"BUILT-UP WOOD" 



1411 



too clearly the increasing scarcity of high-grade structural 

 timber and the necessity of providing built-up substitutes 

 that will be practically as serviceable as the solid material. 



A third factor — now more potential than immediate in 

 its influence but which in the long run will undoubtedly 

 exercise great pressure — is the growing economic neces- 

 sity of making the national wood supply go further by 

 utilizing material now wasted and by adopting more eco- 

 nomical forms of construction and use. 



The airplane exemplifies more than any other one 

 thing the possibilities of built-up wood. It represents 

 accomplishment under the propulsion of necessity and 

 intensive application. During the early days of the war 

 and, in fact, even after America's entrance, it has been-. 



BUT LITTLE IS KNOWN AT PRESENT CONCERNING THE EFFI- 

 CIENCY OF BUILT-UP AXLES AND BOLSTERS SUCH AS THOSE 

 SHOWN IN THE ILLUSTRATION. THEY WOULD UNDOUBTEDLY 

 BE STRONG ENOUGH TO DO THE WORK EXPECTED OF THEM, 

 BUT NO DATA IS AS YET AVAILABLE TO SHOW HOW MUCH 

 RESISTANCE THEY WOULD HAVE AGAINST EXPOSURE TO THE 

 WEATHER AM) THE SHOCKS INCIDENT TO USE. 



said thai ISo per cent of the French propellers had to be 

 rejected before use because strains and stresses in the 

 wood brought about by changing moisture conditions had 

 rendered them practically useless. The propeller proba- 

 bly represents the most refined requirements of glued-up 

 wood from the standpoint of manufacturing practice. It 

 is essential that the propeller be so perfectly manufac- 

 tured and finished that changing weather conditions will 

 not pull it apart, weaken it, or even throw it out of bal- 

 ance or trackage to an infinitesimal degree. By the close 

 of the war, these difficulties had been largely overcome 

 through intensive studies of glues, protective wood fin- 

 ishes, and the effect of moisture upon wood. 



The wing beam of an airplane illustrates another major 

 problem in the use of glued-up wood because it must meet 

 very precise strength requirements. Despite this fact, it 

 was found by experiments that laminations of spruce, 

 glued-up with strong waterproof glue, made a beam which 

 was equal in strength requirements to a solid beam of the 

 same dimension. The United States, England and France 

 had actually approved such beams in their specifications. 

 While laminated beams of many different designs were 

 used to a limited extent by Germany and the Allies during 



the early years of the war, the advantages of such beams 

 became so apparent towards the end of the war that sev- 

 eral of the Allies specified them to the exclusion of solid" 

 beams. While there are at present no glues available that 

 are equal to wood in tensile strength, it is possible to join 

 wood so that it will resist tension satisfactorily by making 

 long scarf joints, the area of which is much greater than 

 the cross-sectional area of the pieces to be glued. Like- 

 wise, scarf joints are used satisfactorily in beams, where 

 both tension and compression stresses must be resisted. 

 There is, of course, more wastage of material in the scarf. 

 It will be apparent that the solution of the problems 

 involved in aircraft manufacture has general application 

 in many other directions and the successful development 

 of glued-up wood for exterior use under exacting air- 

 craft requirements forecasts with seeming certainty its 

 ultimate application to the diversified wood-using indus- 

 tries. There is, however, one very vital problem not 

 encountered in airplane manufacture, and that is success- 

 ful protection against bacteria, to which glued joints are 

 now particularly subject, especially when exposed to 

 conditions of dampness. Recent experiments, however, 



IN THE MANUFACTURE OF LAMINATED BOWLING PINS THE 

 MATERIAL OF THE PROPER SIZE AND KIND IS FIRST SUR- 

 FACED ON TWO SIDES AND THEN GLUED UP INTO A BLOCK AND 

 SET ASIDE FOR A WEEK OR LONGER TO ENABLE THEM TO 

 REACH A STATE OF EQUILIBRIUM. 



have yielded results which indicate quite conclusively that 

 it is possible to make a glue which will be both waterproof 

 and bacteria-proof without decreasing its strength prop- 

 erties. 



The successful use of large built-up columns, trusses, 

 and structural timbers of similar character is more un- 

 certain, on account of the difficulty of designing satisfac- 

 tory joints and fastenings to meet the tremendous strains 

 to which they must be subjected. Another problem at- 

 tending their use is the shrinking of the wood after they 

 are put in place and the consequent loosening of bolts and 

 joints. Further refinements in drying practice, however, 



