IN THE WORKSH P. TUNING NOTES FOR THE AMATEUR MECHANIC.
TH. being the beginning of another racing season, many engines are dismantled undergoing modifications to get more speed and power, and are therefore in a ripe state for special work on the internals which will be impossible when assembled.
A very important point in the preparation of any high speed engine is the matter of polishing of the various parts. Most owners are familiar with the need for polishing all valve ports and gas passages to ensure an unrestricted flow, but there is another and equally important side to the polishing question which is not so well attended to, and even when done, is often done without anyone being very clear about the reason.
What I am referring to is the polishing of all moving parts which are subject to stress. Many readers who have acquired ex-racing machines may have noted the fact that all con-rods, crankshafts, flywheels, and other moving parts shine like silver on every surface, and may imagine that this has merely been done for appearance and to find work for idle mechanics.
This, however, is not the case, the real reason being that this work has a very important bearing on the reliability of the engine. All crankwebs and other parts which are found left in the rough forged state, or even if machined but not finished, should be buffed to a high state of polish, and all sharp corners, lines, and surface cracks removed.
This is not, as many believe, to eliminate “oil-drag,” though it may have some beneficial effect in this direction, but to increase the factor of safety against breakage.
The reasons for this effect are two-fold. The primary object of polishing out all surface marks and scratches is to make sure that they really are surface scratches, and not cracks going into the metal and materially reducing the cross section, and therefore the initial strength of the part in question. These will be liable to cause failure even if the crack is small enough not to affect seriously the actual strength of the part, and the failure may not occur until very much later. To understand this it is necessary to grasp what happens to a piece of steel under repeated stresses. Steel, like most metals, is elastic, that is to say, when the shape is altered under stress, it will return to its original shape when the stress is removed. This is all, provided that the alteration of shape is kept within certain bounds. If the stress is increased, the “yield point” is reached, when the change becomes permanent, and if increased still further will finally break. Neither of these last two occurrences should take place in an engine, but there is a further unfortunate phenomenon in steel, known as fatigue. In fact if it is repeatedly
stressed and released within the elastic limit, the nature of the material—which is crystalline—changes, the change taking the form of coarsening of the crystal structure. This hardens the metal but unfortunately reduces the elastic limit to the point where the metal becomes brittle.
In any engine under load, all shafts, connecting rods, etc., must flex to a definite though small extent, and the question of fatigue is very important.
The resistance of steel to fatigue can be enormously increased by the addition of various elements, such as nickel, chromium, and. vanadium, forming the alloy steels which are used in high class automobile construction. At the same time the tendency to fatigue remains, and the greater the stress at any point, and the strain (which means change of size and not the load) at that point, the more liable is the part to fracture.
In the case of a part with smooth polished surfaces and small or only gradual change of section, the strain will be spread fairly evenly over the whole, whereas a sudden change of section caused by a small fissure will cause a greatly increased local strain at that section which may result in local fatigue at that point and consequent breakage, without the rest of the metal being affected. By careful polishing out of all marks and small cracks such local fatigue can be almost eliminated.
The logical development of this idea is to avoid all sudden changes of section in the designed shape, as well as the accidental. I was recently assisting a friend to dismantle the engine of his competitioncum-touring vehicle, the act being rendered necessary by the crankshaft having broken, and an inspection of the accident gave a practical illustration to the foregoing remarks. The shaft had broken across one of the webs adjacent to the centre main
bearing, which is where one would expect it in the type, 4 cylinder, 3-bearing without balance weights. An interesting point was that the break had started at the join of the journal and the web where there was a sharp corner. Careful examination of the rest of the shaft showed that cracks were started in similar places on several of the other journals, showing that if it had not broken where it did it would have very soon broken somewhere else. The shaft was old, having been in use for some five years, but the greatest factor in its undoing was, I should think, the fact that all the webs were rough, as forged, and the radii on the meeting of webs and journals were extremely
Fortunately no other damage was caused to the engine, and a new shaft, incidentally with slightly different webs, has now been fitted. If the shaft had been polished all over and the radii increased, it would probably have been running to-day and not unduly fatigued.
I had very marked proof of the importance of gradual changes of section in an engine I was working with some years ago. It was a special edition for racing purposes, and we had made it slightly more special, which probably caused the trouble.
It had a built up shaft, and we had trouble with breakage at the main journal at the timing gear end, due presumably to the whipping of the shaft which always occurs. This was fitted into the web by a taper, and owing to the congestion in the timing gear due to this all running in ball bearings for the first time, there was no room for an adequate size main roller race, and consequently for a large enough shaft to cure the trouble. The shaft was made of *first class nickel chrome steel, and therefore when it broke just before an event, some thinking was required. The normal material was not available, so we made the new shaft from a piece of Model T Ford axle. The break occurred at the change of section between the journal and the tapered part, as shown in the figure, which was very sharply radiused. It was found that with a certain amount of wangling the main roller race could be drawn slightly to give room for a larger radius on the change, and this was accordingly done. No further trouble was experienced with the shaft, and now, its competition days over, this machine is still giving good service, though the present owner does not know on what a trifle his engine depends I
All of which goes to show that careful attention to such minor details although, rather a bother at the time, is often well repaid.—B.
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