If you thought all elevators ran with cables, you wouldn’t be alone. These types of elevators, called traction elevators, were the original configuration for passengers needing a “lift” as far back as the third century BC. There were steady improvements over time, but the roped elevator took a giant leap forward in 1852 when Elisha Otis cut the cable on an elevator platform suspended at the New York World’s Fair. The elevator did not crash due to a new device he invented called a roped safety. Thus the modern elevator was born.
Since then tens of thousands of traction elevators have been installed and they have become the image that pops into our heads when we think about how elevators work. This maybe because they are the ones we see in movies. Roped elevators are suspenseful when the cables are down to their last thread and about to break, threatening to drop the hero/heroine down a 50 story shaft. The scene usually involves dramatic music, fire and a longer than necessary goodbye or a steady stream of one liners that make elevator people cringe.
However, if you have the chance to look at the top of an elevator car with a certified elevator technician, in a two to five story building you would more than likely not hear any dramatic music, see flames or tearful goodbyes, but you would also not see any ropes or cables. It is because for low-rise buildings, hydraulic elevators make more sense than their traction counterpart.
Hydraulic elevators first came into use with cargo and limited passenger use in the early 1800’s and did not function with hydraulic oil. Instead they used water to push the piston and car up and released the water when they wanted it to go down. In the 1970’s the use of hydraulics took off and now approximately 70% of all elevators sold for new building construction are hydraulic as they have become the preferred vertical conveyance for low-rise projects. For shorter buildings they are less expensive to install, and require less maintenance over time due to fewer moving parts.
When first introduced, the oil-hydraulic elevators required a piston in the ground, centered under the elevator car. A power unit, which is a big tank with a pump, motor, and valve in it, pumps oil into the piston to raise the car, and lets the oil back out into the tank to lower it.
The piston needs to be as long as the total distance the car travels, so you can see that as you add more floors, the piston or jack, can get unwieldingly long. Likewise the hole the jack would be encased in, would need to be unwieldingly deep. You can extend the reasonable height by using a 2-stage jack, which telescopes up in 2 stages, and thus requires only half the hole of a single-stage jack. Three and four stage jacks are also available, but at some point they just get too expensive, and traction elevators makes more sense economically.
Also in the 70’s hydraulic elevator technology improved with the creation of jacks that don’t require a hole. Holeless jacks come in pairs, one for either side of the car. Instead of pushing up from below the car, they attach to the sides of the car sling. When you get over 15-16′ of travel, the holeless jacks get too long (they poke way up above the car), so those telescope into 2, 3 or 4 stages as well. As with in-ground jacks, holeless telescopic jacks eventually get more expensive than traction equipment.
And that’s as complicated as hydraulic elevator equipment gets. The “drive system” part of these elevators is a jack (or two), connected via piping to a big power unit tank, that fills the jack to raise the car. There is other equipment in the hoistway and on the car, but it’s the same equipment that every elevator has. We’ll post soon about traction elevators, which are more complicated, but can also go higher and faster.
One last piece of advice: if you get to take a peek at the top of your elevator car, with the help of your certified elevator mechanic, don’t be disappointed if you don’t see ropes, or flames. Be relieved.