Web Exclusive: Escalator Step Band Analysis
by Dr. Ali Albadri
The step band in a heavy-duty escalator is subjected to higher levels of stress and strain compared to that of a step band in a light-duty escalator. Heavy-duty escalators are often designed and built for public-transportation systems, which service high volumes of passengers, particularly during peak travel times. In addition, heavy-duty escalators often have high rises (i.e., 10 m or higher), and operate for long hours (i.e., 20 hr. per day).
An escalator consists of two shafts — top and idler shafts (Figure 1). The top shaft is driven by a chain connected to a motor through a gearbox. The idler shaft is placed in a carriage, which is pulled linearly against the trusses using spring and tension rod mechanisms. As the carriage is pulled backward, it keeps the chains/step band under continuous tension.
Authoritative policies and signage such as “Stand on the right” do not encourage uniform wear in the chains over the width of the machine, because one chain is subjected to a higher force and more wear than the other. The chains in the step band enable the steps to maintain safe and practical clearances between each other, and between the steps and skirting. The chains in a heavy-duty escalator are much larger than those in a light-duty escalator. They have wheels located at a pitch distance approximately equal to the width of the step. The two chains are linked with cross axles, where the steps are hinged. The steps are free to rotate around the axle (Figure 2).
The chains are carefully matched in length and accuracy when they are new. The two chains run with high precision on the two sprockets for the top and idler shafts (Figure 1). The high precision between the chains and the sprockets ensures the steps remain correctly aligned with each other and consistently in perpendicular orientation to the centerline of the machine.
The idler shaft is in the carriage in the bottom landing to keep the chains in continuous tension. The tension force should be equal in both chains, but, in reality, this does not happen during the operation of the escalator. The loads imposed by passengers are not applied uniformly over the step band, as they are often directed to stand on the right side. High wear in one chain allows the steps to skew to one side of the machine. Larger chains and higher rise mean more severe wear, which causes more skew in the steps. The skewing problem becomes serious when the misaligned steps try to pass through the combplates. This can have safety implications for passengers, especially when considering the possibility of the step hitting the combplate or the steps colliding.
High wear in one chain can cause the chain to have a kink, especially as it comes out of teeth on the idler sprocket (Figure 3). The creation of a kink generates significant stresses in the chain and steps. Tangent check track can stop the chain from kinking, as the chain would be forced to stay in a horizontal position. However, the force on the steps and chain could remain high. In the space between the lower landing and bottom sweep of the return side, the steps rotate to accommodate the differences between the heights of the tracks. These differences allow the steps to tilt on an angle to ease their movement at the top sweep return side. This maneuver subjects the steps to additional stress.
The impact of chain wear has highlighted the contrast between steps with stub axles and steps with cross axles. In the former, each side of the step is affected by the amount of wear in the chain nearby. Therefore, the body of the step will take the burden of accommodating the differential in the forces at both sides of the step. The steps with chain axles do not expose the body of the step to differential force, as the axle absorbs it.
Running a high-rise escalator in the up or down direction for an extended amount of time causes more chain wear than does frequently reversing the escalator. Frequent use of such monitoring devices as the smart step can be a significant and positive contribution in identifying the critical locations along the escalator, especially for the creation of maintenance schedules. New escalator design concepts could provide better operating conditions, reduce refurbishment costs and improve safety.
Dr. Ali Albadri is a chief engineer at Tube Lines Lifts and Escalators Services in the U.K. He has a PhD and MPhil in Materials Science from the University of Manchester Institute of Science and Technology (UMIST) and University of Sheffield, respectively, and BSc degrees in Nuclear Engineering from Baghdad University and Mechanical Engineering from Technology University. Albadri has published papers about materials science and concrete design instruments. Prior to joining the vertical-transportation industry, he worked for UMIST, Brunel University and Oxford University, in addition to Cookson Group, ABB, Olympus and Hydronix Ltd.