Top Drive vs. Bottom Drive
wally
22 Nov 2006
OK, OK .... I give in, No hair then, No hair now........................... 48
(my avitar pix was taken in 1990)
(my avitar pix was taken in 1990)
wally
26 Nov 2006
Yes it has gone somewhere - however........
"Getting old has its own tensions"
"Getting old has its own tensions"
snoloco
30 Aug 2013
This is my first post.
I read earlier in the skilifts.org glossary that a top drive is slightly more efficient than a bottom drive. I am guessing that the reason for this is because on a bottom drive the motor has to pull down the down-going side and pull all of the slack out of it before the up-going side moves. A top drive pulls directly on the up-going side. I have noticed that on the Kaatskill Flyer at Hunter Mountain which is a Leitner-Poma six pack with a bottom drive that along the lower portion of the lift there is up to 15 feet of cable sag. It is also a rough ride over the towers, much rougher than most LP's I have ridden. On the Bear Peak Express at Mountain Creek, which is a Doppelmayr hi-speed quad with a top drive, there is almost no sag in any parts of the lift, even when it is fully loaded.
I read earlier in the skilifts.org glossary that a top drive is slightly more efficient than a bottom drive. I am guessing that the reason for this is because on a bottom drive the motor has to pull down the down-going side and pull all of the slack out of it before the up-going side moves. A top drive pulls directly on the up-going side. I have noticed that on the Kaatskill Flyer at Hunter Mountain which is a Leitner-Poma six pack with a bottom drive that along the lower portion of the lift there is up to 15 feet of cable sag. It is also a rough ride over the towers, much rougher than most LP's I have ridden. On the Bear Peak Express at Mountain Creek, which is a Doppelmayr hi-speed quad with a top drive, there is almost no sag in any parts of the lift, even when it is fully loaded.
liftmech
31 Aug 2013
Welcome to the forum.
You're correct about the efficiency of a top drive. There are several other factors involved as well, of course. A top-drive lift can run at lower tension because gravity assists in holding the rope tight against the bullwheel. This also allows the lift to use slightly less power as it isn't trying to overcome the added friction of a super-tight line. There are many places where a top drive isn't practical, though; consider access to power or practicalities of morning lift start-up.
You're correct about the efficiency of a top drive. There are several other factors involved as well, of course. A top-drive lift can run at lower tension because gravity assists in holding the rope tight against the bullwheel. This also allows the lift to use slightly less power as it isn't trying to overcome the added friction of a super-tight line. There are many places where a top drive isn't practical, though; consider access to power or practicalities of morning lift start-up.
Peter Pitcher
01 Sep 2013
I believe that the efficiency has more to do with the location of the tension terminal not the drive. I could be wrong but I think the most efficient lift is a bottom tension and drive.
aug
01 Sep 2013
Economical or efficient ? top drive, bottom tension is the most efficient IMO. It has the lowest cwt. weight or hyd. pressure , and the top drive has the most tension. this setup is the easiest engineering wise.
mthornton
02 Sep 2013
Top drive is most efficient in terms of minimizing the required horsepower for both driving and braking. Gravity assisted tension at the top-drive bullwheel means lower tension at the bottom return, and thus overall. Having the return station provide tension means there is a lot less machinery that has to "move" on the tension carriage, making it cheaper to build.
Bottom drive requires higher tension on the bottom driving bullwheel (so the rope doesn't slip). This means that the tension at the top is much higher (gravity), and all that extra overall tension means way more friction, requiring more power to drive.
A bottom-drive lift that must provide 100% downloading is worst-case. Think of the tension required at the bottom bullwheel so the rope doesn't slip on the liner in the event of a 100% download E-Stop.
The electrical energy required to turn a top-drive ski-lift is often ridiculously small, considering the size of the machine. I also work in electric utility energy measurement, and many lifts require more energy to heat the machine-room & machinery, than to actually turn the lift. Building an energy-efficient lift may thus be more associated with designing and building a heat-energy-efficient structure for the drive-station machinery.
Bottom staging areas are often prime real-estate, so the smaller and quieter the bottom, the better. The capitalization required to run a power-line to the top (say $100/meter, or less) can often be justified by having a quiet & small bottom return station down in the village.
Bottom drive requires higher tension on the bottom driving bullwheel (so the rope doesn't slip). This means that the tension at the top is much higher (gravity), and all that extra overall tension means way more friction, requiring more power to drive.
A bottom-drive lift that must provide 100% downloading is worst-case. Think of the tension required at the bottom bullwheel so the rope doesn't slip on the liner in the event of a 100% download E-Stop.
The electrical energy required to turn a top-drive ski-lift is often ridiculously small, considering the size of the machine. I also work in electric utility energy measurement, and many lifts require more energy to heat the machine-room & machinery, than to actually turn the lift. Building an energy-efficient lift may thus be more associated with designing and building a heat-energy-efficient structure for the drive-station machinery.
Bottom staging areas are often prime real-estate, so the smaller and quieter the bottom, the better. The capitalization required to run a power-line to the top (say $100/meter, or less) can often be justified by having a quiet & small bottom return station down in the village.
snoloco
03 Dec 2013
I have noticed that there has been a trend in where the drive and tension are located based on the age of the lift. The oldest lifts were always bottom drive top tension. I think that they chose this because they did not know how to tension a drive terminal yet and they didn't know how to put a drive terminal at the top. Then they figured out that bottom tensioning was more efficient and made most lifts bottom drive bottom tension. Then they figured out how to make a top drive and the choice was the customer's as to where the drive and tension was. Another thing I have noticed is that tensioning on a lift is almost always at the bottom. The only times it won't is if the lift is a bottom drive and the design requires tensioning at the return, or if it is a top drive and tensioning is required at the drive. Am I correct about these things?
