The gearless traction elevator design permits car speeds in excess of 500 feet per minute. The counterweight, sized to equal the weight of the car plus half the weight of a car full of passengers, reduces the load on the motor. Car speed is a function of motor RPM and sheave size, typically two to four feet in diameter. To achieve proper car speed, this huge sheave turns at 50 to 200 RPM.
FIGURE 2-1 Traction elevator motor and drive. (Judith Howcroft)
In addition to the multiple lifting cables shown in Figure 2-2, safety is provided by car brakes that are engaged if the car were to begin falling at greater than a specified speed or if for any reason tension is lost on the hoisting ropes. A clamp closes on the steel governor cable and this causes brakes to engage the guide rails, stopping the car not too abruptly, but quickly enough so that it does not gain excessive momentum.
If the maximum vertical travel is greater than 100 feet, a system known as compensation is used. This consists of an additional set of cables or a steel chain, one end of which is attached to the bottom of the car and the other end is attached to the bottom of the counterweight. As the car rises, more of this chain or cable is lifted to balance the shorter amount of hoist rope between the car and sheave, and as it descends and the counterweight rises, this weight is added to the counterweight. This equalizes the amount of work required of the motor.
When compensation cables are used, an additional sheave at the bottom of the shaft keeps them in place. If they take the form of chains, they are guided by a horizontal bar.
FIGURE 2-2 In a traction elevator, steel ropes engage deep grooves in the sheaves. (Renown Electric)
In traction elevators there are several possible roping configurations. Suspension ropes attach to a hitch plate above the car, or they are underslung below, loop over the sheave, and pass down to the counterweight. Depending on the size of the car, there may be as many as eight, sometimes more, of these hoisting ropes, typically ⅝ inch diameter.
Low-speed elevators with geared motors generally have a single-wrap arrangement, where the rope passes over the sheave once and connects to the counterweight. Double-wrap is used with higher speed elevators having gearless motors.
1:1 roping consists of rope that is connected to the counterweight and travels as far as the car travels, in the opposite direction. It is used with geared traction systems and high-speed elevators. 2:1 roping consists of a sheave attached to the top of the counterweight. The rope moves twice as far as the car. This configuration is used on machine room-less, bottom-drive traction, gearless traction, and freight elevators. When higher capacity is required, 4:1 roping is used. The rope moves four times as far as the car.
In all cases guide rails are necessary. Otherwise the car, suspended at the end of the cable, would swing from side to side, hitting the walls of the shaft. In a new installation or making changes, wiring (in conduit), junction boxes and the like can be mounted on the shaft walls. It is important that they do not protrude beyond the guide rails, so they are not in the path of the car. There may be very little clearance in this area.
In today’s world, saving energy is a focus in building services. In an elevator installation, a regenerative drive accomplishes this by using the electric motor as a generator to return electrical energy to the facility and/or utility when a full elevator, which is heavier than the counterweight, descends or when an empty elevator ascends.
A traction elevator consists of steel ropes that raise and lower the car at a measured rate. The steel ropes lie in grooves milled into the sheave. These grooves serve two purposes—they keep the ropes separate and in place so that they don’t bunch up and tangle, and they provide much greater traction than if the ropes were to wrap about an ungrooved cylinder, in which case there would be only a single line of contact.
Naturally, with all the fast starts and stops, heavy loading, and continuous use, the steel ropes have to be replaced and the sheaves need to be regrooved periodically. How often depends upon various factors. In a group installation, you may be able to take one car at a time out of service in order to have the regrooving done and/or ropes changed. If the building has only a single car, you’ll need to do some careful planning and scheduling. For maximum rope and sheave life, it is important that all ropes have equal tension. Ropes should be checked frequently for signs of wear. All ropes should protrude an equal amount from their respective grooves. Worn sheaves cause premature wear on ropes, which then accelerates sheave wear, creating a mutual destruction scenario. Frequent inspection and maintenance as required are essential. An advantage of the traction elevator is its very safe and efficient braking system. The brake, as in automobiles, may consist of a large drum with brake shoes or a smaller disc with pads. The brake assembly is located close to the motor and/or gearbox. It is electrically actuated. Power is required to keep the brake disengaged, and when power is interrupted, the brake is applied. Thus, in a power outage, the brake is applied so the car cannot move.
Power is applied and interrupted simultaneously at the motor and at the brake. Power is interrupted in an outage when the motion controller senses a serious fault or shuts down for some other reason, when a human shuts off the disconnect, or when the car is intended to stop at a landing. If the motor were to stop without the brake being applied, the net weight of the car and counterweight would probably, depending on the gearing and loading, cause the motor to spin, allowing the car to move. Conversely, if power were to be interrupted at the brake but not the motor, the brake would rapidly heat with all that would entail.
In limited applications, the simplicity and user-friendly nature of hydraulic elevators makes them the preferred choice.
The most common installation, shown in Figure 2-3, involves a large hydraulic piston that extends underground beneath the building to a depth equal to the amount of car travel, measured at the bottom of the car floor. In practice, this type of installation is limited to not much over four floors, so it is never seen in high-rise buildings. That being said, they are often preferred in low-rise buildings where there is no bedrock. There are boring techniques that permit a hydraulic cylinder to be installed in a retrofit situation under an existing building.
An underground oil leak, particularly where there is a nearby aquifer, is a major environmental catastrophe. For this reason, some manufacturers have discontinued hydraulic elevators, while others have developed high-performance cylinder liners to contain any oil that could otherwise escape.
FIGURE 2-3 A below-grade hydraulic piston drives the car to a maximum height of about four stories.
In low-rise applications, hydraulic elevators are widely used and there are a great many currently in service. Some have above-ground cylinders, which solves a few problems but gives rise to others, such as dedicated space that is required, and greater complexity due to the nature of the hybrid traction and hydraulic design. Some hydraulic designs have telescoping