433A Mechanical Drives and Bearings 4 — Questions and Answers
Question 1: What is the purpose of crowning on a conveyor pulley?
- To increase the pulley's load-carrying capacity
- To track and center the belt on the pulley (Correct answer)
- To reduce belt tension requirements
- To improve the coefficient of friction
Correct answer: To track and center the belt on the pulley
A crowned (slightly larger diameter at center) pulley causes the belt to self-center by creating a tracking force. The belt naturally migrates toward the highest point of the crown.
Question 2: When coupling two shafts with a jaw coupling, what is the purpose of the elastomeric spider insert?
- To lock the coupling halves together permanently
- To provide electrical insulation and absorb shock and vibration (Correct answer)
- To increase the torque rating of the coupling
- To prevent the coupling from overheating
Correct answer: To provide electrical insulation and absorb shock and vibration
The elastomeric spider insert in a jaw coupling provides electrical isolation between shafts, absorbs shock loads and vibration, and accommodates minor misalignment while transmitting torque.
Question 3: What condition is indicated by a blue discoloration on a bearing race?
- Normal wear pattern
- Overheating has occurred (Correct answer)
- The bearing has been properly lubricated
- Corrosion from moisture exposure
Correct answer: Overheating has occurred
Blue or straw-colored discoloration on bearing races indicates that the bearing has been subjected to excessive heat, typically above 200°C. This overheating compromises the bearing's hardness and load capacity.
Question 4: In a chain drive system, what is the effect of excessive chain sag on the slack side?
- Increased power transmission efficiency
- Chain whip causing vibration and accelerated wear (Correct answer)
- Better sprocket engagement
- Reduced noise levels
Correct answer: Chain whip causing vibration and accelerated wear
Excessive chain sag on the slack side causes chain whip — a whipping motion that generates vibration, increases dynamic loads on sprockets and shafts, accelerates wear, and can cause the chain to jump off the sprockets.
Question 5: What is the purpose of a shaft seal (oil seal) adjacent to a bearing?
- To increase bearing speed capability
- To retain lubricant and exclude contaminants from the bearing (Correct answer)
- To provide additional radial support to the shaft
- To cool the bearing during operation
Correct answer: To retain lubricant and exclude contaminants from the bearing
Shaft seals serve the dual purpose of retaining lubricant within the bearing housing and preventing external contaminants such as dirt, moisture, and process materials from entering and damaging the bearing.
Question 6: When replacing a V-belt on a multi-belt drive, what is the correct practice?
- Replace only the worn belt to save cost
- Replace all belts as a matched set (Correct answer)
- Replace the two outermost belts
- Replace belts one at a time over several days
Correct answer: Replace all belts as a matched set
All belts in a multi-belt drive must be replaced as a matched set because new and old belts have different lengths and stretch characteristics, causing uneven load distribution and premature failure.
433A Mechanical Drives and Bearings 4
What is the purpose of crowning on a conveyor pulley?