Arizona republican., 1921-10-23


 requirements are about as near 'ab- solutely" as one can reach. We allow variations of only 0.001 inch. On top of all this, main bearings cannot be out of round nor can they very from specifications more than 0.001 inch. A trifle more clearance is allowed on the throw bearings, but that lis neither here nor there; the long and short of all these hair splitting measurements is that the crankshaft is a mighty carefully con- structed part.
 "Now what is the relation between these hair-s splitting measurements and case hardening? It is this: case hardening requires heat and heat tends to warp CT twist or change the form of metal; so, if after the soft crankshaft has been all nicely made, and conforms exactly with every measurement specified. the heat caused the crankshaft to twist and get out of shape, the attempt at case hardening would naturally be fail ure! And on the face of things, the task of exosing metal to high tenn peratures without warping and twisting troubles did appear diff cult.
 "But it really required only a little calm judgment and common sense, as the engineers now point out, to lick these problems. They decided this: if crankshaft shrinks as the result of exposing it to heat, we'll just make the green shaft a little larger than is required: after the shrinkage takes place, we'll have a regular normal shaft. You know how the laundry can shrink a wool sweater so you can't get it on. Well, if the sweater had been too large in the first place. the shrinking accident at the laundry would really have been an accom- modation to you. That's how this shrinking problem was handled by the Franklin engineers-they learned just how much the shrinkage would be under certain temperature condi- tions, and then the green crankshaft was made this much too large in the first place.
 "Shrinkage, however. was only one of the obstacles to be avoided There was warping, twisting, distortion and a change in the crankshaft ma 1 terial. The distortion nuisance was | attacked in the same daring manner i To avoid distortion, a rigid holder known as a hardening die, was made | This hardening die is used just after ! furnace Temperature of 1450 de- grees) and before the crankshafi gets chucked into a tub of water. | This operation is called the bath | and as the hot shaft has to get its | bath immediately after coming oui of the furnace, no time can be wasted l in putting on the hardening die. S6 holders that can be fixed on fin 2 ! jiffy were attached-in fact it take: only p seconds from the time the ! crankshaft comes out of its hot be6 until it gets it's bath.
 Another obstacle to be avoided 1T ! the case-hardening operations was | to get only the wearing surfaces hardened and keep the rest of tht ! material in it's soft original state. l This means that the crank shaf would retain it's original bending l strength; you know a case hardened metal has little bending strength | though it may have great wearing | qualities So before the crankshaf f ] goes into its bed of carbon and ther ! into the furnace, those parts that art not to be carbonized are copper ! coated.
 Then, after the whole carbonizins procedure has been completed, tn, crankshaft goes back to the crank shaft room to be finished up. w, have mentioned the hardening die t, prevent distortion. It is true tha there is no distortion. and that W, have avoided the problem of shrink age-but you can guess that it i absolutely impossible to subject piece of metal to a temperature o 1450 degrees, and have it come ou 1 exactly as 1t went in. Even thoUgl | oN- ylI I-Gh huf at f[,5 requirement before going into tn furnace, there would be some varia tions when it came out.
 So it is up to the crankshaf grinders and experts to clean up tn case hardened shaft. get it per fectly aligned and so forth. And an grinding on the bearing surface afte the case hardening has been com pleted is SOME job. You recal that we stated a file made little i'm pression on the case hardened sur face. Well. it is the job of tn grinders to grind down and take
 small bit off the bearings after tn shaft comes from the case hardenin room. It is slow, tedious grindine and great care must be taken. for a cut or impression is made to deeply on the case hardened surface it cannot be taken out as in the cas of soft metal"
 According to Hunsicl1, there ar two grinding operations after th crankshaft comes from the case
 hardening room. There is a cer tain amount of aligning to be don' and the crankshaft has to be teste for standing balance. All thi handling of the crankshaft mean that it is likely to get out of align ment-for as mentioned, the part very delicate. Aviation experts wer | .I l. eI1