A Guide To Pallet Storage Design In The Context Of Industrial Storage And Materials Handling

Principles of Pallet Storage Design

One of two no-nonsense guides to safe storage from Redirack

A guide to the basic principles of pallet storage design, in the context of modernindustrial storage and materials handling

THE PRINCIPLES OF PALLET STORAGE DESIGN

Foreword Page 4

What is a store? 5

How wide? 8

Which pallet? 9

How high? 12

Pallet rack layout 15

Which pallet, which rack? 18

How much can be stored? 19

Alternative pallet storage layouts 22

CONTENTS

FOREWORDIncreases in the cost of square footage, the high cost of building materials, higher labour charges, expensive equipment, and stiff penalties for late deliveries and damaged goods. The key to success in the storage and materials handling business is, to a great part, knowing how to get over these hurdles.

The science of storage and materials handling is a fast-growing science. New systems and techniques come thick and fast. But a close look at the ‘science’ will reveal a basis of simple fundamentals and good old fashioned common-sense.

The title of this booklet, ‘The Principles of Pallet Storage Design’, may sound much like that of a complicated textbook. Far from being complicated, it is like the science it is about, mostly common-sense. Although only a very concise guide to basic principles, this booklet will provide you with the basic formulae necessary either for designing a working storage installation, or for improving an existing one.

It won’t, of course, turn you overnight into a leading storage and materials handling expert. But it will, we hope, help you over some of those hurdles that exist in the business of building and operating a successful storage installation.

Before we can begin to analyse stores, storage methods, materials handling and so forth, we must know what a store is. And we must identify the unique characteristics which make it a store.

To help us arrive at a defi nition, let us take a simple store, found in every household. The most common storage item in a house is the cold water storage tank, found normally in the roof space.

Water comes into the tank from the mains, the water is held in the tank and is then drawn off through various taps.

Immediately we can see three of the basic characteristics of a store - water comes IN (goods come in) the water is HELD (goods are stored), the water goes OUT (goods go out).

IN HELD OUT

The next distinguishing features of a store is why the water is held.

Let us look at what happens when we draw water off. Sometimes no water is drawn off. One may turn on a tap. A toilet is fl ushed, and somebody runs a bath. These things can happen individually, at any time, or all together, or, one single demand can be combined with any other demand. In other words, the demand for water in a household varies from nothing, to all taps running at the same time.

However, water comes in from the Main at a constant rate. So it would be impractical to expect the water from the Main to meet this widely varying demand. At times, if all taps were being used, the pressure would be very low and each user would get very little water.

So the water tank acts as a BUFFER between the fl uctuating demands for water, and the rate of supply of water. Notice we have built up the defi nition of a store - A STORE HOLDS GOODS AS A BUFFER BETWEEN THE FLUCTUATIONS IN SUPPLY OF THE GOODS AND THE DEMAND FOR THE GOODS.

Stores throughputAlthough the function of a store is to hold goods, the way In which goods fl ow through a store affect the method in which they are held. In particular, goods come in and go out in the following manner:

BULK IN BULK OUTINDIVIDUAL IN INDIVIDUAL OUT

SETS IN SETS OUT

Any alternative in the way the goods come in,shown in the fi rst column, can be combined with

any alternative in the way goods go out, shown in the second column. To make matters more complicated, in any store there can be several of these combinations operating at once.

Take for example, a factory where at the end of the production line, coils of wire are stacked on to a pallet of half a ton or one ton capacity. They are moved into the store on a pallet, so we have BULK IN. Ideally despatches to customers are made in pallet loads, so we get BULK OUT.

BULK IN BULK OUT

However, there are times when an individual coil has to be despatched to a customer, and this is a case of BULK IN - INDIVIDUAL OUT.

Take another factory where pneumatic tools are assembled. Bulk supplies of components come in from suppliers. In the stores different components are selected and put together on to a tray to make up a completed tool, so here we have BULK IN - SETS OUT.

If we consider a pallet racking beam or a pallet racking frame as one item, then at the end of the production line individual items are transferred to the store where they are put together to meet an order, so we have INDIVIDUAL IN - BULK OUT.

Of course some items are either too large or too valuable to be moved other than in individual form, e.g. large diesel engines that go into mining equipment. These are moved into, and out of store individually.

Under normal circumstances it is rare to get a SETS IN - BULK or INDIVIDUAL OUT. This most common occurrence of sets is in the outward movement, but the above is theoretically possible.

