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# ? for the Master Physicists

OK guys, I have a question for you. Stowe installed a new Dopplemayer HSQ this season. One difference between it and the old loft is that it has the drive motor at the top instead of the bottom. Dopplemayer says that top drive takes less horsepower because it is easier to pull the chair up than to push them from below. I don't see how there could possibly be any difference, but I assume that Dopplemayer has built enough of these to know that it truly is so. How can this be true?

Regardless of where the lift motor is, the chairs get lifted by tension on the cable.  There is tension on every part of the cable, but the tension varies with the load due to balancing all the forces going  up and down on any section of cable. The thing is, if the motor's at the bottom you have to carry more tension on the cable going up from the bottom to the bull wheel and around it to reach the load; where as if you have the motor at the top the tension on the the part of the cable carrying the empty chairs down will be lower.  Less tension in that part of the cable means less friction loss.

I was thinking that the tension has to be the same at both ends doesn't it?

I don't see how there could be a difference in the work required to "push" or "pull".  As far as I know you can ONLY pull a cable to transmit force, if you push it you just wind up with a pile of coiled cable.

Quote:
Originally Posted by Alveolus

I don't see how there could be a difference in the work required to "push" or "pull".  As far as I know you can ONLY pull a cable to transmit force, if you push it you just wind up with a pile of coiled cable.

Exactly, when the motor is at the bottom, it's pulling the empty side down the mountain, not pushing the loaded side up.

Quote:

Originally Posted by Ghost

Regardless of where the lift motor is, the chairs get lifted by tension on the cable.  There is tension on every part of the cable, but the tension varies with the load due to balancing all the forces going  up and down on any section of cable. The thing is, if the motor's at the bottom you have to carry more tension on the cable going up from the bottom to the bull wheel and around it to reach the load; where as if you have the motor at the top the tension on the the part of the cable carrying the empty chairs down will be lower.  Less tension in that part of the cable means less friction loss.

Posted before I read that.  Makes some sense but I'll have to think about it for few minutes.  Not sure how much the tension in the cables varies from point to point (ie how much friction there is at the wheels - the more there is the more force/tension is diverted into the towers and bullwheels)

Considering that one of the limiting factors for how much load you can carry is going to be friction between the bullwheel and the rope, it seems like the tension at the bullwheel would have to be quite high. I'm going to see if I can get a tour of the new lift take pics and ask questions. What would you want to know if they'll meet with me and I can ask some questions?

I can say that in a theoretical, loss less world as depicted in physics books there would be no difference the the work required for motor top vs motor bottom.  Unfortunately this is a real world problem and all those messy/annoying factors can't be ignored unless you are happy with an answer that is accurate to "an order of magnitude or two."

Think of a bicycle chain.  If you had to pull the chain at the bottom from near the back wheel, there would be a lot of tension on the bottom bit of chain between the point where you pull it and front sprocket, around the sprocket and all the way to the back sprocket, but when you pedal, that part of the chain can be slack and the tension starts at the front sprocket.  The friction at the top sprocket/pully is much more with the motor at the bottom because you have two fully-loaded full tension forces pulling it down (one on the loaded chair cable, one on the empty chair cable) than what it would be if you only had the loaded chair side fully-loaded up with full tension.  Of course you still need tension in all of the cable, but the difference in tension in that cable on either side of the motor is equal to how hard the motor is pulling.

Quote:

Originally Posted by epic

Considering that one of the limiting factors for how much load you can carry is going to be friction between the bullwheel and the rope, it seems like the tension at the bullwheel would have to be quite high. I'm going to see if I can get a tour of the new lift take pics and ask questions. What would you want to know if they'll meet with me and I can ask some questions?

I'd like you to ask them "Why the heck/exactly does it take less work with the motor at the top?"   Although my undergrad is in engineering, I have been in medicine for 20 some years.  As I was thinking about this I heard grinding noises in my head and rust fell out of my ear.

I wonder if you can put some kind of straingauge in the rope and measure this in realworld terms

Ghost...

Would it be accurate to say that by placing the motor at the bottom, you will loose significantly more work to friction as that applied force (tension) has greater losses by going through all the uphill (empty chair side) guides and peak bullwheel before being available to do its job on the full chair side?

When the cable is under tension it will stretch out. Pulling from the bottom may necessitate using a heavier gauge cable to prevent excessive stretch since much more cable will be out between the motor and the load. Just a thought.

