HINTS ON MAINTENANCE.
Page 30
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How to Get the Best Out of a Vehicle, to Secure Reliability and to Avoid Trouble.
CONTRIBUTIONS. are invited for this page from fleet managers, drivers,, garage foremen, and mechanics, works staff and draughtsmen, and will be paid for on a generous scale. Every system, make, and type of commercial motor vehicle will be dealt with, and the matter should be written with a view to the disclosure of workshop and garage practice in the maintenance of a vehicle—practices which, whilst they may be quite normal, are peculiar to the particular vehicle arid may not be generally known to those responsible for its running. Expedients and suggestions for overcoming roadside and other troubles are covered in the following page, dealing with letters from our driver and mechanie • readers. Communications should be addressed to " The Editor, The Commercial Motor, 7-15, Rosebery Avenue, London, E.C.1."
245.—Obtaining Increased Power from Brakes.
Certain commercial vehicles, when turned out by their manufacturers, will be found to have the brake shoes almost completely covering the area of the drum. This is not. correct from the point of view or. design, and also detracts from the efficient working of the brakes. The correct pressure on the brake drums depends on the position of the centre of pressure of each shoe or shoe lining. It is desirable that the centres of pressure should he divided by 180 degrees and be positioned at the centre of each shoe. This prevents undue wear on the axle bearings by
assisting in keeping the brake drum concentric with the axle, and gives the friction material a longer lease of life.
The accompanying illustrations show the right and -wrong methods of arranging the brakes. Fig. 1 shows that the leverage represented by L is about equal to the distance from the centre of the brake shoe fulcrum to the centre of the brake cam—i.e., one to one.. Fig. 2 shows how the leverage on the brake shoe is increased. The distance L is possibly half that from the centre of the fulcrum to the centre ( of the cam, giving a leverage of one and a half to one.
If it is desirable to alter existing brake, shoes, when renewing the Ferodo friction material it is better to place short. pieces in the centre of the shoe, as shown in the second diagram, rather than to cover the complete surface. A 27-in, brake drum will serve quite well if the length of lining in contact is about 12 ins., whilst with q 10-in, brake drum 9 ins, is about the right length. The slipping effect is much the same, whether the shoes are arranged as in the wrong method, or correctly.
246.—Big-end Trouble on Tylor Engines.
With reference to our Hint on Maintenance No. 222, giving particulars of the lubricating system of the Tylor engine, we have received information from a company who have had trouble with the big-end 'ena...ings of these engines. They overcame the
1346 trouble by-removing the release valve and replacing it by a screwed plug, thus entirely eliminating the former. It appears that this procedure has been adopted with over 200 engines of Tylor make, and since that time no big-end troubles have been experienced. At the seine time, whenever an engine needs overhaul, while this is being performed scraper-ring pistons are already fitted, if not already in use.
247.—Replacing Ball Thrust Bearings.
If any appreciable amount of wear or slackness is observed in ball thrust bearings, it is nearly always inadvisable to take up this by fitting packing washers or shortening existing distanceapieees, unless this slackness is due to some initial fault in the design and not caused by wear.
Wear usually removes most, if not all, of the casehardened skin of the races, and, even if adjustments be made, the comparatively soft core of the races will wear rapidly. Therefore, it is preferable to fit new bearings. It is important to remember that a thrust race which is worn to any appreciable extent is liable to break up, and chippings of the hardened surfaces or roughness of the ball races will very quickly damage the balls, leading to the possibility of a sudden failure of the bearing as a whole, probably causing damage by the lodging between teeth, eto., of pieces of the broken balls.
248.—Wear on Gear Type Oil Pumps.
After a certain amount of use, the pinion type or oil pump is liable to a reduction in its efficiency, due to wear. This is not BO much by reason of wear on the teeth of the two gearwheels, but because the slight amount of end •thrust wears the faces of the end cover and the pump body respectively. The cover is particularly prone to wear, and as a result the oil, instead of being carried round between the teeth of the pinions, is merely churned at each side of them, the loss or slip being proportionate to the amount of wear.
The particular points where Wear is liable to occur in this mannerare the corners formed by the pump casing and cover, and the only method of curing the defect, apart from fitting a new pump or cover, is to skim the cover in a lathe and remove the portions which have not been worn until the cover will again fit close to the pinions, which should have only .004 in. to .005 in. clearance when the cover is screwed down.
TO make a good joint, it is usually necessary to grind the cover and bodytogether with fine emery, although if care is taken in the, turning and the tool cuts are not deep, a brown paper packing betwoeri the two is sometimes sufficient. In treating a' pump in this manner, care should be taken to note that the small setscrews which hold the cap in position .do not bed down into the bottom of their holes before the two parts are tight, and, similarly, that the bearing basses of the driven pinion are not too long to clear the ends of their holes in the casing
and cover respectively. •