The patent by James Cooper of Chicago, Illinois, describes an Elevator Safety Device designed to prevent an elevator car from being started whenever the elevator door is open.
Invention and Mechanism
The safety device is a mechanical lock integrated with the elevator’s motor control system, actuated by the opening and closing of the elevator door.
1. Control and Lock Interface
- Controlling Mechanism: The car’s movement is typically controlled by ropes/cables (A, A) that operate a cross-piece (A1) and a linkage (A3,A5,A4) connected to the valve rod (B). This rod (B) controls the motor (shown as an ordinary hydraulic type, but the rod would be attached to a rheostat arm for an electric motor).
- Locking Rod: The valve rod (B) is the moving element that controls the motor. It is provided with a single notch (B3) (see Fig. 3).
- Locking Pin (B4): A pin housed in a tube (B2). It is positioned to come opposite the notch (B3) only when the valve is closed (i.e., the car is stopped).
- Normal Position: A spiral spring (B7) tends to keep the pin (B4) out of engagement with the notch (B3).
2. Door-Actuated Locking System
- Connecting Cord (C): A flexible cord extends from the top of the shaft and is connected to a movable frame (B5) housing the locking pin (B4).
- Door Arms (C3): Each elevator door is provided with an arm (C3) through which the cord (C) passes.
- Coil Spring (B8): A coil spring is attached between the frame (B5) and the cord (C).
- Function (Open Door): When a door is opened, the arm (C3) carries the cord (C) with it, shortening the effective length of the cord. This pulls the frame (B5) and the pin (B4) forward. Since the car is stopped (valve closed), the pin (B4) enters the notch (B3) in the valve rod (B). The rod is thus held rigid, preventing the valve from being moved to start the car. The coil spring (B8) allows the door’s motion to continue even after the pin has engaged the notch.
- Function (Closed Door): As the door is closed, the cord (C) returns to its normal position. The tension on the frame (B5) is released, and the spiral spring (B7) pushes the pin (B4) out of the notch (B3), freeing the valve rod and allowing the car to be operated.
3. Core Principle
The system is a fail-safe mechanical interlock that connects the physical position of the door to the motor’s control mechanism, ensuring the motor is locked stationary if a door is not fully closed.
Historical Significance and the Inventor
James Cooper’s 1895 patent addresses the single most dangerous failure point in early elevators: starting the car while the door is open or a person is entering/exiting.
- Elevator Safety: By the late 19th century, elevators were becoming common in tall commercial and residential buildings. Early hydraulic and electric elevators, however, were plagued by accidents, often caused by operator error (starting the car before the door was fully closed). Cooper’s device is a classic example of an early automatic safety interlock.
- Mechanizing Safety: The system is entirely mechanical, using springs, levers, and a simple rope/cord to transmit the status of the door to the control valve. This was necessary before modern electronic interlocking became feasible, proving that crucial safety features could be implemented using robust, low-tech mechanics.
- The Inventor (James Cooper): Cooper, residing in Chicago, was operating in a city rapidly building up with skyscrapers, placing him at the center of the demand for improved vertical transportation and safety devices.
Relation to Current Items
Cooper’s interlock is the direct conceptual ancestor of all modern elevator safety systems:
- Door Interlocks: Every modern elevator uses electrical door interlocks that rely on switches, not ropes and pins, but the function is identical: the car’s drive system is physically and/or electrically locked out unless every hoistway door is fully closed and locked.
- Single-Point Locking: The design ensures the lock is only effective when the valve rod (B) is in the “stop” position (notch B3 opposite pin B4). This concept—that a safety device only needs to engage when the machine is stopped or moving slowly—is fundamental to modern overspeed governors and safety brakes that activate only under specific, critical conditions.
- Safety as a System: The invention demonstrates the principle of system integration, where a peripheral safety condition (door status) directly overrides the primary machine control (valve rod), a core tenet of modern industrial safety engineering.
