” HOTTING-UP ” A STANDARD MODEL FOR COM PETITION WORK. (PART II). NOTES ON THE TREATMENT OF BEARINGS.
ONE of the most curious things connected with refitting a set of bearings is the fact that the actual scraping of the surfaces is by no means the most important part of the business. Indeed, with the exercise of care and skill, it is quite possible to take up a set of bearings without having to resort to scraping at all, the latter often consisting in little more than correcting faults consequent ur on lack of judgment when letting the halves of the bearings together.
A Safe Working Axiom. Before commencing any work on a set of bearings, one is well advised in assuming nothing to be absolutely correct, and thus attention will be directed to all the details of the business from the very fundamentals. Unless this is done there is the danger of overlooking
something likely to upset the proceedings as they advance. In the previous article the matter of journal accuracy was discussed, so that we will assume that we are now starting operations with a true crankshaft and a set of bearings which are characterised by a certain amount of wear but no other damage, such, for example, as loose or broken white metal, badly scored or” pickedup” surfaces, or badly distorted shells. If any of the latter defects exist, the amateur will be well advised to invest in a new set of bearings, rather than undertake the process of re-metalling the old shells, as this will mean constructing some form of re-metalling jig, which, for a single set of bearings, is hardly worth while.
Locating the Looseness in Bearings.
Contrary to what one might suppose, the looseness in a bearing is often occasioned by other factors than
wear between the bearing surface and that of the journal, and the mere letting together of the halves without reference to other conditions which may exist will not be sufficient to ensure a sound and workmanlike job. The bearing shells may be loose in their keeps, or they may be adjusted so that, while gripped, they lack proper support, and are thus liable to distortion of the kind which rapidly causes the white metal to break away from the shells. These are mentioned as two defects, though there are a number of others liable to produce equally serious results. Our first step, therefore, must consist in locating the looseness of the bearing, which can be done by the simple and effective method of using marking. Mechanics have various fancies as to the composition of marking, but for bearing work a mixture of lampblack
powder and thin oil, reduced to a thin paste, will be found quite suitable. This should be applied to (a) the crankshaft journal, (b) the edges of the bearing shells, (c) the edges of the bearing keep, and (d) the inside of the keep housing. To make the explanation quite clear to beginners at bearing work, these points are shown by the shaded areas in Pig. 1. The next step is to fix the bearing in place on the journal, tightening up the bolts to the fullest extent, during which process the marking will be transferred so as to indicate how the various surfaces make contact with each other. On removing the bearing and separating the shells from the keeps, we may now proceed to examine the condition of the fit, for by the mottled appearance of the marking it will be easy to distinguish those surfaces that have been pressed together by the action of the bolts from those which have
“HOTTING-UP” a STANDARD MODEL for COMPETITION
either cone into contact but lightly, or possibly not at all.
In the first place we shall probably find that the marking has been transferred from the journal to the inner surface of the bearing as an irregular smear, which need not concern us at the moment. What we really want to know is : Does the keep grip the shell ? And, if so : Is the grip free from distorting stresses ?
To explain matters more clearly let me refer to the diagrams reproduced as Fig. 3, which shows three examples of big-end fitting A, B and C. A represents a correctly fitted bearing, for it will be observed that the two halves of the shell butt together, as do the cap and body of ithe connecting rod respectively. Here no distortion is possible, for the whole is held together by the bolts as a solid mass. At B, however, things are different, for while the bolts clamp the two parts of the connecting rod together, it will be noticed that a gap exists between
to call for the use of a file, special care will be needed, and on no account should the faces of the body of the rod be touched, for, as shown in Fig. 2, a very slight irregularity in removing the metal will throw the rod considerably out of square. In easing away any metal from the faces of the shells or caps particular attention must be paid to the important matter of alignment, and constant checking, during the whole process, will be necessary by means of a surface plate and scribing block, as shown in Fig. 4.
In bringing a bearing from either condition shown at B and C in Fig. 3 to that of A, it is necessary to make alternative use of the file or emery sheet, with the process of marking as described above, but the process may be complicated by the introduction of yet another factor, i.e. the gripping of the bearing on the journal, which brings us to the subject of scraping.
The Development of a Bearing Surface.
