OBVIATING BIG-END TROUBLES.
Page 63
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Useful Contributions From Our Driver and Mechanic Readers.
AMONO the various letters that have been written on this very important subject, we have not before seen a suggestion for the way in which shims should be fitted similar to that which is sent us by a correspondent from Reading, "H.J.P.," whose experience points to the fact that although shims ought to be used they should not extend between the brasses. He says :— "I have had experience in the fitting of big-ends of various makes, and have had opportunities of observing their behaviour under severe working conditions. After trying most a the plans in use, I have come to the conclusion that the method adopted for, the big-ends• of the Saurer lorry is one of the best, both from the point of view of tbe fitter and from the satisfaction they give in use.
" The plan is as follows :—Thc brasses are made to meet each other, and the shims do not extend between them; the faces of the rod and its capare so machined that there is room for a fairly thick shim. The method of fitting is an easy one, as no unknown allowances have to be made for the relative tightness of the brasses or the shim. The brasses are fitted to the journal, and the bolts tightened up just sufficiently to hold them securely in place, but not enough to distort thecap. "When the brasses are ready, the spaces between the surfaces of the rod and cap are measured, and shims reduced in thickness until they will just pass between them. The bolts are then removed, the shims put in place and the bolts pulled up to their proper tension. ,
" It is true that only the dowel-pin Can be relied upon to prevent the brasses from moving in their place, but if they are well bedded into the rod and cap, and a good pin fitted, there is no fear of their working loose."
Oil-channelling.
A LETTER on this subject has been received from " F.0.11.," of Richmond, Yorkshire. He does not agree with " E.F.F.," of Bermondsey, in the matter of the cutting of oil-ways for use with pressure-fed lubrication systems, and says that he cannot see how the hole in the crankpin, from which the oil is delivered, can have time while passing the oil-way to deliver sufficient oil to be of use. He advocates the cutting of a channel on the lower half of the brass, as shown in one sketch, where the groove runs for 'half a revolution in the direction in which the hole travels, thus allowing ample time for a full supply of oil to escape front the crankpin. He then suggests that in the upper half of the brass a groove should be cut at an angle ; this, be says, will distribute the oil from the chamfer all along the journal. He aSsures us that he has had experience with this procedure on a type of engine that had ,a weakness for running out its big-ends, and that since this method has been adopted no more trouble has occurred.
He disagrees with "E.F.F." in the matter of fitting shims made 'of vulcan
ized fibre, as he points out that such materials are much affected by atmospheric conditions, and that under heat they may shrink and become loose, thus allowing the big-end to hammer. He finds thatbrass shims, if properly fitted, are quite satisfactory.
Four-bolt Big-ends.
THE following useful hints regarding bolts and split-pins are given by "J.K.," of Barnsley :—
"First as to oil grooves. Any competent mechanic should know that No. 1 sketch (accompanying the letter from E.P.F.') is the correct way with pressure feed, but if splash feed and dippers be used it is best to take the grooves to the sides so that the bearing has two ways of being lubricated. If a big-end bearing runs out without any previous warning being given, it is best first to find outthe reason for the failure. It may be that the oil-ways are clogged up, and these should be cleaned out with a paraffin squirt.
"If the bearings be rernetalled at home care should be taken when boring same ; they should be placed in connecting rods, not put into the lathe with the halves soldered together, as when they are nipped up in the chuck
it is liable to loosen the metal. It takes more time putting the connecting rods up true in the lathe, but when finished they are better jobs and there is not much chance of the connecting rods being out of line.
"As for shims, both brass and mild steel have been used without trouble. Care should be taken to fit them properly, so that they are nipped all over, otherwise they will come out.
"When tightening big-end bolts these should be pulled up tight, but it is not necessary to use a 2-ft. tommy bar as is sometimes employed, for this merely stretches the bolts and often means one breaking and causing considerable damage. Why all makers do not fit four-bolt connecting rods is difficult to understand, as it is seldom that a fourbolt bearing comes adrift."
How to Fit Shims.
FURTHER testimony as to the right method of fitting big-end bearings comes from " J.P.," of Maryport, Cumb., who says:— " If the bearings be properly fitted and the shims made the correct shape and size, sheet-brass or any soft, strong alloy is satisfactory to use, as it cannot damage the crankshaft, being of a softer nature.
"By being properly fitted I mean that the edges or joining faces should be squared and shims of the correct thickness inserted.
"In tightening up, great care should be taken to make sure that it is the big-end and not the cap that is tight, and the pins should be a tight fit in the locking holes.
"The best method of tightening is to screw up the bolts, and if the holes be not in line with the split-pin hole a little should be filed off the face of the nut to allow it to go farther on."
Shim Material.
FROM Mortlake " C.J.F. " writes as follows:— " A reader of Bermondsey quotes that brass shims between big-end caps are unsatisfactory, owing to the hammering they receive. .At present I am mechanic to a large fleet of Leylands and have never yet taken a bearing adrift with a faulty brass shim. If a bearing be correctly fitted it cannot hammer the shim ; the most common trouble is, the brasses are found to be floating in the caps.
"To fit a hearing correctly both brasses and caps must be dead square. Also great care must be taken in fitting the brasses to the caps: they should be fitted as well as the bearings to the journals. The brasses must thon butt together on the shaft, and when the cap is attached there should be a clearance of .005 in. with the big-end bolts partially tightened. When the final nip is applied the faces will be drawn together, making it impossible for the brasses to float or to hammer the shims.
"In types where the big-end depends upon oil from the mains, as in Leyland engines, this method is vital, for if the brasses do float the oil. is cut off owing to the holes coming out of line with those in the crankshaft."