Pallet Performance: What's Needed for Today's Automated Equipment
| Presented to DBM WoF99 Conference on Innovative Technologies in Distribution and Logistics |
|
| by John W. Clarke |
|
June 1999 |
Palletized unit loads have traditionally been moved and stored using "manual" equipment, such as forklifts, pallet jacks, and conventional racks. In general, pallets have been manufactured to meet the requirements of this type equipment. Today, automated "operator-free" equipment is replacing many manual systems to improve unit load handling efficiency. Pallets may now pass through both manual and automated equipment. Under such circumstances, pallets should be designed to fit the equipment representing the strictest "functionality" requirements. In most cases, this will be the automated components in the systems. The level of pallet performance required for use in automated systems, however, had not been documented.
The Center for Unit-Load Design at Virginia Tech assisted the Material Handling Industry (MHI) and the National Wooden Pallet & Container Association (NWPCA) in developing performance specifications for pallets to be used in automated material handling equipment. The relationship between pallet performance and operating efficiency of various types of automated material handling equipment was evaluated.
This was accomplished through:
- industry surveys
- field observations of pallets and automated equipment in use
- laboratory testing of pallets on automated equipment
No pallet material was specified, just performance. New, used, and repaired pallets were evaluated.
Surveys of pallet manufacturers, automated unit-load material handling equipment manufacturers.
A preliminary set of performance criteria was mailed to industry manufacturers for comment. In general, timber and plywood pallet manufacturers were willing to meet any dimensional tolerances, but at an unknown cost. Green (moisture content > 30%) timber pallets will shrink approximately 1/4-inch in height after drying (moisture content 10-15%). If more restrictive height tolerances are required, timber pallets must be manufactured of predried lumber or manufactured 4% overheight when green and allowed to dry to the target height before introducing into the system. Timber shrinkage along the pallet length and width is minor (approximately 1/16-inch).
Most plastic pallet manufacturers surveyed were concerned with strength and stiffness requirements and dimensional tolerances. Plastic pallets expand and contract between temperature extremes. Over a 100° F temperature change, polyethylene pallets may vary ±1/2-inch along the pallet length and width. More restrictive length and width tolerances would curtail the use of many plastic pallets.
Machinery manufacturers were concerned with bottom deck designs and several requested boards oriented in both pallet directions (such as perimeter or cruciform bases), or pallets conveyable in both directions. They also felt that pallet deflection should be minimized. Other concerns included inadequate pallet strength, inadequate pallet connections, excessive variation in pallet size and squareness, non-flat bottom decks, inadequate bottom deck coverage, excessive variation in size and location of pallet components, excessive pallet deflection under load, excessive deformation of pallet components, low coefficient of friction, exposed fasteners, and pallets repaired with companion stringers.
Field studies of existing, automated unit-load material handling systems.
The authors visited nine sites to observe unit loads being handled in various automated unit-load material handling equipment.
Areas Observed
| Pallets |
Sites |
Automated Machinery Systems |
Sites |
| Timber |
9 |
Deep bay, shuttle cart, dynamic rack |
1 |
| Plastic |
3 |
AS/RS systems |
4 |
| Plywood slave |
2 |
Dynamic skate wheel rack |
2 |
| |
|
Dynamic push-back rack systems |
1 |
| |
|
Roller conveyor systems |
9 |
| |
|
Chain conveyor systems |
8 |
| |
|
Palletizers |
9 |
| |
|
AGV's |
1 |
In addition, all sites handled pallets with conventional forklifts and pallet jacks. Factors such as types of automated equipment, pallets, load weights, pallet deflections, equipment clearances, etc., were recorded.
