How Elevators and Lifts Work: Traction, Hydraulic and MRL

An elevator looks simple from the inside. You press a button, wait, step in, step out. Behind the doors sits a quiet machine built from a few plain ideas, repeated with a great deal of care. This page explains the three common ways a car is moved (ropes over a wheel, a hydraulic piston, and ropes with the machine tucked into the shaft itself), then names the parts that guide, stop and supervise it. We use the traction elevator as the running example because its parts are the easiest to name, and the other types are variations on them. In the UK and much of the world the same machine is called a lift, and the two words are interchangeable here.

A few terms first. The car is the box you ride in. The hoistway (or shaft) is the vertical space it travels through. A landing is any floor where the car can stop and the doors open. Everything else on this page hangs off those three words.

Traction elevators: ropes, a sheave and a counterweight

In a traction elevator, steel ropes pass over a grooved wheel called the sheave. The car hangs from one end of the ropes. A counterweight, a stack of heavy metal blocks, hangs from the other. Washington State's Department of Labor and Industries defines the type in a rulemaking document as an elevator "in which the friction between the hoist ropes and the drive machine sheave is used to move the elevator car."

That word, friction, is the whole trick. The ropes are not wound onto a drum. They grip the grooves, and the weight of the car and counterweight presses them into place. A student report from MIT's course 2.972 draws out the consequence: power is needed only to move the unbalanced load between car and counterweight. A seesaw works the same way, and so does a well-balanced elevator.

The balance is chosen on purpose. A paper from the U.S. Mine Safety and Health Administration, which covers mine elevators and is therefore a specialized case, describes a counterweight sized to equal the car's weight plus about 45 percent of its rated load. Under that arrangement a car carrying a little less than half its rated load nearly balances the counterweight, and the motor mostly corrects the difference. Other designs pick other fractions; this is one published example, not a rule.

Geared and gearless machines

The motor that turns the sheave comes in two arrangements. In a geared machine, the motor drives the sheave through a gearbox. In a gearless machine, the sheave is attached directly to the motor. A 2019 University of Mississippi honors thesis on elevator safety design notes that geared machines typically cannot reach the speeds and heights that gearless ones can, which is why most high-rise buildings use gearless traction.

The MIT report fills in what a geared machine does. It describes a motor plus, most commonly, a worm gear reducer (a gear pair that turns rotation through a right angle), which lowers rotational speed and raises torque. It gives typical reduction ratios of 12:1 to 30:1, and a typical motor-and-gearbox efficiency of about 60 percent. That efficiency figure comes from a worked example of a residential-sized car, not from a survey of buildings, so read it as an illustration.

The same report describes the brake that holds the machine still. A compression spring applies it, and a solenoid (an electromagnet) pulls it away from the shaft when energized. Put plainly, the brake sets when power is removed. The motor itself may be alternating or direct current; the MIT report says direct current gives good starting torque and easy speed control, while alternating current is more common for ruggedness and simplicity. Taller buildings and the machines that serve them are covered further in tall buildings and modern elevator innovations.

Hydraulic elevators: a piston, a pump and oil

A hydraulic elevator trades ropes and counterweight for fluid pressure. The Mississippi thesis describes it as plunger-driven: a piston below the car pushes it up, and an electric motor forces oil (hydraulic fluid) into the piston's cylinder. The moving piston is often called the jack or plunger. To lower the car, oil is released back through a valve, a description that appears in a City Tech course page drawing on a building-systems textbook by Wendell Edwards.

The same course page lists the usual variants. An in-ground cylinder extends below the pit as far as the car rises, which means deep excavation. A hole-less design uses telescoping pistons beside the car and is typically limited to under 40 feet of travel. A roped hydraulic design lifts the car with a rope pulled by pistons, much as a traction machine does. The page gives typical hydraulic use as low-rise buildings up to about 50 feet or five stories, at speeds of 25 to 150 feet per minute.

Because there is no counterweight, the lifting energy is not recovered on the way down, so hydraulic machines are less efficient than traction machines according to the course page. In exchange, the Mississippi thesis says they are usually cheaper to install and maintain, though not favorable for high-rise buildings. A machine room holds the oil tank and pump; the thesis places it on the lowest floor next to the shaft. Older installations and local rules vary, so treat these as typical descriptions, not guarantees about any one building.

