When iron is heated it expands, and upon being cooled it contracts to about its original size. This property is utilized in doing what is known as shrinking. Fig. 144 shows a collar shrunk on a shaft. The collar and shaft are made separate, the hole through the collar being slightly less in diameter than the outside diameter of the shaft. The collar is then heated red hot and the heat causes the collar to expand, making the hole larger in diameter than the shaft The collar, while still hot, is then placed on the shaft in proper position, and cooled as quickly as possible by pouring water on it. As the collar is cooled it contracts and squeezes, or locks, itself firmly in position. This principle of shrinking is used to a large extent where a firm, tight fit is wanted, the only objection being that it is rather difficult to take a piece off after it has once been shrunk into place.
Fig. 144. Collar Shrunk on Shaft.
Red, lowest visible in dark
Red, lowest visible in daylight
Red, light (scaling heat)
Orange, light Yellow, full
In doing work in the blacksmith shop it must be constantly remembered that the work is larger when being worked than it is when cool. Allowance must, therefore, always be made for shrinkage. As the pattern-maker allows for the contraction of the molten metal to the cold casting, so the blacksmith must allow for the contraction of the hot iron or steel to the cold forging.
From the scale of iron heats at the several colors, given in Table IV, it will be seen that the temperature at which forgings are finished under the hammer, should be at about 900° Fahrenheit. When these same forgings are cold their temperature will be from 60° to 70° Fahrenheit. There is, therefore, a difference of at least 840° between the working and the finished temperature. The expansion of iron may be taken to average about .00000662 of its length for each increase of one degree Fahrenheit in its temperature. If a bar of machine steel exactly 2 feet long when cold is heated red hot and measured, it will be found to have increased nearly \ inch in length. Taking the temperature of the red heat as 1370° Fahrenheit, and that of the cold bar as 70° Fahrenheit, the increase in length would be 1300X.00000662X24 (length in inches) =0.206 inches. This expansion must be allowed for when measuring forgings red hot.
*Table IV is based on temperatures given by Messrs. Taylor and White, Transactions American Society of Mechanical Engineers, Vol. XXI.