulic fluid property most affects the performance of a system at low temperatures?
- Flash point
- Viscosity (Correct answer)
- Specific gravity
- Acid number
Correct answer: Viscosity
Viscosity increases significantly at low temperatures, reducing pump efficiency and increasing pressure drop across lines and valves.
Question 3: A hydraulic cylinder has a bore diameter of 4 inches and a rod diameter of 2 inches. Which side produces more force at equal pressure?
- Rod side
- Bore (cap) side (Correct answer)
- Both sides produce equal force
- Force depends only on stroke length
Correct answer: Bore (cap) side
The cap (bore) side has a larger effective area since no rod occupies it, so at equal pressure it produces more force than the rod side.
Question 4: What is the primary reason for using a hydraulic filter with a bypass indicator?
- To signal when fluid temperature is too high
- To alert the technician when the filter element is clogged and fluid is bypassing filtration (Correct answer)
- To indicate that pump pressure is too low
- To monitor reservoir fluid level
Correct answer: To alert the technician when the filter element is clogged and fluid is bypassing filtration
When the filter element clogs, a bypass valve opens to maintain flow; the bypass indicator (pop-up or gauge) signals that the element needs replacement before contamination damages components.
Question 5: Aeration in hydraulic fluid appears as which symptom during system operation?
- Blue smoke from the reservoir
- Foamy or milky fluid with spongy actuator response (Correct answer)
- Sudden pressure spike above relief valve setting
- Increased actuator speed
Correct answer: Foamy or milky fluid with spongy actuator response
Entrained air creates foam and causes erratic, spongy actuator movement because air is compressible unlike hydraulic fluid.
Question 6: What does ISO cleanliness code 16/14/11 represent in hydraulic fluid analysis?
- Fluid viscosity grade
- Particle contamination levels at 4, 6, and 14 micron sizes (Correct answer)
- Operating temperature range
- Fluid acidity levels
Correct answer: Particle contamination levels at 4, 6, and 14 micron sizes
ISO 4406 cleanliness codes represent the number of particles per milliliter at three size thresholds (4µm, 6µm, 14µm), indicating contamination severity.
CPT CPT Fluid Systems & Hydraulics 2
In a hydraulic system, what does a directional control valve control?