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.
1 回答
5.1.1 托盘存储
托盘通常是仓库里最大的标准作业单元,它由坚固的材料构成,支撑货箱在其上堆码。主要是木制,也有耐用塑料的。
托盘的质量与成本各有不同,从低到高,以纵梁托盘>四向托盘>周底托盘这样的顺序排列。用于自动化设备处理的质量会要求更高。而供应链越长,则人们越有越动力采用质量高的托盘。
托盘规格也是多种多样,表5.1列出最为主要几种规格。其中,最常见的托盘在北美是1219*1016毫米(48*40英寸)托盘,也被称为食品加工协会或GMA托盘(1000*1200毫米托盘一般可与GMA托盘互换。)
有些托盘是为特殊用途而设计的。例如,澳大利亚的托盘尺寸是为了提高铁路车厢空间利用率而设计的;而欧标托盘是为了方通进出门设计的。但是这两种在ISO标准集装箱内的空间利用率不好,而GMA托盘与集装箱就很配。
双向托盘可以让标准叉车或托盘搬运车的货叉从40英寸长的两边插入,而四向托盘则连48英寸的两边也能插入并叉举起来。四向的托盘成本要稍贵点,但在操作时间和空间会能有所节省。只需较少的机动调整就能叉起托盘,即使在有限的空间都能从任意方向作业。
关于托盘允许堆码的高度并不存在什么标准。
最简单的按托存储的方式是地面堆码,通常按货道来排列,每货道为一个货位,每排货位由两两背靠背的货道面对着通道组成,横向平铺的托数为货道深度,纵向堆码的托数为货道高度,堆码托数则因托上货物的重量、易损程度、每托码放的箱数不同而异。请注意,单个货道只能由一个SKU占用,无论是否存满。这个规则是普世通用的,因为如果在一个货道里存放多过一个SKU,在存取时就有可能需要挪货,导致多次处理同一货托,那么花费时间成本要明显大于因混货而节省出来的空间成本,而且混货也法准确地跟踪SKU到底存在哪里。因此,每个货道只能用于一种SKU,因此会出现有货道未被存满,造成了空间上的损失,它被叫做蜂窝效应。 托盘货架用于托盘化的货品按托存取的(图5.1)。托盘的长宽要最好统一,而货架上库位的长宽要与其匹配,因托盘码放高度不一致,货位的高度应可以调整。
因为货架每一层都有独立支撑,因此其载荷能力会更大,可能会比地面堆码在纵向取得更高的堆码层数。
通常主要有以下几种货架:
横梁式货架或单进深货架:如图5.1所示,只存单托深度。每托货都能独立存取,对通道需要求比较高。 双深度货架:本质上是由两个单深货架紧贴着前后设立,在深度上能存放两托。每个2进深货位都是可以独立存取的,因此每层的每个货位都能存放任何SKU,但是为避免多次挪托,则每个货位原则上只允许存放同种SKU,当SKU的货量为奇数量,则有一托的存储空间被闲置了。另外,还存在需要花费额外的时间来存取居于货位里面的托盘的不便。但是,双通道会因为对通道的需求下降而带来更高的存储能力,能处理更多的货品的好处。当然,存取后部的托盘也需要配置专门的叉车。
后推式货架:你可以把它当成2进深货架扩展成3-5进深,但是位于后部每个托盘都能存取,托盘可以如抽屉那样可以被拉出来,它每一层的每一个货道都是能独立存取的。 驶入式或驶出式货架:允许叉车驶入货架里面存取里面的货托,为了不发生挪货,同列货位所有层只允存同种SKU。驶入式货架的存取都在同侧通道;而驶出式货架,则可以从一边存入,另一边取出,可很好地实现FIFO先入先出。驶入/驶出式货架可以视做是地面堆垛,只不过没有托盘直接叠放。它不能如其它类型的托盘货架那样地可以灵活存取。此外,由于每个托盘只是架放在两侧的货架边缘,对于托盘的强度需要额外的关注。而且,在这样的货架里作业的工人需要有额外的经验,相对于来说成本将会更高。 流利式托盘货架:托盘被存放在带有角度倾斜和辊轮的多进深货架上,但位于外部的托盘被取走时,后面托盘会因为重力自动滑向前面来。它允许从一边存入,从另一面取出,因为重量的原因,通常最多为8托进深。这种托盘用于高吞吐量的仓库。
在货架上存取托盘,除用自动存储系统(AS/RS)之外,都需要专门叉车来完成存取,而特殊的货架会需要配特殊的叉车。 平衡式叉车:这是最通用的叉车。坐式的需要宽为12-15英尺(3.7-4.6米)的通道,举升高度为20-22英尺(6.1-6.7米),行驶速度为每分钟70英尺(21.3米/分钟)。站式的需要10-12英尺(3.1-3.7米)宽的通道,举升高度最多20英尺(6.1米),行驶速度约为65英尺/分钟(19.8米/分钟)。
前移式或双伸式叉车:它有一个能够将货叉伸展进行存取托盘的机构。而双伸式的还能让叉车存取位于双深货架里面的货托。叉车需要7-9英尺(2.1 - 2.7米)宽的通道,举升最高为30英尺(9.1米),行驶速度约为50英尺/分钟(15.2米/分钟)。叉车一般由“外伸臂”支撑,外伸臂在叉下,支撑货叉向前伸展。为了容纳这些外伸臂,货架底层托盘的通常需要离地几英寸(约10厘米),以便让外伸臂可以通过。
转叉式叉车:它通常可以让货叉进90度的向左或向右转向。因为叉车无需在通道整体转向,其通道仅需要5-7英尺(1.5-2.1米),最高举升在40-45英尺(12.2-13.7米),行驶速度为75英尺/分钟(22.9米/分钟)。因为通道是如此地窄,它需用额外的引导装置,如导轨、线缆、胶条等。它只能用于单深度的货架,而且地面也要求非常平,因此用它对仓库会有额外的投入。这种叉车在货架外是不好机动的。
AS/RS堆垛机:是AS/RS中的存取组件,它可以最高举升到100英尺(30.5米),被安装在屋顶或地板上的轨道来引导。通道宽度比搬运单元宽约6-8英寸(0.15-0.20米)。通常每台堆垛机只工作于一条巷道内,如果要让堆垛机能移动另一条巷道,则需要额外费用和机构。
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