Issue link: https://cp.revolio.com/i/738411
ista views • October 2016 • www.ista.org Systems Based Design of Global Supply Chains > CONTINUED FROM PAGE 15 2.Pallets are also springs. Weigel and White (1999) demonstrated that pallets have multiple modes of vibration as unit loads move through supply chains. A principal vibration mode is vertical displacement of the pallet deck. The unit load can be modeled as a spring/mass system with a series of resonant frequencies. Figure 2 shows the relationship between forcing frequency, transmissibility from bottom to top of the pallet, and pallet deck stiffness (EI). Notice the frequencies at which maximum accelerations occur, tends to overlap frequencies that will occur during shipping. As the stiffness of the pallet deck increases the level of amplification of input accelerations decreases. Pallet design can be used to increase unit load stability and reduce dynamic forces that would potentially damage product. 3.Pallet deck stiffness affects the compression of packaged product. When packaged product is to be unitized, a primary packaging design criteria is compression strength. How much packaged product is compressed within a unit load, depends on the stiffness of the pallet deck. As shown schematically in Figure 3, the compression stress on packaged product is not uniform across the pallet deck. The lower the pallet deck stiffness the higher the stress concentrations. White (2015) and Han et.al.(2007) have shown that these stress concentrations can be 5 to 10 times greater than the average compression stresses on the packaged product. White further showed that changing the stacking pattern of packaged product on a pallet, can reduce compression levels by as much as 30% without any packaging or pallet design change. 4. More automated unit load handling systems require smarter unit loads. Computers still have an IQ of zero. They are less tolerant of variation in the unit load than rider on board handling equipment. Unit loads used in automatic systems must be very consistent and uniform in geometry. They must be more stable to resist deforming during handling and they must be very stiff and not sag as beams. Generally these smarter unit loads cost more but this is necessary to achieve the targeted performance level of the automated warehouse or DC. How the pallets, packaging and unit load handling equipment mechanically interact as product moves through supply chains 1.Pallets and unit loads are beams. Reducing spans reduces beam cost and increases beam strength. Spans in supply chains include storage rack spans, Spans between fork lift tines, Spans between supports on an AGV deck, Spans at AGV docking stations, spans between skate wheels and chains in conveyors. Designers of unit load handling equipment should use low cost ways to reduce these spans rather than increasing spans to reduce equipment cost. Equipment is a one-time, capital expense. Packaging and pallets are re-occurring operating expenses. Figure 2: Vibration interactions during shipping Stiffer pallets result in more stable unit loads and better protect certain vibration sensitive products. Pallet Deck Stiffness EI (N-M 2 ) 202 860 2391 Resonant Frequency (Hz) 13 16 17 Transmissibility Output g's Input g's 2.4 1.8 1.6 Figure 3: Compression stress concentrations at the interface between the pallet deck and distribution packaging ng agi k ac P k ec d t e l al P RQ ´1 -XQ HVV U W ´6 W GLV HVV U W V P RU QLI LRQVµ DW U FRQFHQW LRQµ LEXW U W tress S t en conc ess ation tr k dec t e all P on cti le def 16