Why Do Ski Lift Pylons Lean? - Colin's Physics Query

[Colin Charman]

I am in Austria on a walking holiday, Why are the pylons on ski lift's not vertical? I only have A level physics but surely centre of gravity and gravitational force must put tremendous pressure on the back of the pylon, as it will naturally want to lean fwd. My friend says this is just good mechanics and physics and that it wouldn't fall fwd because of tension in wires that pull cars. That don't make sense to me. Have they just engineered it so it won't fall over i.e. Big foundations, the lean of the pylons makes pulling cars easier up the hill? Any help appreciated
Colin

 

[berkeman]

I am in Austria on a walking holiday, Why are the pylons on ski lift's not vertical? I only have A level physics but surely centre of gravity and gravitational force must put tremendous pressure on the back of the pylon, as it will naturally want to lean fwd. My friend says this is just good mechanics and physics and that it wouldn't fall fwd because of tension in wires that pull cars. That don't make sense to me. Have they just engineered it so it won't fall over i.e. Big foundations, the lean of the pylons makes pulling cars easier up the hill? Any help appreciated
Colin

Welcome to the PF.

Can you post a picture? I looked on Google Images, and most of the ski lift towers appear to be vertical.

 

[Colin Charman]

Thanks and thanks for replying

Sorry no pics and I'm quite annoyed now that I haven't taken one. I walked under the Maiskogel cable car lift yesterday, pylons and concrete base of vertical by 10 -15 degrees, today went up the 3 Kitsteinhorn lifts to summit, on steeper slopes, pylons again 10 - 15 degrees of vertical. I will get picture tomorrow hopefully

 

[PhanthomJay]

As berkeman notes, many are vertical. I believe inclined tower pylons may be used on steep inclines in order to keep the resultant cable loading axial along the inclined axis of the pylon, resulting in a less expensive tower and foundations that are subject to reduced overturning moments. On a steep incline, horizontal cable tensions are unbalanced at the support, since the cable direction changes sharply. This puts a resultant non-vertical (inclined) load on the supports. By orienting the tower in the direction of this resultant load, the tower and its foundations are subject primarily to axial loads with minimal transverse loads that place overturning moments on the foundations. There may be other reasons I am not aware of.

 

[jerromyjon]

Hi, I don't think there is any way to "gain" any advantage by the orientation of the cables, it still takes the same work to pull the loaded chairs up to whatever elevation the people get off at and the car and cables proceed back down the other side balancing out all but the riders and friction. I worked on top of a ski resort 25 years ago and you'd think I would have some recollection of the pylon configuration, but I only rode them at the time.

 

[berkeman]

Hi, I don't think there is any way to "gain" any advantage by the orientation of the cables, it still takes the same work to pull the loaded chairs up to whatever elevation the people get off at and the car and cables proceed back down the other side balancing out all but the riders and friction. I worked on top of a ski resort 25 years ago and you'd think I would have some recollection of the pylon configuration, but I only rode them at the time.

Maybe draw the FBD so you can see what PhantomJay is saying...

 

[jerromyjon]

Maybe draw the FBD so you can see what PhantomJay is saying...

I don't know what you mean by FBD, but I understand what Jay wrote.

the lean of the pylons makes pulling cars easier up the hill?

This is why I posted, because there is no mechanical advantage to be gained by the angle of the pylons or the angle of the cables they support.

 

[berkeman]

FBD is Free Body Diagram. They are used to work out forces and moments in structures. As PhantomJay was saying, it looks like they are trying to keep the loading on the pylons axial (and not bending)...

 

[PhanthomJay]

I don't know what you mean by FBD, but I understand what Jay wrote.
This is why I posted, because there is no mechanical advantage to be gained by the angle of the pylons or the angle of the cables they support.

Generally speaking, I agree, although perhaps there may be some geometric advantages (avoiding interferences) in the traveler attachment support details at the pylon arms. But the main function of the inclined towers would be for the foundation design to avoid shears and overturning moments( which with vertical pylons could nevertheless be overcome with proper foundation design).

 

[jerromyjon]

The inclined plane reduces stress on the cables, and therefore the supports, also reducing friction, but you're still doing the same work.

 

[insightful]

Since the cables go over rollers at the pylons, the only force they can exert on the top of the pylon is perpendicular to the cable. Leaning the tower away from the hillside keeps the force more axial down the pylon (as stated above).

