Building Services/Vertical Transportation/Traction Lifts
Traction Lifts
Traction lifts are the oldest and most versatile type of lift. They basically consist of a lift car and a counterweight attached to separate ends of a cable which travel over a large pulley wheel called a Sheave at the top of the lift shaft. The counterweight and the lift are intended to balance each other out which means that only a small amount of power is needed to move the lift car up and down. When the lift car goes up, the counterweight goes down and vice versa.
The lift car and counterweight are usually mounted on guides which are like rails that run vertically inside the shaft to ensure they do not stray from their path (otherwise mischievous passengers could ‘bump’ against the sides or back of the lift and make it swing away from its intended course. A dual purpose of these rails is for braking in an emergency (see emergency procedures).
While there are numerous disaster movies about lift cables snapping, in reality this is extremely unlikely as the suspension cables are made of high tensile steel and designed to be ten times stronger than they actually need to be. As long as lifts are inspected regularly, any damage to cables can be identified and remedied. Even if the cable was to snap, there is an overspeed governor and buffer at the bottom of the lift shaft which would stop the lift from plummeting – see emergency procedures for more information.
The lift is moved using a large electric motor that is connected to the Sheave. The design of this arrangement varies depending on the application of the lift. The traditional method has been to use a DC (Direct Current) motor attached to a worm gear. The motor runs much too fast for the purpose it is intended for so the worm gear significantly reduces the speed before transferring the motion to the sheave. An electrically operated disc brake is also connected to this axle, which is generally used for keeping the lift car stationery while not moving as while the counterweight is designed to balance the load, the actual balance is dictated by the load in the lift car (generally the counterweight is slightly heavier than the lift car to compensate for the fact that the car is usually loaded while in motion). This brake is designed to fail closed so in the event of a power failure, the brake will operate and hold the lift car where it is. See emergency procedures for more information.
In this configuration, the DC motor is fed by a generator that uses a three phase AC (Alternating Current) motor to drive it, thus converting the standard three phase AC supply that is available in most lift motor rooms to the DC that is needed to supply the motor.
An increasingly popular alternative to this ‘geared’ configuration, particularly for express lifts is a ‘gearless’ configuration where the incoming current to the AC motor is increased or reduced to adjust the output speed of the DC motor. The motor spins at the required speed, hence no worm gear is required although this configuration still features a brake.
A third alternative that has relatively recently stared being used is an AC motor, usually in a geared configuration. This eliminates the need for a generator and any DC equipment.
The motor and brake are controlled by a complex control system that is the ‘brain’ of the lift. This will be explained in the ‘controls’ module.
Traction lifts generally require a machine room at the top of the lift shaft. This is typically higher than a standard room as the top of the lift shaft protrudes into the room, terminating in a platform about 1m higher than the floor of the room that houses the motor, gears, sheave, safety governor and floor selector. Lift machine rooms must be kept locked as unauthorised persons could interfere with the equipment or sabotage it or injure themselves. A lift motor room features fast moving parts and high voltage electricity. Machine rooms should be well ventilated and heated if there is a chance of cold weather damaging the equipment and they should also have a lifting beam attached to the ceiling and a hatch in the floor so that heavy machinery can be winched out and new machinery winched in. With lift machinery having a seemingly ever decreasing lifespan, it is important to ensure that all the equipment can be replaced easily. Machine rooms should ideally be accessible by stairs although many machine rooms are only accessible by a ladder. Machine rooms should not be used for any purposes other than housing the machinery required for the lift, when lift machinery is located in a general service penthouse, it should be located in a separate and locked room.
In some cases, the machine room is located below the lift shaft rather than above it, in this case, the lift cable is a complete chain with an unpowered sheave at the top of the haft and a powered sheave at the bottom, located in a machine room.