How Long Should Mobile Bases Be?

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So let’s take a six pallet level high system with three pallets between uprights (1m side facing). This equates to 18 pallets on a single side and 36 on the double and at 1.2 ton per pallet its 43 tons per bay. Multiply that by 10 bays and you’re already at 430 tons and about 35m long at 3.5m upright centres.
At Barpro / Storax, mobiles are designed to handle up to 400 tons per mobile and a maximum of 10 or 11 bays long. Should there be a need for longer mobiles due to warehouse configuration, the company splits the mobiles in two halves and harmonises them to run end to end in tandem.
Benefits
The benefits of this approach are increased life time of the mobiles, decreased downtime and lower maintenance costs over the long term.
In theory long mobiles should operate as affectively as short mobiles, but the operating environment affects mobile racking performance and has to be taken into consideration.
Factors:
housekeeping may not be ideal
loading of the mobile base may be uneven
forklift impacts do occur and maintenance may not be ideal
floor settlement
Each of these factors places increased demand on the mobile base. The impact is greater for longer mobile bases. A longer mobile base carries more weight and it has more drive power, so the stresses on the mobile base structure are greater.
Results
Jams occur when housekeeping is not done properly and debris gets wedged under the wheel units. Repeated or prolonged jamming causes distortion of the mobile bases, fixings are loosened as bolts are smaller than bolt holes and wheel units become misaligned and don’t travel in line with the rails. This puts extra stress on the guide wheels as friction builds up between guide wheel and guide rails and places additional stress on mobiles. More so on long mobiles than short ones.
A mobile base that is not running parallel to the rails is likely to reduce the available aisle width for the fork trucks. A long mobile base will tend to reduce the aisle width more than a short mobile base.
Increased wear and stress of the mobile base can also be caused by uneven loading of the mobiles ie, more or heavier pallets being loaded at one end. This affect is negligible on short mobiles but can be significant on long ones.
Even the most careful sites suffer from some damage, and planned maintenance can be delayed or missed. The effect can only be to add to the burden on a mobile rack. If a problem results in a section of mobile base dragging, then a long mobile base will be more severely affected than a short base.
Misalignment of photoelectric cells on long mobile bases is possible if mobiles bow (bend) in the middle. This can cause safety concerns.
Furthermore, if the leading edge of the mobile base is convex (bowing outwards), then there is also a risk that the straight line between transmitter and receiver units will lie underneath the base behind the leading edge, which means it may not detect objects in the closing aisle.
Differential floor settlement affects rail levels. A mobile rack will gravitate towards the lowest point, wherever it is. The guide rail system will restrain the mobile rack. However there will be increased side thrust, as the flanges of the guide wheels rub more heavily on the sides of the guide rail. This sets up the drag on the mobile base and again, a long mobile base will suffer greater force and greater drag than a short mobile base.
A further advantage of a split mobile base is that it allows the floor designer to use smaller floor slabs, since he or she can include a movement joint along the line of the split in the mobile bases. This results in a floor that is easier to construct, leading to a potential saving on floor cost.
Design features
Direct geared drive, from motor to drive wheel
There are usually more drive motors per mobile base with direct drive when compared with a shaft drive system. Consequently, a problem with one drive is less detrimental to a Storax mobile base.
A complete wheel unit fabrication on all wheel units
The wheel unit is the principal structural member on the mobile base. Storax wheel units are all single-piece fabrications. This results in a more rigid base structure, compared with a base where wheel units are split into two halves.
Side members
Storax bases have side members. These are distinct from load carrying bottom beams (orange) and add extra strength to the base structure.
Diamond pattern bracing
The diamond pattern bracing is stronger and more rigid than cross pattern bracing, owing to its four short members, compared with the two longer members that are used in cross bracing. The bracings are strong hot rolled angle profiles.
Each side member/wheel unit connection is dowel pinned on assembly at the joints and ends
Unlike a bolt, a dowel is a dead fit whichprevents the mobile base from stretching.
Barpro has in excess of 250 systems in Southern Africa that have stood the test of time over a 25 year period. Systems are designed to maximise storage capacity, equipment up time and availability, warehouse operational efficiency and minimise running costs over the long term.
Barpro Storage Sysytems, Email: info@barpro.co.za