 

[Colin Charman]

Thanks everyone for getting the physics minds working on this, here is a pic of a small chair lift pylon, my walk yesterday unfortunately did not go near any of the bigger cable cars, some of which are the same - off vertical pitch (Sorry photo posted at 90 degrees -couldn't figure out how to reorientate from my phone). I sketched a diagram on a napkin the other day of the diff loads and I still think there is more forces when added together acting to pull the pylon structure down the hill than if the pylon was vertical. Do they just super engineer the base do it can take this extra force, the tower at an angle then helps the cars travel up hill?
Sorry if I am being naive and obvious!
Colin

 

[jerromyjon]

Since the cables go over rollers at the pylons, the only force they can exert on the top of the pylon is perpendicular to the cable. Leaning the tower away from the hillside keeps the force more axial down the pylon (as stated above).

The pylon appears roughly perpendicular to the cable, which makes sense to me.

 

[jerromyjon]

As for the FBD I just play cut the rope in my mind and it's obvious. Cut the pylon and which way would the cable sag.

 

[jerromyjon]

It also appears the design was chosen to minimise the impact of free-standing structure weight compared to cable weight.

resulting in a less expensive tower and foundations that are subject to reduced overturning moments

 

[PhanthomJay]

Thanks everyone for getting the physics minds working on this, here is a pic of a small chair lift pylon, my walk yesterday unfortunately did not go near any of the bigger cable cars, some of which are the same - off vertical pitch (Sorry photo posted at 90 degrees -couldn't figure out how to reorientate from my phone). I sketched a diagram on a napkin the other day of the diff loads and I still think there is more forces when added together acting to pull the pylon structure down the hill than if the pylon was vertical. Do they just super engineer the base do it can take this extra force, the tower at an angle then helps the cars travel up hill?
Sorry if I am being naive and obvious!
Colin
View attachment 84702

Nice photo , those hills of Austria are impressive! I can't see the cable that well, but on the right of the pulley traveler, it more or less departs horizontally, while to the left of the pulley, it departs sharply downward. Since the cable tension on each side of the pulley is more or less equal, but acting in different directions, it's resultant force acts downward along the bisector of the obtuse angle between the cables. By inclining the pylon 'tower' along this bisector, the
Cable load from the resultant force, which can be quite large due to the high tension in the cable (which is a function of cable weight, equipment weight, etc.) acts along the inclined axis, eliminating overturning forces along line. The tower is quite strong across line due to the spread truss design, in order to resist wind forces and other across line loads, but along line, the tower has little strength due to its narrow base, so these along line forces on the tower are minimized by orienting the tower as shown. It is true that the tower weight itself now tends to produce overturning downhill, but the tower weight is rather small by comparison. I can't tell if the tower sits on a foundation or whether it is directly buried in the ground with a grillage to resist uplift and bearing forces, but in any case, costs of foundation design is smaller than the vertical tower case.

 

[Colin Charman]

Comprehensive Phantomjay, thank you for your thoughts an effort. This photo wasn't a usual cable car pylon, just a wee ski lift up a tiny grassy slope, most of the cable car systems went up really steep slopes and had several very large towers that were all on the piss or sloped, would that make any difference to loads? (no horizontal section of catenary in cables) .

By the way Austria is a fine fine place for walking and beer x

 

[PhanthomJay]

Comprehensive Phantomjay, thank you for your thoughts an effort. This photo wasn't a usual cable car pylon, just a wee ski lift up a tiny grassy slope, most of the cable car systems went up really steep slopes and had several very large towers that were all on the piss or sloped, would that make any difference to loads? (no horizontal section of catenary in cables) .

By the way Austria is a fine fine place for walking and beer x

es muss ein Stiegl sein!. Pity I never tried it.
Even when the tower is on a steep slope with an adjacent tower downhill and the other adjacent tower uphill, there is still an unbalanced horizontal tension component uphill. Depending on span lengths and cable sags, the departure angle downhill is steep, and the departure angle uphill may not be horizontal, but it is much less steep since the low point of the cable sag on the uphill side is close to the tower (and conversely, the low point of the sag on the downhill side is far from the tower). So you still get a resultant load acting along the axis of the tower appropriately inclined along the approximate bisector
of the angle between the downhill and uphill cable.

 

[dragoneyes001]

the pylons are set to take the down pressure the cables create on them that is not going to be perfectly straight down but more in line with the surface they are on which is a slope and the direction the cables are traveling in towards the next pylon. the most efficient position needs to take the cable travel as well as the surface into consideration.the other forces like the cables movement and the cars riding on the cables add to the direction and angle the pylon is set at. in a building a post is most efficient supporting a horizontal slab when its vertical. the cable is similar to a slab but its not horizontal the slop it follows changes as it follows the mountains shape. the best way to transfer the forces to the footing are to set the pylons at angles which take the angle of the cable into consideration.

 

[Mathi-429]

I pretty sure the reason is to help distribute the pressure the best it can, mean while making sure the rope hits the pylon wheels as flush as it can.

 

[dragoneyes001]

on most lift pylons the wheels are on swivel mounts because the cars and cable moving changes the angle they need to be at.