Machine-room-less (MRL) traction

Traditional traction elevators keep the motor, brake and controller in a dedicated machine room, usually above the shaft. A machine-room-less design, abbreviated MRL, is still a traction elevator, but it puts the compact machine inside the hoistway so no separate machine room is needed. The Mississippi thesis calls this a good choice for mid-rise buildings because it saves space and energy while offering similar operation and reliability to a traction elevator with a machine room.

The thesis adds two useful cautions. First, a machine room is more common in the United States because building codes in many places do not allow the drive system inside the hoistway, and those codes are slowly being updated. Second, even where there is no machine room there is still a control room, which the thesis places on the top floor and within 150 feet of the machine, housing the control boxes. So MRL does not mean the machinery vanishes. It means the machinery lives somewhere else.

Which of these three systems a building gets depends on height, budget, space and local code, and those decisions belong to designers and the authority that enforces the code where the building stands. Nothing here is a recommendation for any particular building.

What the car and hoistway contain

Guide rails

Two sets of vertical steel tracks run the height of the shaft. The Mississippi thesis lists car guide rails, which guide the car, and counterweight guide rails, which guide the counterweight. They keep both from swaying and give the safety gear something to grip.

The governor and the safety

The overspeed governor is a speed monitor. In the thesis's description, flywheels inside it lock when the speed passes a set limit, and that action pulls levers on the safety gear attached to the car. The safety gear is designed to grip the guide rails and stop the car with enough deceleration to halt it but not so abruptly that riders are hurt.

The thesis also sorts safety gear by speed. Below a rated speed of 0.76 metres per second, an instantaneous type, which bites quickly with no flexible element, is acceptable. Up to 2.5 metres per second, instantaneous gear with a buffering effect may be used. Above 2.5 metres per second, or for any duplex (two-part) arrangement, progressive gear that limits the retarding force must be installed. The MSHA paper adds history: it credits the instantaneous type to Elisha Otis's 1853 design, and the progressive type to later work for cars still held by intact ropes. If you wonder what actually happens when a rope fails, elevator myths: cable snaps and free fall takes that question on directly, and a short history of the elevator covers where the safety brake came from.

Buffers

At the bottom of the hoistway, beyond the lowest point the car and counterweight normally reach, sit buffers. They absorb or dissipate the kinetic energy (energy of motion) of a car or counterweight that travels too far. The thesis lists spring, oil and polyurethane types. They are a last backstop, not a routine stopping method.

Doors and the door interlock

Each landing has hoistway doors, and the car has its own. The thesis notes that car doors are usually fitted with a sensor to avoid trapping people or objects, and that doors should open only when the car is stationary at a floor. Behind that rule sits the door interlock, a combined latch and electrical switch. A U.S. patent background on the subject states that the hoistway doors must be locked closed when no car is at the floor, and that when a door is open the control circuit prevents the car from moving away from that floor. Interlocks are safety devices for qualified people to service. This page describes what they do, never how to defeat one, and riders should never try to open doors themselves; elevator, lift and escalator safety explains the protective layers in more detail.

Operating panels

At each landing there is a call panel (a button, or a pair for up and down). Inside the car is the car operating panel, with floor buttons, door controls and an emergency phone or call button. These panels are the only parts most riders ever touch, which is why they are designed to be simple.

Basic control: call, dispatch and leveling

When you press a call button, the request goes to the controller, the computer-like unit that runs the elevator. The Mississippi thesis describes the controller as a processing unit that performs diagnostics, analyzes traffic and controls the elevator's functions, located in the control room or the machine room if there is one.

Dispatch is the controller's choice of which car answers which call. In a single-car building the choice is trivial. In a bank of cars it becomes a scheduling problem, and a good deal of effort in tall buildings goes into solving it well.

Leveling is the final adjustment that brings the car's floor into line with the landing floor. It matters for a plain reason: the thesis lists misalignment between car and floor as a hazard that can lead to passengers tripping or falling. The controller, drive and brake work together to stop the car precisely, and the brake described earlier holds it there while the doors operate.

Who checks all of this, how often, and under which code, differs by place. Inspection, maintenance and regulation explained covers the general structure. Codes, enforcement and rules differ by jurisdiction, and the local authority and qualified professionals have the final word on any real building. For anyone actually stuck in a car, the emergency phone and emergency services come first, not anything on a page like this.

The short version

Most elevators move a car by one of three methods: ropes over a sheave with a counterweight, a hydraulic piston, or a traction machine mounted inside the shaft. Guide rails, a governor and safety gear, buffers, interlocked doors and a controller surround the car whichever method is used. Local codes and qualified professionals decide what any given building must have.