The following observations were typical of the equipment observed:
- The most common roller configuration was 2.5" diameter rolls on 6" centers
- A bottom deck design that effectively spanned roller centers was more important to functionality than the percent or amount of bottom deck coverage
- Pallet or component deflection of 1/2" or greater may interfere with handling on roller conveyors
- Pallet and component deformation under load should be less than ¾-inches on chain conveyors
- The most critical requirement for dynamic storage rack systems was a smooth bottom deck design. Any broken components, extreme thickness variation, protruding fasteners or components, etc., in the bottom decks interfered with handling efficiency
- Gaps between pallets and AS/RS crane lifting arms varied from 0.875 to 2.0 inches. One system exhibited only 1/2", side to side, clearance in stringer notches. Proper pallet performance in AS/RS systems was essential due to the higher costs of system shutdown for jammed pallets
- Pallet component placement and length/width tolerances within 1/4" were often required
Laboratory Tests of Pallets in Automated Material Handling Equipment
The Center for Unit-Load Design used three types of automated material handling devices to evaluate the performance of various pallet designs.
The following automated material handling equipment was used:
- Roller Conveyor
- High-Density Dynamic-Storage Rack
- 4-deep Push Back Rack
This equipment was used to evaluate the performance of fifteen (15) 48x40-inch pallet designs manufactured of either solid timber, plywood, or plastic. The suitability of a given pallet design for a given type of equipment was based on the following measurable performance factors:
- Rate of pallet movement
- Consistency of pallet movement
- Pallet deflections
In addition, other factors such as static coefficients of surface friction, rotation of pallets on roller conveyors, pallet damage, handling equipment damage, product damage, and load stability were observed and recorded. Three loads were used, a 600 pound, interlock-stacked, boxed paper load, a 1500 pound, interlock-stacked, bagged load, and a 2800 pound, interlock-stacked, bagged load. Pallets were also tested with no load.
Testing with the Roller Conveyor
Loaded and unloaded pallets were tested for speed of conveyance, deviation or rotation from a straight line, and smoothness of conveyance on the 2.5" diameter, full length, rollers. Pallets were tested on both 3-inch and 6-inch roller centers. In general, if the bottom deck components spanned the roller spacing, the rate of conveyance and deviation from a straight line was unrelated to other factors such as pallet design, pallet direction, and unit load weight. Timber pallets with non-flat bottom decks, protruding fasteners, and broken bottom boards displayed "rough" or "semi-rough" conveyance. Pallet surface coefficient of friction levels as low as 0.15 were adequate for movement on the roller conveyors.
High-Density Dynamic-Storage Rack
The Dynamic-Storage rack consisted of two skate wheel rails, with paired skate wheels on each rail, each lifted and dropped by an inflatable air bladder. Tubes are inflated to allow pallets to convey (uptime). Tubes are deflated to halt pallet movement (downtime). The controls allow changes to uptime and downtime to adjust pallet flow for individual applications. During testing, the uptime and downtime remained constant at approximately 1.0 seconds and 2.0 seconds, respectively.
Pallets with uniformly flat decks above the skate wheels (plywood and timber perimeter base designs) exhibited better flow than pallets with less flat, low quality timber or plastic mesh bottom deck designs. All plastic pallets with mesh or ribbed bottom decks would convey on single and double row skate wheels when empty and at relatively low load weights. The loss of conveyance supporting heavier loads was a result of localized bending of the mesh or ribbed structure around skate wheels, and was not related to overall pallet deflection or bottom deck coverage. Interactions between the mesh or rib spacing, bottom deck stiffness, unit load weight, and skate wheel configuration determined the ability to convey. One potential solution is to incorporate solid regions at least 3" in width in all bottom deck components. Pallet deflections less than 1 inch did not significantly affect conveyability. Localized component deflections greater than 0.25", however, did reduce conveyability. Pallets users should be aware that pallets with mesh or ribbed bottom deck components may experience difficulty in conveyance on skate wheel conveyors.
These results suggest that bottom decks for use on skate wheels should be flat and portions of the bottom deck over the wheels should span the wheel centers.
Push Back Rack
The operation of this equipment is affected most by the tendency of the pallet
to slide on the push back carts. The pallets were tested empty and under load.
In general, all pallets functioned adequately supporting the given test loads
in this rack system. Pallet slippage, or the overhang past the outer rack
edge, was measured after pallets rolled for approximately 48 inches.
