CHAPTER III 

 MATERIALS AND THE STRENGTH OF MATERIALS 



A knowledge of the materials used in the construction 

 of farm machinery and the strength of these materials 

 will be helpful in the study of farm machinery. 



65. Wood. At one time farm machinery was con- 

 structed almost entirely with wooden framework, but 

 owing to the increase in the cost of timber and the re- 

 duction in the cost of iron and steel, it has been super- 

 seded largely by the latter. Progress in the art of work- 

 ing iron and steel, making it more desirable for many 

 purposes, has also been a factor in bringing about the sub- 

 stitution of iron and steel for wood. The woods chiefly 

 used in the construction of farm machinery are hickory, 

 oak, ash, maple, beech, poplar, and pine. It is not possi- 

 ble to discuss to any length the properties of these woods. 

 The wood used in the construction of machinery must be 

 of the very best, for there is no use to which wood may 

 be put where the service is more exacting or severe. 

 Wood used in farm machinery must be heartwood and 

 cut from matured trees. It should be dry and well sea- 

 soned, and protected by paint or some other protective 

 coating. Moisture causes wood to swell, and for this 

 reason it is difficult to keep joints made of iron and wood 

 tight, for the iron will not shrink with the wood. 



Excessive moisture in wood greatly reduces its 

 strength, and wood subjected to alternate dryings and 

 wettings is sure to check and crack. Wood is especially 

 well adapted to parts subject to shocks and vibrations, as 