As remarked earlier in the article, the first transference of the marking from the journal to its shell will
the halves of the shell. This condition permits the latter to hammer under the repeated thumps of the explosion, which loosens the metal, distorts the shell, and soon causes the journal itself to wear oval if the defect is not brought to light sooner by a collapse of the bearing.
The state of affairs depicted at C is just as serious as those described, for while the shell is kept to its work, and cannot get loose, the gap between the joint of the connecting rod throws a great deal of extra load on the bolts, and, in addition, imposes considerable stresses upon the shell, which it is not designed to withstand. The inevitable result is distortion, loosening of the metal, possibilities of fractured connecting-rod caps, and sheared bolts.
From the above remarks it will be appreciated that scraping does not by any means constitute the whole art of fitting bearings.
Means of Adjustment.
to B 3, means ment is obviously to ease away a certain amount of metal to reduce the gap. If the amount is so great as
probably take the form of an irregular smear, but now that the pressure is increased, the marking will become more regularly defined. Instead of easing away the high places indiscriminately, this part of the work should be done so that the finished surface shall comprise a number of evenly distributed bearing areas, which, when run for a while, will merge into one even and accurate bearing surface.
The production of a good bearing surface depends almost entirely upon the manner in which the scraper is used. If the strokes follow semi-circles spaced from one end of the bearing to the other, the bearing surface will be formed in annular corrugations, and a large proportion of the bearing area will be lost. The proper way to use a scraper is to work the cuts diagonally from one corner of the bearing to the other, changing the direction of the stroke at frequent intervals by working from the opposite corners. Fig. 5 shows how this method produces a regular and evenly distributed bearing surface. For high speed engine work it is advisable to ease away the edges of the bearings to the extent of a few
thousandths of an inch to provide a clearance. As an example, on a 2 in. diameter bearing this clearance should be about in. on either side, or in. at each edge of the shells. This can be cleared away before the bearing is finally fitted by the aid of a heavy scraper.
Many people seem to be under the impression that the nuts on bearing bolts require careful adjustment, which is a very great mistake. These nuts must be dead tight, and nothing else will do. If the last quarter turn on the last nut causes the bearing to grip, then the surface must be scraped a little more, slacking the nut back one castellation is nothing more or less than a criminal act. If a bearing is fitted properly, blows on its side with a hammer will not ease it should it happen to be tight. I was once working in a shop with a first class bearing fitter, and when he had completed a set of connecting rods one day he called the foreman to inspect them. The foreman thought one was a little tight, picked up a hammer, and struck the rod a blow on the side. In a flash the fitter seized the hammer from the foreman’s hand and threw it through the window. I thought he was going to throw the foreman after it, but the timely intervention of the works manager prevented the more serious assault. The fitter did not get the “sack,” as
it happened the works manager was a practical engineer, and appreciated the action of a man who knew his job thoroughly, and resented ill-treatment of his handiwork. (To be continued.)
A 1,500 c.c.
The new Thomas Special, which has a 1,500 straight eight engine, with a supercharger, has entered for the Grand Prix of the Automobile de France. J. G. P. Thomas is establishing an racing stable at Linas-IVIontlhery, and has been fixed for seven different of machine.
A Good Performance by an Alfa Romeo.
On 25th January an Alfa Romeo car, driven by Massias, put up an excellent performance by climbing the redoubtable Cote du Pailladou from a standing start at a speed of 73 kilometres per hour. The car was fully equipped for touring, and a new record was established for all touring classes.
A “P. & M.” Appointment.
Mr. C. T. Ashby, the winner of the 500 c.c. class of the Championship Meeting at Brooklands, and fourth in the 1924 Junior T.T., has been engaged to ride the Panther in all races for the 1925 season. Captain F. C. Townshend is remaining in charge of the company’s experimental and tuning staff at Brooklands. A Notable ” Triumph “
In the French Six Days’ Winter Trial, over a course of 1,149 miles, which took place on the 3rd-8th inst. M. Pinney, riding the recently introduced Type ” P ” 4.94 h.p. Triumph, completed the most difficult course without loss of marks. By the fourth day all riders had retired with the exception of M. Pinney, in the 500 c.c. class.