Unintended Car Movement Protection vs. Dual Plunger Brakes
Unintended Car Movement Protection vs. Dual Plunger Brakes
Unintended Car Movement Protection vs. Dual Plunger Brakes
๐ vs ๐
New York City recently updated its elevator codes to improve the safety of elevators with single-plunger machine brakes. Owners now have two upgrade options. From NYC’s appendix K section 3.8.4.1:
- Convert single-plunger assemblies to dual-plunger type
- Comply with Unintended Car Movement Protection (UCMP) under ASME A17.1 Section 2.19.2
Both options improve safety, but they do so in very different ways. So, we want to talk about whatโs the same and whatโs different.
Dual Plunger Brakes โ Mechanical Redundancy
A dual-plunger upgrade is purely mechanical. It reduces single points of failure by:
โข Preventing a single failed part (pin, arm, shoe, pad, rivet, nut, bolt, spring, coil, sleeve, or housing) from causing a loss of braking by adding redundancy
โข Using two mechanically separated individual brakes so one sticking part, like a plunger, does not compromise the other brake
โข Assuming only one side has failed but not the other, giving mechanics a chance during routine maintenance to detect a failure on one side before the other side fails
Protects against:
โข Certain common failures within the brake assembly
โข Loss of braking due to one mechanical component within the brake assembly
Does not protect against:
โข Failures that occur after the machine brake in the coupling, shaft, or gearing
โข Failures that affect both brakes
โข Electrical or control system malfunctions
Unintended Car Movement Protection (UCMP) โ Mechanical + Electrical Safety
UCMP adds another layer of safety by combining a mechanical emergency brake with an electrical monitoring system.
It includes the mechanical redundancy of a second brake, as with a dual-plunger assembly, but differs in the brake location and electrical monitoring, allowing it to prevent many additional failures.
This ensures protection against unintended movement from:
โข The emergency brake location allows it to catch motor, brake, coupling, shaft, or gearbox failures
โข Electrical monitoring allows it to catch control system malfunctions, where no mechanical component has failed but the fault comes from an electrical component or the controller
โข Additionally, while traction sheaves are excluded from the A17.1 code, UCMP systems with rope brakes can cover these as well
Failure modes UCMP prevents that dual plungers cannot:
โข Simultaneous brake pad wear
โข Coupling or rod failures
โข Gearbox component failures
โข Lubrication or contaminants on the drum (if the emergency brake does not act on the drum)
โข Controller faults or crashes
In short, UCMP provides comprehensive protection against unintended car movement, covering mechanical, electrical, and control system failures, while a dual-plunger conversion adds mechanical redundancy to single-plunger brakes.
