Dual-Trolley Tandem Lifting: How to Prevent Crane Skew
📋 Key Summary
When two trolleys lift a single beam, any speed mismatch causes the beam to tilt, creates uneven load distribution, and can even lead to a dropped load. Multi-mechanism synchronization control keeps all drives moving in step—through master-slave control, speed following, position closed loop, and skew detection. This article explains why multi-mechanism systems drift out of sync, how synchronization control works, and how to implement dual-trolley tandem lifting in practice.
A beam dozens of meters long, suspended from two trolleys—one at each end—is being lifted together. If the two trolleys differ even slightly in speed, one end of the beam rises higher than the other, shifting the load to one side. In mild cases, the lift loses accuracy; in severe cases, the load drops.
This is exactly what multi-mechanism synchronization is designed to solve: keeping two trolleys—or even the hoist, crane bridge, and trolleys together—moving in perfect step. Below, we break down how to prevent drift and keep the lift on track.
Why Multi-Mechanism Systems Drift: Speed Variance and Uneven Loads
Multi-mechanism synchronization is difficult because mechanisms are "naturally out of sync."
Each of the two trolleys has its own motor, reducer, and brake. Even when given the same speed command, their actual speeds will differ slightly. Motor characteristics, gearbox backlash, wheel diameter wear, and load distribution—any of these variables will cause the two trolleys to gradually drift apart.
Once drift occurs, a vicious cycle sets in: misalignment causes uneven load distribution, which in turn widens the speed gap, and the deviation keeps growing. This is why multi-mechanism synchronization cannot rely on each mechanism running independently—active synchronization control is essential. FEM 1.001 Crane Design Standard sets requirements for operating stability. Kelude treats multi-mechanism synchronization as the core control strategy for large-tonnage dual-trolley tandem lifting.
How Synchronization Control Works: Master-Slave, Speed Following, Position Closed Loop
Multi-mechanism synchronization control is built on a "follow-and-correct" framework.
Master-slave control designates one mechanism as the master and the rest as slaves. The master follows the operator's command, while the slaves track the master rather than executing commands independently. This master-slave structure provides a reference point for synchronization.
Speed following means the slave mechanisms continuously track the master's speed to eliminate any speed difference. If the slave runs fast, it decelerates; if it lags, it accelerates—keeping speeds matched.
Position closed loop uses encoders to measure each mechanism's position in real time. When a position deviation appears, the system feeds back and corrects it. Speed following eliminates speed differences, while the position closed loop removes accumulated displacement error. Both working together are required to prevent drift.
Implementing Dual-Trolley Tandem Lifting: Skew Detection and Automatic Correction
Synchronization in dual-trolley tandem lifting comes down to an executable sequence.
Step one: master-slave assignment. One trolley is set as the master, the other as the slave. The master follows the operator's command, and the slave tracks the master.
Step two: dual closed-loop following. The slave tracks both the master's speed and position deviation measured by encoders—correcting speed differences and displacement errors simultaneously.
Step three: skew detection as a safety net. A skew detection device is mounted on the lifting beam to measure the beam's inclination angle in real time. If the tilt exceeds the limit, the system triggers an alarm or automatic correction. In its dual-trolley tandem lifting systems, Kelude applies master-slave assignment, dual closed-loop following, and skew detection as three layers of protection. GB/T 28264 Safety Monitoring and Management System requires synchronized operation to be logged and traceable.
Most Common Mistakes in Implementing Synchronization Control
Mistake one: running independently with no master-slave relationship. Both mechanisms execute their own speed commands with no tracking relationship, so speed differences accumulate into drift. Synchronization requires a master-slave reference.
Mistake two: following speed only, without correcting position. Speed following eliminates speed differences, but accumulated position deviation goes uncorrected—over time, drift returns. Both speed and position must be in a dual closed loop.
Mistake three: no skew detection as a safety net. When synchronization control fails and there is no skew detection to raise an alarm, the beam can tilt without anyone knowing. Kelude uses skew detection as the final safety net—if control fails, the system still alarms and stops the crane.
Synchronization Control Methods: A Comparison
| Method | Principle | Problem Addressed | Implementation Complexity | Applicability |
|---|---|---|---|---|
| master-slave control | Master-Slave Follower | SynchronizationReference | Medium | dual-trolley tandem lifting |
| speedFollow | Follow Master Speed | EliminatespeedDifferential | Medium | Coordinate with Master-Slave |
| position closed loop | encoder feedback | Eliminate Displacement Differential | Medium-High | HighAccuracySynchronization |
Quick Reference of Standard Clauses for Synchronization Control
| Standard | Key Clause Points | andSynchronization ControlRelationship with |
|---|---|---|
| FEM 1.001 Crane Design Standard | crane design specification | operating stabilityReference |
| GB/T 28264 Safety Monitoring and Management System | safety monitoringTraceabilityrequirements | SynchronizationOperational Traceability |
| ISO 24617 | intelligent control system for cranes | Synchronization ControlFramework |
FAQ: Multi-Mechanism Synchronization in Crane Operations
Q: Where should I start troubleshooting travel deviation on twin trolleys?
A: First, check whether a master-slave control scheme is configured. If each trolley operates independently with no follower relationship, travel deviation is inevitable. Next, inspect the position closed loop — if it only regulates speed without correcting position, cumulative deviation will continue to grow. Finally, examine the encoders and skew detection system; inaccurate feedback signals or a failed skew detection mechanism will render the correction system ineffective. The troubleshooting sequence is: master-slave setup, position closed loop, then sensors.
Q: How do I know if my operating conditions require multi-mechanism synchronization?
A: Look for scenarios where multiple mechanisms must operate in a coordinated manner. Twin trolleys lifting a single girder, simultaneous lifting at multiple lifting points, or coordinated hoisting, crane bridge, and trolley movements all demand synchronized control to keep mechanisms in step. Standalone mechanisms operating independently do not require this. The key indicator is whether you have a situation where multiple mechanisms must move together without drifting apart.
Q: Why are multi-mechanism systems inherently prone to travel deviation?
A: Because no two mechanisms are perfectly identical. Differences in motor characteristics, gearbox backlash, wheel diameter wear, or load distribution will cause the actual speeds of the two trolleys to diverge — even when given the same command, they will gradually drift apart. This misalignment then creates uneven load distribution, which further widens the speed gap, creating a vicious cycle. That is why active correction through master-slave control combined with a position closed loop is essential — relying on independent operation will not work.
Multi-mechanism synchronization is often implemented alongside variable frequency drive retrofits. For a reference on synchronizing three mechanisms, see "Harbor Portal Crane Custom VFD Retrofit Solution: Commissioning Synchronized Hoisting, Luffing, and Slewing Mechanisms".
When two trolleys share the load of a single girder, synchronization is a safety prerequisite. Kelude employs three layers of protection — master-slave configuration, dual closed-loop tracking, and skew detection — to keep multi-mechanism operations in step and automatically correct any travel deviation that occurs.
