Introduction
Categories of materials stored by manufacturing firms:
- product-related:
o raw material
o purchased parts
o WIP
o Finished products
o Rework and scrap
- process-related:
o refuse
o tooling
o spare parts
- overall support of factory operations:
o office supplies
o plant records
These require different storage methods and controls.
Manual storage: inefficient (HR, floor space, material control, etc)
Automated: higher efficiency
11-1 Storage System Performance:
1- Storage Capacity
a- total Space Available
b- Total number of storage compartments (pallets, boxes, containers, etc.)
2- Storage Density
- Volumetric space available for actual storage relative to total storage volume.
The larger the density, the better the storage system.
- Sometimes floor area is used to calculate the density because it is easier to visualize and draw
using top views, but volume is more accurate
3- Accessibility
- capability to access the desired item
- inversely proportional to storage density
4- Throughput
- The hourly rate at which the system receives loads and/or retrieves and delivers them.
- It is related to the time to perform a S/R transaction which includes:
o Storage: (pick up load – travel to storage location – place load into location – travel back)
o Retrieval: (travel to storage location – pick item – travel to output location – unload at
output location)
- There are two main types of cycles:
- Single command cycle: either retrieval or storage transaction
- Dual command cycle: combined storage and retrieval transaction
For automated storage systems:
5- Utilization
- proportion of time the system is being used
- For automated systems, 80-90% is the optimum utilization range.
- Less than that it would be very low for its cost
- Higher than that, there wouldn’t be any place for rush times
6- Availability
- Proportion of time the system is capable of operating (not down or on maintenance).
- Stoppages occur due to computer failure, human errors, maintenance, load jams, mechanical
breakdowns, etc.
11-2 Storage Location Strategies:
Each item type is stored in warehouse in a “stock-keeping-unit” (SKU).
1- Randomized Storage:
- SKU’s are stored at any location
- Retrieval on a first-come-first-go basis
- Minimal space required
2- Dedicated Storage:
- SKU’s are assigned to specific locations according to:
a- P/N
b- Activity level
c- Activity-to-space ratio
- Highest throughput, but needs larger space
3- Class-Based Dedicated Storage:
- Storage according to class division according to activity level
- Within each class, randomized strategy is used in storage to minimize space within each class
11-3 Conventional Storage Methods and Equipment
1- Bulk Storage: see fig.
2- Rack Systems:
- Pallet Rack: see fig.
- Cantilever rack:
- Portable racks: box frame with single pallet load on top of each other like bulk
- Drive-through racks (drive-in)
- Flow-through racks
3- Shelving and Bins
4- Drawer Storage: see fig.
11-4 Automated Storage Systems
11-4-1 ASRS:
- custom designed for each application
- degree of automation varies
- One or more storage aisles
- Each has a S/R machine (crane)
- The aisles have storage racks
- The S/R machine has an input-output station called pickup-and-deposit (P&D) station
ASRS Types:
1- Unit load AS/RS: see fig 11.5 - This is the generic AS/RS
2- Deep-lane AS/RS: for large quantity (higher density) unit load systems but low number of SKU’s
3- Mini-load
4- Man-on-board
5- Automated item retrieval system
6- Vertical Lift Storage Modules (VLSM)
WIP Storage:
- In job shop and batch production, AS/RS are efficient in storing material between processing
steps.
- In high production rates, conveyors are used for storage (buffers) and transport
Reasons justifying the use of AS/RS for WIP:
- Buffer storage in production (between two different processes where the first one has a higher
production rate
- Supporting Just In Time delivery (to buffering reduce some of JIT Risks
- Kitting of parts for assembly
- Compatible with Automatic Identification Systems (see CH12)
- Computer control and tracking of materials
- Support of factory-wide automation
AS/RS components and features:
See fig. 11-5
1- Storage structure (frame)
2- S/R machine [horizontal movement of the mast, vertical movement of the carriage,
and shuttle transfers between the carriage and storage compartments
3- Storage modules (compartments, pallets)
4- Pickup-and-deposit stations
5- Control systems
11-4-2 Carousel Storage System:
- chain conveyor revolving around a long oval rail system
- human worker at the load/unload station
- if carousel is very big, higher capacity but higher cycle time
Types of Carousels:
1- Horizontal Carousel: more common
2- Vertical Carousel: less floor space, but also less capacity because of height limitation (ceiling)
Carousel Applications:
- high throughput
- alternative to mini-load AS/RS
- relatively low cost
- Versatility
- High reliability
Carousel Applications:
1- Storage / retrieval
2- Transport and accumulation (sorting)
3- WIP [especially in Electronics Industry]
11-5 Engineering Analysis of Storage Systems:
AS/RS:
- Sizing the structure
- Throughput
Carousel:
- Storage Capacity
- Throughput