The one exception to these defi nitions of fl ow is when the holding operation is the prime function and the fl ow through is insignifi cant. An example of this is where the goods are held for an aging process, where for example, whisky may be kept in store from fi ve to fi fteen years.

‘Measuring’ performanceThere are four criteria we can use in measuring the ‘performance’ of a store.1. Use of volume. 2. Ease of selection.3. Ease of handling. 4. Preservation of the quality of goods.The ‘Use of volume’ is a statement of how much cubic capacity is effectively being used out of the total amount available, i.e. the volume of the goods

WHAT IS A STORE?

stored compared with the volume of the building length x breadth x height.

‘Ease of selection’ - indicates all those factors which go to the identifi cation of the location of the goods stored, both for putting away into the stores, and for getting them out to meet despatch requirements.

‘Ease of handling’ describes those factors that affect the handling of goods, whether mechanical handling is required, whether double handling is involved, the speed of handling, etc.

‘Preservation of the quality of goods’ relates to deterioration of the goods, either by accidental damage, age, as with vegetables for example, or deterioration because goods go out of style, e.g. fridges, cookers (which although fairly robust in themselves do have changing styles ).

Now these factors are all inter-related. Any action to improve one factor will most likely adversely affect the others.

A simple theoretical example can demonstrate this. Imagine you have a simple cubic building with one door into it. If you fi ll this building up completely, you will make excellent use of volume. However, if you have more than one type of item stored in there, then it would be extremely diffi cult to select the item you want. So what happens?

Do you start creating gangways to improve the selection of goods? This will immediately reduce the use of the volume.

If you require mechanical handling the gangways need to be even wider, reducing the volume, and there is always a danger that with mechanical handling the rate of damage to goods increases.

Of course, you might fi nd that by block stacking the lower items will be crushed, so you may decide either to store only one-high with the consequent reduction in volume, or put the goods on racks, which also reduces the amount of volume effectively used.

The most common plea from someone investigating an effi cient store is that he needsmore space. In practice this is rarely so. There is usually enough volume to cater for the goods he stores. It is more likely that the requirement is infact, for improved handling, improved selection ofgoods (whether putting away or despatching), or forincreased protection of the goods. This is why great care must be taken to identify the more important factors relating to the optimum effi ciency of the store.

For a simple example relating to the use of volume, imagine a building 30 m long by 14m wide by 7m high to the inside of the ceiling obstructions, that is 2940 cubic metres.

Storage capacity of layout:

1 block 22 pallets x 6 pallets x 4 pallets high = 528 pallets2 blocks 10 pallets x 5 pallets x 4 pallets high =400 palletsTOTAL: 928 pallets

Each pallet uses 1.5 cubic metres.

Therefore 1392 cubic metres of storage is utilisedin 2940 cubic metres available.

=47.35% usage!

That allows only 42 different lines to be stored and assumes every single space can be used, which will not be the case in practice.

PALLET AND LOAD DETAILS

When considering storage equipment to be used with uprights we need to go back to basics. The function of Adjustable Pallet Racking is to store goods on pallets. Therefore, the physical parameters of the pallets affect the racking used. The weight-carrying capacity of pallet racking is a specifi c instance of a general principle. The affects of the weight of the load can be traced through all the structural members of the racking down to the fl oor, and every structural member must be considered as follows:

At this stage we are considering wooden pallets only. Pallets are objects which permit goods to be moved mechanically by fork lift trucks. These goods may be single units or a multiple of items, but normally the load is such that it cannot easily be carried manually, and the use of a fork lift truck effects cost savings.A pallet must have a deck on which to carry goods, a space into which forks can enter, and a support on the ground. Because the thickness of forks entering the pallet is of the order of 50 mm the

gap between the top and bottom decks is usually 100mm.

When used in conjunction with racking, pallets normally rest on beams (they of course could rest on solid shelving but this would be an expensive way of carrying the load). Consequently the underside of the pallet, which is the interface between pallet and beam, is most important. Pallets are frequently described as two-way or four-way entry, which means that the forks can either enter on two sides or four, as the sketch shows.

In the following section, WHICH PALLET?, we take a closer look at pallets and pallet types, the choice of which will ultimately dictate to a larger extent, the width of racking to be used.

GENERAL PRINCIPLE

LOAD

SHELVING

BEAMS (IF USED)

UPRIGHTS

FLOOR

PALLET RACKING

LOAD

PALLETS

BEAMS

FRAMES

FLOOR

HOW WIDE?

