Within the warehouse the largest standardized material-handling unit is generally the pallet, which is just a rigid base on which cartons can be stacked. Most are made of wood, but some are made of durable plastic.
    Pallets are available in a range of qualities and prices. In general order of quality and price,
they include string pallets, block pallets, and perimeter base pallets. Any pallet expected to be
handled by automation will generally have to be of high quality. As supply chains get longer, there is an incentive to use higher quality pallets.
    There are several standards, the most important of which appear in Table 5.1. Of these, the most common pallet in North America is the 1219*1016 mm (48*40 inch) pallet, also known as the Grocery Manufacturer’s Association or GMA pallet. (The 1000*1200 mm pallet is generally
interchangeable with the GMA pallet.)
    Some pallets are designed with special uses in mind. For example, the Australian pallet was
designed to be space efficient in Australian railroad cars; and the EURO pallet was designed to
fit through doorways. Neither is very space efficient in ISO standard shipping containers, while
the GMA pallet fits well.
    A 2-way pallet allows forks from a standard forklift or pallet jack to be inserted on either of the
40 inch sides. A 4-way pallet also has slots on the 48 inch sides by which it can be lifted by fork
lift. The 4-way pallets are slightly more expensive, but the extra flexibility in handling can save
both time and space. Less maneuvering is required to pick up such a pallet, and it can be oriented in either direction even in confined space.
    There is no standard height to which a pallet may be loaded.
    The simplest way of storing palletized product is floor storage, which is typically arranged in
lanes. The depth of a lane is the number of pallets stored back-to-back away from the pick aisle.
The height of a lane is normally measured as the maximum number of pallets that can be stacked one on top of each other, which is determined by pallet weight, fragility, number of cartons per
pallet, and so on. Note that the entire footprint of a lane is reserved for a SKU if any part of
the lane is currently storing a pallet. This rule is almost always applied, since if more than one
SKU was stored in a lane, some pallets may be double-handled during retrieval, which could offset any space savings. Also, it becomes harder to keep track of where product is stored. For
similar reasons, each column is devoted to a single SKU.

This loss of space is called honey-combing.
    Pallet rack is used for bulk storage and to support full-case picking (Figure 5.1). Pallet length
and width are reasonably uniform and pallet rack provides appropriately- sized slots. The height
of slots can be adjusted, however, as pallet loads can vary in height.
    The advantage of rack storage is that each level of the rack is independently sup- ported, thus
providing much greater access to the loads, and possibly permitting greater stack height that might be possible in floor storage.
    The most common types of rack storage are:
Selective rack or single-deep rack      stores pallets one deep, as in Figure 5.1. Due to rack  supports each pallet is independently accessible, and so any SKU can be retrieved from any pallet
location at any level of the rack. This gives complete freedom to retrieve any individual pallet
but requires relatively more aisle space to access the pallets.

Figure 5.1: Simple pallet rack. (Adapted from “Warehouse Modernization and Layout Planning Guide”, Department of the Navy, Naval Supply Systems Command, NAVSUP Publication 529, March 1985, p 8–17).

Double-deep rack      essentially consists of two single-deep racks placed one behind the other, and so pallets are stored two deep. Due to rack supports each 2-deep lane is independently accessible, and so any SKU can be stored in any lane at any level of the rack. To avoid double-handling, it is usual that each lane be filled with a single SKU, which means that some pallet locations will be unoccupied whenever some SKU is present in an odd number of pallets. Another disadvan- tage of deep lanes is that slightly more work is required to store and retrieve product. However, deep lanes have the advantage of requiring fewer aisles to access the pallets, which means that the warehouse can hold more product. A special truck is required to reach past the first pallet position.

Push-back rack.      This may be imagined to be an extension of double deep rack to 3–5 pallet
positions, but to make the interior positions accessible, the rack in each lane pulls out like a
drawer. This means that each lane (at any level) is independently accessible.

Drive-In or drive-through rack allows a lift truck to drive within the rack frame to access the
interior loads; but, again to avoid double-handling, all the levels of each lane must be
devoted to a single SKU. With drive-in rack the put-away and retrieval functions are
performed from the same aisle. With drive-through rack the pallets enter from one end of the lane and leave from the other, so that product can be moved according to a policy of First-In-First-Out (FIFO). Drive- in/through rack may be thought of as floor-storage for product that is not otherwise stackable. It does not enable the flexibility of access that other types of pallet rack achieve. In addition, there are some concerns; for example, in this rack each pallet is supported only by the edges, which requires that the pallets be strong. In addition, it requires a more skilled forklift driver to navigate within the lane, and such a person will be more expensive.

Pallet flow rack is deep lane rack in which the shelving is slanted and lines with rollers, so that when a pallet is removed, gravity pulls the remainder to the front.This enables pallets to be put-away at one side and retrieved from the other,Because of weight considerations, storage depth is usually limited to about eight pallets. This type of rack is appropriate for high-throughput facilities.

Except for automated storage-and-retrieval systems (AS/RS), some type of lift truck is required to access the loads in pallet rack; and specialized racks may require special- ized trucks. The most common type of lift trucks are:

Counterbalance lift truck is the most versatile type of lift truck. The sit-down ver- sion
requires an aisle width of 12–15 feet (3.7–4.6 meters), its lift height is lim- ited to 20–22 feet
(6.1–6.7 meters), and it travels at about 70 feet/minute (21.3 meters/minute). The stand-up version requires an aisle width of 10–12 feet (3.1–3.7 meters), its lift height is limited to 20 feet (6.1 meters), and it travels at about 65 feet/minute (19.8 meters/minute).
Reach and double-reach lift truck is equipped with a reach mechanism that allows its forks to
extend to store and retrieve a pallet. The double-reach truck is re- quired to access the rear
positions in double deep rack storage. Each truck re- quires an aisle width of 7–9 feet (2.1–2.7
meters), their lift height is limited to 30 feet (9.1 meters), and they travel at about 50 feet/minute (15.2 meters/minute). A reach lift truck is generally supported by “outriggers” that extend forward under the forks. To accommodate these outriggers, the bottom level of deep pallet rack is generally raised a few inches (approximately 10 centimeters) off the ground so that the outriggers can pass under.
Turret Truck uses a turret that turns 90 degrees, left or right, to put-away and retrieve loads.
Since the truck itself does not turn within the aisle, an aisle width of only 5–7 feet (1.5–2.1 meters) is required, its lift height is limited to 40–45 feet (12.2–13.7 meters), and it travels at about 75 feet/minute (22.9 meters/minute). Because this truck allows such narrow aisle, some kind of guidance device, such as rails, wire, or tape, is usually required. It only operates within single deep rack and super flat floors are required, which adds to the expense of the facility. This type of truck is not easily maneuverable outside the rack.
Stacker crane within an AS/RS is the handling component of a unit-load AS/RS, and so it is
designed to handle loads up to 100 feet high (30.5 meters). Roof or floor- mounted tracks are used to guide the crane. The aisle width is about 6–8 inches (0.15–0.20 meters) wider than the unit load. Often, each crane is restricted to a single lane, though there are, at extra expense,
mechanisms to move the crane from one aisle to another.