wherein the regulating power was obtained from the electric current itself.

An application of this principle to a gas engine is shown at figs. 96 and 97, which consists of a pair of solenoids A, with laminated iron cores B, having a connecting stud C, crank lever D and rod E, directly attached to the pecker F, which is actuated by a cam G and lever H. The solenoids being wound to regulate a current of varying quantity are connected as a shunt from the main current, the power required being about 120 watts.

The engine is started in the usual way, the speed and current increasing until the solenoids are converted into a powerful electro-magnet, the iron cores being magnetically drawn or sucked within the coils, and there held in suspension. Should the normal voltage be exceeded, the cores rise slightly and draw the pecker away from the gas valve J, and the engine receiving no impulse, and decreasing in speed, the cores fall until the pecker is brought into action again. Should the driving belt break or fly off the pulley, or the main circuit be broken, the safety electro-magnet K releases the roller L, and the cores drop to their lowest point, and, bringing the pecker out of action, the engine is brought to a standstill.

This is a very delicate form of governor, as the slightest change in current causes an instantaneous adjustment of the gas inlet.

It is not an unusual practice even now to find gas engines which are used for driving dynamos with the governor out of action, and the gas cock regulated to suit the load. This has been rightly described as a 'one-man automatic regulator—i.e. a man sitting with his hand on the gas cock handle and his eye on the voltmeter.'

CHAPTER X

CRANK SHAFT BEARINGS

THE crank shaft bearings form such an important part of an engine that they require special care both in design and construction.

The general practice is to use the simple form having a twopart brass, adjusted by a cap and bolts; the division line of brass in this type is usually at an angle of 45° to the centre line in horizontal engines. The practice of manufacturers differs as to the position of the cap to resist the explosion; in some cases it is thrown upon the cap and bolts as in fig. 11, where the cylinder is placed to the right hand of the bearing. The method generally followed, however, is to place the cylinder to the left hand, as in fig. 5, this being the preferable position, comparatively little strain being thrown on the cap, as the force of explosion is taken directly by the main bed casting. The bottom brass has a toggle N, fitted in a recess of the main casting, to prevent the brasses revolving with the shaft. In this form of bearing the brasses should be bored 0.05 per cent. larger than the diameter of the shaft, to prevent them closing in and pinching the shaft should they become warm.

The table of sizes in figs. 98 and 99 are from actual practice, and may be taken as typical sizes used by leading makers in this form of brasses. In vertical engines the division line is usually placed horizontally, the strain of the explosion being taken up by the cap if the cylinder is below the crank shaft, and by the bed if the cylinder is above. This makes a very efficient bearing and gives uninterrupted bearing surface. In some designs of beds, especially for the larger size of horizontal engines, the bearing shown in fig. 98 is unsuitable, and as it is not permissible for the division line of brasses to be on the horizontal centre line of the engine, it becomes necessary to use brasses in four parts, as shown in figs. 100 and 101, which are taken from a large size Robey engine, as they are adjustable both vertically and horizontally; the vertical adjustment being effected by the cap and bolts, and the horizontal adjustment by means of wedges drawn up by nuts coming through the cap. These wedges are only placed on the side nearest the cylinder, so that there is no strain on them from the force of the explosion. In this type of bearing the steps are of cast iron lined with Babbitt metal, which gives excellent results. The flanges of the two side steps are made with projecting lips to butt against the flanges of the top and bottom steps, by which means

H

K

by a massive cap provided with lugs at each end, fitting tightly throw the bearing out of line. The gap for the bearing is bridged

FIG. 98

have been eased the required amount, making it impossible to they form one rigid bearing, and prevent adjustment until they

112

1447400

on the bed casting, and effectually tying the bed together at this point. In bearings having four-part brasses, 0·08 per cent. larger than the shaft is necessary.

A good practice in determining the size of crank shaft bearings is to allow a maximum pressure of 400 lbs. per square inch of rubbing surface. Assuming that the maximum pressure of the explosion is transmitted direct to the crank pin.

PxA

Dx L

=

= Pressure per square inch of rubbing surface. Where D diameter of shaft in inches; L = length of bearing in inches; P= maximum pressure of explosion; A area of cylinder; the rubbing surface being taken as approximately one-third of the circumference-i.e. diameter x length.

Owing to the effect of the inertia of moving parts (as shown in fig. 61), the maximum pressure per square inch on piston does not reach the main bearings. Then the above formulæ may be used, and the allowable maximum pressure will be 180 lbs. per square inch of rubbing surface.

Some makers prefer to speak of the pressure on bearings based on the average pressure per square inch on piston, but as already explained this is not to be commended.

The Lubrication of Bearings

The lubrication of main bearings is of great importance, and efficient means should always be provided for the distribution of the oil, or solid lubricant if such is used instead of oil.

A very common method is shown at fig. 98, a piece of wick being the means, by capillary attraction, of feeding the oil; although this is somewhat wasteful, practice has proved it to be very reliable. There are many compounds of greases, sold in a semi-liquid and solid form, suitable for main bearings; but these have to be used with great care, as the extra width of oil grooves necessary for their use reduces the bearing surface of the brasses, and in addition to which, the varying temperatures of the engine-room and the susceptibility of the greases to liquefy by heat and run through, cause many heated bearings.

These solid lubricants should never be used for crank necks or moving parts.