Incorrect upthrust track specification setting causes escalator steps to pile into combplates

Incident Investigation

Incorrect upthrust track specification setting causes escalator steps to pile into combplates

April 1, 2018

Elevating Devices

Reference Number:


Incident overview

Escalator pile up, steps piled into combplates at the top end.

Regulated industry sector

Elevating devices - Escalator or moving walkway



Investigation conclusions

Site, system and components

Inside an escalator, a pair of chains wraps around two pairs of gears that are driven by an electric motor. The motor and chains sit within a metal structure known as the truss, which extends between the floors served by the escalator. Within the truss the escalator houses steps and tracks, which allows the conveyer like rotation to travel through the truss with precise alignment and precision.

Failure scenario(s)

Licenced mechanic failed to properly investigate grinding noise.

Upthrust track was not set to the correct specification which could allow the step rollers to ride higher before entering into the combplates.

The hold down track uses a clip on the step roller to hold the step down. When the track is not set to the correct specifications, this could allow the step to rise more than 1mm. The combination of the hold down track and the upthrust track outside of specifications could allow the step to rise and come into contact with the combplates and upper landing deck.

Facts and evidence

Onsite investigation:

  • Unit had two service call backs prior to the escalator pile up. See figure 1.
  • The proper adjustment for the upthrust track is 1mm. This escalator had the upthrust track set to 6mm. (5mm above maximum allowance)
  • The hold down track was allowing the steps to rise 4mm. (3mm above maximum allowance)
  • Through conversation the Field Mechanic made the Safety Officer aware that he did not have the upthrust track specifications for allowances. Field Mechanic was not provided the documentation to maintain those specifications.
  • Licenced contractor after discussion would be providing training and documentation to maintain specifications.
  • Field Mechanic was not trained or checking for these allowances at this time as they were not provided the specifications.

Causes and contributing factors

It is highly likely that the upthrust track and the hold down track where not set to the correct specification after a recent step chain and step replacement, which lead to in correct tolerances causing the pile up.
The contributing factor was that the Licenced contractor failed to provide field mechanics with the escalator specifications and tolerances therefore leading to the mechanics not setting the tracks to proper OEM specifications.


  • Injury
    • Qty injuries: 0
    • Injury description: None
    • Injury rating: None


  • Damage description: 3 damaged steps, step axial, upper comb plates, up thrust track, hold down track upper landing plate and step chain.
  • Damage rating: Major
  • Incident rating: Major



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