Notes for FSP0023 – Elevators Part 1
Mostly I’m talking about the modern automatic passenger or freight elevators we commonly see in commercial and residential buildings, although the basic concepts might apply to any kind of elevator.
Types of elevators
There are 2 basic types of elevators (Traction and hydraulic) (although in the last few years there have been developments of what we might call a third type, which is an elevator that uses electromagnetic propulsion (I’m recording this in late 2019).
Other than the details of the mechanisms of vertical movement (which I’ll talk about), most elevators have the same basic parts, so I’ll talk about the parts first.
Parts of an elevator – basic idea of an elevator is that we have a box that people ride in, and that box moves up and down an elevator shaft (also called a hoistway) to bring the people to the different floors on the building.
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Car (maybe cab or carriage) – that’s the box part of the elevator that people ride in. This is a box with doors on it that moves up and down. There are some controls inside the allow the riders to choose their floor. You’ve likely seen and been in elevators of various shapes and sizes and made out of different materials. Most cars are rectangular metal boxes, although some are round or other shapes and some are made out of other materials, like glass to give the riders a view of whatever might be outside the elevator. There is usually an access hatch on the top of the car, although contrary to what you see in the movies, it is not meant to be opened form the inside and is usually locked or latched from the outside.
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Hoistway or elevator shaft is the space in which elevator car moves (a vertical column of space). Inside the hoist way are the car, the guide rails on which the car is mounted, and various switches, sensors, and safety devices. At each floor there will be some mechanism to indicate to the elevator controller that the car is approaching the floor (so it should slow down) and also to indicate when the car is level with the floor and should stop. At the top and bottom of the hoist way there might be some limit switches to tell the controller that the car can’t travel any further.
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Guide rails – Other than the mechanical device that moves the elavator up or down (the hydraulic piston or the cables), the cab isn’t attached to anything (if it was, it wound’t be able to move up and down). Instead it’s captured on guide rails with rollers that keep the car from swinging or swaying from side-to-side so that it’s always lined up properly in the hoist way. In some elevators (mostly traction elevators, the kind where the car hangs from cables), the guide rails also provide a surface for a braking device to prevent free fall of the elevator cab if the cables were to break.
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Doors, usually 2 different types of doors
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Car doors are part of the car assembly. They keep the occupants of the elevator from falling out or getting hurt or crushed by things outside the car as the car moves through the hoist way\
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Hoistway doors at each floor. These doors are to keep people in the hallway from falling into the open elevator shaft or getting crushed or otherwise injured when the car moves past.
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Safety – generally automatic elevators are set up with various locking and interlocking mechanisms so a person inside the car can’t open the car doors if the car isn’t properly lined up with a set of hoist way doors at a landing. Also, the hoist way doors generally can’t be opened by normal means if the car isn’t lined up properly at that particular landing (again in order to prevent someone from opening the doors and falling in.
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Pit – At the very bottom of the hoistway, we have what is called the pit. This is extra space below the bottom landing where the elevator will never travel under normal circumstances. The pit provides space for various elevator components including parts that might be hanging from or attached to the bottom of the car. There are a few things you might find in an elevator pit:
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There is a last resort safety device in the pit called a buffer. The buffer is some type of spring mechanism designed to catch the car if the car were to fall into the pit. The buffer mechanism would be designed or selected based on the energy expected from a falling car (which would be based on the expected weight and maximum speed of the car as it fell into the pit). The idea is that the buffer will prevent people in the car from being killed by the impact, although my understanding is that they might be injured.
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Also in the pit, you would find the hydraulic piston and hoses or piping for a hydraulic elevator. You might also find other machinery or machine components depending on the exact elevator design.
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Water – unfortunately you might also find water in the pit. That’s generally a bad thing and should be fixed as soon as possible.
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Machine room – Next we have the elevator machine room. The machine room provides space for the machinery that actually moves the elevator. You will also typically find the elevator controller in the machine room.