Pallet components - standard terms and defi nitionsTWO-WAY ENTRYHere the bearers permit the entry of forks or fi ngers from two opposite directions only. The sides through which the forks enter are called “entry sides”.FOUR-WAY ENTRYThe bearers (or blocks) permit entry of forks or fi ngers at each end and at each side. The sides through which the load wheels of a pallet truck can pass without leaving the ground are known as “free entry sides” . The other sides, where the load wheels have to pass over the bottom slats are termed “restricted entry sides”.BEARERSMembers under the top deck or separating the top and bottom decks, which provide a space for the entry of forks or fi ngers. They may consist either of longitudinal members or distance pieces (usually known as blocks).

DECKThe top or bottom fl at surface which may be solid or otherwise.ENTRY MEMBERSMembers forming the outside edges of decks on the sides in which the forks or the fi ngers enter.

STRINGERA horizontal member connecting the bearers and supporting the deck.ENTRYSpace permitting the entry of forks or fi ngers in a particular direction.WINGSThose parts of the deck or decks which project beyond the bearers.

CHAMFERA rebated angle in the structure of a pallet, in particular on the edges of the bottom deck, to facilitate the passage of the load wheels of a pallet truck.

STANDARD SIZES FOR FLAT PALLETSThe British Standards Institution has selected the following plan dimensions for standard pallets: 800 mm x 1200 mm, 1000 mm x 1200 mm, 1200 mm x 1200 mm, 1200 mm x 1800 mm (B.S. 2629 1967)

The International Organization for Standardization has also recommended the above sizes and proposed the following plan sizes as international standards for maritime use:

1200 mm x 1600 mm and 1200 mm x 1800 mm

These standard plan sizes take into account the majority of transport vehicles and are designed to give the best practicable loading on general purpose vehicles, provided the pallets themselves are loaded to capacity.

TWO-WAY ENTRY, SINGLE DECKEDA pallet with only a top deck. Suitable for items which can withstand the concentrated loads imposed by the bearers, during stacking, e.g. wooden boxes and drums. Also used where little stacking is required and for storing unit loads in pallet racking when the bearers are supported by the rack framework. Shown here with an Open boarded (slotted) deck.

WHICH PALLET?

(a) Through chamfer

(b) Stopped chamfer

TWO-WAY ENTRY, REVERSIBLEA pallet with similar top and bottom decks, either of which will take the same load. This gives even distribution when stacking. Shown here with a Close-boarded deck. Note: Not suitable for use with pallet trucks.

TWO-WAY ENTRY, NON-REVERSIBLEFor multi-tiering. The bottom slats add to the strength of the pallet and provide a larger surface area, therefore giving a less concentrated load distribution. Shown here with an Open-boarded (slotted) deck.

TWO-WAY ENTRY, REVERSIBLE, WINGEDThe decks extend beyond the outer bearers for lifting purposes, e.g. by crane with spreader-bar slings. Similar top and bottom decks either of which will take the same load. Even load distribution when stacking. Shown here with a Close-boarded deck. Note: Not suitable for use with pallet trucks.

FOUR-WAY ENTRY, NON-REVERSIBLEA pallet having bearers which permit the entry of forks or fi ngers from either side or end. This can be of advantage in block stacking operations. Only suitable for items which can withstand the concentrated load imposed by the bearers when stacking. Shown here with a Close-boarded deck.

TWO-WAY ENTRY, PERIMETER BASEThe bottom slats are so arranged as to present a level bearing surface, thus giving a less concentrated load distribution when stacking. Shown here (inverted) with a Close-boarded deck.

FOUR-WAY ENTRY, PERIMETER BASEFour-way entry, having the bottom deck so arranged as to present a level bearing surface compatible with hand pallet truck usage, and giving a less concentrated load distribution when stacking. Shown here (inverted) with a Close-boarded deck.

FOUR-WAY ENTRY, CRUCIFORM PERIMETER BASEThe ‘cruciform’ design of the bottom slats adds to the strength of the pallet and improves load distribution when stacking. The most widely used type of four-way entry pallet. Shown here (inverted) with a Close-boarded deck.

FOUR-WAY ENTRY, REVERSIBLEA pallet with similar top and bottom decks, either of which will take the same load. Gives even load distribution when stacking. Shown here with an Open-boarded deck.

