Storage and retrieval unit
A storage and retrieval machine (SRM) (Engl. Stacker crane (STC) or storage and retrieval machine - S / R machine ) is a rail-guided, single-track vehicle to operate the goods in a high-bay warehouse . The construction height of an SRM starts at approx. 6 meters and goes up to a maximum height of 46 meters. There are SRMs that can serve the entire warehouse via a switch system (curved SRMs for lower capacities), or change the aisle with a transfer device and those that only operate in one aisle (aisle-based SRMs for high capacities). The degree of automation ranges from manual to fully automated, depending on the application.
An SRM is moved in the following three axes:
- x = longitudinal aisle direction (drive unit)
- y = vertical direction (lifting unit)
- z = transverse aisle direction (load handling unit)
History and Development
With the arrival of mass production in industry , the demands on the internal flow of materials and thus on the warehouse technology also increased. The requirement to be able to store in a small area, more and more, which emerged in the 1950s block storage . The block storage facilities were served by stacking cranes , which required much less space for the aisles and reached heights that were not possible with a forklift or reach truck .
In the 1960s, the first stacker cranes were built, which, in contrast to the stacking cranes, were aisle-bound and therefore did not require a portal to drive over the entire hall . This not only increased storage capacity due to increased space utilization, but also increased performance, as a separate SRM was now available for each aisle. In the beginning, the SRMs drove like small portal cranes on the hall ceiling and were guided on the floor. But they soon switched to not introducing the force via the shelf or the hall ceiling, but via the hall floor, as this was much easier to control mechanically. The single-track storage and retrieval machines on the ground were now able to achieve ever higher mileage.
Up until now, the storage and retrieval machines were operated manually by a driver , but the development of information technology in the 1980s enabled the extensive automation of storage and retrieval machines.
This led to strong growth in the industry from the 1990s onwards. In the following years, the development of the software (LSR (warehouse control computer ) and LVR (warehouse management computer), see high-bay warehouse ) was to become increasingly important. Mechanically, the SRMs were challenged by the ever higher performance, but the basic concept has remained to this day.
Mechanical structure of an SRM
The storage and retrieval unit is not a combination of industrial truck and hoist, but rather a typical hoist due to the top and bottom guidance, which moves itself in the direction of travel (X-axis) and the lifting carriage in the lifting direction (Y-axis). The storage and retrieval unit never appears alone, but always in combination with a so-called load handling device, which manipulates the load directly or the so-called loading aids, which act as carriers of the load (in the Z direction).
As a rule, a stacker crane is installed for each rack aisle. Changing the shelf aisle would require a considerably more complex construction and considerably increase the access times to a shelf; nevertheless they are manufactured (mostly referred to as 'curve-going' SRMs). If storage and retrieval are separated from one side to the other, pairs of storage and retrieval units for each aisle are also obvious. The choice of solutions is not only determined by the desired operating time, but also payloads, building heights, storage strategies, etc.
landing gear
The single-track chassis connects the two wheels with the mast or the frame. The running wheels are guided on rails and are rotatably mounted on curved SRMs. Depending on the type of rail ( hot-rolled profiles such as U-profiles, I-profiles and railroad tracks ) and wheel load, steel, plastic or Vulkollan wheels ( steel hub with cast elastomer tread) are used in single or double wheel housings. Depending on the power requirement, one or both wheels are driven.
mast
The mast (column) connects the chassis with the head cross-member. Depending on the application, one or two mast versions (frame units) are possible. The lifting carriage is guided along the mast. But the mast includes other components such as the lifting gear with the rope or chain drive , the main cabinet , platforms and ladders with the personal protective equipment (PPE), power supply lines to the main cabinet and lifting over contact lines or cable chains .
Lifting carriage
The lifting carriage primarily carries the load to be transported and is equipped with devices for picking up and dropping off the load, the so-called load handling device.
With automatic SRMs, there is usually an emergency control station on the lifting carriage (for troubleshooting). Manual SRMs often have a cabin with more or less extensive equipment (PPE, seat, shelves, PC, scanner, fire extinguisher ...). The design of the escape route is also an important issue here.
The lifting movement takes place via a rope, belt or chain drive. So that the lifting movement is automatically switched off in the event of a mechanical blockage of the lifting carriage, safety switches are installed in the suspensions to detect slack rope or overload. There are devices on the lifting carriage to prevent falls in the event of a rope or chain break. This safety gear is especially important when people can travel with the SRM.
Head traverse
The top traverse contains the upper chassis and, if necessary, connects the two masts. The upper chassis consists of guide rollers that are guided in a rail on the rack yoke (upper connecting structure of the rack rows). The head traverse can even be omitted in the case of single-mast devices that cannot handle curves.
The head traverse is particularly important when there are several curved SRMs in a rail system. In this case, a collision must be prevented. The anti-shock devices are built into the head cross-member, which also serves as a buffer.
Drive and performance
The travel and hoist drives are predominantly speed-controlled electric motors today , with the driving performance becoming ever higher in order to reduce access times and increase system performance. Hydraulic drives are hardly used any more because of the high risk of contamination, especially for the goods.