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For a hydraulic elevator, the machine room contains the hydraulic power unit. This consists of a tank of hydraulic oil with a pump to pump oil into the hydraulic piston. The machine room for a hydraulic elevator has to be located close enough to the elevator pit to effectively pump oil into the piston. Typically you’ll find the machine room near the elevator at the lowest floor.
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For a traction elevator the machine room contains the traction machine (a motor and a sheave that moves the cables in order to move the car, I’ll talk about that a little more in a minute). The machine room for a traction elevator is going to be directly above the hoistway.
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Some elevators systems don’t have a machine room at all and instead include everything inside the elevator shaft. We call these machine-room less elevators. Generally these designs put the hydraulic power unit (the pump, oil reservoir) in the pit under the elevator car or the traction machine at the top of the hoistway, although I have seen at least one traction-type design with the traction machine in the pit under the cab. This machine-room less concept is enabled by advances in miniaturization of motors and is generally done for space-saving purposes.
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Controls and indicators- Elevators have various controls and indicators
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We have controls inside the car to allow riders to select the floor they want to travel to, to close the door or hold it open and to signal for help in the case of entrapment or emergency. We also have various key switches to allow service personnel or firefighters to put the elevator into special operating modes. The control panel inside the car is called the car operating panel (often shortened to COP).
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We also have indicators inside the elevator to tell the riders which floor they are on and which direction the are traveling in.
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In the hallway we have controls to call the elevator and indicators that tell us where the elevator is and which direction it is moving in. For groups of elevators we might also have an indication of which elevator will be servicing our call. It’s also common to find a firefighters key switch at a hall station which allows a firefighter with the proper key to take control of an elevator in case of emergency.
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There is also usually a control panel on top of the car to allow a service technician to operate the car from up there.
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The elevator controller receives input from all of the various controls and sensors and uses that information to decide how to move and start and stop the elevator and which calls to service and in which order. The controller logic will also decide which car to dispatch in a group elevator system. Modern elevator controllers are electronic and computerized and programmable. Older controllers might use relay logic which makes them less flexible.
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Elevator types – Now let’s talk a little more about the different types of elevators (traction, hydraulic)
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traction (hoisted from above by cables, often called “ropes” in the elevator industry)
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The traction machine (elevator motor and sheave) is at or above the top of the hoistway.
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Traction elevators have a counterweight (possibly multiple counterweights depending on shaft geometry) that balances the weight of the car. The counterweight has a weight equal to the elevator car plus some percentage of the elevator’s capacity.
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The cables are attached to the car, they they go up to the top where the motors are, pass over a sheave (a wheel with a channel around its perimeter for the cable to lay in…if you think of a pulley, the thing you might pass a rope through to change the pulling direction of the rope, the wheel in the pulley that rope passes over is a sheave), the go back down the shaft to attach to the counterweight. Basically, you can think of a long cable with the elevator car at one end and the counterweight at the other end. That whole cable assembly is slung over a sheave at the top of the hoist way. The motor turns the sheave to put more cable on one side or the other to raise or lower the car
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For safety reasons there are multiple redundant cables in this type of elevator. Generally the car will still be carried safely even if most of the cables fail.
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The counterweight and the elevator car with passengers approximately balance each other on average. The motors turn the sheaves to let the car or the counterweight fall in a controlled way, bringing the other one up. Of course, the counterweight and the car will rarely, if ever, exactly balance, because the weight of the passengers will vary widely. The difference between the weights is made up by the friction between the cables and the sheave. If there were no friction, the heavier piece would just free-fall to the bottom and stay there. The fact that this type of elevator relies on this friction is why it is called a “traction” elevator. The systems uses traction (friction) of the sheave wheel on the cables to move the elevator and to hold it in place when we don’t want it to move.
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The counterweight mechanism allows a traction elevator to be fairly energy efficient. The motors only have to move a weight that is the difference between the car and the counterweight rather than the whole weight of the car and its passengers. Gravity moves the rest.
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Some more modern traction elevators do even better by building generators into the system. Whenever the heavier half of the system is moving down (it’s basically falling in a controlled way), the elevators can use the energy from the controlled fall to turn a generator and generate electricity. It’s similar to the concept of regenerative breaking in electric cars if you’ve ever heard of that.
