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How to make elevators safer and maintenance-free

28 February 2016

Phil Burge takes a closer look at the various types of bearings that have particular relevance to lifts and elevators, in terms of machine construction, safety and maintenance.

The 530m tall CTF Finance Centre in Guangzhou, China, which is due for completion later this year, will house the world’s fastest lift. At 20 metres per second (m/s), it travels as fast as an Olympic cyclist, reaching the top floor in just 43 seconds.

However, speed is not everything. Lifts must be compact and safe, and require little or no maintenance. Whether they are operating in skyscrapers or two-storey apartment blocks, the systems underpinning them – ropes, pulleys, bearings and seals, for instance – must be robust and reliable.

Geared traction machines are traditionally used in elevators, and are typically quite slow, with speeds of around 2.5m/s. They require relatively small motors and are aimed at heavy duty, low speed or mid-rise applications.

A high speed electric motor drives a worm gear, which then turns the hoisting sheave to move the elevator car. A worm gear operates with two shafts: the primary (input) shaft, which ensures largely axial loads; and the secondary (output) shaft, which withstands mainly radial loads. Both must operate with minimal noise, vibration and wear. Unless the worm gear is sealed, metallic particles in the lubricant can affect performance – reducing bearing service life and machine reliability.

Several SKF bearing and sealing products can help to solve some of these challenges, whether they are new installations or system upgrades. For example, an SKF hub bearing unit, comprising a double row of angular contact ball bearings equipped with cassette seals and special grease, can be integrated with a geared traction machine’s primary shaft. It accommodates heavy axial loads caused by gear meshing and carries radial loads by supporting the shaft. It is preloaded to provide the necessary axial stiffness and maximise bearing service life. The integrated seals prevent bearing contamination, while a ‘silent running’ grease minimises noise.

One leading elevator manufacturer has used the unit to eliminate particle ingress on the primary shaft bearing of its worm gear traction machine. It protected the bearing against fine metallic particles in the gear mesh lubricant, which had been entering the bearing.

Pulleys and sheaves
Elevator pulleys and sheaves are the rolling pivot point between the elevator car and its counterweight. Linked by traction rope, they rise and fall as the motor turns the sheave one way or the other – moving the rope along with it.

The motor produces enough torque to get the sheave moving, after which the counterweight does the rest. Depending on the elevator application and load, a different number of pulleys can be used to create roping ratios of 1:1, 1:2 or 1:4.

One way to increase pulley and sheave lifecycles – and reduce maintenance – is to use SKF deep groove ball bearings with SKF’s ‘Solid Oil’ lubricant. This avoids the need for re-lubrication, while eliminating dangerous grease leakage. Within the bearing, the rolling elements and cage are encapsulated in a polymer matrix saturated with lubricating oil. As the polymer rotates with the cage, oil is released – providing lubrication for the rolling elements and raceways during operation.

This arrangement was used by an Asian elevator manufacturer as a way of avoiding initial and periodic re-lubrication in its diverter wheel bearings.

Gearless shift
Modern lifts are more likely to be gearless, which allows faster speeds and more compact designs. A good example is ‘machine roomless’ (MRL) designs, which incorporate the lifting mechanism into the hoistway rather than in a machine room located on the roof above the lift shaft.

Gearless traction machines offer faster lift speeds, but the motors require robust bearings that can handle higher loads while minimising friction, noise and wear. For MRL gearless traction systems that integrate the sheave and motor, the majority of the load will be carried on the sheave side – which places even higher stress on the bearing.

No gearbox means no gear oil, so the bearings that support the rotating shaft must be lubricated regularly. This is difficult and potentially dangerous for workers who must attend to the sheaves in very difficult-to-access locations.

One solution is to install a bearing specifically designed to increase carrying capacity, such as SKF Explorer sealed spherical roller bearings. They cut noise and vibration emissions and require minimal maintenance, which helps to keep repair and replacement costs under control.

After a three-month testing programme, one Asian elevator manufacturer switched its entire production to these bearings in order to achieve low-maintenance and a bearing life of at least 20 years. Similarly, a European elevator manufacturer used them with adapter sleeves when it needed a quickly mountable bearing on its first high-rise gearless traction machine.

Moving away from conventional ball bearings helps manufacturers to increase carrying capacity while maintaining – or even shrinking – bearing arrangements. SKF Explorer sealed spherical roller bearings have integrated seals so are virtually maintenance-free. They handle high radial and axial loads, while correcting for both misalignment and shaft deflection.

Other bearing types can be specified in gearless traction systems: open bearing arrangements in gearless traction systems, for example, can be fitted with radial shaft seals, which feature a rubber-covered outside diameter and can handle high surface roughness and thermal expansion.

Condition-based monitoring (CBM) systems, which provide early warnings of potential problems can help to minimise maintenance in modern elevator systems. While maintenance can never be totally discounted, it can be reduced to well below that of traditional time-based maintenance routines where work is carried out at pre-defined intervals. CBM also helps to reduce unplanned downtime.

Many elevator components can be monitored by CBM. One such system is SKF’s MetroCon, which helps transit operators reduce the cost of system shutdowns. London Underground has installed the system on a number of its escalators and elevators, cutting maintenance and inspection by 40 percent, and power consumption by 15 percent.

Elevator maintenance has always been challenging due to inaccessibility, the need for manual lubrication and the risks these pose to worker safety. Incorporating CBM, and systems like automatic lubrication, can help to overcome some of these traditional problems.

Phil Burge is with SKF


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