Performance of an SRM
Typical performance data of an SRM for Euro pallets with 1,000 kg payload, Ly = 30 meters high and a total weight of 20,000 kg are summarized in the following table:
| axis | Speed v [m / min] | Acceleration a [m / s²] | Maximum electrical power P [kW] | comment |
|---|---|---|---|---|
| x = longitudinal direction of aisle | vx = 225 | ax = 1.0 | Px = 150 | |
| y = vertical direction | vy = 90 | ay = 1.0 | Py = 55 | |
| z = load cross aisle ( telescopic fork ) | vz1 = 55 | az1 = 0.5 | Pz1 = 1 | depending on the stability of the goods |
| z = cross aisle direction unloaded (telescopic fork) | vz2 = 120 | az2 = 2.0 | Pz2 = 1 |
These values vary greatly with the parameters of the stacker crane height and payload. An SRM in a small parts warehouse can achieve acceleration values of 3–4 m / s² due to its significantly lower mass.
Optimal speed ratio
The speeds in the x and y directions are directly related to the bearing dimensions. The optimum speed ratio exists when the lifting carriage arrives at the point Lx / Ly with simultaneous lifting and travel movement from point x0 / y0, in other words: the speed ratio vx / vy corresponds to the length ratio Lx / Ly. The optimal aisle length (Lx) for the example in the table above is based on this condition:
Lx = Ly * (vx / vy) = 30 * (225/90) = 75 meters
Playing times
The playing time is the sum of all times that are needed for a given movement sequence of the SRM in a high-bay warehouse. It corresponds to the response time of a control system or the access time of a hard disk drive. It allows conclusions to be drawn about the handling performance (number of entries, removals and / or relocations per unit of time) of a high-bay warehouse. These playing times can be measured directly after the project has been implemented and are therefore part of the acceptance; deviations of 6% are permitted. In our example above, e.g. B. 85 single cycles (retrieval) and 50 double cycles (storage and retrieval) per hour.
Mean playing time
When you talk about playing times, the mean playing times are always meant. This is a statistical mean value , provided that all subjects are approached equally within a certain period of time. The exact determination is therefore very time-consuming and impractical. For this reason, an average work cycle is defined and the points P (pallet location), A (retrieval point) and E (storage point) in the actual warehouse are determined depending on its dimensions. Depending on the position of A and E in the bearing, a distinction is made between six different cases (for more details, see FEM 9.851).
Single game
A single cycle of an SRM consists of the sum of all times for storage or retrieval, including all travel, positioning, control and fork cycle times. A single game for retrieval describes the path A → P → A, a single game for storage E → P → E.
Combined game
A combined game (or double game) of an SRM consists of the sum of all times for storage and retrieval, including all travel, positioning, control and fork cycle times. Such a game describes the route E → P1E → P2A → A
Control types
Manual control
With manual control, all movement axes are controlled by the operator on board using a joystick or button. With this type of control, logical and electrical interlocks must prevent all movements at any time during normal operation. Due to the steadily increasing degree of automation, manually operated storage and retrieval machines no longer play an essential role. Man-operated devices are still used, especially for order picking work.
Semi-automatic control
With this type of control, certain movement sequences are automated. It is very helpful e.g. B. the so-called fork cycle, in which the operator approaches the relevant compartment and starts the following cycle by pressing a button:
Extend telescopic fork → lift telescopic fork → retract telescopic fork
Automatic control
With the automatic control, all movements of the storage and retrieval system are controlled and monitored autonomously on the storage and retrieval unit. The movement is coordinated by the order data from the warehouse management system . The data transmission between the functional units can, for. B. via cables, light paths ( infrared ) or via radio .
A manual movement of each SRM is possible via an emergency control station, with which the connection to the warehouse management system can be overridden.
Distance measurements
In order to determine the position in the x-direction, laser measurement technology or rotary encoders that work in incremental or absolute terms are mainly used.
Since around 2005, barcode measuring systems in various designs have mainly been used to measure the X-axis (this applies to the manufacturers LTW and TGW). These are also used for the Y-axis.
Investments
The costs for an SRM depend heavily on the degree of automation, the dimensions, the number of pieces and the performance data. A smaller automatic SRM is in the range of 100,000 euros, for an SRM as in the example above, the investment is in the range of 300,000 euros.
Relevant standards
- EN 528 stacker cranes - safety
- FEM 9.001 Terminology / Dictionary storage and retrieval machines
- FEM 9.101 terminology / storage and retrieval machine definitions
- FEM 9.221 Proof of performance for SRM / reliability, availability
- FEM 9.311 calculation bases for SRM structures
- FEM 9.512 calculation bases for SRM drive units
- FEM 9.753 Safety rules for storage and retrieval machines
- FEM 9.851 Proof of performance for storage and retrieval machines - playing times
- FEM 9.831 Calculation bases for SRMs in the high bay warehouse: tolerances, deformations, free dimensions in high-bay warehouses
- FEM 9.832 Calculation bases for SRMs in the small parts warehouse area: tolerances, deformations, free dimensions in the automatic small parts warehouse
Web links
- helpful animations at http://www.tgw-group.com/at-de/produkte/lagersysteme/regalopergeraete-fuer-akl/
- Playing time calculation according to FEM 9.851 at http://www.logscout.de/faces/modules/lager/lagerleistungs/fem9851/eingabe.jsp
swell
- ↑ http://www.mhia.org/learning/glossary/s
- ↑ FEM 9.001, Terminology - Dictionary storage and retrieval machines , page 7
- ↑ FEM 9.831 calculation bases for stacker cranes, tolerances, deformations and free dimensions in high-bay warehouses , page 3
- ↑ FEM 9.851, Terminology proof of performance for storage and retrieval machines - playing times