Note: Not suitable for use with pallet trucks. TWO-WAY ENTRY, NON-REVERSIBLE, WINGED A normal two-way entry pallet but with the deck extending beyond the outer bearers for lifting purposes, e.g. by crane with spreader-bar slings.

Shown here with an Open-boarded (slotted) deck.FOUR-WAY ENTRY, REVERSIBLE, WINGED Similar top and bottom decks either of which will take the same load. Even load distribution when stacking. Both decks extend beyond the outer bearers (blocks) for lifting purposes, e.g. by crane with spreader-bar slings. Shown here with Close boarded deck.Note: Not suitable for use with pallet trucks.

The advantages of storing pallets with the front to back runners resting on the beams enables the front and rear, left to right cross members to act as a locking device so that pallets cannot be accidentally dislodged. However, it does mean that the front to back members must be substantial, which virtually eliminates four-way entry pallets.

Locating uprights on the beams as shown in Sketch 2 permits the pallets to be enclosed within the perimeter of the rack. However, the width of the beam is only 50 mm so if the fork lift truck driver fails to put the pallet fully into the rack there is a danger he will miss the back beam, causing the goods to be dislodged, or if he puts the pallet 50 mm too far in he will miss the front beam level, again causing the goods to be dislodged. One can visualise the damage that would ensue to the racking.

85% of all wooden pallets in the UK measure 48 in x 40 in (1219 mm x 1016 mm, or the European standard equivalent, 1200 mm x 1000 mm).The next 10% of pallets are 40 in x 40 in (1016 mm x 1016 mm, or the European standard equivalent 1000 mm x 1000 mm). In the former case with the fork entering the 1200 mm side, 95% of all pallets enter into the racks with 1000 mm dimension front to back.

Consequently considering the two methods of loading pallets into racks as detailed before, the required beam centres are shown as follows. It can be seen why the 900 mm and 750 mm wide frames have been established as standards.

To determine the height of the rack, one has to combine factors relating to the fork lift truck, the goods, the pallet and the building.Fork truck capabilityBelow is a sketch showing the height of the lift as generally described by fork lift truck manufacturers. The thing to note is that they defi ne the height of the lift as the height to the top of the forks, when the mast is fully extended, down to the ground.

Let us translate this into practicalities. The pallet sits on the forks. The forks support the underside of the top timbers, thus there is a 125 mm of pallet hanging below the top of the forks. In addition, allowance must be made to clear the beams. This allowance increases with the beam height. The actual amount to be added on can be seen in column B on the chart on page 18.This means that the sum produced must be taken off the truck manufacturers maximum lift height to calculate the height at which you can put in the top beam.

However, fork lift trucks have parts extending above the height of the forks. The forks are supported by the mast. Sometimes there are load guards fi tted on top of the mast to prevent accidentally dislodged good falling back onto the driver. Loads rarely exceed the height of the parts of the fork lift truck extending above the top level of the forks. When working out a layout careful note must be made as to whether the pallet and its load is higher than those parts above the forks.

Trucks with extending mastsMany fork lift trucks have extending masts, usually denoted by the term double or triple mast, or duplex or triplex mast. They also have what is known as ‘free lift’, when used with these masts. What this means is the forks travel up the mast for some height before the mast starts extending.

Now it is possible that a user may have a fork lift truck with a double or triple extending mast, but is using it in a comparatively low ceiling situation. On these occasions it is quite possible that the mast itself will be higher than the load and care must be taken about this point.

HOW HIGH?

Most fork lift trucks used with pallet racking areeither reach-trucks or narrow-aisle trucks (which are based on the reach-truck construction). In conjunction with pallets care must be taken when calculating the clearance from the ground to the underside of the fi rst beam level. When the mast is retracted the pallet must sit on top of the wheel casings, so it is already above the ground at thispoint. As the mast is extended forward to put the

load into the rack the pallet will travel at this height above the ground. It is not until the pallet is fullyenclosed within the rack that the forks can be lowered to the ground. So, in addition to the clearance between the top of the load and the underside of the beam, clearance must be allowedfor the wheel casing, plus the height of the palletabove the wheel casing, as shown in the sketch.

Of course, it is possible for the forks to beextended with their load in the gangway, before thepallet it placed within the rack. This increases the width of the gangway because in effect the truck is being used as a conventional counter-balancetruck. The whole point of the reach truck principle islost on these occasions. Normally increasing theheight from the ground to the underside of the fi rstbeam level is a most cost effective way of using theavailable space.

Door sizesThe height of doors through which trucks will travel is obviously critical. Frequently, even when the forks are in a closed position, the height from the ground to the top of the mast can be in excess of 2,400 mm. Many doors are only 2,000 mm or 2,100 mm high.Roof heightsThe other factor that must be considered inchoosing the height of the top beam, is the buildingitself. There must be a clearance beneath the roof, the roof truss, or any obstruction such as steam pipes, lights etc, and the top of the pallet load. This clearance is not only required in the racks but alsoin the full width of the gangway. Clearance requiredis shown in column B of table on page 18. Errors

PLAN VIEW SHOWING PALLET SITTING ON WHEEL CASING

SIDE ELEVATION SHOWING PALLET SITTING ON WHEEL CASING

LOAD + PALLET 850mm BEAM 90mmCLEARANCE 75mm

FLOOR TO BOTTOM BEAM 1015mm

LOAD + PALLET 850mmBEAM 90mmCLEARANCE 75mmHEIGHT OF WHEEL CASING SAY - 400mm

1415mm

ResultantBeamHeight

1050mm

1425mm

are frequently made here - the following sketch shows what can happen.

The other clearance in the roof needed is that required for the sprinkler system. Normally, insurance companies like the top of the goods to be at least 300 mm clear beneath the bottom of the sprinkler head. They estimate that with this clearance, water can be thrown far enough to put out any fi re. Failure to meet this means that the insurance premium can be increased signifi cantly.

If you are measuring up a partially completed warehouse, remember that sprinkler systems, steam pipes, electric trunking etc, all tend to be among the last of the services put into a building. So it’s possible that when you measure up the building these services will not yet have been installed. (These comments apply to control boxes located on walls and columns, too.)

And on lower beam levels, 75 mm min. clearance is required between the top of the pallet and load to the underside of the beam above.

So how high?If a survey was carried out on all the fork lift trucks available it would be found that the height of the lift for fork lift trucks with a simple, single mast, would be in the range of 3 m to 4 m, the trucks with a double or duplex mast would be in the range of 5 m to 6 m and trucks with a triple or triplex mast would be in the range of 7 m to 8 m. However, before one can determine the height of racks one has to consider the load and the building.

Unless a building has been purpose-made for warehousing it is rare indeed that the height to the eaves is greater than 5 m. During the post-war boom in industrial building the standard height was about 4 m. In practice, over 95% of all buildings used for storage are less than 5 m high, and the majority of these are less than 4 m high to the eaves. Assuming that a load plus the top of the pallet is about 1 m high, and that we use a simple mast truck, the height to the top of a fully lifted load will be in the order of 4 m to 5 m. So it can be seen that depending upon the height of the load, the most common height to the top shelf is 2.4 m, 3 m and 3.6 m.

PALLET RACK LAYOUTIn any rack layout there are four components: 1. The space occupied by the goods and the

equipment on which they may be stored, with the necessary clearances.

2. The aisleways between the goods, for direct access to the goods.

3. The gangways, at the right angles to the aisleways.

4. Any other areas, devoted to such things as marshalling of goods, loading and unloading of lorries, fi re escape routes, clearances round heating systems etc.

Normally, the goods are stored within the perimeter of the pallet. If not, the comments about pallets will apply to the area occupied by the goods.

Aisleway widthsThe width of the aisleway is the clearance between the pallet, goods or rack, whichever clearance is the smallest:

Width of aisleway is determined by the turning circle of the fork lift truck. This is usually given by the manufacturer assuming a 1200 mm x 1000 mm pallet is being used. But check, sometimes they assume a 1000 mm x 1000 mm pallet.If the pallet being used by the customer is larger, a bigger aisleway is required.

ALWAYS INCREASE THE MANUFACTURER’S RECOMMENDED AISLEWAY BY AT LEAST 200mm.

Why?Warehouse layouts invariably involve a compromise between the use of the available space and the speed of materials handling. Working to the manufacturer’s minimum aisleways is similar to parking in a very tight parking space. It can be done, but it takes time, and there is considerable wear and tear on both the vehicle and the driver. (Imagine how fatigued you would be after 8 hours spent maneuvering in an out of tight parking spaces). It is reasonable to expect between 20 and 30 pallet movements per hour. This throughput will be halved with restrictive aisleways.

