In industrial material handling operations, the 20-ton overhead crane is one of the most frequently used pieces of equipment due to its ability to move heavy loads efficiently, safely, and precisely. Among its main structural and mechanical components, the trolley system plays a pivotal role in enabling smooth and controlled movement of loads across the working area. Understanding how the trolley system functions in a 20-ton overhead crane offers valuable insight into the mechanical principles, design, and control mechanisms that make such cranes reliable and effective.
Understanding the Trolley System
The trolley system of a 20 ton overhead crane is the mechanical assembly that supports and moves the hoist along the bridge girder. It consists of wheels, a frame, drive motors, brakes, and control mechanisms that allow the hoist—and consequently the suspended load—to travel horizontally.
In a typical overhead crane, there are two main directions of motion:- Bridge travel – the movement of the entire crane structure along the runway rails.
- Trolley travel – the movement of the trolley along the bridge girder.
The trolley system provides lateral motion, enabling the crane to position the hoist directly above the load or target location. When combined with the bridge travel and the hoist’s vertical lifting motion, the trolley enables full three-dimensional material handling within the crane’s operational area.
Components of a 20 Ton Overhead Crane Trolley System
A well-designed 20-ton trolley system includes several key components that work together to achieve smooth and reliable operation:
1. Trolley Frame
The frame provides the main structural support for the trolley. It houses the hoisting mechanism and supports the wheels, motor, and gearboxes. For a 20-ton crane, the trolley frame is built from high-strength steel to withstand heavy loads while maintaining rigidity and stability.
2. Trolley Wheels
The trolley moves along rails fixed on top of the bridge girders. These wheels are made of forged or cast steel and often incorporate precision bearings for smooth travel. Depending on the design, the trolley may have two or four wheels, with one or more of them being powered by a drive motor.
3. Drive Mechanism
The trolley drive mechanism typically consists of an electric motor, a reduction gearbox, and drive shafts or couplings that transmit torque to the wheels. For a 20-ton overhead crane, the motor is usually an electric squirrel-cage induction motor that provides sufficient torque for acceleration and deceleration under heavy loads.
4. Braking System
Brakes are crucial to ensure accurate stopping and holding of the load. The trolley drive system is fitted with electromagnetic or hydraulic brakes that engage automatically when power is cut off or during emergency stops.
5. Control System
Modern overhead cranes often use variable frequency drives (VFDs) to control trolley speed. This allows for smooth acceleration and deceleration, reducing load swing and improving precision in positioning. The operator can control the trolley movement via pendant control, remote control, or cabin control.
6. Power Supply System
Power for the trolley system is typically provided through festoon cable systems or conductor bars running along the bridge. These supply electricity to the trolley motor and hoist, enabling continuous operation across the entire length of the crane bridge.
Working Principle of the Trolley System
The functioning of the trolley system in a 20-ton overhead crane can be broken down into a sequence of coordinated actions involving electrical, mechanical, and control components:
- Command Input – The operator sends a control signal (via pendant or remote control) to initiate trolley movement.
- Motor Activation – The control system energizes the trolley motor, which begins to rotate.
- Power Transmission – The motor’s rotation passes through the reduction gearbox, reducing the motor’s speed while increasing torque to drive the wheels.
- Wheel Rotation – The powered wheels rotate along the rail tracks mounted on the bridge girder, causing the trolley to move horizontally.
- Hoist Movement – As the trolley moves, the hoist (and the suspended load) also moves horizontally, allowing precise load positioning.
- Controlled Stopping – When the operator releases the control or applies braking, the VFD gradually decelerates the motor, and the brakes engage to stop the trolley smoothly at the desired position.
The combination of electric motor control and braking ensures smooth, stable, and accurate travel, even when handling heavy loads such as steel beams, machinery components, or precast concrete elements.
Types of Trolley Systems
The design of the trolley system may vary based on the configuration of the overhead crane. The two main types are:
1. Single Girder Trolley
In a single girder overhead crane, the trolley runs on the bottom flange of the bridge girder (also known as an underrunning trolley). It is typically compact and lightweight, suitable for moderate loads. For a 20-ton crane, this type is less common but may be used in applications where space and headroom are limited.
2. Double Girder Trolley
In a double girder crane, the trolley runs on rails mounted on top of both girders. This configuration is standard for 20-ton cranes since it provides better load stability, greater lifting height, and improved rigidity. The hoist is mounted between the girders, allowing higher hook travel and efficient use of vertical space.
Motion Control and Safety Features
The trolley system of a 20-ton overhead crane integrates several advanced control and safety features to ensure reliable and secure operation:
1. Variable Frequency Drive (VFD) Control
The VFD system allows the operator to control trolley speed precisely. This not only improves load positioning accuracy but also reduces mechanical wear by avoiding abrupt starts and stops.
2. Anti-Sway Technology
Modern cranes use electronic anti-sway control systems that automatically adjust trolley acceleration and deceleration to minimize load swing, improving safety and operational efficiency.
3. Limit Switches and Buffers
Limit switches are installed at the ends of the bridge girder to prevent the trolley from overrunning. Rubber or polyurethane buffers further absorb impact if the trolley reaches the limit of its travel path.
4. Overload and Safety Monitoring
Sensors integrated into the hoist and trolley can detect overloads or irregular movements. The system can automatically cut power or issue alarms to prevent accidents or equipment damage.
Maintenance and Troubleshooting
Proper maintenance of the trolley system ensures long service life and safe operation. Key maintenance activities include:
- Lubricating bearings, gears, and wheel assemblies to reduce friction.
- Inspecting wheel alignment to avoid uneven wear or derailment.
- Checking electrical connections and cables for wear or damage.
- Testing brakes and limit switches periodically for proper function.
- Examining rail conditions to ensure smooth and level trolley travel.
Common issues such as jerky movement, unusual noise, or overheating motors usually indicate problems with wheel alignment, gear lubrication, or motor control settings. Timely maintenance prevents such faults from escalating into serious breakdowns.
Importance of the Trolley System in Crane Performance
The trolley system directly influences the operational precision, efficiency, and safety of a 20-ton overhead crane. A well-engineered trolley enables seamless lateral movement, allowing operators to position heavy loads exactly where needed. It enhances productivity in workshops, warehouses, and manufacturing plants where materials need to be transported quickly and safely.
Moreover, with advancements in control technology, modern trolley systems now integrate smart diagnostics, energy-saving drives, and automation features, aligning with the growing demand for intelligent and efficient lifting solutions.
Conclusion
In summary, the trolley system is the heart of horizontal motion in a 20-ton overhead crane, combining mechanical engineering, electrical control, and safety mechanisms to ensure precise load handling. From the motor and wheel assemblies to the VFD and anti-sway systems, every component plays a critical role in achieving reliable and controlled movement. Proper design, regular maintenance, and intelligent control integration not only extend the lifespan of the trolley system but also significantly enhance the overall performance of the overhead crane—making it a cornerstone of modern industrial lifting operations.