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Generally the elevator is hoisted with some number of steel cables, and the traction machine (that’s the motor assembly with the sheave and the friction and all that) is generally at the top of the hoist way, but there are many possibilities. Some elevators use steel belts instead of cables (the belts are said to be more efficient), or even a combination of belts and cables, and sometimes the traction machine is at the bottom of the hoistway in the pit rather than at the top (generally for smaller elevators I think).
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Traction elevators can be built to travel several hundred meters. I’m not sure exactly what the limit is, I think I’ve seen 500 meters given as a limit somewhere. Browsing around, the highest elevator I was easily able to find advertised for sale was 300 meters. The limit has at least partially to do with how long we can feasibly make steel cables.
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For very tall elevators, we start having to deal with the weight of the ropes. Steel cables are very heavy and longer cables obviously weigh more. When, in a very tall elevator, most of the rope is on one side or the other, the entire system can become unbalanced and overcome the traction mechanism. This problem has a solution called rope compensation. For this we hang an extra set of ropes from the bottom of the car. These ropes pass through a sheave in the pit. This sheave is passive, by which I mean it isn’t turned by a motor, it just acts to change the direction of the ropes. The compensation ropes pass through that sheave and go back up to attach to the bottom of the counterweight, basically mirroring the arrangement of the hoisting ropes. The compensation ropes move back and forth as the car and counterweight move so that there is always the same amount of rope on either side of the traction machine. In this way the weight of the ropes is removed from the traction balancing equation. Of course the structure and the ropes themselves still have to be able to support all the weight.
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hydraulic (pushed up by hydraulic piston from below) – limited height (usually 2-5 floors, with 8 pretty much the max) for the reason of piston sizing.
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Very simple machine, low complexity and easier installation and maintenance relative to a traction elevator
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Less energy efficient than traction elevator because hydraulic pump has to move entire weight of cab + passengers against gravity (traction gains considerable efficiency due to counterweight system).
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The hydraulic piston is in the pit underneath the car. Sometimes the piston is in a hole under the pit and sometimes it is fully contained in the pit. Depending on the height of the building, there can be just one piston or multiple pistons that operate in stages.
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Limited to 8 or so floors. After that the hydraulic piston system gets too big or too complex to be reasonable and we use a traction elevator instead.
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Magnetic. – a fairly new concept that uses electromagnets in the hoistway to move the car up or down. Since this doesn’t require any ropes or pistons, there is effectively no limit to how high a magnetic elevator can go. They can also conceivably move sideways or horizontally. I’m fairly certain these exist in the wild, but I don’t think they’re widely used yet (this is in late 2019).
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While I said there were the 2 basic types (hydraulic and traction) there are many different elevator designs, some of which even combine elements of the hydraulic and traction concepts.
If you are responsible for an elevator, you should
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Make sure the elevator is properly inspected and certified by qualified professionals based on the appropriate standards and at the appropriate time interval (usually annually but twice a year in some places).
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Have the phone number of a reliable and qualified elevator service company in case somebody gets trapped in your elevator and you need help getting them out.
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Have a set of all the keys necessary to access and service your elevator including the appropriate hoist way door release tool (drop key or whatever applies to your elevator). These should only be used by the trained service personnel. While most elevator service technicians and properly trained firefighters can probably get into your elevator without your help if there’s an emergency, it will be easier, safer, faster, and also less likely to damage your elevator if you can provide quick access to the keys.
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Not allow anybody without the proper training to work on the elevator or access the hoistway or pit. Modern elevators are very safe if used properly. People that don’t know what they are doing can hurt themselves and also potentially cause damage to elevator components making the elevator unsafe for passengers.
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Call for help if somebody gets trapped in your elevator. Rescuing people trapped in an elevator can be a very dangerous operation, so in general, you want only trained personnel to attempt the rescue. In a non-emergency situation, you’ll want to call your favorite elevator technician, and in an emergency situation you’ll also want to call your local fire department (In addition to the elevator technician).