CMRT CMRT Mechanical Systems and Alignment 2 — Questions and Answers
Question 1: What is the difference between static (residual) unbalance and dynamic unbalance in a rotating component?
- Static unbalance exists only at rest, while dynamic unbalance only appears above 3,600 RPM
- Static unbalance is a single heavy spot in one plane, while dynamic unbalance involves heavy spots in two or more planes that create a couple (Correct answer)
- Static unbalance applies only to overhung rotors, while dynamic unbalance applies only to between-bearing rotors
- Static unbalance is corrected by adding weight, while dynamic unbalance is corrected by removing weight
Correct answer: Static unbalance is a single heavy spot in one plane, while dynamic unbalance involves heavy spots in two or more planes that create a couple
Static unbalance is a mass imbalance in a single axial plane causing the shaft to bow, while dynamic unbalance involves imbalance in two or more planes that creates a rocking couple during rotation.
Question 2: What is the purpose of a keyway and key in a shaft-hub connection?
- To allow the hub to slide freely along the shaft for axial adjustment
- To transmit torque between the shaft and the hub by providing a positive mechanical drive (Correct answer)
- To prevent the hub from vibrating by adding mass to the assembly
- To allow fine angular position adjustment of the hub on the shaft
Correct answer: To transmit torque between the shaft and the hub by providing a positive mechanical drive
A key inserted into matching keyways on both the shaft and hub creates a positive mechanical connection that transmits torque while preventing relative rotation between the two components.
Question 3: What does 'interference fit' (press fit) mean when installing a bearing or coupling hub on a shaft?
- The hub or bearing bore is slightly smaller than the shaft diameter, requiring force or heating to assemble (Correct answer)
- The hub or bearing bore is slightly larger than the shaft, allowing hand assembly
- The shaft and hub are the same size, requiring no force to assemble
- The hub is threaded onto the shaft to create a tight mechanical connection
Correct answer: The hub or bearing bore is slightly smaller than the shaft diameter, requiring force or heating to assemble
An interference fit (press fit) means the bore is smaller than the shaft diameter so the component must be pressed or thermally expanded for installation, creating a tight grip that transmits loads without slipping.
Question 4: Which seal type is most appropriate for preventing lubrication leakage from a rotating shaft in a pump or motor?
- Gasket seal
- Mechanical face seal (mechanical seal) (Correct answer)
- O-ring static seal
- Spiral wound gasket
Correct answer: Mechanical face seal (mechanical seal)
Mechanical face seals (mechanical seals) use two precision-lapped faces — one rotating and one stationary — pressed together to create a dynamic seal around a rotating shaft.
Question 5: What is the primary function of a flexible disc coupling in a high-speed turbine-generator set?
- To absorb torsional shock loads by flexing the disc packs under load reversal
- To transmit torque while accommodating small amounts of angular and axial misalignment without requiring lubrication (Correct answer)
- To provide a mechanical fuse that shears if the generator experiences a short circuit
- To allow for large parallel offsets between the turbine and generator shafts
Correct answer: To transmit torque while accommodating small amounts of angular and axial misalignment without requiring lubrication
Disc couplings transmit torque through thin flexible metal disc packs that accommodate angular and axial misalignment without lubrication, making them ideal for high-speed, high-power turbine applications.
Question 6: What is 'thermal growth' in rotating machinery alignment, and how is it accounted for?
- The expansion of bearing housings due to grease heat that must be compensated by re-greasing
- The dimensional change in machine components as they heat up from cold installation to operating temperature, requiring 'cold offset' corrections during alignment (Correct answer)
- The increase in vibration amplitude as a machine warms up to operating temperature
- The elongation of bolts under thermal cycling that requires periodic re-torquing
Correct answer: The dimensional change in machine components as they heat up from cold installation to operating temperature, requiring 'cold offset' corrections during alignment
Thermal growth is the predictable dimensional change (expansion) of machine components as they reach operating temperature; alignment is performed cold with pre-calculated offsets to achieve correct alignment when hot.
CMRT CMRT Mechanical Systems and Alignment 2
What is the difference between static (residual) unbalance and dynamic unbalance in a rotating component?