MARINE ENGINES.

519

No part of the piston rod, such as the sectional area at the partwhere cotters are used, should be made smaller, so that the rod maysafely take the greatest tensional strain imparted by the full steampressure plus the vacuum. When two piston rods are used forreturn connecting-rod engines, add one-fourth more area; this isnecessary as the Vibration of long rods is rather severe at the highrate of piston speed now adopted. The depth of the stuffing box,from inside of the cylinder end to the face of the flange, equals thediameter of the piston rod multiplied by 2; the breadth of the packingspace equals the diameter of the rod divided by 4; and the thicknessof the stuffing-box flange equals the rod divided by 3. When fouror more bolts are used for tightening up the gland, their combinedarea is found by dividing the piston rod area by 6.

Slide-valve rod .—To find the sectional area of the slide-valverod, multiply the full area of the valve by the full pressure, as for thepiston rod (i.e. steam and vacuum), to which add the weight of thevalve in lbs., and divide the result by 2240; one-sixth of the quo-tient equals the area of the valve rod in square inches at the weakestpart. Sometimes two rods are adopted, passing over the shaft onthe return principle; in such cases each rod is but little reducedfrom the single-rod arrangement. To prevent the rods bending orspringing, guides are fitted at about the middle of their length.

Eccentric strap, rod, and link .—The dimensions of the bolts forthe eccentric strap are found by multiplying the total area of theslide valve by the steam pressure, adding the weight of the valvein lbs. as for the slide-valve rod, and dividing the result by 4000;one-sixth of the quotient divided by 2 will give the area of eachbolt at the bottom of the thread. Find from a table of areas thecorresponding diameter, to which add the depth of the two threads,which will give the full diameter; this may be made a trifle moreto enable the thread or screwed part of the bolt to pass easilythrough the hole in the strap. The sectional area of the gun metalstrap where it is bolted together at the lugs should be three times thearea of the bolt at the bottom of the thread, increasing in thicknessall the way round; its breadth equals about twice the diameter ofthe bolt. The thickness of the snugs should be one-fourth more thanthe thickness of the strap at the lugs. The thickness of the eccen-tric ring equals one-eighth of the diameter of the shaft, and the metalround the boss for securing the eccentric to the shaft equals one-seventh of the diameter. The eccentrics are generally cast open,