WTMA Hydraulics and Fluid Dynamics 5 — Questions and Answers
Question 1: A hydraulic system uses oil with a specific gravity of 0.9. Compared to water, the oil weighs:
- 10% less per unit volume (Correct answer)
- 10% more per unit volume
- The same per unit volume
- 90% more per unit volume
Correct answer: 10% less per unit volume
Specific gravity of 0.9 means the oil is 90% as dense as water, so it weighs 10% less per unit volume.
Question 2: In a hydraulic system, what is 'back pressure'?
- Residual pressure on the return side of an actuator opposing its motion (Correct answer)
- The pressure created by a pump starting up
- Pressure that pushes fluid backward through a check valve
- The maximum rated pressure of the system
Correct answer: Residual pressure on the return side of an actuator opposing its motion
Back pressure is the pressure that exists in the return line and acts against the moving side of a cylinder, reducing net force.
Question 3: A check valve is installed in a hydraulic line. What does it do?
- Allows fluid to flow in one direction only (Correct answer)
- Regulates flow rate in both directions
- Opens at a set pressure to relieve excess
- Divides flow equally between two branches
Correct answer: Allows fluid to flow in one direction only
A check valve contains a ball or poppet that blocks reverse flow while allowing free flow in the intended direction.
Question 4: When a hydraulic hose bursts suddenly, the most immediate danger is:
- High-pressure fluid injection through skin, which can cause severe injury (Correct answer)
- Loss of lubrication to the engine
- System pressure rising to dangerous levels
- Air entering the system and causing cavitation
Correct answer: High-pressure fluid injection through skin, which can cause severe injury
High-pressure fluid can penetrate skin and cause serious internal injury even through small pinhole leaks.
Question 5: A hydraulic motor converts:
- Fluid pressure and flow into rotational mechanical output (Correct answer)
- Mechanical rotation into fluid flow
- Electrical energy into fluid pressure
- Fluid flow into linear motion
Correct answer: Fluid pressure and flow into rotational mechanical output
A hydraulic motor is the reverse of a pump — pressurized fluid enters and drives a shaft to rotate.
Question 6: Why do hydraulic systems typically use oil rather than water as the working fluid?
- Oil lubricates components and resists corrosion, unlike water (Correct answer)
- Oil is cheaper and more widely available than water
- Oil is more compressible, allowing energy storage
- Oil has lower viscosity, reducing pump effort
Correct answer: Oil lubricates components and resists corrosion, unlike water
Oil provides necessary lubrication to internal moving parts and does not cause rust or corrosion as water would.
Question 7: If hydraulic system pressure is measured at 2,000 psi and a cylinder has a 5 sq in piston area, what is the maximum force it can exert?
- 10,000 lbs (Correct answer)
- 5,000 lbs
- 2,000 lbs
- 400 lbs
Correct answer: 10,000 lbs
Force = Pressure × Area = 2,000 psi × 5 sq in = 10,000 lbs.
WTMA Hydraulics and Fluid Dynamics 5
A hydraulic system uses oil with a specific gravity of 0.9.
Compared to water, the oil weighs: