Lecture Notes - Week 5 Material Handling - Storage

Industrial Automation BMFA 3443

MATERIAL HANDLING - STORAGE

To understand and explain the storage

system performance and location strategies.

To differentiate between

conventional and automated storage

and identify the reasons for

automated storage.

To understand the analysis method of

storage system.

Chapter Objectives

©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems,

and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Categories of Material Handling Equipment

1. Material transport equipment - to move materials inside a factory, warehouse, or other facility

2. Storage - to store materials and provide access to those materials when required

3. Unitizing equipment - refers to (1) containers to hold materials and (2) equipment used to load and package the containers

4. Identification and tracking systems - to identify and keep track of the materials being moved and stored



©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book

Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Storage Systems

Sections:

1. Storage System Performance and Location Strategies

2. Conventional Storage Methods and Equipment

3. Automated Storage Systems

4. Engineering Analysis of Storage Systems



Storage Systems

Function – to store materials (e.g., parts, work-in-process, finished goods) for a period of time and permit retrieval when required

Used in factories, warehouses, distribution centers, wholesale dealerships, and retail stores

Important supply chain component

Storage Systems

Current situation • manual methods for

storing and retrieving items

• Inefficient• Human resources• Factory floor

space• Material control

Alternative solution• Automation

• to improve efficiency





Storage System Performance

Performance measures for storage systems: Storage capacity - two measures:

Total volumetric space Total number of storage compartments (e.g., unit loads)

Storage density - volumetric space available for storage relative to total volumetric space in facility

Accessibility - capability to access any item in storage System throughput - hourly rate of storage/retrieval

transactions Utilization and availability (reliability)



Storage Location Strategies

Randomized storage - incoming items are stored in any available location Usually means nearest available open location

Dedicated storage - incoming items are assigned to specific locations in the storage facility Typical bases for deciding locations:

Items stored in item number sequence Items stored according to activity level Items stored according to activity-to-space ratios

Each item type stored in a warehouse is known as Stock-Keeping-Unit (SKU)



Inventory Level as a Function of Time

Example: No. of SKU to be stored = 50 SKUs

Comparison of Storage Strategies

Randomized storage strategy Dedicated storage strategy The number of locations required

for each SKU is equal to the average inventory level of the item, since incoming orders are scheduled each day throughout the 50-day cycle

This means that when the inventory level of one SKU near the beginning of its cycle is high, the level for another SKU near the end of its cycle is low.

Thus, the number of storage locations required in the system is

50 SKUs x 60 cartoons = 3000 locations

The number of locations required for each SKU must be equal its maximum inventory level

Thus, the number of storage locations required in the system is

50 SKUs x 110 cartoons = 5500 locations





Comparison of Storage Strategies

Less total space is required in a storage system that uses a randomized storage strategy Dedicated storage requires space for maximum

inventory level of each item Higher throughput rates are achieved in a system that

uses dedicated storage strategy based on activity level The most active items can be located near the

input/output point Compromise: Class-based dedicated storage

Items divided into classes according to activity level Random storage strategy used within each class



Conventional Storage Methods

Bulk storage - storage in an open floor area Problem: achieving proper balance between storage

density and accessibility Rack systems - structure with racks for pallet loads

Permits vertical stacking of materials Shelving and bins - horizontal platforms in structural frame

Steel shelving comes in standard sizes Finding items can be a problem

Drawer storage - entire contents of each drawer can be viewed



Bulk Storage

Bulk storage arrangements: (a) high-density bulk storage provides low accessibility, (b) bulk storage with loads forming rows and blocks for improved accessibility



Pallet Rack System

Pallet loads placed on racks in multi-rack structure



Drawer Storage



Automated Storage Systems

Mechanized and automated storage equipment to reduce the human resources required to operate a storage facility

Significant investment Level of automation varies

In mechanized systems, an operator participates in each storage/retrieval transaction

In highly automated systems, loads are entered or retrieved under computer control



Objectives and Reasons for Automating Storage Operations

To increase storage capacity To increase storage density To recover factory floor space currently used for WIP To improve security and reduce pilferage To reduce labor cost and/or increase productivity To improve safety To improve inventory control To improve stock rotation To improve customer service To increase throughput



Types of Automated Storage System

1. Automated Storage/Retrieval System (AS/RS) Rack system with mechanized or automated crane to

store/retrieve loads

2. Carousel Storage System Oval conveyor system with bins to contain individual

items



AS/RS

Unit load AS/RS with one aisle



AS/RS Types

Unit load AS/RS - large automated system for pallet loads Deep-lane AS/RS - uses flow-through racks and fewer

access aisles Miniload AS/RS - handles small loads contained in bins or

drawers to perform order picking Man-on-board AS/RS - human operator rides on the

carriage to pick individual items from storage Automated item retrieval system - picks individual items Vertical lift storage modules (VLSM) - uses a vertical aisle

rather than a horizontal aisle as in other AS/RS types



AS/RS Applications

1. Unit load storage and retrieval Warehousing and distribution operations AS/RS types: unit load, deep lane (food industry)

2. Order picking AS/RS types: miniload, man-on-board, item retrieval

3. Work-in-process storage Helps to manage WIP in factory operations Buffer storage between operations with different

production rates Supports JIT manufacturing strategy



Carousel Storage Systems

Horizontal Operation is similar to overhead conveyor system used

in dry cleaning establishments Items are stored in bins suspended from the conveyor Lengths range between 3 m and 30 m Horizontal is most common type

Vertical Operates around a vertical conveyor loop Less floor space required, but overhead room must be

provided



Horizontal Carousel Storage System

Manually operated horizontal carousel storage system

Vertical Carousel Storage System

Example of Vertical carousel storage

system





Carousel Applications

1. Storage and retrieval operations Order picking Kitting of parts for assembly

2. Transport and accumulation Progressive assembly with assembly stations located

around carousel

3. Work-in-process WIP applications in electronics industry are common

4. Unique applications Example: time testing of electrical products



Engineering Analysis of Automated Storage Systems

1. Automated Storage/Retrieval Systems Sizing the AS/RS AS/RS throughput analysis

2. Carousel storage systems Storage capacity Throughput analysis

Sizing an AS/RS System

The total storage capacity of one storage aisle depends on how many storage compartments are arranged horizontally and vertically in the aisles

Capacity per aisle can be expressed as: Capacity per aisle = 2nynz ny = number of load compartments along the length

of the aisle nz = number of load compartments that make up

the height of the aisle 2 is the constant, accounts for the fact that loads

are contained on both sides of the aisle©2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems,


Top and Side Views of a Unit Load AS/RS

ny= 9 nz= 6

W

H

Pick up & Deposit (P & D) station

P & D station

Storage compartments

Bottom rail

Overhead rails

Assuming a standard size compartment Let x and y = the depth and width dimensions of a unit load, z =

the height of the unit load The width, length, and height of the rack structure of the AS/RS

aisle are related to the unit load dimensions and number of compartments as follows: W = 3 (x + a) L = ny (y +b) H = nz (z + c)

Where W, L, and H are the width, length, and height of one aisle of the AS/RS rack structure (mm, in);

x, y, and z are the dimensions of the unit load (mm, in); and a, b, and c are allowances designed into each storage compartment

to provide clearance for the unit load and to account for the size of the supporting beams in the rack structure (mm, in). Recommended values a= 150mm (6 in), b = 200mm (8 in), and c=250 mm (10 in)



Example: Sizing an AS/RS System

Each aisle of four-aisle AS/RS is to contain 60 storage compartments in the length direction and 12 compartments vertically. All storage compartments will be the same size to accommodate standard size pallets of dimensions: x= 42 in and y= 48 in. the height of a unit load z = 36 in. Using the following allowances a= 6 in, b=8 in, and c= 10 in determine: How many unit loads can be stored in the AS/RS The width, length and height of the AS/RS



Answer

(a)Storage Capacity Capacity per aisle = 2(60)(12) = 1440 unit Total for 4 aisles = 4(1440) = 5760 unit

(b)Structure dimension W = 3(42 + 6) = 144 in = 12 ft Overall W = 4(12) =48 ft L = 60(48 + 8) = 3360 in = 280 ft H = 12(36 + 10) = 552 in = 46 ft



AS/RS Throughput Analysis

Defined as the hourly rate of

Storage / Retrieval

transactions that the automated storage system

can perform.

Several methods are available to compute AS/RS cycle times to estimate throughput performance. The next following method assumes:• Randomized storage of loads in

the AS/RS• Storage compartments are of

equal size• The P & D station is located at

the base and end of the aisle• Constant horizontal and vertical

speeds of the Storage/Retrieval machines

• Simultaneous horizontal and vertical travel

Material Handling

Institute (MHI) methods



Travel Trajectory of The Storage/Retrieval Machines

Single Command

Cycle

Dual Command

Cycle




Single Command CycleThe load to be entered or retrieved is assumed to be located at the center of the rack structure

Thus, the S/R machine must travel half the length and half the height of the AS/RS, and must return the same distance



Cycle Time

The single command cycle time:

Where: Tcs = cycle time of a single command cycle (min/cycle)

Tpd = pickup-and-deposit time (min) L = length of the AS/RS (m, ft) Vy = velocity of S/R machine along the length, L (m/min, ft/min)

H= height of the rack structure (m, ft) Vz = velocity of the S/R machine along vertical direction (m/min,

ft/min)




Dual Command CycleThe S/R machine is assumed to travel to the center of the rack structure to deposit a load,

Then, the S/R machine must travel ¾ the length and height of the AS/RS to retrieve the load

Thus, the total distance is ¾ the length and height, and back



Cycle time

The dual command cycle time:

Tcd = cycle time of a dual command cycle (min/cycle)

Tpd = pickup-and-deposit time (min) L = length of the AS/RS (m, ft) Vy = velocity of S/R machine along the length, L (m/min, ft/min)

H= height of the rack structure (m, ft) Vz = velocity of the S/R machine along vertical direction (m/min,

ft/min)



System throughput analysis

Depends on the relative numbers of single and dual command cycle performed by the system

Let, Rcs = number of single command cycles performed per

hour Rcd = number of dual command cycles performed per

hour At a specified or assumed utilization level, U, the amount

of time spent in performing single command and double command cycles each hour is as follow: The total

number of minutes of

operation per hour

System throughput analysis

To solve the previous equation, the relative proportions of Rcs and Rcd must be determined, or assumptions about these proportions must be made.

Then, the total hourly cycle rate is calculated as follow:

Where, Rc is the total S/R cycle rate (cycles/hr).

Since there are two transaction accomplished in each dual command cycle, the total number of storage and retrieval transactions per hour will be greater than this value unless Rcd = 0,

Let Rt = the total number of transactions performed per hour, then:

Example: AS/RS Throughput Analysis

Consider the AS/RS in previous example which an S/R machine is used for each aisle. The length of the storage aisle = 280 ft and its height = 46 ft. suppose horizontal and vertical speeds of the S/R machine are 200 ft/min and 75 ft/min, respectively. The S/R machine requires 20 sec to accomplish a P & D operation. Determine: (a) the single command and dual command cycle times

per aisle, and (b) throughput per aisle under the assumptions that the

storage system utilization = 90% and the number of single command and dual command cycles are equal

Answer

(a) Cycle times Tcs =2.066 min/cycle

Tcd = 3.432 min/cycle

(b) Throughput Rcd = Rcs = 9.822 cycles/hr

Rt = 29.46 transactions/hr

For 4 aisles, Rt = 4(29.46) = 117.84 transaction/hr



Example: AS/RS Throughput Using a Class-Based Dedicated Storage Strategy

The aisles in the AS/RS of the previous example will be organized following a dedicated storage strategy. There will be two classes, according to activity level. The more active stock is stored in the half of the rack system that is located closest to the input/output station, and the less active stock is stored in the other half of the rack system farther away from the input/output station. Within each half of the rack system, random storage is used. The more active stock accounts for 75% of the transactions, and the less active stock accounts for the remaining 25%. Assuming the utilization is 90% and the number of single command cycles equal to the number of dual command cycles.

Determine the throughput of the AS/RS, basing the computation of the cycle times on the same kinds of assumptions use in MHI method.

Answer

Cycle times Class A

TscA = 1.366 min

TdcA = 2.382 min

Class B TscB = 2.766 min

TdcB = 3.782 min

Throughput RcsB = RcdB = 3.035 cycles/hr

RcdA = RcsA = 9.105 cycles/hr

Rt = 36.42 transactions/hr

For 4 aisles, Rt = 4(36.42) = 145.68 transaction/hr

Carousel Storage System

Due to its construction, carousel

systems do not possess nearly the

volumetric capacity of an AS/RS.

Usually, a typical carousel system is

likely to have higher throughput rates than

AS/RS



Individual bins or baskets are suspended from carriers that revolve around the carousel oval rail.

The circumference of the rail is given by:

Where, C = circumference of oval conveyor track (m, ft), and L and W are the length and width of the track oval (m, ft)

Storage Capacity

The following figure shows a typical carousel storage system



Storage Capacity

The capacity of the carousel system depends on the number and size of the bins (or basket) in the system.

Assuming standard size bins each of a certain volumetric capacity, then the number of bins can be used as a measure of capacity. From figure where nb is the number of bins hanging vertically from each carrier and nc is the number of carriers around the periphery of the rail. Then the total number of bins is nc x nb .

The carriers are separated by a certain distance to maximize storage density yet avoid the suspended bins interfering each other while travel around the ends of the carousel. The carrier spacing sc (m/carrier, ft/carrier) can be calculated using the following relationship: sc x nc = C.



System Throughput Analysis

The storage/retrieval cycle time can be derived based on the following assumptions:• Only single command cycles

are performed• The carousel operates at

constant speed vc.• Random storage is assumed• The carousel can move in

either direction

Under the last assumption of

bidirectional travel, it can be shown that the mean travel distance

between the load/unload station and a bin randomly located in the carousel is C/4.

thus the S/R cycle time is given as:



System Throughput Analysis

Where Tc = S/R cycle time (min)

C = carousel circumference (m, ft) Vc = carousel velocity (m/min, ft/min)

Tpd = the average time required to pick or deposit items each cycle by the operator at the load/unload station (min)

The number of transaction accomplished per hour is the same as the number of cycles and is given by the following



Example: Carousel Operation

The oval rail of a carousel storage system has length = 12m and width = 1 m. There are 75 carriers equally spaced around the oval. Suspended from each carrier are six bins. Each bin has volumetric capacity = 0.026m3. Carousel speed = 20m/min. Average P&D time for retrieval = 20 sec. Determine (a) volumetric capacity of the storage system (b) hourly retrieval rate of the storage system



Answer

(a) Total number of bins nc x nb = 75 x 6 = 450 bins

Volumetric capacity = 450(0.026) = 11.7 m3

(b) Hourly retrieval Circumference, C = 2(12 - 1) + 1 = 25.14 m Cycle time per retrieval Tc = 25.14/[(4)(20)] + 20/60 =

0.647 min Throughput, Rt = 60/0.647 = 92.7 retrieval

transactions/hr



Summary

Storage system performance is measured through capacity, density, accessibility, system throughput and availability.

Two storage location strategies which are randomized storage and dedicated storage.

AS/RS and Carousel Storage System are two automated storage system.



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Lecture Notes - Week 5 Material Handling - Storage