circumference of the outside diameter of worm over the points of teeth or threads; join B C, and the angle this line makes with A B equals the angle of thread in worm.

Fig. 116 shows the same construction applied to the wheel. We now construct figs. 114 and 115, setting off D E equal to A C, and D F equal to width of worm and wheel (in this case 21 inches), and having divided D E into the number of teeth in worm and wheel respectively, draw the centre lines of teeth parallel to BC. If we mark off half the width of teeth at points x y (fig. 110) on each side of these centre lines, and fasten these two

strips on the outside of the wheel blocks (as in fig. 118), they will represent the true angles of the points of teeth for the worm and wheel. By making a template of the spaces between the wheel teeth (see fig. 117) we can by its aid cut out by hand the teeth spaces, holding the template in the position of normal pitch. After having worked two teeth to template, it is advisable to mark out the profile of teeth on the two faces of blocks, which must be taken from the actual teeth, as owing to the angle of teeth their profiles on faces of wheels are not sym

metrical.

In machine-cut wheels it is only necessary to make the cutter to the shape of the tooth spaces (see fig. 117).

CHAPTER XII

VALVES

WITH very few exceptions, all the makers of gas engines adhere strictly to the mushroom type of valve, experience having proved that this form embodies the most advantages, as it works exceedingly well and admits of inexperienced men grinding them when necessary to their seats with an ordinary joiner's brace and flour of emery.

The author has known an engine in constant work in which the exhaust valve was not re-ground in until it had worked for sixteen months, and the air valve was not taken out within the first two years; this practice is not to be commended, but is only quoted to show how long an engine working free from dust will run without attention, so far as the valves are concerned.

The length of time to which an engine should be allowed to run before the air and exhaust valves are cleaned and ground in depends altogether on the quality of the gas and the oil used. Where both are good and the engine-room free from dust, the air valve should be taken out once in three months, and the exhaust valve once a month.

Fig. 119 is a type of exhaust valve very largely used.

The method of lifting the valve by means of a pivot does not permit the versed sine of the lifting lever being taken up without throwing some strain on the valve spindle; and as the arrangement prevents the valve from turning on its seat, making it difficult to keep it tight, this principle should be either well designed or not used.

Fig. 120 is another type of valve and lifting arrangement both for air and exhaust valves. When used as an exhaust valve great difficulty is experienced from the spring becoming heated and losing its elasticity, although the valve is permitted to turn on its seat. The position for lifting when the valve is under the greatest pressure tends to strain the valve spindle. The method of adjustment in case of wear is very simple.

Fig. 121 shows a method of lifting the valve by means of a

spool and central spring attachment. Owing to the small amount of lifting lever surface contact, very little strain is thrown on the valve spindle. As the opposite end of the lever is always heavier, the end on the spool-lifting lever need not bear on the bottom of the spool. The adjustment for wear is made by a fine thread and four flats on the spindle, so that very fine adjustments can be made.

In fig. 122 a distinct departure is made both in the construction of the valve and the lifting arrangement, and was introduced by the author in 1892.

It is well known that where a large engine is working at full power great difficulty is experienced from the exhaust valve fusing. To overcome this the head is made of 'cylinder liner' metal and attached to a steel spindle, the valve seat being also made of the same metal as the head. It is not good practice to arrange the exhaust valve in a loose box, especially on large engines. There is not only the difficulty of taking the box out, but there is a great tendency for the box to become distorted, owing to the comparatively large hole which must be in the box for the exhaust gases to escape in proportion to the sectional area of the box. To mitigate this a rib is placed in the centre of the opening.

The lower seat (as shown in fig. 122) admits of the water space being brought well up to the casing, and is at once a good mechanical job. The method of lifting is arranged so that in all positions of the lift the lifting die is at right-angles to the valve spindle. The valve spindle guide is loose, the joint on the under-side of the combustion chamber being a metallic one. Renewal of the guide or bushing is by this method considerably cheapened. This arrangement has been used with marked success by Messrs. Robey & Co. on their larger engines.

The pressure on the head of an exhaust valve in large engines amounts to about two tons, and the strain thrown on the worm gear and lifting mechanism is very great. To overcome this two exhaust valves have been used, one valve having twice the area of the other, the smaller valve being arranged with a 'lead' of the larger one.

An equilibrium exhaust valve arranged with one-half the

usual lift, and which would have the pressure equal to the difference between the two areas, is practical, and will, no doubt, be used on very large engines in the future.

Although valves working in a vertical position give good results, great difficulty is found in lubricating the spindle; and owing to the high temperature, and the amount of moisture contained in them, a leakage past the spindle into the engineroom is not unusual after twelve months' hard work. · To overcome this one maker has arranged a hole in a spindle guide, leading to the air pipe, to allow the escaping exhaust to mingle with the incoming air.

Another means of overcoming this unpleasant leakage is to arrange a bowl on the valve spindle, surrounding the top part of the guide, which very effectually prevents the exhaust rushing down the spindle guide.

The diameter of the air and exhaust valves vary considerably with most makers; some on small and medium-sized engines use the same diameter for air as the exhaust, whilst others prefer the air valve larger than the exhaust. A common and good practice is to arrange the air valve for 100 and the exhaust for 80 feet per second.

CHAPTER XIII

GAS VALVE AND COCK

THE size of the gas inlet varies from 450 to 650 feet per second through the valve. There being no fixed rule for the diameter of the gas supply, the difficulty usually experienced is to get sufficient in; the same size is often used for three or four sizes of engines, the difference being made in the lift given to the valve and the size and number of holes in the air valve box.

A very neat form of gas cock and valve is shown at fig. 123. The plug is held in its place by three screws, two of them being used for determining the position of handle to 'open' and 'shut.' The gas valve can be readily taken out.

A good rule for the taper of the plug is to allow a taper of 1 inch in 8.