A NEW MAGNETO
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of practical design
Some Particulars of a Robust B.T.-H. Product of the Rotating-magnet Type, which Embodies a Number of Interesting Features in its Construc RECENT advances in combustionchamber design have brought in their train the need for a magneto capable of developing a voltage higher than was,sufficient not long ago. At the same time, operating methods call for an even higher standard of reliability than was at one time considered to be satisfac-. tory. In order to meet these demands the ,British Thomson-Houston Co., Ltd., of Rugby, is now producing a new magneto designed specifically to suit commercial-vehicle engines with four or six cylinders. Between the two types,
designated H.E.4 and the only differences are in the distributors. Externally, these new magnetos have every indication of being robustly structed, and this impression is confirmed by an examination of 'the component parts. It is also clear that much thought has been expended to ensure reliability, electrically as well as mechanically, and to enable such maintenance work as may be required to be carried out as easily as possible. For example, the machine consists of a number of units, any of which can be easily removed for examination or adjustment.
A 25 per cent. nickel-steel shaft carries a cobalt-steel magnet with laminated iron poles. This shaft is mounted in two bail bearings and the magnet poles are arranged to rotate between the laminated armature poles. The armature itself is stationary and forms a separate unit mounted above the magnet on extensions from the armature poles, which are cast into the main housing of the magneto. A mica condenser is attached to the armature winding unit and is thus easily accessible.
The make-and-break is conventional in principle and has a two-lobed central cam. In detail, however, it is nineworthy in more than one respect.
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Duralumin is the material selected for the rocker arm, and the resultant lightness of this member avoids "fling" at high speeds, without the need for a particularly strong spring. The rockerpivot bearing is of the metal-to-metal variety, and the absence of a fibre brush eliminates the tendency for the rocker arm to stick. A small wick is provided for the lubrication of this bearing and it requires about one drop of thin oil every 2,000 miles or so. • • It has been found that an oillubricated metal bearing is satisfactory 'only if it be relieved of all current flow. In order, to ensure this state of affairs, the single-blade steel spring is backed up by a phosphor-bronze blade :which acts simply as a conductor between the moving contact and the body of the magneto. This arrangementeliminates any tendency to carbonize the oil in the pivot bearing. .
Mounted on the main, spindle, between the make-and-break and the magnet, is a steel pinion meshing with the distributor gearwheel, which is made of a special moulded material. This is used chiefly to reduce the weight of that part, for it has.been found that wear of the teeth is due mostly to a' hammering effect, which is reduced if the distributor and its wheel have only a small inertia.
The distributor itself il of the jumpspark type, as this has certain advantages, particularly if the sparking-plug insulation be at all poor. Solid nickel is used for the distributing arm and for the segments, which are of generous section and can be renewed without difficulty. Like the main shaft, the revolving portion of the distributor is mounted on ball bearings, and the assembly forms an easily removable unit.
As to the attention which this magneto requires in service, a small oil well is provided for each bearing, and this should be filled ' with light oil every 3,000 miles, or so. . Should too much lubricant be put in, the surplus is drained off, automatically .so that it can do no harm. At similar intervals a couple of drops of oil should be applied to the cam-lubricating pad and, even less freqnently, the brush holder and the make-and-break should carefully be
cleaned. .
Here, as in other parts, the leakage surfaces have been made as long as possible, so that the natural accumulation, of dust shall have little effect on the operation of the magneto. Because of these and other features of the electrical side of the design, RT.-IL-type H.E. magnetos are able to produce a satisfactory spark even in circumstances similar to those produced by using a plug with poor insulation. According to laboratory tests, this holds good over a wide speed range from about 50 r.p.m. upwards.