Pallet Stacker

What is a Pallet Stacker

A pallet stacker, also called a stacker truck, walkie stacker, or pedestrian stacker, is a material-handling machine that lifts a palletized load on a vertical mast and travels it short distances within a warehouse. It is operated by a walking operator who controls travel, steer, lift, and lower from a tiller arm at the rear, rather than from a seated cab. The defining feature is the mast: unlike a pallet jack, which only lifts the load high enough to clear the floor, a stacker raises the forks to rack-beam height so loads can be stored vertically rather than only moved horizontally.

Functionally the stacker occupies the middle of the material-handling ladder. Below it sits the manual pallet jack and platform trolley, which move loads but do not stack. Above it sits the counterbalanced forklift and the reach truck, which carry a riding operator, lift heavier loads, and reach higher, but cost several times more and need wider aisles. The stacker exists because a large share of warehouse work, replenishing pick faces, putting away light cartons, double-stacking on the floor, loading trailers, does not need a full forklift and is cheaper and safer to do with a narrow pedestrian machine.

A typical stacker handles 500 to 2,000 kg and lifts from about 1.6 m up to roughly 5.4 m, depending on mast and chassis. The chassis is narrow, often under 800 mm wide, and turns within an outer radius below 2 m, so it works in aisles a 2.5 t forklift cannot enter. Travel is electric or, on the lightest models, fully manual via a hand-hydraulic pump. Because the operator walks, travel speed is intentionally limited to roughly 4 to 6 km/h, and the machine is designed for flat, improved indoor floors only.

Historically the stacker evolved from the hand pallet truck. The hand pallet truck, invented in the early twentieth century, gave warehouses a way to move pallets without a forklift. Adding a hydraulic mast turned that jack into a manual stacker that could lift a pallet onto a low rack. Electrification of the lift, and then of the drive, produced the semi-electric and fully electric stackers that dominate modern warehouses. Today the powered walkie stacker is one of the highest-volume classes of industrial truck, sold by every major forklift maker alongside a long tail of value-brand OEMs.

Four engineering metrics determine whether a stacker is right for a job: rated capacity at the working load center, maximum lift height, chassis footprint and turning radius, and drive type with its duty-cycle endurance. These four interact. Pushing lift height up reduces residual capacity; adding straddle legs improves stability but constrains pallet and rack compatibility; choosing manual drive saves money but caps throughput. The rest of this guide unpacks each of these trade-offs so the selection is made on physics, not on a sales brochure.

Stacker Types and Chassis Geometry

Pallet stackers are classified along two independent axes. The first is the power source: manual (hand-hydraulic), semi-electric (electric lift, manual push), and fully electric (electric lift and drive). The second is the chassis geometry that resists the forward tipping moment of an elevated load: straddle leg, counterbalance, walkie reach, and fork over. A complete model description names both, for example a fully electric straddle stacker or a manual fork-over stacker. The table below summarizes the four chassis geometries.

Straddle stackers carry two outrigger legs that extend forward along the floor and straddle the pallet. The legs put a wide, low base under the load, so the truck resists the forward tipping moment without a heavy rear counterweight. This makes straddle stackers light, energy efficient, and capable of high lift. The constraint is geometric: the straddle legs must pass under or around the pallet and clear the racking feet, so they suit open-bottom pallets and racks with compatible foot spacing. Some models offer adjustable-width legs to handle a range of pallet and stillage sizes.

Counterbalance stackers remove the legs and add a rear counterweight, like a miniature forklift. Because nothing protrudes ahead of the forks at floor level, a counterbalance stacker can drive its forks straight into closed-bottom pallets, box pallets, stillages, and roll cages, and can set loads flush against a wall. The price is weight and turning radius: a counterbalance model is heavier and turns wider than a straddle truck of the same rating, and rates lower for a given chassis size. It is the right choice when pallet types are mixed or floor-stacking is required.

Walkie reach stackers add a pantograph (scissor) mechanism that extends the forks forward past the straddle legs. This lets the truck place a load into single-deep racking whose beams sit beyond the leg footprint, combining the tight turning of a straddle stacker with the placement flexibility of a reach truck. Yale's MRW series and similar models target exactly this duty. The pantograph adds cost and a maintenance point but unlocks rack configurations a plain straddle stacker cannot serve.

Fork-over stackers raise the forks on a carriage that rides over the top of the straddle legs, so the forks sit above the legs rather than between them. This is the simplest, lowest-cost high-lift geometry, used on machines such as the Crown ES 4000 and Jungheinrich EJC fork-over range. It works only with open-bottom pallets, because the straddle legs occupy the pallet's bottom-board zone. On the power axis, manual hand-hydraulic stackers suit very light, occasional duty; semi-electric models add a powered lift to spare the operator's effort; fully electric models add powered travel for sustained throughput and are the workhorse of modern distribution centers.

Mast and Drive Technology

The two subsystems that most affect a stacker's performance are the mast (which sets lift height, collapsed height, and rigidity) and the drive train (which sets throughput, runtime, and total cost of ownership). Both are spec-sheet line items that buyers routinely under-specify or over-specify. The table below compares the three common mast configurations.

