1258
THEORY AND PRACTICE OF ENGINEERING.
Book IT.
will give the diameter in inches for a piston-rod of wrought-iron: when cast-iron is used,divide by 42, and when steel, divide by 72 for the result instead of 45 : when rods of air-pumps are to be considered, the pressure of the atmosphere and the diameter of the pumpmust be taken instead of the force of the steam and the diameter of the cylinder.
Beams, their Strength .—When of a uniform thickness it is not safe in cast-iron to makeit less than fa of its depth, when the velocity is the same as that of the piston: D*F/ =2\5bd % ; and when 166 equals <1, and 12/ equals n I), it becomes
/1*34P«\3 ,
( alH
E
D represents the diameter of the piston in inches; <1 the depth of the beam in inches, andthe breadth ^ of that depth ; n , the number of times the diameter is contained in thelength, from the point where the force is applied to the centre of motion ; 1\ double theforce of the steam in the boiler, in pounds per circular inch : half the depth at the centreof motion should be given to the depth at the end, keeping a uniform breadth : whenwrought-iron is employed, substitute 240 for 212, and for wood 64.
Cranks, their Strength _They should embrace the shaft, so that their depth should
be l£ times the diameter of the shaft ; thus, if SD be the diameter of the shaft, the depthof the crank will be 1 *5 S D ; hence D* PL = 212 bd 2 ; we have
Vi P /
2-25S 8 x212“477S*“ *
Cylinders and Metal Pipes, their Strength .—To determine the thickness of a cast-ironcylinder, the metal of which at an equal temperature is to bear a given stress ; multiply2*54 times the internal diameter of the cylinder by the greatest force of the steam on acircular inch, divide by the tensile force the metal will bear without alteration, and youhave in the result its thickness in inches. The cylinder is supposed to have equalresistance throughout its length, and the stress upon an inch of that length is equal to thediameter in inches multiplied by the greatest possible force on a square inch, and theresistance as twice the thickness of the cylinder, by one-fourth of the tensile strain of themetal, the tension being unequal on the resisting part: thus, for a cast-iron cylinder of 60inches internal diameter, for a pressure not exceeding 3*2 pounds per circular inch, iuaddition to the atmospheric pressure ; to determine its thickness
2*54 x 60 x 30
= 0*305 inches:
twice the force is here taken, or 30 pounds on the circular inch, and the resistance of cast-iron at 15,000 pounds the square inch. To determine the thickness of a working cylinder,where wear and other causes of pressure exist, multiply four times the clastic force ofthe steam in pounds per circular inch by the diameter in inches, and divide by 6000; thisresult, multiplied by the quotient arising from dividing the diameter by the diameter less2 2, is the thickness for strength, to which £ inch may be added for wear.
Pistons, their Construction. — It is most important that they should not admit of anyleakage, and 3Ir. Watt found that mutton tallow* was the best adapted to keep thepiston tight; w*hen cylinders were new and imperfectly bored, the grease soon was wantingand the piston left dry; he afterwards mixed blacklead dust, but it was found that thisapplication wore away the cylinder, but they are now made so true as not to requireit, or at least only* for a short time. Pistons are usually of metal, and are formed soas to have some elasticity: the common piston is a double cone of wood havingsecured around it by strong nails a couple of bands of leather ; when of metal, they areusually turned out of brass, and made to lit the cylinder in which they are to move, so thatwithout much resistance they will slide to and fro ; on this as well as below is a plate,between which and the cylinder of brass is inserted two leathers cupped, with the edgeschamfered oif to an angle of 45°.
The piston for the atmospheric engine is a plate of cast-iron, about £ inch less in diameter than the cylinder, and1^ inch in thickness, with a rim about 4 inches from theedge ; beyond this a flat ring is fitted, and they are screwedtogether, after hemp mixed with tallow has been insertedbetween them to form the packing.
The hemp-packed Piston is that in general use : the bottomis accurately fitted to the cylinder, and the upper part iscut away to admit of gasket or unspun hemp or soft ropebeing wound evenly* round it, which forms the packing;this is compressed by a plate by means of the screws: thepiston-rod is usually* attached to the bottom of the piston, andit is secured by a screw-nut inserted between the top and Fi S- 1995 - hemp-packed piston.bottom; melted tallow is applied by a funnel on the top of the cylinder lid to the piston.