When calculating minimum aisleway widths fork truck manufacturers assume that pallets will be located as far back as possible on the fork.

And that pallets have been placed on racks accurately.

Quick Method of Assessing LayoutFor calculating the number of runs of pallet locations that can be positioned into the width of the room, a module consisting of one aisleway + two pallets + the clearance between the back of the

pallet, can be used e.g.

GangwaysGangways are used to gain access to aisleways.The absolute minimum width of one gangway should be 1800 mm. This permits only one fork lift truck at a time to use it. Although this is more than suffi cient for the truck to drive down, the space isneeded for turning in and out of aisleways.If more than one fork lift is being used on the above the gangway should be 3000 mm.

Gangways are basically dead space. They should be kept to a minimum. Consequently, the best utilisation of space is usually achieved when the racks run in the same directions as the longest

THIS

NOT THIS

THIS

NOT THIS

Aisleway + 2 pallets + Clearance = Module2500mm + 2 x 1000mm + 100mm = 4600mm

WIDTH OF ROOM ÷ MODULE = NUMBER OF AISLEWAYS FLANKED BY SINGLE ENTRY RACKS

EXAMPLE:

37 METRES ÷ 4,600 = 8 Aisleways(or two single entry and seven double entry racks i.e 16 x single entry racks)

dimension of the space. This rule holds true in 90% of cases.The number of gangways is frequently determined by the requirements of the Fire Offi cer and Fire Escape routes, e.g. the optimum layout may be:

This would be rejected because there are aisleways with access from one end only, causing dead ends or blind alleys. Thus there should normally be gangways at the end of aisleways.

It is possible, of course, to convert a gangway into an aisleway like this:

The number of gangways required - (which then determine the length of the racks) is determined by a number of factors such as the amount of accessrequired, number of forklift trucks in use, position of marshalling-area. However, it is desirable that the racks should not exceed 30 metres length. Longer than this the fork lift truck drivers’ effectiveness decreases, caused by things such as diffi culty in locating pallet positions, greater distance penalties for choosing wrong aisleways etc.

The beam length is determined by number of pallets per beam x left-to-right dimension of pallet, plus at least 75 mm clearance between each pallet, and 75 mm clearance between pallet and frame.So taking the most common pallet sizes used in this country, 48 in (1219 mm) x 40 in (1016), and itsEuropean equivalent 1200 mm x 1000 mm, two pallets side by side.

2 x 1219 = 2438 + 3 x 75 mm clearance. = 2438 + 225 = 2663 mm.2 x 1200 = 2400 + 3 x 75 mm clearance. = 2400 + 225 mm. = 2625 mmOur STANDARD is 2650 mmThe length of the rack is determined by adding the width of the upright + clearance to the length of the beam x number of bays + one upright (add 90 mm to beam length for our SD uprights, 110 mm for our HD uprights).e.g. 4 bays, SD Uprights, 2650 beam 90 + 2650 = 2740 x 4 = 10960 + 90 = 11050, overall length of run.

Width of frames, clearance between back-to back loads, pallet overhang, height clearances. These are all key considerations in planning a rack. Fortunately, the SEMA Code of Practice provides an easy way of calculating the necessary dimensions. They’re tried and tested

recommendations, and you should be surprised if your supplier suggests anything different. If he does, and he hasn’t got a convincing reason for doing so - use these tables to work it out for him...

Pallet locations end elevation2 WAY ENTRYtypical dimensions

x y z

750 600 100

900 700 100

1000 750 150

1200 900 150

4 WAY ENTRY

x y z

750 700 75

900 800 75

1000 900 100

1200 1100 100

x - overall depth of pallety - dimensions over outside of beamsz - back to back clearance between pallets or loads

Pallet locations front elevationtypical dimensions

BEAM HEIGHT

a b

3000 75 75

6000 75 100

9000 100 125

12000 100 150

Beam Height is the dimension from fl oor to top of beama - clearance between adjacent pallets or loads, and

pallet/load and upright.b - clearance between underside of beam and top

of load or pallet where no automatic height selection is used.

WHICH PALLET-WHICH RACK?

HOW MUCH CAN BE STORED?The illustrations on the following pages show layouts and capacities for various types of storage systems with different types of fork lift trucks.

The table on page 21 shows the theoretical and practical capacities of goods stored in each of these layouts. These are based on a building 60 m long by 32 m wide by 7 m headroom (clear).When considering the practical capacity of a store one must bear in mind the criteria for measuring the performance of a store (see page 5). These are:1. Use of volume2. Ease of selection3. Ease of handling4. Preservation of the quality of goods (quality

preservation)To get an exact answer to the question: ‘how

much can be stored?’ one would have to consider all the factors relating to a specifi c warehouse.

The notes below are designed to help you apply the generalised facts and fi gures to your specifi c application. In some cases they cover in detail points made earlier in the text.Ease of SelectionAs we’ve said, it is impossible to operate a store assuming 100% use of available capacity.

Normally it is desirable that goods of different types are not mixed together. Pallets of item B should not block access to pallets of item A. Therefore when assessing the practical capacity of a store for say drive-in racking, or for block-storage or live storage, one has to assume that the lane of storage is for one item only. Thus, in a drive-in or block-storage situation, four pallets deep front to back, by four pallets high, then all the 16 pallets should be of one type of goods.

If the goods are mixed, all that is happening is thecost space and storage equipment is being transferred to the cost of selection handling and quality preservation. Obviously if one has to remove and replace two or three pallets to get at the item required, then it can be seen handling will take considerably longer. There is a very good chance that the goods to be selected will be hidden by other items, so selection would take longer, and every time goods are moved the likelihood of damage increases.

‘LIFO’ and ‘FIFO’ are terms that you will come across in warehousing, they mean ‘Last In, First Out’ and ‘First In, First Out’. If deterioration in the quality of goods is important then a warehouse normally has to operate under FIFO principles, consequently block storage and drive-in storage are not practical because their use tends to be restricted to a LIFO situation.

If the principle of non-mixing goods is adhered to, then the variety of goods that can be kept will depend upon the number of distinct different storage locations available. In layout 5, showing block storage, the theoretical capacity is 5076 pallets. However, there are only 94 lanes so that particular store can accept only 94 different types of goods or lines. To establish whether the extra capacity would be useful, one should ask ‘How often would we get a situation where all 94 lines will each fi ll every single space available?’ On our table of capacities of the various layouts we have shown the number of ‘picking locations’ available against each lane.Picking Locations‘Picking locations’ is an important term, which needs explaining.

Basically there are two ways of locating stock. One can either locate goods that come in into the fi rst available space, i.e. Random Location, or specifi c locations can be allocated for certain types of goods, i.e. Specifi c Location.

In Specifi c Location the item numbered 1 will go in the fi rst location and the item numbered 2 will go into the second location, and so on. By using the item stock number obtained from a catalogue orcomputer printout the location holding the stock of that item can be found. The potential disadvantage of such a location system is where the size of the goods stored varies considerably. It is used extensively for example in certain sub sections of mail order warehouses, e.g. shirts, pyjamas, dresses etc. are packed in boxes all of roughly the same size. However, in sayan engineering spares or service store, item 1 could be a complete tractor engine, and item 2 a 12 mm washer. Here it can be seen that variation of size can cause great wastage of space if a Specifi c Location system is used.

As a consequence most stores do tend to work on a Random Location system. Unfortunately it often happens that no systematic method of enumerating the locations is used and these warehouses tend to rely on the memory of a storekeeper as to the location of a particular item A systematic method of pin-pointing a location is to use a numerical or alpha numerical system of the fi rst digit numbering the rack, the second digit numbering the bay and the third digit numbering the shelf level. So working on a six digit code, a location might be numbered as follows: 05 1402, which means the fi fth rack, the 14th bay and the second location from the fl oor assuming goods are stored on the fl oor, i.e. fi rst shelf level. All that is needed now is for the warehouse keeper to record on a card where he is putting the goods using this

numerical location system and when the goods are called for despatch they can be located very easily.

This brings us to the term ‘Picking Location’which is one frequently used in mail order andwholesale operations which tend to store goods on a Specifi c Location basis. When a storekeeper is assembling a multi-item despatch order he walks around the various locations and picks the goods required, hence the term Picking Location. The corollary of that term is ‘Putting Away Location’. Ease of Handling

If a fork lift truck has to drive down a narrow aisle it will go slowly. It is for this reason that installations using narrow aisle trucks have guide rails so that the speed of the fork lift truck operation can be increased. However, guide rails are not normally supplied with drive-in installations or block storage installations so in these cases speed of operation is reduced dramatically.