Simplex (single-stage) masts use one channel and one lift cylinder. They are the most rigid and give the clearest view of the fork tips, but their collapsed height almost equals their lift height, so reach is limited to roughly 2.5 m. They suit low-bay storage and applications where ceiling and doorway clearance is not a constraint and visibility matters most.

Duplex (two-stage) masts add an inner moving channel inside a fixed outer channel. Many duplex masts provide free lift, the ability to raise the forks a useful distance before the mast itself begins to telescope. Free lift lets a stacker work inside a trailer or under a low ceiling where the mast cannot extend, then telescope only once clear. Duplex masts reach roughly 2.5 to 3.6 m while keeping a moderate collapsed height.

Triplex (three-stage) masts add a third channel, reaching about 3.6 to 5.4 m yet collapsing low enough to pass through standard industrial doorways. Full free-lift triplex masts raise the forks to the top of the inner section before any stage telescopes, the configuration needed for double-stacking inside a height-restricted trailer. The cost of every added stage is reduced rigidity and reduced residual capacity, because the higher the lift the larger the tipping moment, so a triplex truck rated 1,500 kg low may permit only 1,000 kg near full height. Do not specify a triplex mast where a duplex suffices.

On the drive side, the choice begins with power source. Manual stackers lift with a foot or hand hydraulic pump and are pushed by the operator, with no battery to maintain, suited to a few lifts per shift. Semi-electric stackers power the lift from a small battery but still rely on the operator to push the chassis. Fully electric stackers power both lift and travel and are the only practical choice for sustained throughput.

Within powered models, the drive motor is increasingly AC rather than DC. An AC drive motor, around 1.2 to 1.5 kW on a 1.5 t class truck, has no brushes to wear, supports regenerative braking, and gives smoother low-speed control, which matters when inching a load into a rack. Battery chemistry is the other major decision. Lead-acid (flooded or AGM) batteries are cheaper to buy, deliver roughly 1,000 to 1,500 cycles, and need watering, equalizing, and ventilated charging. Lithium-ion batteries, typically LiFePO4, cost more upfront but are maintenance-free, accept opportunity charging during breaks, hold voltage to near empty, and last about 3,000 to 6,000 cycles, often a lower cost per cycle in multi-shift duty. Travel speed is governed to roughly 4 to 6 km/h, and gradeability is modest, near 5 percent laden, because the wheelbase is short and the machine is meant for flat indoor floors.

Capacity, Load Center, and Standards

The single most misunderstood concept in stacker selection is that rated capacity is conditional. A stacker plated 1,500 kg does not lift 1,500 kg in every situation. Rated capacity is stated at a defined reference load center, usually 500 or 600 mm, and at a defined lift height. As soon as the real load center moves further out on the forks, or the mast extends higher, the safe (residual) capacity drops, because both increase the forward tipping moment that the chassis must resist.

Load center is the horizontal distance from the fork face (heel) to the center of gravity of the load. A standard 1,000 by 1,200 mm pallet loaded evenly has a load center of roughly 600 mm. A load that is deeper, overhangs the forks, or has its weight biased forward shifts the load center outward and reduces residual capacity. This is why a capacity plate always names both the load center and the height at which the rating applies, and why a separate mast-mounted capacity chart or table covers the full operating envelope.

The table below is an illustrative residual-capacity pattern for a mid-size electric straddle stacker. It is for orientation only: every truck carries its own certified plate, and you must read the actual chart for the exact model and mast before lifting. The inverse relationship between height and permitted weight is the point to internalize.

Standards exist precisely because exceeding the chart causes forward tip-over, the dominant stacker accident mode. For pedestrian-propelled stackers, manual and hand-hydraulic, the governing safety standard is ISO 3691-5:2014; the parallel U.S. design standard for manually propelled trucks is ANSI/ITSDF B56.10. For self-propelled (electric) pedestrian and stand-on stackers, ISO 3691-1 applies internationally, and ANSI/ITSDF B56.1-2020 is the U.S. safety standard for low-lift and high-lift powered industrial trucks.

Stability, the property that the capacity chart encodes, is verified to ISO 22915-4:2018, which covers stability tests for stackers, double stackers, and order-picking trucks with operator-position elevation up to 1,200 mm. Driverless and automated stacker variants additionally fall under ISO 3691-4:2023. In the European Union these trucks must meet the Machinery Regulation and carry the CE mark, while in the United States they fall under OSHA powered-industrial-truck rules that reference the ANSI/ITSDF B56 series. When buying, always confirm that the conformity certificate names the exact model, mast height, and battery configuration, because a certificate for a different variant does not transfer.

Key Specification Parameters

A stacker datasheet lists dozens of dimensions, but a manageable set drives the selection decision. Reading them correctly separates a truck that fits the aisle and the rack from one that arrives and cannot do the job. The parameters below are the ones to extract from every quote and compare like for like.

