HINTS ON MAINTENANCE.
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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 he 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 and 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 mechanic readers. Communications should be addressed to "The Editor, The Commercial Motor, 7-15, Rosebery Avenue, London, E.C.1."
203.—The Foden Regulator.
Hint No. 196 referred to a method of curing leakage of steam from the regulator by grinding the valve face and seat together with fine earborundurn paste.
'We have received from a contributor a criticism of this method. He states that using corn-pound for grinding this valve is a somewhat daring innovation, because, if any lumps have been formed on the valve, these will,in turn, make recesses on the valve seating.
He states that the most efficient method of tackling the job is first to bed the valve to a surface plate, and then to bed the valve seat to the valve, the bedding or scraping being done with the aid of red lead and proper scrapers. He concludes his letter with a further hint referring to the setting of the valve.
He states. it ia convenient to know that the Foden engine expands about in. from regulator to valve seating when hat. When the valve is being set, this expansion must be considered. Of course, the fitter can set the valve before steam is up if the expansion is known. To prove the amount, it is only necessary to utilize a piece of mild steel with each end bent at right-angles and sharpened. The length between the points should equal the distance between the pivot stud of the regulator and the stuffing-box of the starting valve when the engine is cold, A centre-punch mark should be put on the stud and on
the staffing-box. When setting the h.p. and 1.p. valves, it is essential to know the correct expansion, and this little tip of utilizing a measuring bar will show exactly what is required,.
204.—The Function of the Magneto. Condenser.
Trouble with the magneto condenser is a contingency which seldom arises ; in fact, itis the least frequent trouble in the ignition system. This is satisfactory up to a point, but it renders the trouble very diffieult todetect when something really does go wrong. The condenser is what might be termed the " buffer" of the magneto, and it prevents surging of the low-tension current.
The more rapidly the primary current is broken at the contact-breaker, the stronger will be thecurrent induced in the high-tension winding, and the con denser ensures this gudden stoppage at the moment of breaking contact by absorbing the cuigent that would continue to flow across the air gap, and which would otherwise be evident in the form of a lowtension spark. Secondly, when the low-tension CUTrent is broken, a reverse current is introduced. This tendsto weaken the high-tension current. The condenser's next anty,therefosesis to absorb this current.
From this it will be manifest that one of the. symptoms of a faulty condenser will be excessive sparking at the .contact-breaker points, with. consequent burning, pitting and general diseolouration of the latter. The other symptoms vary in degree, according to the magnitude of the condenser trouble. They may take the form ,of difficulty in starting, feeble sparking at the plugs, missing in a manner suggestive of a choked jet, or the. engine may stop altogether. If any of these symptoms exist in conjunction with excessive sparking at the contact-breaker points, there is no need to test for other possibilities. Such a trouble is not a job to be tackled by a novice. It should only he attempted by a man who is an expert.
205.—The Foot Brake on the J-type Thornycroft.
The foot brake on the J-type Thorny-croft chassis should. be kept properly adjusted and balanced. Two a-in. bolts and nuts are. placed at each side of the brake shoes to enable this to be done.. When the shoes are off, they should rest on the heads of the bolts as, by these, the clearance between the shoes and the brake drums is equalized, and thus. no dragging occurs. The shoes are fitted with detachable cast-iron pads, and when worn these can be renewed easily, as they are each held by four set-screws. Attention should always he given to the bottom pivot pins. They.should be kept well oiled to prevent rusting up. Do not. use the foot brake only during emergencies. Make a practice of using the side brake.
206.—The Rear Spring Bolts on the 2-3-ton
The rear springs of the 2-3-ton Dein/Tee are each held down on to the axle seat by four long bolts with nuts top and bottom. It pays occasionally to run oil down these bolts where they enter the axle casting. If this he not. done, it may be found that the bolts have rusted so fast into the axle that tightening them u-p by means of the bottom nuts is impossible. The top. ends of the bolts are threaded for only a sufficient distance to accommodate the tints, so that little tightening can he done there, unless the nuts arefuse removed and spring washers utilized. This, however, is troublesome, as the bolt ends are always riveted over the nuts.
It is important that the rear springs should be kept absolutely tight,, because this lorry has no torque tube, and the springs have to resist the drive and brake reactions. Any looseness, therefore, may result in the bottom leaf of the spring beeaking.