CPM Bearings, Seals, and Shaft Couplings 2 — Questions and Answers
Question 1: A mechanical seal in a centrifugal pump creates its primary seal between which two components?
- The shaft sleeve and the stuffing box bore
- A rotating seal face and a stationary seat (Correct answer)
- The impeller back shroud and the back plate
- The pump casing halves at the split line
Correct answer: A rotating seal face and a stationary seat
A mechanical seal prevents leakage through two lapped, highly polished mating faces — one rotating with the shaft and one stationary in the housing.
Question 2: Compared to compression packing, the primary operational advantage of a mechanical seal is:
- Lower initial purchase and installation cost
- Simpler installation requiring no special tools or training
- Near-zero leakage during normal operation (Correct answer)
- Tolerance of much greater shaft runout and deflection
Correct answer: Near-zero leakage during normal operation
Mechanical seals are engineered to produce virtually no leakage, whereas packing requires a controlled drip rate for lubrication and cooling of the packing rings.
Question 3: A radial lip seal (rotary shaft seal) should be installed with the sealing lip oriented:
- Away from the fluid being retained
- Toward the fluid being retained (Correct answer)
- Perpendicular to the direction of shaft rotation
- In the direction opposite to shaft rotation
Correct answer: Toward the fluid being retained
The garter spring and lip face the fluid side so that fluid pressure assists the lip in maintaining contact with the shaft, improving the seal.
Question 4: When replacing compression packing in a pump stuffing box, adjacent packing rings should be staggered by:
- 45° so joints alternate every other ring
- 90° so no two ring joints are aligned (Correct answer)
- 120° distributing three joints evenly
- 180° placing joints directly opposite each other
Correct answer: 90° so no two ring joints are aligned
Staggering each ring 90° from the previous ensures no two ring joints align, eliminating a straight leakage path through the packing stack.
Question 5: What is the purpose of a lantern ring installed within a packed stuffing box?
- To add structural rigidity to the packing stack under high pressure
- To distribute flush fluid or lubricant evenly around the shaft circumference (Correct answer)
- To act as the primary sealing element if the packing fails
- To center the shaft and dampen vibration within the stuffing box
Correct answer: To distribute flush fluid or lubricant evenly around the shaft circumference
A lantern ring is a hollow spacer with radial ports that routes seal flush water or lubricant uniformly around the shaft inside the stuffing box.
Question 6: An O-ring installed in a groove that is too narrow (undersized width) may result in:
- Rolling and twisting of the O-ring during dynamic motion
- Extrusion of the O-ring material into the clearance gap under pressure (Correct answer)
- Insufficient compression leading to immediate leakage
- Accelerated chemical degradation from increased contact stress
Correct answer: Extrusion of the O-ring material into the clearance gap under pressure
Over-compression in a too-narrow groove forces excess rubber to extrude into any available clearance gap under pressure, causing nibbling and seal failure.
Question 7: Which type of seal is most commonly used to prevent leakage past a reciprocating piston inside a hydraulic cylinder?
- Mechanical face seal
- Labyrinth seal
- U-cup piston seal (Correct answer)
- Rotary lip seal
Correct answer: U-cup piston seal
U-cup seals are designed for reciprocating linear motion; their lips conform to the cylinder bore and seal in both directions of travel.
CPM Bearings, Seals, and Shaft Couplings 2
A mechanical seal in a centrifugal pump creates its primary seal between which two components?