• Rated capacity: the maximum load at the reference load center and reference height, in kilograms. Always paired with the load center (500 or 600 mm). Meaningless without both qualifiers.
• Load center: the reference distance from the fork heel to the load center of gravity at which capacity is rated, typically 500 or 600 mm. Confirm your real load center does not exceed it.
• Maximum lift height: the top of the fork travel, governing which rack level the truck can reach. Pair it with the residual capacity at that height, not the plated low-lift figure.
• Lowered (collapsed) mast height: the closed mast height, which must clear doorways, trailer roofs, and mezzanine soffits. A triplex mast can reach high yet collapse low; a simplex cannot.
• Free lift: the fork travel before the mast telescopes, essential for working inside trailers or under low ceilings.
• Fork length and width: must match the pallet. Common fork lengths run 1,150 to 1,220 mm for a standard pallet; over-fork or under-fork length both cause instability.

Beyond the load-handling figures, the chassis and drive parameters decide whether the truck physically fits and how hard it can work. The following are the dimensional and electrical specs to verify against the site.

• Overall width and turning radius: the chassis width (often under 800 mm) and outer turning radius determine the minimum aisle. Compare the required Ast (working aisle width) against the planned rack layout.
• Straddle leg width and inner clearance: on straddle and fork-over models, the legs must straddle the pallet and clear the rack feet. Adjustable legs add flexibility at extra cost.
• Drive type and motor: AC drive (around 1.2 to 1.5 kW on a 1.5 t truck) for brushless control and regenerative braking, versus older DC drive.
• Battery chemistry and capacity: lead-acid versus lithium-ion, rated in Ah and V, sized to the shift pattern. Lithium suits multi-shift and opportunity charging; lead-acid suits single-shift.
• Travel speed and gradeability: roughly 4 to 6 km/h and about 5 percent laden. Stackers are not built for ramps or outdoor yards.
• Standards and certification: ISO 3691-1 or ISO 3691-5, ANSI/ITSDF B56.1 or B56.10, ISO 22915-4 stability, plus CE marking in the EU.

One spec that buyers routinely overlook is service weight and floor loading. A high-lift electric stacker with a heavy lead-acid battery can exceed a tonne of dead weight, which matters on mezzanines, suspended floors, and inside trailers rated for limited point loads. Always check that the loaded truck weight plus the load stays within the floor or trailer rating before committing to a model.

Selection Decision Factors

The right stacker falls out of the application data when the decisions are taken in the correct order. Most selection mistakes come from fixing on a capacity number first and discovering too late that the chassis does not fit the rack or the aisle. The sequence below works from the constraints that cannot be changed (the building and the racking) toward the ones that can (drive type and battery).

1. Lift height and rack profile: measure the highest beam the truck must reach, then read residual capacity at that height, not the plated low-lift figure. The rack layout also dictates whether straddle legs or a reach pantograph are needed.
2. Load weight and load center: take the heaviest real pallet and its true load center. Choose a truck whose residual capacity at your height and load center exceeds the load with margin, never the truck whose nameplate merely equals it.
3. Pallet and stillage types: open-bottom pallets allow straddle or fork-over geometry; closed-bottom pallets, box pallets, and roll cages require a counterbalance stacker that drives forks straight in.
4. Aisle width and turning radius: compare the truck's working aisle (Ast) and outer turning radius against the narrowest aisle it must serve, including the end-of-aisle turn into the rack.
5. Floor and travel conditions: confirm flat, improved indoor floors. If ramps, expansion joints, or outdoor sections exist, reconsider, since stackers offer only about 5 percent gradeability and are not for rough ground.
6. Drive type and duty cycle: manual for a few lifts per shift, semi-electric for moderate lift effort, fully electric for sustained throughput. Match motor and battery to hours of actual lifting and travel, not to peak.
7. Battery and charging: lead-acid for single-shift with overnight charging; lithium-ion for multi-shift, cold-store, or high-utilization duty where opportunity charging removes downtime.
8. Standards, certification, and total cost of ownership: verify the ISO 3691 and ANSI/ITSDF B56 conformity for the exact model, then weigh purchase price against service-network density, spare-part availability over 10 years, and battery cost per cycle.

One dimension that does not appear on the spec sheet but governs long-term cost is manufacturer serviceability: local spare-part inventory, dealer service coverage, mast and seal kit availability, and how long the maker supports a model after the sale. Premium brands such as Crown (ES 4000 fork-over and M and SH walkie series), Toyota (6BWS and 8BWS industrial walkie stackers), Raymond, Jungheinrich (EJC fork-over and ETV ranges), Linde, Yale (MRW walkie reach stacker), Hyster, and Mitsubishi (PWT straddle stacker) justify their price through dense service networks and long part support, which suit multi-shift core fleets. Value brands such as EP Equipment, Hangcha, Noblelift, and H-Lift supply 500 to 2,000 kg manual, semi-electric, and electric stackers at a fraction of the price, which suit occasional duty and backup units. Match the brand tier to the duty cycle, not to the brochure.