The smaller the number of aisleways and gangways the smaller the number of fork lift trucks that can be used to operate the warehouse. So in the warehouse where there is fast movement of the goods this can be a disadvantage.

Where the border between the aisleway and the goods is not clearly defi ned this will lead either to an increase in the damage to goods or to slowing of handling.

As mentioned before if different types of goods are mixed together this automatically slows down the handling. One of the most important principles that should be applied to handling and particularly to the place where goods are kept is Pareto’s rule:

“In any series of elements to be controlled, a selected small fraction in terms of numbers of elements almost always accounts for a large fraction in terms of effect. This tendency has also been expressed as the concept of the ‘vital few and the trivial many’ and is commonly known as the ‘80 -20 Rule’” . This means in warehouse terms that 20% of the lines account for 80% of the volume of goods despatched, volume of space taken up, volume of goods coming in. Ideally therefore if these can be identifi ed they should be located near to the goods inward and despatch areas to reduce the amount of fork lift truck travel.

In block storage and drive-in it is essential that the width of the fork lift truck is smaller than the width of the pallet, otherwise extra space has to be allowed to accommodate the fork lift truck in the storage lanes.Use of VolumeAnother term one comes across is ‘Lead Times’. This is the time taken from the order being placed to the delivery of the goods. Usually suffi cient goods have to be stored to accommodate the despatch demands during the lead time. In other words, if it takes four weeks for goods to be delivered after ordering, it is advisable to have at least fi ve weeks

stock, to ensure that after the purchase order has been placed demands on the store can still be met. This of course has a serious consequence on practical use of volume.

If goods go out on an average of one pallet a day and the lead time is 20 working days, then goods would be re-ordered normally when there is 25 working days stock. At that stage there are 25 pallets in stock. However, by the time the fresh supplies of goods have arrived this stock holding is down to 5 pallets. So one can see that on average during this time one has 15 pallets in stock.

If the minimum quantity that can be ordered is say 30 pallets then at the end of four weeks the stock holding is 35 pallets.

Consider the consequence on the effective use of volume on bulk storage systems such as block stacking or drive in. When the purchase order is delivered 35 to 40 pallets are in stock, so 35 to 50 spaces must be available. Just prior to that delivery only 5 pallets are in stock. This is a situation that is repeated throughout a warehouse, but with different phasing. On average therefore there are 23 pallets occupying 40 pallet spaces, or 57.5% of the usable space.

This is a situation that will apply in all warehouses using the Specifi c Location system, and, because of the constraints of not mixing goods bulk storage systems must operate to a Specifi c Location system. Basically, on average, in the examples given in the bulk storage situations (drive-in, block stacking, and live storage). it is estimated the practical use of the space available is just over 50%.

Now of course this problem will not apply to Random Stock location systems because as stocks go down in one item the space freed can be fi lled with different goods. It has been known that a Random Stock selection system has been operated up to a 92 or 93% effi ciency, but this demands extensive systems-control of the warehouse. For practical purposes a fi gure of 80% should be used.

The amount of money tied up in stock would be exorbitant if on every single occasion any customer placed an order it could be met out of stock immediately. Most warehouses aim to achieve a 90% satisfaction level. In other words in any 10 items ordered 9 will be in stock.

By judicious use of the Pareto’s rule, and ensuring that the 20% popular lines meeting the 80% demand are always in stock a 99% customer satisfaction level can be achieved.GeneralIt tends to be treated as a bit of a platitude, but the situation is that the only constant in business is change and that any warehouse or store designed for today’s business is likely to be obsolete next year or the year after. Frequently warehouses are

designed to allow for the maximum of fl exibility to meet changing conditions even if this means a loss in the use of volume.

Bearing this in mind an optimum warehouse

for any particular business would normally contain a conglomerate of different types of storage equipment, be it block stacking, conventional racking, drive in, live storage or powered mobile.

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ALTERNATIVE PALLET STORAGE LAYOUTS1. Reach Truck

2. Reach Truck

3. Counter Balance Truck

4. Drive-in

5. Block Storage

6. Powered Mobile

8. Live Storage

7. Powered Mobile

9. Narrow Aisle

10. Pallet Converter

Plan view

N.B. Uprights encroach on pallet space thus reducing capacity

Redirack Limited, Wharf Road, Kilnhurst, Mexborough, South Yorkshire. S64 5SU.Tel: +44 (0) 1709 584 711 Fax: +44 (0) 1709 589 821 Web: www.redirack.co.uk