An elevator is not a single machine; it is a collection of subsystems that each have one job. When a car stops between floors, the fastest diagnosis comes from knowing which subsystem could physically produce that symptom – not from starting at the controller and working outward.
The parts, in power order
| Subsystem | Job | Typical failure you will see |
|---|---|---|
| Traction machine | Moves the car and counterweight by driving the sheave | Noise under load, brake drag, gearbox oil loss, encoder drift |
| Ropes / belts | Couple car and counterweight to the sheave | Uneven wear, broken wires, tension imbalance, belt rib damage |
| Counterweight | Balances roughly the car plus 40-50% of rated load | Guide shoe wear, tie-rod slack, weight stack movement |
| Car frame and platform | Carries the load to the sling and safeties | Loose sling bolts, platform deflection, isolation pads |
| Guide rails and shoes | Constrain travel in both axes | Roller flat spots, rail joint misalignment, oil starvation |
| Governor and safeties | Detect overspeed and grip the rails | Sticking safety, dry governor rope, wrong pull-through tension |
| Buffers | Arrest the car or counterweight at the pit | Oil loss, oil contamination, switch not resetting |
| Doors and interlocks | Control access and prove the hoistway is closed | The single largest source of callbacks: alignment, gibs, closed switches, operator cams |
| Controller | Sequences motion, speed and levelling | Mostly reports faults caused by other subsystems |
Why “it stopped between floors” is usually not a controller fault
A controller stops the car deliberately when a safety input opens. The productive question is not “what did the controller do” but “which input opened, and why”. Most between-floor stops trace to one of four families:
- Door circuit — an interlock or closed switch dropped out in motion, often a worn contact rather than a mis-adjusted door.
- Safety chain — pit switch, governor switch, or a slack-rope device operated.
- Drive fault — overcurrent, position loss, or encoder feedback error.
- Power — a phase loss or voltage sag that the controller recorded as a fault.
Traction versus hydraulic: the split that changes everything
If the building has a hydraulic unit, add the following to the list before touching the controller: oil level and temperature, jack seal condition, valve behaviour, and the pump/motor coupling. Hydraulic faults are frequently thermal – the car is fine cold and drifts or sinks when the oil is hot, which is a valve or seal problem, not a control problem.
Where to read more
Component-level questions are covered in the library entries on control systems, door systems, and the safety chain. If you are chasing a live fault on a specific unit, the forum has threads on most major controllers – start with controller fault.