Pallets with timber bottom decks slid from 0 to 2.25 inches. Pallets with
plywood bottom decks moved between 0 and 1.19 inches. Plastic pallets slid
between 0.50 and 1.94 inches. No measurable load shifting on any of the test
pallets was noticed. Pallets with bottom deck static coefficient of friction
levels of at least 0.15 did not exhibit excessive movement.
Performance Specifications for Pallets Used in Automated Unit-Load Material
Handling Equipment
The minimum performance specifications for pallets to be used in automated
unit-load material handling equipment are listed in Table 1. In general,
to function in automated equipment, pallets must be consistent in size,
shape, and geometry. They must be stiff and contain flat surfaces with a
minimum static coefficient of surface friction of 0.15. Note that these
specifications apply to all pallets intended for use on automated equipment,
including repaired pallets. No pallet material is specified, just performance.
These are minimum criteria, and represent an attempt to balance economy
with functionality. The pallet quality requirements of automated equipment
varied among the devices studied. There is no guarantee that pallets conforming
to the performance criteria will function in all automated handling equipment.
In addition, pallets manufactured to a lower quality level may function
in some equipment. Pallets are but one part of the handling system. For
example, one can alter the roller spacing, spans between rack supports,
etc., and change the equipment sensitivity to variations in pallet quality.
Conversely, stiffer, stronger, and more uniform pallets can be designed
for a more sensitive handling system. This study confirmed that there is
a synergism between the design of the pallet, the design of the handling
equipment, and the performance of the entire material handling system. When
pallets are captive, it may be more economical to use less sensitive, higher
performance pallets in more sensitive, less costly automated handling equipment.
However, if pallets are passed through the equipment only once, it may be
more economical to use less sensitive, higher performance handling equipment
with a lower cost pallet. This study developed a performance specification
for pallets that is adequate for the majority of the automated handling
equipment currently in use. Future studies will develop multiple levels
of performance (sensitivity) for pallets and handling equipment in order
to help designers achieve the most economical and efficient unit load material
handling system.
Performance Specifications for Pallets Used
in Automated Unit-Load Material Handling Equipment. 1,2,3 |
|
| Pallet Size and Shape Variation |
|
| Length |
+0.125/-0.250" |
| Width |
+0.125/-0.250" |
| Height |
+0.125/-0.375" |
| Squareness |
diagonals shall be within 0.50" |
| Flatness of Decks (supporting no load) |
within 0.25" of target level 4
|
Minimum Static Coefficient of Surface Friction:
Bottom Surface of Top Deck or
Bottom Surface of the Bottom Deck against conveying equipment |
0.15 |
| Component Placement Variation |
within 0.25" of specifed location |
| Maximum Deflection of the Pallet Under Load |
0.50" |
| Minimum Clearance between Pallet Under Load and Handling Equipment |
0.50" |
| Maximum Deflection of the Pallet Components Under Load |
0.25" |
Notes
- No standard pallet design is implied by these recommendations. Pallet
design shall be specified by the system designer.
- In addition, the following requirements must also be met:
- no broken or loose components
- no exposed fasteners
- no loose stretch wrap, packaging, etc. that may interfere with equipment
- no companion stringers, half stringers, or plug stringer repairs
- metal repair plates are allowed if covered with dark, non-reflective
paint
- bottom deck components should span the center to center distance between
rollers and skate wheels
- pallets with mesh or ribbed bottom deck components may experience
difficulty in conveyance on skate wheel conveyors. Interactions between
the mesh or rib spacing, bottom deck stiffness, unit load weight, and
skate wheel configuration determine the ability to convey. One solution
is to incorporate solid regions at least 3" in width in all bottom
deck components.
- pallets with low bottom deck compression strength (typically plastic
post designs) may experience difficulty in conveyance on rollers
- Pallets are in compliance if 99% conform to the criteria.
- Intentional protrusions exceeding these limits may be acceptable by
mutual consent of buyer and seller.
