Buffers, Pit Equipment, and Terminal Stopping Devices

Buffers, Pit Equipment, and Terminal Stopping Devices

The Purpose of the Elevator Pit

The pit is the portion of the hoistway that extends below the lowest landing. It is not a storage area or a shortcut to anywhere — it is a safety-critical work zone. Every time you enter a pit, lockout/tagout (LOTO) must be applied to the pit stop switch and any other energy sources before you step off the ladder. Confirm with a co-worker, inspect the pit lighting, and never rush. The pit stop switch is a red, mushroom-head switch located near the pit ladder at the bottom of the hoistway; opening it de-energizes the elevator control circuit so the car cannot move while you are below. ASME A17.1 (the Safety Code for Elevators and Escalators) establishes the minimum pit depth, lighting level, and equipment requirements you will encounter on every job site.

Standard Pit Equipment

During a pit inspection you should locate and verify the condition of the following items:

• Car buffer — absorbs kinetic energy if the car travels below the lowest landing.
• Counterweight buffer — absorbs energy if the counterweight descends too far (which happens when the car is at or near the top of travel).
• Pit ladder — provides safe access from the lowest landing to the pit floor; must be fixed and extend above the sill.
• Pit stop switch — mushroom-head, red, manually opened switch that removes power from the control circuit when you are in the pit.
• Pit lighting — code-required fixed lighting with a switch accessible from the ladder; keeps hands free while descending.
• Sump pit (sump pocket) — a recessed area in the pit floor that collects water; some jurisdictions require a sump pump.

Record every item on your pit inspection checklist as you confirm it. A missing or non-functional item is a deficiency that must be reported before the elevator is returned to service.

Oil (Hydraulic) Buffers

An oil buffer uses a hollow steel cylinder filled with hydraulic fluid and a plunger. When the car strikes the plunger, fluid is forced through metered orifices, converting kinetic energy into heat gradually — a controlled stop rather than a crash. Oil buffers are required by ASME A17.1 when rated speed exceeds the threshold that permits spring buffers (generally above 200 ft/min; verify with the current code edition).

Checking oil level: Every oil buffer has a sight glass or a dipstick. The oil level must be at or above the minimum mark. If it is low:

1. Obtain the buffer manufacturer's specification for fluid type and viscosity.

2. Use a clean measuring cup to add the correct hydraulic buffer oil — do not substitute brake fluid, motor oil, or any non-specified fluid.

3. Re-check the sight glass after filling. Overfilling is also a deficiency.

4. After a buffer is compressed (struck by the car), it must return to its fully extended position before the elevator is returned to service; the return time is a code-checkable parameter.

Always wear chemical-splash safety glasses and nitrile gloves when handling hydraulic fluid.

Spring Buffers

A spring buffer stores energy in a coil spring and releases it — essentially bouncing the car or counterweight back. ASME A17.1 permits spring buffers for elevators operating at or below the code speed threshold. Spring buffers require no fluid maintenance, but the spring must be inspected for corrosion, cracking, and permanent set (a spring that does not return to its uncompressed height is defective). Measure the free (unloaded) height with a steel tape and compare it to the manufacturer's data plate value.

Terminal Stopping Devices

Terminal stopping devices prevent the car from running past the top or bottom terminal landing. There are two layers of protection:

Normal Terminal Slowdown Switch (NTS) / Normal Terminal Stopping Device (NTSD): These switches are actuated by cams on the car or by the selector system before the car reaches the terminal landing. They interrupt or reduce speed through the normal control circuit, bringing the car to a controlled stop at the landing. If the NTS works correctly, the car stops at the floor — the passenger never knows anything happened.

Final Limit Switch (FLS): If the car passes through the NTS zone without stopping, the final limit switch opens the control and safety circuits, cutting power to the motor and brake. The FLS is a last-resort device and should never be the first thing that stops the car. On a wiring diagram, the FLS contacts appear in series with the control circuit; when open, no current can flow and the car must be manually recovered.

When tracing these circuits on a hoistway wiring diagram:

1. Find the terminal strip or device legend to identify switch designations (e.g., 1LS, 2LS, UNTSD, DNTSD).

2. Trace the wire from the switch contact through the diagram to the load it protects (motor contactor coil, safety relay, etc.).

3. Determine whether the switch is normally closed (NC) — opens on actuation — which is the standard configuration for limit switches.

4. Note that upper and lower terminal devices mirror each other: upper devices act when the car is too high; lower devices act when the car is too low.

Safety Summary

• LOTO before pit entry — no exceptions.
• Wear a hard hat, safety glasses, and steel-toed boots in the hoistway and pit.
• Use the correct fluid specification for oil buffers; contaminated or wrong-viscosity fluid changes the buffer's stopping performance.
• Never bypass or jumper a final limit switch to run the car during troubleshooting without explicit, supervised, code-permitted procedures.
• Report all deficiencies in writing on the inspection checklist before the unit is returned to passenger service.