How to Calculate when A Roll Back Would Occur on a Ski Lift

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In summary, the conversation discusses the concept of a roll back on a chairlift due to a loss of friction between the haul rope and the bull wheel. The individual is seeking help in determining when a roll back would occur and how to calculate the needed frictional torque. However, it is noted that overloading the chairs may not necessarily cause a roll back and that the angle of the cable also plays a role. The video referenced in the conversation shows a roll back caused by an incorrect brake application, not a failure of frictional contact.
  • #1
Chairliftop
Hello!

First off, here is a video of what a roll back is:

Here's my question:

How could I determine how much of a load the lift is carrying for there to be a loss of friction between the haul rope (the cable) and the bull wheel (the main wheel at both the top and bottom terminals), causing the lift to run backwards?

Here's my attempt:

I know I am going to have to find the friction coefficient between the rubber lining of the bull wheel and the haul rope.

The chairlift I am talking about has the following:

- 169 Chairs (82 are loaded at one time)
- Each chair has a 6 person capacity
- Assuming the average person weights 137 pounds

Besides that, I need help figuring out when a roll back would occur due to a loss of friction between the rope and the bull Wheel! Help!

Note: I have never taken physics before, so an explanation in basic terms would be great!

Thanks!
 
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  • #2
The maximum frictional torque available should be proportional to the tension in the cable. That increases as the payload increases. So it is not apparent that merely overloading the chairs should lead to such a failure. More likely, it would lead to snapping the cable or toppling a support.
The frictional torque needed is proportional to the difference between the tensions in the cable on the two sides of the bull wheel.
The tension added to the cable when the load on a chair increases depends on the angle between the cable on the uphill side and the cable on the downhill side of the chair. If the cable is initially too slack, the chair hangs low and the weight on the chair doesn't add so much tension. That is how I can imagine getting a rollback in normal operation.
In the video, I did not understand the details, but it sounds like the rollback occurred because the wrong brake was applied, not because the frictional contact with the bull wheel failed.
 

1. What factors determine when a roll back would occur on a ski lift?

The main factors that determine when a roll back would occur on a ski lift are the weight of the passengers, the speed of the lift, and the angle of the lift's incline. Other factors such as wind and weather conditions may also play a role.

2. How is the angle of the lift's incline calculated?

The angle of the lift's incline is calculated by dividing the vertical rise of the lift by the horizontal distance. This can be done by using a protractor or a clinometer, or by using trigonometric functions if the exact measurements are known.

3. What is the maximum weight a ski lift can handle to prevent a roll back?

The maximum weight a ski lift can handle to prevent a roll back varies depending on the specific lift and its design. However, most lifts are designed to handle the weight of several fully loaded chairs plus additional weight from wind and other factors. The maximum weight limit should be clearly marked on the lift and should be strictly adhered to for safety purposes.

4. Can a roll back be prevented or predicted?

Roll backs on ski lifts are rare and usually occur due to unforeseen circumstances such as extreme weather or equipment failure. While preventative measures such as regular maintenance and weight restrictions can decrease the likelihood of a roll back, it is not always possible to predict or prevent them from happening.

5. What should passengers do if a roll back does occur on a ski lift?

If a roll back does occur on a ski lift, passengers should remain calm and follow the instructions of the lift operator. In most cases, the lift will automatically stop and reverse to the previous tower, allowing passengers to safely disembark. If the lift does not stop, passengers should be prepared to jump off the lift onto a designated safe area below. It is important to always listen to and follow the instructions of the lift operator for the safest outcome.

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