SOME PETROL-ENGINE REPAIRS.
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Useful Hints and Tips from our Driver and Mechanic Readers.
SPARE engine and other bearings and bushes are occasionally supplied by the manufacturers without the essential oil grooves, it, being in souse cases preferable that these should be cut when the said bearings or bushings are being fitted in their respective places. For this and other reasons it is essential that the mechanic, or the driver who effects his own repairs and overhauls, should be acquainted with some of. the principles underlying the lubrication of bearings, and that he should understand how grooves, for various types of hearing ought to be fitted. Some useful ''guidance in this matter is afforded by "JAY.," of Lincoln, to whom we are awarding the 15s. prize this week.
After remarking that it is not sufficient merely to groove and channel bearings without proper consideration, and that the oil will, as likely as not, fail to reach the vital parts of bearings so cursorily treated, he proceeds, by use of a simple example, to explain the governing principle of bearing lubrica tion, so far as it is effected by the location and formation of the oil grooves.
He invites us to imagine a metal block, sliding over a face plate on which there is a film of oil. If the edges of the block at A, in Fig. 1, are sharp and square, then considerable difficulty. will be encountered in trying to permade the oil to pass between the block and the plate ; the tendency will be for the block to scrape the oil off the plate. If, however, the edges -of the block are bevelled slightly, as is shown in exaggerated form in Fig. 2, then the oil will pass readily under the block, which will then ride easily upon an. oil film.
This is the circumstance which must be kept in mind when considering how the grooves of a bearing should be cut. In many cases these grooves are formed with a sharp edge, corresponding to that in Fig. 1. In such eases, if the bearing be a reasonably tight fit., the oil will not be able to pass to those parts of the bearings where its presence is most needed.
Many mechanics, not realizing wherein the trouble lies, ease off the bearing a little, making it a slack fit, so as to enable the oil to pass. • This, of course,
is wrong; the bearing should not be slack on its journal, and the proper course is to bevel the edge of the groove in the direction opposite to that in which the journal surface is travelling; so that the oil can get between the two surfaces. Preferably, the bevelling should be carried out on -both sides of the groove, as shown in Fig. 3. In that ease oil enters at B, and flows along the bevelled-edge groove, which is cut to within a short distance of the end of the bearing. If the latter be in halves, then the edges of the half bearings should also be bevelled, as shown, and in this case, too, the bevelling should cease within a short distance of the ends.
In the case of a shaft which slides to and fro in a bush—as, for example, the valve spindle of a steam wagon-then the grooving should be as depicted in Fig. 4.
An example of the correct method of grooving a bearing to which the lubricant is supplied ernder pressure is shown in Fig. 5. The groove (C) can be cut with the bearing in a lathe, the top half being placed temporarily in the reverse position for that purpose. The object of staggering this groove in the fashion shown is that of obviating the formalion of grooves in the shaft.
IN the opinion of "H.A.B.," of Rother
ham, the use of red-lead " raddle," as a means of showing the extent to which a journal is bedding in its bearing, is now being superseded, in case of motor. vehicle engine bearings, and in other cases of equal delicacy, by other and more efficient means, in particular by the employment of fine lead wire.
When about to bed a journal, using such wire, make sure, as a preliminary, that the rounded ends of the bearing are not hard on the corresponding fillet on the journal. Examine the grooves in the bearing, too, and see that they are not so shallow as to be likely to be scraped away in. the course of the work. It may be assumed that the bearing is to be fitted so that the journal makes contact in a manner familiarly known as a crown bed.
The wire must be placed over tha journal as shown in the sketch, the bearing put on and tightened up. Take out the journal and " mike " the wire. If it be of uneven thickness, the bearing must be scraped at the parts corresponding to those places where the wire is thinnest, and the process should be continued until the.wire, when removed, is of uniform thickness. Replace once more and tighten up the bearing bolts, marking the nuts so that when the wire is finally removed they can once more be screwed up to the same place.
THE following useful hint referring to a means of preparing rings for the pistons of a Daimler engine is before us. The description of the method employed does not carry with it the name or address of the sender, and perhaps when he reads this he will communicate the necessary information to the Editor.
He remarks that most Daimler engines have the piston rings dowelled so as to keep them from rotating on the piston and to prevent the slots in the rings from getting into line one with another, and thus reducing the effectivenea of the piston and rings. When about:tea/it new rings, it is usually necessary to slit them in order that they may engage the dowel pins which are actually fitted in the piston. To facilitate this process a simple jig can easily be made in the following manner— Take an old inner sleeve valve and put it in the lathe by the .end carrying the lugs. Part off about 6 ins, from the base of the sleeve, and before removing the rest from the lathe scribe a line round it about a in. from the edge. Drill and tap six equidistant holes in this scribed line to accommodate k in. cheeseheaded screws, which should' be put in from the interior of the sleeve. Any rings which are to be slotted can be sprieng into the sleeve, being supported there .by the screws, and the required Ant can be made with a round file.