How Rail Car Doors and Lead Screws Keep Commuters Safe

Rail car door systems are historically one of the most complex, inconsistently performing, and potentially unsafe components in rail transit systems worldwide. Yet safety is of utmost importance when human lives are at stake in accidents that may result in injury and/or death.

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Rail car door systems are historically one of the most complex, inconsistently performing, and potentially unsafe components in rail transit systems worldwide. Yet safety is of utmost importance when human lives are at stake in accidents that may result in injury and/or death. Key system design considerations affecting rail car door operation include:

  • Safety;
  • Performance/reliability;
  • Environmental/climatic considerations;
  • Equipment availability;
  • Functionality;
  • Service life;
  • Deteriorated service conditions;
  • Maintenance/troubleshooting/repair;
  • Adjustment and component replacement requirements;
  • Replacement parts availability.

Design and construction of lead-screw-driven rail car door systems need to prevent any single point, component-level malfunctions that create unsafe conditions. In addition, door systems require safety interlocks that prevent the train from developing traction if any train doors are not fully closed, latched, mechanically locked, and isolated. Another key requirement is that the door system operates safely over a wide range of operational, environmental/climatic, and varying rail conditions.

Incorrect opening of train doors are a huge safety problem that promotes the use of a high-lead back driving screw to thwart injuries and death. Research conducted by the Transportation Research Board (TCRP Project J-6, Task 62, “Field Door Survey Project”) seven years ago that identified train-door failures as the biggest single cause of delay and disruption of rail transit service bolsters this claim. Combining all of the aforementioned elements with sound engineering design practice and experience yields a safe effective solution with profitable results.

rail-car-doorFigure 1 illustrates one possible lead screw design solution for this challenging and demanding door system application. From a mechanical standpoint, this is an application rich with linear motion components arranged to attain a cost-effective scheme to a dilemma as old as rail transportation.

Lead Screw Rail Car Door System

Right and left hand individual lead screws (or a single twin-lead screw) with machined ends are connected through a flexible coupling and supported in simple or fixed bearing mounts. Proper wet or dry lubricated lead screw(s) are critical for this type of application and are achieved with either PTFE dry Teflon coating, spray lube, or grease. Lubrication intervals are driven by the application and require a frequency that ensures a thin film covering of lubricant on the screw. Wide arrays of lead screw nuts manufactured from standard polymers such as PEEK, Torlon®, Acetal, Polypropylene, Turcite®, and custom engineered polymers that incorporate fillers like PTFE, PFPE, silicone, carbon fibers, graphite, glass fibers and molybdenum disulfide provide great latitude in custom engineered door rail system solutions.

These components employ off-the-shelf readily available linear motion components. Both standard and custom flangesare threaded on lead screw nuts and securely pinned to prevent disengagement during service. A drive hanger containing linear bearings and supported by a hardened and ground shaft is mounted to each of the doors and each is translated by the lead screw nuts.

Consistently performing mechanical components including lead screw(s), nuts and flanges, bearing supports, linear bearings, and shafts provide safety and cost-effective, time-proven door rail system designs.