RUNNING, UPKEEP AND OVERHAUL.
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Interesting Contributions from Maintenance Engineers, Drivers and Mechanics.
The " Collaring " of Crank-pins. TF we may judge from the bulk of the .1correspondence we have had at vol.ous times on that apparently veryinteresting subject, "Big-end Bearings,". we should say that the diagonally disposed oil channel is' the most highly favoured by all those who have had actual experience on maintaining en
gines. Ira repair work it is often not used because, to the average mechanic,' it is thought to require some special machine to produce it; we therefore welcome the suggestion of " S.B.F.," of Wakefield, who points out bow such grooves can be easily cut in any ordinary screw-cutting lathe.
Our contributor says: "The bearing should have the chamfers made on the edges and the central oil-hole drilled in one half in the usual manner. The bearing should then be placed in the chuck of a screw-cutting lathe and change gears such as would give a thread pitch of lin. utilized. That is to nay the saddle should move along the bed 1 in. to every revolution of the lathe. If the lead screw be one of font. threads per inch, 20 on the screw and 80 on the mandrel would be the wheels, but if the screw be two threads to the inch, 40 on the screw and 80 on the mandrel would do. In such cases it is often found that the gearing up of the screw to such a high ratio in. relation to the mandrel will render it impossible to revolve the screw by turning the mandrel. This can be overcome by fixing a handle on to the end of the screw, so that the screw turns the mandrel.
A suitable tool to make the groove should be fixed in the rest and set so that in its travel it passes the oil-hole. By winding the screw backwards and forwards and withdrawing the tool from its groove during the reversal, a grcrove can be cut from chamfer to ebtunfer with ease and accuracy. The pitch of 1 ill. will cause the saddle to travel * in, at each half revolution of the lathe, so if the oil-hole be * in.. in diameter the groove will leave the hole uncovered for a sufficiently long period to allow oil to escape, hut will prevent collaring on the shaft.
Trouble with Overhead Valves.
WE recently published a letter from a correspondent who turned grooves in the stems of some of his valves, into each of which he inserted a spring ikng to prevent the valves from dropping into the cylinder if the cotter should become displaced. A correspondent, "R.N.," of Spennymore, tells us that he has even a better way of dealing with the trouble. He drills a very small hole through the valve stem, through which he passes a split-pin, taking care that the Valve is allowed its full movement without the pin coming in contact with the valve guide, The same .correspondent claims to have discovered a reason why. nuts on disc wheels have rightand left-hand threads. Apparently he does not agree with the theory advanced by " Shim " that the nuts age blown loose by the wind as they revolve—which disagree meat does not surprise, us—but offers as a suggestion that the reason for fitting right-hand threaded nuts on the right side and left hand on the lef thand side, is that by doing this it acts as a guide to an inexperienced person to whom the task of changing a wheel may be entrusted, for to loosen such nuts the brace must be turned towards the rear of the vehicle in both cases, whilst to tighten them the brace must be turned towards the front of the vehicle. We think, however, that "J.M." is nearer the truth when he suggests creeping as the real reason. It would be interesting to hear from the designer of a wheel of this kind his actual reason for the procedure, if he really had one, or if he merely fitted right and left threads because he had seen others do so.
Another Dissentiert.
THE writer of the letter which commenced this argument on wheel nuts—" A.R.P.," of Horley---also disagrees with the view of " Shim," and says that had it not emanated from Glasgow he should have looked upon it as a joke. He says that it is far beyond his elementary mathematics to work out the difference of wind pressure owing to the distances from the centre of revolution of the two sides of a i-in. nut. The drawing shown in our issue of August 7th was incorrect, as the wheel of a lorry does not revalve on its own centre as shown, but is moving through curves struck from the point of contact of the rim ef the wheel and the ground, which would reduce the difference of wind pressure to something so minute that it would be difficult to calculate.
Brake Adjustment.
WE hear from "J.G.P.," of Wisbech,
that he found that the adjustment of the transmission brake of his Dennis lorry appeared to have corns to an end, as he was unable to tighten the nut any farther. An examination of the linings showed that there was plenty of useful life in them.
Thinking that the screwed part of the adjusting bolt did not extend 'far enough to allow the nut to tighten, he took the parts to pieces, but found all in order, there being plenty of thread to allow for further adjustment. Further investigation showed that the coils of the spring eloied so that the shoes could not be brought any closer. A few coils cut off the end of the spring made further adjustment possible,
A Bracket for Pyrene Fire Extinguishers,
wE are told by one of our cor
respondents, " W.M.C.," ofFrampton-on-Severn, that the bracket supplied with the Pyrene fire extinguisher, although satisfactory on a private ear which is fitted with pneumatic tyres, has not proved so successful on his lorry, where rough roads and solid tyres cause considerable vibration.
Recognizing the importance of at all times having his extinguisher ready for immediate use, he made a bracket to his own design, which lie has had in use for some 'considerable time, and which has proved to be quite successful.
A piece of iron or steel plate, 3 ins. by * in., was bent to the shape shown hi sketch. The bending was done cold so as to retain the natural spring of the material, by holding part of it in a vice and bending and hammering the remainder to a right angle. Holes were drilled to enable the bracket to be bolted to any convenient part of the cab, whilst a taper hole was made to receive the end of the extinguisher,