Vertical lift modules can be ordered with internal, external, or dual delivery bays, and the choice changes both throughput and footprint requirements, not just operator comfort. Modula lists internal/external, single or dual delivery bay options across its Lift, Flexibox, and Pallet lines, with bay heights of 27.36 in, 35.23 in, and 50.98 in, and tray payloads up to 2,200 lb [S1].
The mechanical difference is small but real. An internal bay sits within the unit's envelope, so the VLM footprint stays at roughly 66 in / 1.6 m deep for slim models; an external bay protrudes from the unit opening and creates an independent operator workstation that can host laser pointers, picking scales, or a crane-assist arm [S1][S4].
Internal Bay: When the Footprint Is the Constraint
An internal bay is the default on slim and single-column VLMs because it adds zero projection beyond the storage envelope. Modula's slim module ships at 66 in / 1.6 m depth, runs on 240 V single-phase, 240 V three-phase, or 48 V three-phase, and is built for installations where aisle width is the binding constraint [S1].
Throughput tops out around 140 trays/hour for single-elevator designs, which is sufficient for slow- to medium-pick operations such as MRO stores and hospital pharmacy replenishment [S1][S3]. White Systems describes this "Golden Zone" pick window as 37-40 in high, the ergonomic band where the operator neither reaches nor bends [S3].
For cold-chain or outdoor installations, internal bays also keep the operator inside the conditioned envelope while the storage unit lives in an exterior climate-controlled enclosure, a layout Kardex documents for reclaiming 100% of interior floor space [S2]. Read the baseline on vertical lift module architecture before sizing bay type against ceiling height.
External Bay: Throughput, Ergonomics, and Heavy Item Handling
An external bay is a free-standing operator console attached to the VLM opening, which physically separates the pick zone from the storage column. Olpin Group notes that the external bay "offers the best possible ergonomic workspace for operators" and is the right pick when the operation requires crane assist or other mechanical manipulators for heavy trays [S4].
Dual-external configurations are where throughput climbs. Modula Flexibox combines an external bay with a second internal delivery point to reach 180 bins/hour, and White Systems' Compact Twin stages a second elevator behind the active pick face, so tray retrieval happens while the operator is still picking, pushing past 100 tray presentations per hour on standard-sized trays [S1][S3].
Practical limits matter. External bays add roughly 18-30 in of aisle projection depending on tray depth, and they need a flat, level floor with a defined load rating for the console footprint. A 2,200 lb tray delivered to an external bay is still a 2,200 lb dynamic load on that floor zone, so spec the slab, not just the unit [S1][S4].
Decision Matrix: Internal, External, or Dual Bay
Three criteria usually decide the call: available aisle depth, target throughput in trays/hour, and whether any SKU requires mechanical pick assist. Internal bay wins when aisle depth is under 8 ft and pick rates stay below 100 trays/hour. External bay wins when operators need crane assist, vision pick-to-light, or a large work surface, and dual bay wins on multi-shift order fulfillment above the 100 trays/hour line [S1][S3][S4].
Bay height is the hidden variable. Modula's three heights of 27.36 in, 35.23 in, and 50.98 in map to typical SKU profiles, small parts, medium cartons, and oversized items respectively. Spec the largest SKU height first, then the bay opening, because retrofitting a taller bay usually means a new extractor and new safety light curtains [S1].
Operators also need to compare access options against wider materials-handling choices such as selective vs drive-in pallet rack when the VLM is feeding a pallet zone rather than a bin zone, since pallet VLM and rack VLM solve different density problems.
Who Should Pick Internal vs External Bay
Pick internal bay if the facility is a leased space under 10,000 sq ft, ceiling height is below 24 ft, and the SKU mix is dominated by small-to-medium parts under 50 lb. Pick external bay if any SKU exceeds 100 lb, pick rates are sustained above 80 trays/hour, or the workcell will host a collaborative robot arm, scanner tunnel, or label printer that needs a flat external surface [S1][S4].
Pick dual bay if the operation runs two shifts and the same VLM feeds both an order fulfillment window and a replenishment window, or if the unit sits between two production cells and each cell needs its own pick face. Modula Flexibox supports up to 3 bays across the front, rear, and upper floors of a single unit, a layout worth modeling for multi-story mezzanines [S1].
For facilities already running automated storage alongside other plant assets, the same maintenance discipline applies across systems, and a CBM vs PdM vs RCM framework gives the reliability side a comparable structure to the throughput side.
Failure Modes and Spec Pitfalls
The most common internal-bay failure is thermal. A sealed internal bay with high-cycle picking can trap heat from the extractor motor and the operator's body, which drifts tray-positioning sensor calibration. External bays vent this heat into the aisle, but they pick up dust and forklift-propelled debris, so specify IP54 on the bay control panel and a positive-pressure air curtain if the aisle is dirty [S1][S3].
External bays are also the weak point for floor flatness. A 0.25 in deviation across the bay footprint is enough to mis-trigger the tray-present optical sensor on some models, and shimming a 1,500 lb console after commissioning is a service call nobody budgets. Pour a flat, reinforced pad under any external bay before delivery [S4].
Software side, dual-bay VLMs require WMS or WCS logic that knows which bay requested which tray; running both bays off one pick list leads to crossed orders. Modula's Copilot console and White Systems' MILO software both handle dual-bay logic natively, but the host system must be configured to publish separate pick-to-tote tasks per bay [S1][S3].
Sourcing, Standards, and Vendor Due Diligence
There is no single ISO or EN standard that governs VLM bay geometry, so the spec is built from mechanical and electrical references: ISO 12100 for general machine safety, IEC 60204-1 for electrical equipment of machines, and EN 528 for the broader class of ASRS including VLMs, which sets requirements for access openings, light curtains, and emergency stop coverage. The bay opening itself should be guarded to PL d per ISO 13849-1, and any outdoor enclosure must meet local wind and snow load codes plus the IP rating called out in the project specification [S2].
OEM disclosures to verify in any quote: tray payload (up to 1,102 lb for Modula Next, up to 2,200 lb for Lift and Pallet lines), throughput figure with its test SKU weight (140 trays/hour is rated at a specific load class, not all SKUs), bay height list, and drive type. Rack-and-pinion drives do not stretch over time and need less maintenance than belt drives, per White Systems' design notes, but they are also heavier and louder [S1][S3].
Plan a pilot with a single VLM in a non-critical cell for 60-90 days before committing to a fleet, and capture pick-rate, error-rate, and MTTR data in that window. For plants also weighing VLM against carousels or horizontal carousels for slow movers, the same access-control logic used in access control and ergonomic-height workstation design applies, since the limiting factor is rarely the machine and almost always the operator interface.
Track the following signals over the next two quarters: OEM disclosures of dual-extractor throughput at sustained 80%+ load (currently cited at "over 100 trays/hour" without a continuous-duty figure), and any updates to EN 528 or ANSI MH16.1 that touch VLM bay guarding, since the bay opening is the single most prescriptive interface in the entire machine.
Component reference pages worth checking: access scaffold.