When does an industrial trackless electric transfer cart require four lifting support legs?

Square lifting frame = A purely rigid load-bearing frame without a self-locking twist-lock mechanism; it requires steel wire ropes and manual locking components. Semi-automatic container spreader = Equipped with four sets of automatic twist-locks; specialized lifting equipment for containers that allows for mechanical locking with a single action.
I. Structural Differences
1. Square Lifting Frame (Rectangular Lifting Beam / Simple Lifting Frame)
Essentially a hollow rectangular steel frame; it features only upper lifting lugs, lower attachment points, and reinforcing ribs, lacking twist-locks, linkage mechanisms, or guide claws.
Only lifting points are provided at the four corners; it requires the additional use of four steel wire ropes, shackles, and manual container twist-locks.
It has no mechanical linkage or self-locking structures; it is a simple, rigid frame designed solely to bear the load.

2. Semi-Automatic Container Spreader
Consists of an integrated frame, four independent twist-lock assemblies, an internal linkage mechanism, and guide/positioning claws.
Uses the spreader's own weight during lifting/lowering to trigger a mechanical ratchet, allowing the four twist-locks to rotate 90° synchronously to lock or unlock the container; a fully integrated unit requiring no extra accessories.
Includes built-in limit mechanisms, interlocks to prevent accidental operation, and status indicators.

II. Locking Mechanism (Key Difference)
Square Lifting Frame
After the frame lands on the container top, personnel must climb onto the container to manually insert and rotate each of the four independent twist-locks into the corner castings.
Manual unlocking and detachment are required after the container is set down.
The frame itself has no inherent locking capability; it merely serves to space out the four lifting slings and prevent them from crushing the container roof.
Semi-Automatic Container Spreader
Purely mechanical linkage; requires no external electrical or hydraulic power. Crane lowers the spreader → twist-locks insert into container corner castings. As lifting begins, the spreader's weight triggers the linkage, automatically rotating all four twist-locks 90° to lock onto the corners.
After setting the container down and fully unloading the tension, lifting again automatically unlocks the mechanism; the entire cycle is controlled by the crane operator from the cab, eliminating the need for personnel to climb onto the container. III. Comparison Table of Key Parameters
Table
Comparison Dimension    Square Lifting Frame (Simple Lifting Beam)    Semi-automatic Container Spreader
Locking Method    Manual locking of 4 twistlocks individually; requires personnel to work at heights for both locking and unlocking    Automatic mechanical synchronization of locking/unlocking via hoist movement; no personnel needed on the container
Required Accessories    Requires additional steel wire ropes, shackles, and 4 independent container twistlocks    Complete, integrated unit; ready for use immediately upon attaching to the crane hook
Container Compatibility    Fixed size (20ft or 40ft only); non-adjustable    Available in fixed single-size or telescopic (universal 20ft/40ft) models
Operational Efficiency    Very low; requires manual locking/unlocking at height for each lift; long cycle times    Very high; operator handles continuous lifting independently; suitable for high-frequency operations
Deadweight    Light; low frame weight    Heavier; includes integrated twistlocks and drive mechanisms
Safety Features    Manual work at height; risks of falls or incomplete locking; slings prone to slipping    Mechanical interlocks prevent lifting if not fully locked; guide arms ensure automatic alignment, eliminating spreader misalignment
Cost    Very low purchase price; negligible maintenance costs    Higher price; requires only periodic lubrication of linkages and twistlocks
Certification    Standard lifting beam certification only; lacks specialized CCS container certification    CCS (Classification Society) certification available; compliant for port and maritime use
IV. Classification of Applicable Scenarios
✅ Scenarios Suitable for Square Lifting Frames
Low-frequency, sporadic container lifting within a factory (e.g., a few lifts per day) where efficiency is not a priority;
Cranes used for lifting a mix of containers, equipment, and steel components; the frame can be adapted to lift other types of loads;
Limited budget or temporary/emergency lifting needs; no desire to purchase a specialized container spreader;
Small inland waterway terminals or storage yards with non-standardized, simple operations. ✅ Application Scenarios for Semi-Automatic Container Spreaders
High-frequency, repetitive container loading and unloading (routine transfers at terminals, railway freight yards, and container depots);
Operations where personnel are prohibited from frequently climbing onto containers for work at heights due to safety regulations;
Export shipping and vessel-based lifting operations requiring equipment certified by classification societies (e.g., CCS);
Dedicated container transfer operations using gantry cranes or overhead cranes where continuous operational speed is prioritized.
V. Summary of Pros and Cons
Square Lifting Frame
Pros: Low cost; high versatility (capable of lifting other components besides containers); lightweight and easy to assemble/disassemble;
Cons: Requires manual work at heights (significant safety risks); very low lifting efficiency; unsuitable for high-volume operations.
Semi-Automatic Container Spreader
Pros: Automated mechanical locking; no need for personnel to climb; fast operation; high safety; standardized for containers; widely used in ports and shipping;
Cons: Higher procurement cost; limited to container lifting; poor versatility for lifting other types of cargo.
VI. Selection Recommendations
Occasional container lifting, with cranes also required to handle other workpieces → Choose the Square Lifting Frame;
Dedicated, long-term, high-volume container loading/unloading; priority on safety and efficiency; requirement for marine certification → Choose the Semi-Automatic Container Spreader.