Elevator Traction Sheave
The traction sheave, also known as a hoisting sheave, is an important part of any elevator’s suspension system. The sheave is designed to grip the hoist ropes and provide traction as the sheave turns.
In a traditional gearless traction elevator, the drive sheave is large (from 0.6 to 1.2 meters [2-4 ft] in diameter), and the motor that runs it must be powerful enough to turn it at 50-200 revolutions per minute. This translates into an energy consumption rate of only half a carload at any given time.
Rope Compensation Systems
In high elevator shafts with a height of 50 meters or more an Elevator Traction Sheave (ETAS) is used to transport the car and counterweight between their extreme upper and lower positions. In the ETAS the hoisting roping and traction sheave are driven by a drive machine in connection with the car sling to move the car and the counterweight backwards and forwards in the elevator shaft along guide rails.
In order to reduce the friction between the hoisting roping and the traction sheave two compensation pulleys are situated in the top part of the elevator shaft, one on each side of the traction sheave and fitted to move in essentially the vertical direction. The hoisting ropes are led to the first of these compensation pulleys and subsequently to the second one after the traction sheave.
The arrangement can be adjusted to suit the technical features of the elevator. The balancing area in which the compensation rope is led could be shifted to a separate shaft running aside of the elevator shaft or somewhere else. In this separated balancing area the run of the compensation rope can be guided much more easily as the travel paths of the car and counterweight do not build obstructions for the technical guiding solutions.
It may also be possible to adjust the structure of the compensation element in such a way that it comprises more than two compensation pulleys, one lower and two higher than the other. The incoming hoisting ropes are led on a case-by-case basis to the lower and the upper compensation pulleys with the corresponding rotation plane of the closest diverting pulley in the bottom part of the elevator shaft, in a manner that the ropes do not disturb each other.
Another advantage of the arrangement is that in addition to the balancing area, the compensation rope can be led directly to a component of the car and the counterweight whereby it does not have to pass around the traction sheave. This can be a very effective solution for safety reasons as the sway of the compensation rope is not disturbed by the travel path of the car and counterweight.
Rope Slippage
When an elevator car moves up or down, its weight and the counterweight exert pressure on hoisting ropes (also known as wire ropes) wrapped around the drive sheave. This forces the ropes into grooves on the sheave, allowing the elevator to travel up and down the shaft.
Ropes can be made of a variety of materials, including wire and synthetic fibres. The most common material is polypropylene, which has many benefits and can be used for a number of applications. However, it is important to Elevator Traction Sheave remember that it can also be susceptible to damage and breakage if the rope is not properly maintained or installed correctly.
One way to ensure the safe installation of elevator cables is to use a cross lay construction. This type of strand construction has a special arrangement that prevents wire crossing in the strand.
This is beneficial for two reasons: It helps reduce the chance of internal wire breaks that can be incredibly dangerous, and it also reduces the amount of wear. Moreover, the cross lay construction also allows for minimal permanent and elastic elongation.
Depending on the type of elevator, this can be accomplished by passing the rope over the sheave once or twice. It is most commonly found on low-speed and mid-speed elevators that have a gearless traction motor, but it can also be used on high-speed elevators with geared traction motors for additional friction.
The most significant factor determining the rate of rope slippage is the difference in sheave groove depth/rope diameter. This can be seen by placing a straightedge across all the ropes and using feeler gauges to measure the gaps over the crown height. If the gap is different on each side, this indicates that either the rope diameter is incorrect or the traction sheave grooves are too shallow.
When the sheave groove is too shallow, the elevator’s traction rope may “slip” and grind the sheave material. This can lead to serious damage, and it should be immediately addressed by equalizing the ropes. This should be done at the beginning of each up or down run. This is especially crucial on high-speed elevators where the rope could potentially damage the sheave itself.
Rope Damage
As with any mechanical component, a rope can be subject to a variety of different types of damage. These can range from minor cosmetic issues to major structural failures, but most of these damages are preventable with proper maintenance and inspection.
Abrasive Wear
Abrasion can be caused by dragging the rope over rough ground, which can cut the inside fibers of the strands and reduce the overall strength of the rope. Similarly, abrasion from chemical exposure can also damage the rope. Consult the manufacturer for specific information regarding chemicals that can be used on a particular rope type.
Corrosion
Wire ropes that have been exposed to corrosion may exhibit a pitted surface or rust patches on the outer wires. This is an indication that the interior wires have been affected and need to be relubricated.
Fatigue Breaks
Fatigue breaks are the result of repetitive bending and torsional stresses in the wire ropes that move over sheaves. This can be accelerated by the repeated rubbing against other wire ropes and sheaves, as well as vibrations, whipping and slapping.
Internal Wire Breaks
A wire rope that works on a plastic sheave is more likely to experience internal wire breaks than a wire rope that runs on a steel sheave. This is because the plastic sheave has a lower pressure between the wire elements than the steel core. This allows the wires to nick each other during normal operation, resulting in internal wire breaks that are difficult to detect with a non-destructive test.
Kinks
A kink is a point where the wire rope has broken and will not return to its original position. These kinks can be caused by improper installation procedures, incorrect rope design or excessive torque during the rope winding process.
Sheath Slippage
A sheath that has slipped noticeably is a sign that it has become overused and must be replaced. This is especially common at gyms, where people use shared top-ropes and the overuse of the rope will cause a section to sag or bunch.
Rope Burns
If a rope is subjected to extreme friction Elevator Traction Sheave or abrasion, it can burn and compromise its strength. This is particularly true for natural fiber ropes, which are more susceptible to heat than synthetic ropes.
Rope Wear
When the rope runs over a traction sheave, the wires inside it are exposed to a complex of stress factors – tension, flexure, compression and abrasion – that are all responsible for material fatigue. In addition to this, corrosive media can affect the wires as well.
As a result, wire ropes should be tested regularly for proper tension and wear patterns. This can be done by examining the length of a rope that hasn’t been subjected to any bending work, or by comparing a rope section with a similar length to one that was subjected to some bending.
This is important because it ensures that the rope can be replaced before any serious damage occurs. It also makes it easier to determine the remaining service life of a rope.
If the strands of a wire rope become flat on the outside due to abrasion (which is normal), the rope should be replaced immediately, as this can cause the sheaves to wear prematurely and reduce the overall life of the sheaves and the elevator. This is why many manufacturers of wire ropes recommend that the outer wire diameter of the rope be checked for wear.
The ropes in traction drive elevators are subjected to very high levels of contact pressure when they run over the grooves on the traction sheave, which can lead to uneven wear. In a rope construction with an outer wire layer that is particularly thin, the resulting high contact pressure can even lead to internal wire breaks in the rope.
In these circumstances, the optimum rope construction can only be achieved with special design considerations. Such considerations include the cross-sectional area of the strands and the number of wires in each strand as well as the degree of elasticity and the core material.
Polyester, for example, is a very good choice as it has a relatively high strength and stretches little, and can take a lot of abrasion. It is also able to handle a variety of weather conditions, and doesn’t shrink after being wet.
Nylon, on the other hand, is a strong fiber that can stretch quite a bit and can absorb energy. However, it can be very sensitive to temperature. This can be a problem for ropes used in harsh environments, like those found in the oil and gas industry.
