A. Cheapness and convenience of renewal after wear of the contacting surfaces. Q. What may be said of malleable-iron driving boxes? A. They are good if the side next the hub is spotted with babbitt; else, they cut the cast-iron hubs. Q. Should oil cellars be straight or tapering? Q. What per cent of the surface of a bearing, such as a driving box, is figured as supporting the load, and what is the usual pressure allowed per square inch? A. It is now customary to figure the "vertically projected" area from the journal diameter and the length of the bearing as the supporting surface. Given driver bearings 9 x 13 inches, 9 x 13 117 square inches is the projected area. If the load on each wheel is 24,750 pounds, dividing this by 117 gives 211.5 pounds per square inch. = Q. What is the advantage of a cast-steel box with a shell, over a solid one? A. That it may be renewed more cheaply; also it does not close up so readily when worn part way through. Q. What effect will an engine being low on one side be apt to have on the bearings? A. The back end of the connecting-rod brasses, or the crosshead, may heat on that side. Q. What is Babbitt metal properly? A. An alloy of 9 parts of tin and 1 part of copper, for journal bearings; so called from its inventor, Isaac Babbitt, of Boston. Some variations have been made; among the published compositions are: (Another formula substitutes zinc for antimony.) The term is commonly but falsely applied to any white bearing alloy, as distinguished from those in which copper predominates. Q. What is a lead-lined journal bearing? A. One having its inner surface covered with a thin layer of lead, so that it may fit itself to the journal when worn. They are often called Hopkins bearings. (A variety of others are more or less similar, but a greater quantity of lead or Babbitt metal is frequently used.) Q. What is the proper method of packing a journal box? A B Fig. 258. Proper Packing. A. As clearly shown in Fig. 258* the first packing should be made into a tightly twisted roll or pad A and firmly packed in the back of the box against the journal shoulder and dust guard, to keep the oil in and the dust out. Then the lubricating space should be packed with good-sized balls, firmly enough to resist being shaken down by shocks. The side packing should be loose and the strands on the sides where the journal leaves, carefully turned down under the mass to prevent its being pinched between journal and brass. None should be put above the axial line of the journal. The journal should *This and the other cuts in this connection are from "Baldwin Locomotives" for July, 1922. be well behind the collars and on the ends in order that the lateral movement of the journals will not cause the collars to loosen the packing and work it up into the front Fig. 259. Improper Packing. part of the box. When the packing is thus applied behind the collars, a wedge C of packing should be placed against the end of the journals and the end of the box Fig. 260. Improper Packing. to aid in holding it in position. It should not extend above the bottom collar. Q. What precautions should be taken after this has been done? A. To see that no strand is left hanging out; and to close the box tightly. Q. Where is this last especially necessary? A. In dry, dusty countries. Q. How far should boxes thus packed run without renewal? A. About 1,000 miles. Q. What should be done at the end of that time? A. If all is in good condition the packing should be loosened up to prevent glazing, and some oil added each side about half length. Q. What care should be taken in repacking? A. Not to lay the packing on a gritty surface. Q. What is the result of having too much packing in front and not enough at the rear of the box? A. Irregular lubrication; possible heating of the journal at the rear. (Fig. 259.) Q. What is the effect of putting too much packing at the front and not enough at the rear? A. Waste of oil, as seen in Fig. 260. CHAPTER LXIV FRICTION AND LUBRICATION Q. What is friction? A. The resistance that contacting bodies have, to change their relative position; principally caused by interlocking of moderate roughnesses on their opposing surfaces. Q. How many kinds are there? Q. How is friction lessened? A. By substituting rolling for sliding friction; by suitable choice of contacting surfaces; by reducing their mutual pressure, and by interposing between them a layer or film of minute particles, which have less friction among each other than exists between the otherwise contacting bodies. Q. What are the principal lubricating materials? Q. Have balls and rollers come into practical use in locomotive practise, as in automobiles and other machines, to substitute rolling for sliding friction? A. Not yet. Q. What is the coefficient of friction? A. The ratio between resistance to sliding motion to the pressure between the sliding surfaces. Q. Is this ratio the same for all pressures? A. Yes, up to the point where the pressure deforms one or the other of the contacting surfaces, when it may either increase or decrease, according to the change in the surfaces or up to the point where the lubricant is squeezed out. Q. Is the friction between a journal and its bearing "sliding" friction? A. Yes, because only one of the contacting surfaces |