Single-mast stacker cranes are specified for light to medium unit loads between 1.1 and 1.65 tons in installations up to 148 ft tall, while double-mast configurations cover the 5,000 kg payload class and storage heights reaching 42 m [S1][S4].
The two classes diverge on rigidity, footprint, and stability envelope, so the decision is driven by load weight, building height, and aisle width rather than vendor preference [S2][S3]. Körber's product line explicitly groups its cranes into single-mast and double-mast families, each further split into low-bay and high-bay variants for unit load handling [S4].
Structural Definition and Operating Envelope
A stacker crane is a mast-based, rail-guided storage and retrieval machine: the carrier travels along a bottom rail fixed to the floor slab while an upper guide rail attached to the rack profiles absorbs lateral forces during load insertion and extraction [S1][S8]. Unlike overhead cranes that ride on a runway girders, a stacker crane uses a vertical mast to lift and position the load, with the forks mounted in a cradle on that mast [S2][S8].
The mast count is the primary structural variable. A single-mast crane relies on one vertical column to carry the cradle and the load, whereas a double-mast crane uses two parallel columns that bracket the load and share the vertical reaction [S2]. Interlake Mecalux documents a single-mast pallet model range (MT-1 through MT-5) with maximum single-deep heights from 59 ft to 148 ft, single-deep permitted loads of 1.1 to 1.65 t, a 722 ft/min max travel speed, and a 217 ft/min max lift speed [S1]. Double-mast designs in the same vendor taxonomy reach storage heights up to 42 m (about 138 ft) and span 1 kg to 5,000 kg payloads, which is a roughly 3-4x payload bracket on the high end compared to the single-mast range [S4].
Load Capacity, Height, and Sway Performance
Double-mast stacker cranes deliver increased load capacity from dual-mast support, higher lifting heights, and enhanced rigidity that reduces sway and vibration [S5]. Demag notes that a stacker crane for heavy or bulky loads is commonly configured with a double-column layout, exactly like the double-girder analogy in overhead cranes, and that the load mass usually dictates the second column [S2].
Single-mast cranes, by contrast, achieve load stability through three other mechanisms: a high-strength steel mast with bending resistance, a wide heavy base that lowers the center of gravity, and adjustable forks or load-locking clamps that prevent pallet slip during acceleration and turning [S3]. Their control systems layer anti-sway sensing and ramped acceleration on top of the mechanical package to keep the load stationary at the rated 1.1-1.65 t envelope [S3]. Körber's double-mast heavy-class range closes the gap on acceleration and speed metrics at the high-bay end, which is why high-rise and automotive applications are routinely double-mast specified [S4][S5].
Selection Criteria: When to Pick Each Class
Choose a single-mast stacker crane when the unit load is at or below 1.65 t, the building height stays within the 148 ft single-deep envelope, the aisle must remain narrow, and the racking system is a mini-load AS/RS for boxes or small parts [S1][S5]. This class is the default in pharmaceutical, electronics, and high-density small-parts storage where maneuverability and lightweight structural stress on the rack matter more than raw lifting height [S5].
Choose a double-mast stacker crane when the payload reaches the 5,000 kg class, the building height extends to 42 m, the load is bulky enough to require a cradle between the columns, or the application involves high-rise, deep-lane, automotive, heavy manufacturing, or die storage [S2][S4][S5]. SMB International notes that double-mast stacker cranes are compact enough to automate pallet warehouses starting at 150 storage locations or stored-goods heights up to 7.5 m, which is a useful lower bound for when a double-mast unit becomes economically justified [S6]. Single-mast units remain the right answer for cold-storage or ambient facilities down to -22 °F, since both classes share the same temperature-rated components and WCS-driven 24/7 operation [S1].
Comparison Matrix: Single-Mast vs Double-Mast
Across four decision criteria, the two classes line up as follows. Payload: single-mast 1.1-1.65 t, double-mast 1 kg-5,000 kg (with 5,000 kg being the heavy-class ceiling) [S1][S4]. Maximum height: single-mast 148 ft (about 45 m) single-deep, double-mast up to 42 m with comparable or higher high-bay capability [S1][S4]. Stability mechanism: single-mast uses high-strength steel mast, wide base, and load-locking clamps; double-mast uses dual-column geometry to share vertical reaction and cut sway [S3][S5]. Typical applications: single-mast for pharmaceutical, electronics, and mini-load AS/RS; double-mast for automotive, heavy manufacturing, high-rise warehouses, and die/coil handling [S2][S5]. Engineers comparing the two at the same site should run a check on stacker crane selection criteria before locking the spec.
Integration with Deep-Lane Storage and AS/RS
Both classes can pair with single-, double-, and triple-deep rack extraction systems, which trades retrieval agility for storage density [S1]. Single-deep configurations optimize FIFO throughput for e-commerce and retail, while double- and multi-deep configurations serve cold storage, large distribution centers, and bulk storage where LIFO rules apply [S5]. The same mast-count logic transfers directly to vertical lift module access bays, where single-mast vs twin-mast extractor selection follows a closely related decision tree on load and height.
For warehouses comparing mast-based automation to belt-based or crane-based alternatives, the broader pallet stacker family and crawler crane reference data are useful cross-checks when load and aisle constraints push the spec toward non-rail equipment. Engineers evaluating mast-based extraction as part of a wider intralogistics upgrade should also review crane scale integration when payload verification is a regulatory or invoicing requirement.
Limitations and Failure Modes to Watch
Single-mast cranes have an inverse height-to-load relationship: the taller the crane, the lighter the pallet, which sets a hard 1.1 t ceiling on the MT-4 and MT-5 models above 118 ft [S1]. Exceeding that envelope introduces mast bending risk that no anti-sway control system can fully mask. Double-mast cranes consume more aisle width, cost more in steel and drive hardware, and require the racking system to anchor both upper guide rails, which raises civil works on retrofits [S5]. SMB International's 150-location and 7.5 m minimum thresholds are a useful economic gate below which a double-mast unit is hard to justify [S6].
Across both classes, cold-storage operation down to -22 °F and 24/7 WCS-driven cycling are standard, so environmental and duty-cycle limits rarely force a class swap on their own [S1]. The mast-count decision is overwhelmingly a payload, height, and rigidity question, and the data above gives a defensible cutoff for each spec range.