Hoisting Mechanisms & Safety Devices
Hoist components, upper and lower limit switches, two-blocking prevention, load brakes, drum wraps, and VFD creep speed operation.
- Identify the major components of an overhead crane hoisting system
- Explain the purpose and function of upper and lower limit switches
- Describe load brake function and rated load indicator operation
- Understand drum wrap requirements and VFD creep speed control
Leçon 1
Hoist Components - Drums, Sheaves & Load Brakes
The Hoisting System
The hoist is the heart of an overhead crane - it raises and lowers the load. Understanding its components is critical for safe operation and effective daily inspections.
Key Hoist Components
| Component | Function |
|---|---|
| Hoist motor | Provides power to raise and lower the load |
| Hoist drum | Cylindrical spool that stores wire rope |
| Wire rope | Flexible steel cable connecting drum to hook block |
| Sheaves | Grooved pulleys that guide wire rope and provide mechanical advantage |
| Hook block | Lower assembly containing sheaves and the lifting hook |
| Load brake | Controls descent speed and holds load |
| Holding brake | Stops and holds hoist motion when control is released |
Load Brake Purpose
The "load brake" purpose is to control descent and hold load in hoist. A load brake (also called a Weston brake or mechanical load brake) is an automatic brake that engages whenever the load tries to lower. It provides controlled lowering speed and prevents the load from free-falling if the holding brake fails.
Critical Safety Device
The load brake is a critical safety component. It controls descent and holds load in the hoist. Without a functioning load brake, the load could free-fall if the holding brake fails.
Rated Load Indicator
A rated load indicator on some cranes displays hoist load and may include overload protection. This device measures the actual weight on the hook and displays it for the operator. Some models include an alarm or automatic shutoff when the rated capacity is approached or exceeded.
The load brake controls descent and holds the load in the hoist. A rated load indicator displays the hoist load and may include overload protection. These are critical safety devices that must function properly.