Rail Joint Step on a New MRL Installation
A new six-stop machine-room-less traction elevator is being commissioned in a hotel. Rated load is 1,000 kg and contract speed is 1.75 m/s. During initial runs the car sways laterally between floors 3 and 5; the sway is worse at contract speed and with about 75% load. There are no faults. Guide shoe clearance is 1.0 mm on the left side and 0.5 mm on the right at the car top. Rail records show plumb within 1.5 mm over 5 m, but the rail joint at floor 4 has a 1.2 mm step and a visible kink. The bracket spacing at that joint is 3.8 m, while the installation drawing specifies 3.0 m maximum, and one bracket is shimmed with washers. One suspension rope is 15% below the average tension. The general contractor wants to hand over the elevator in five days.
1. How should you prioritize the possible causes of the sway?
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Prioritize the rail joint and bracket spacing first, then rope tension, then guide-shoe clearance. The joint step and kink are local geometric defects that the guide shoes must follow; at contract speed and under load, the car is pushed sideways each time it crosses the joint. The 3.8 m bracket spacing exceeds the 3.0 m maximum, so the rail is less restrained exactly where it is already disturbed. The washer shim is also a warning that the bracket was fitted without a proper shim pack and may not be stable under dynamic load.
The rope tension imbalance is a contributing cause because uneven rope lengths load the car unequally and can produce a rhythmic sway, but it is less likely to be the primary cause when the symptom is concentrated at one rail joint. Guide-shoe clearance is also relevant: 1.0 mm on one side versus 0.5 mm on the other means the car is not centered in the guide path, so a rail defect will translate into car motion. The five-day schedule is not a reason to accept a dynamic rail defect; handover should follow correction and retest.
2. What correction sequence would you require, and what should be checked after each step?
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Require the rail to be corrected before rope tensioning is used to chase the sway. First, re-align the joint to remove the step and kink, and correct the bracket spacing to the drawing, using proper shims and confirming bracket tightness. Check rail plumb and joint alignment after the work, not just the original record. Second, re-tension the suspension ropes to equal tension and verify that the car is centered and the guide-shoe clearances are within the manufacturer's range on both sides. Third, run the car at inspection speed through the affected zone, then at contract speed empty and with 75% load, and observe whether the sway is gone. If a residual sway remains, check guide-shoe alignment, rail lubrication, and car balance before considering any further adjustment. Do not use controller gain or ride-quality settings to hide a mechanical defect.
3. What acceptance evidence should be provided before the general contractor takes handover?
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Provide a close-out package that ties the correction to the original symptom. Include the rail joint alignment readings before and after correction, bracket spacing and shim details, rope tension readings before and after, guide-shoe clearance measurements, and the date and names of the personnel who performed the work. Include ride-test results at the affected floors with empty and 75% load at contract speed, and note whether the sway was reproduced. If the manufacturer has a ride-quality or vibration criterion, record the measured values against it. Also include a statement that no controller tuning was used to mask the mechanical defect. Handover should be conditional on the loaded ride test being acceptable, not merely on the elevator completing a few empty runs.