Tracing Those Elusive Electrical Faults
Page 29
If you've noticed an error in this article please click here to report it so we can fix it.
SAVING LABOUR IN MACHINE TOOL MAINTENANCE
IN a large shop, many man-hours may, be occupied in cleaning and replenishing sumps for cutting coolants in machine tools. To reduce this labour wastage, a self-contained pumping outfit has been introduced by St. George's Engineers, Ltd., Ordsall Lane, Manchester, 5.
The equipment consists of a tank and an electric motor-driven pump mounted on a Crompton Parkinson standard TU.20 Electriear industrial truck. Both the truck-driving motor and the pump motor work from one battery.
The D.4 Mono pump is driven by a 11 b.h.p. 30-volt Crompton Parkinson motor. At 1,450 r.p.m, the pump will deliver 1,125,990 and 855 gallons per hour, against 50, 100 and 150 ft.-heads respectively. In practice, the pump will empty an average tank of 10-15 gallons in five minutes.
The capacity of the truck 'tank is about 100 gallons. For cleaning out, a hinged door is fitted, and an inspection port allows for checking the quantity of coolant. A length of 2-in. diameter armouredhose is provided.
ELECTRO-PLATING MADE EASY
-rw0 electro-plating outfits recently I introduced by Messrs. Runbak en Electrical Products should put this useful facility within the range of even the smallest undertaking.
The Midget model, which is a selfcontained plant, is operated off the mains supply-200-250 volts, 40-60 :ycles a.c. The price of the plant suitable for nickel, copper, etc., is £22 10s. The next in the range is the Easidun model, which costs £80 12s. 6d, A third outfit, which is suitable for both chromiumand nickel-plating, is priced at £129. The two larger models also work off the mains. Full information from Burton's Buildings, 71-73a, Oxford Road, Manchester, I.
ADVICE on tracing those intermittent ignition faults which cause hold-ups on the road, but refuse to manifest themselves after the offending vehicle has been brought into the garage, was offered by Mr. K. Sharpies, of Joseph Lucas, Ltd., when he gave a lantern lecture on "Coil Ignition Fault Finding" at a recent meeting of the BradfordCentre of the Institute of the Motor Industry.
Mr. Sharpies said that these elusive faults were usually caused partly by defective electrical fittings, which would function while the engine was more or less cold, but ceased to do so when it became sufficiently hot. This curious phenomenon arose because the resistance of the ignition wiring to the electric current so increased with the rise in temperature of the wiring that the current flow broke down.
To trace such faults, one must first switch on the ignition with the contacts closed. When the current had thus been maintained continuously through the primary coil for about 20 minutes, the ignition circuit would have reached working temperature, and voltmeter tests should be applied at various points in the ignition circuit.
In his introductory remarks, Mr. Sharpies emphasized that the key to maintenance of coil-ignition equipment was in fault location.
Ignition testing—for which a good, but not necessarily expensive, voltmeter was essential—should begin at the battery as the source of current supply, and progress made systematically throughout the circuit.
One of Mr. Sharpies's many hints on testing was that, in battery examination, One should look for a plum-coloured crust which was apt to form between the lug and the pillar. This crust was quite as much a potential cause of trouble as the copper-sulphate ' sponge which tended to form on the lugs. Another hint concerned tinned joint protectors at wiring junctions. Excessive engine heat might cause the solder to melt.
Another possible cause of trouble was accumulation of dust at the rear of the instrument panel. If this became moist, it was liable to cause electrical leakage.
In discussing the testing of contact points, Mr. Sharpies remarked that if one of the contact points remained unusually clean after a vehicle had run several thousands of miles, this suggested a fault just as much as if, in the case of tyres, one tread remained in new condition while the others were worn out. Emphasizing that a file should never be used to clean contacts, he said that contact-breaker files did not figure. among the servicing devices provided by manufacturers of coil-ignition equipment.
Cleaning Contacts The surface of the contacts was slightly domed so that the resultant flexing would provide a slight wiping action which helped to keep the points cleaner for a longer period—an action which would be destroyed by filing. The correct method of cleaning was to wrap a piece of emery paper around a piece of wood, and rub the contact-breaker on the emery with a circular motion.
Weak sparking was frequently caused by a defect in the condenser; on the other hand, it often arose from dirty connections. If no fault were found in the condenser, the trouble might be in the rotor.
In the testing of sparking plugs, coil ignition called for a different procedure from that adopted in the case of magneto ignition. As plugs must be tested under the most severe conditions, the proper procedure with magneto ignition was to test at a slow engine speed, when the magneto armature was dispersing the magnetic field slowly. In the case of coil ignition, however, the most severe conditions were at high speed,
Positive Earth Answering a question as to why a positive earth connection was employed in the ignition systems on some vehicles, Mr. Sharpies said that the use of a positive earth meant that the same size of spark could be obtained with 25 per cent, less voltage.
Another question brought an interesting comment from Mr. Sharpies concerning the use of the insulated return instead of the earth return for electrical „. installations on some vehicles. Explaining that the law enforced the use of the insulated return in certain cases, such as vehicles carrying explosives, petrol, or more than a specified number of passengers, he said that many motor-vehicle experts considered the earth return was • safer. In his view, the insulated earth offered no advantage over the noninsulated return, and it had many disadvantages.