I'm not an physicist, so I can't answer the question in theory, but I know how it will save power in reality. Any morning that there is issue with the motor on starting the lift, they'll have to take a sled all the way to the top, wasting time, then wasting more time because avy control won't be able to start until it's fixed (maybe not an issue at Stowe), causing a significant delaly in opening. Thus there will be less power used as the lift won't be running as many hours in a day. Dumbest design ever for a lift system. Might work well from an engineering stand point (I have no idea if it's better), but obviously not designed by somebody used to waiting in line for first chair on a powder day.

Trying to jog my memory from my high school physics class like 14 years ago. Hmm. Could it perhaps have anything to do with potential energy? Since there is more potential energy stored at the top of any mechanism, such as a hoist, a pulley, or an elevator, then I would imagine that it might have an easier time running the entire lift from the top. In fact, I believe most modern elevators use a motor placed at the top of the elevator shaft. The best example is a roller coaster at the top of the track about to go down. It might be a similar principal here.

I'm thinking that it doesn't make much difference once the lift is in motion and up to speed, but I'm thinking it could make a good bit of difference when starting the lifts back up after stopping them, due to that stored potential energy at the top.

Quote:
Originally Posted by epic

I was thinking that the tension has to be the same at both ends doesn't it?

Pulling from the bottom increases tower friction on the down-going cable, so the overall tension is greater overall for the same up going load.

And the tension has to be the same at both ends of the passive (non-drive) wheel only  if the passive (non-drive) wheel is completely frictionless relative to the cable.

frictionless pulley >> less cable/pulley slip >> lift wheel >> less cable/pulley slip >> bicycle chain

Imagine the cable and uphill wheel as a bicycle chain on a sprocket, and you'll be able to imagine how the return tension can be vastly different from the up pulling tension.

Quote:
Originally Posted by BushwackerinPA

http://peakstoprairies.org/p2bande/skigreen/ch%208%20lift%20op.pdf

They don't actually say it.

Edited by cantunamunch - 10/31/11 at 12:03pm

Picking up the bicycle chain analogy, you can imagine the chain being completely taut and under considerable strain on one side of the sprocket/bull wheel and slack chain actually falling down the other side, so in that picture tension need not be equal all around the circuit that the chain/cable travels. Of course that is with a gear that actually engages the chain securely with its teeth. In this high speed quad you have a cable being "gripped" by a bullwheel under tension. i suppose its a matter of how much frictional grip you can impart on the cable before the cable just slides. Only with a slippery wheel would the tension necessarily equalize on both the up going cable and the down cable.

Does that make any sense?

My head is spinning now so I'm going to put it into a condition of rest.

Quote:
Originally Posted by cantunamunch

Pulling from the bottom increases tower friction on the down-going cable, so the overall tension is greater overall for the same up going load.

And the tension has to be the same at both ends of the passive (non-drive) wheel only  if the passive (non-drive) wheel is completely frictionless relative to the cable.

frictionless pulley >> less cable/pulley slip >> lift wheel >> less cable/pulley slip >> bicycle chain

Imagine the cable and uphill wheel as a bicycle chain on a sprocket, and you'll be able to imagine how the return tension can be vastly different from the up pulling tension.

They don't actually say it.

they do, less haul rope tension mean less friction on all part by 10-15 percent.  they also use a lighter haul rope. its all in there.

Cable is stretching and flexing more when you pull it from the bottom because there is all that extra length of cable (and pulleys, and chairs bouncing and tower shocks) between the motor and the concentration of weight.  Pulling it from the top allows a lower cable length to weight ratio between the motor and the heavy side.

Quote:

Originally Posted by oisin

i suppose its a matter of how much frictional grip you can impart on the cable before the cable just slides.

Which is quite a fair bit, or railroads wouldn't work.

Quote:
Originally Posted by crgildart

Cable is stretching and flexing more when you pull it from the bottom because there is all that extra length of cable (and pulleys, and chairs bouncing and tower shocks) between the motor and the concentration of weight.  Pulling it from the top allows a lower cable length to weight ratio between the motor and the heavy side.

Sort of, but the big lossy friction item is that  pull tension points downwards and therefore compresses the lift towers and wheels downwards, adding to friction thereby.

Friction is proportional to the normal force on the frictional surface, and  down-pulling wheels add to gravity component of normal force/ up pulling wheels relieve gravity component of normal force.

Quote:

Originally Posted by Alveolus
Would it be accurate to say that by placing the motor at the bottom, you will loose significantly more work to friction as that applied force (tension) has greater losses by going through all the uphill (empty chair side) guides and peak bullwheel before being available to do its job on the full chair side?

That is pretty much what I was thinking, though I have no physics knowledge to back it up. Just seems logical that the force would be exerted more efficiently on a load that is closer to the motor on the cable.

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