CET Traction Elevator Mechanics 3 — Questions and Answers
Question 1: What is the significance of the 'T1/T2 ratio' in traction elevator design?
- It defines motor torque to brake torque ratio
- It is the ratio of rope tension on the load side to the slack side, determining traction capacity (Correct answer)
- It describes the car-to-counterweight weight ratio
- It specifies the ratio of up-travel to down-travel speed
Correct answer: It is the ratio of rope tension on the load side to the slack side, determining traction capacity
The T1/T2 ratio compares the heavy (load) side tension to the light (counterweight) side tension and must not exceed the traction capacity of the sheave-rope system.
Question 2: Undercut grooves on a traction sheave are used primarily to:
- Reduce rope wear
- Increase traction by raising the effective friction coefficient (Correct answer)
- Decrease sheave weight
- Improve rope centering
Correct answer: Increase traction by raising the effective friction coefficient
Undercut grooves create a higher contact pressure on the rope sides rather than the bottom, effectively increasing the traction coefficient.
Question 3: What happens to traction if a traction elevator hoist rope becomes oil-contaminated?
- Traction increases due to lubrication
- Traction decreases, risking rope slippage on the sheave (Correct answer)
- Traction is unaffected
- The brake compensates automatically
Correct answer: Traction decreases, risking rope slippage on the sheave
Oil contamination significantly reduces the friction coefficient between the rope and sheave groove, diminishing traction capacity.
Question 4: In a gearless traction machine, what directly drives the sheave?
- A worm gear connected to the motor shaft
- A helical gearbox
- The motor rotor shaft without intermediate gearing (Correct answer)
- A belt and pulley reduction system
Correct answer: The motor rotor shaft without intermediate gearing
Gearless machines connect the sheave directly to a slow-speed, high-torque motor, eliminating gearbox losses and noise.
Question 5: What type of motor is most commonly used in modern gearless traction elevator machines?
- DC series wound motor
- Permanent magnet synchronous motor (PMSM) (Correct answer)
- Induction squirrel cage motor with variable voltage
- Universal AC/DC motor
Correct answer: Permanent magnet synchronous motor (PMSM)
Permanent magnet synchronous motors are favored in modern gearless machines for their high efficiency, compact size, and precise speed control.
Question 6: Which ASME A17.1 rule governs the minimum factor of safety for elevator hoist ropes?
- 7.1
- 8.5
- 10.5 (Correct answer)
- 12.0
Correct answer: 10.5
ASME A17.1 requires a minimum factor of safety of 10.5 for steel wire hoist ropes based on rated load and speed.
Question 7: What effect does increasing the arc of contact (rope wrap angle) on the drive sheave have on traction?
- Decreases traction by reducing rope contact time
- Increases traction by allowing more friction area (Correct answer)
- Has no effect on traction
- Increases sheave wear only
Correct answer: Increases traction by allowing more friction area
A greater wrap angle increases the arc over which friction acts, exponentially increasing the maximum traction capacity per the Euler equation.
CET Traction Elevator Mechanics 3
What is the significance of the 'T1/T2 ratio' in traction elevator design?