Solid Wood Products 263 1 



Stringer design. — Stringer thickness for most standard pallet designs is in the 

 range from I-Va to 1-% inches, and usually not more than I-Va inches. There is 

 some inclination to increase this dimension to 2- 'A inches for the class B woods 

 of medium density listed earlier in this section and to I-Va inches for class A 

 woods of low density. 



E. G. Stem (1979c) compared yellow-poplar, sweetgum, and sweetbay pal- 

 lets with 2-y4-inch-thick stringers assembled with 129 nails to pallets assembled 

 onto l-yg-inch-thick stringers with 111 nails as shown in figure 22-31. On 

 average, pallets with the 20 percent thicker stringers and 16 percent more nails 

 had 7 percent less deflection of pallet sides, 4 percent less deflection at pallet 

 ends from a centered concentrated load, and 6 percent less distortion in comer- 

 wise free-fall tests than did pallets with l-yg-inch-thick stringers. In pallets made 

 with both sizes of stringers, yellow-poplar performed better than sweetgum or 

 sweetbay. 



Stringer strength contributes to pallet longevity, so the design of stringers is of 

 interest. Kurtenacker (1969) found that hickory and yellow-poplar stringer-type 

 reusable 48- by 40-inch warehouse pallets (non-reversible, double-face-flush) 

 performed best when designed for two-way entry only, i.e., stringers not notched; 

 simulated wane on the top edge of stringers weakened them less than notches 

 made for four- way entry. Pallets having notches achieved by nailing on blocks 

 performed better than those with one-piece stringers notched with l-!/2-inch 

 radius at comers. Stringers with notches cut curved at the ends were stronger 

 than those with straight sloped cuts at the ends. Stern (1969b) also found that 

 built-up blocks below continuous upper stringers provided optimum perfor- 

 mance, especially if the blocks were made of plywood. 



Strength and durability computations. — The load that can be safely placed 

 on a pallet, and the service life obtained from it, depend on: 



• Pallet use: the load distribution, the manner in which the pallet is sup- 

 ported on the floor, in stacks, or in racks, and the roughness with which it 

 is handled during empty and loaded transport. 



• Pallet design: pallet size, pallet construction, and fasteners 



• Raw materials: mechanical properties of the pallet parts and fasteners 



For permanent pallets that are used in racks, ultimate bending strength of the 

 notched stringers and deck deflection are critical factors. For expendable pal- 

 lets deck deflection usually govems design. Durability depends to a large degree 

 on the quality and number of fasteners used. 



Readers interested in computations yielding pallet designs based on engineer- 

 ing principles should find useful the procedures outlined by Wallin et al. (1976) 

 and Wallin (1977). 



Computer programs for pallet design are available from Virginia Polytechnic 

 Institute, Blacksburg. 



Test methods. — Engineers needing descriptions of pallet test methods are 

 referred to E. G. Stern (1974b), Wallin et al. (1976), American Society for 

 Testing and Materials Standard D 1185, and to ANSI MHI-1977. 



