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thread C on the cylinder. The same construction is followed out for the twist of thread at pitch circle D and at root of teeth E.

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Knowing the angle of the curve D, all that is now necessary is to find the angle of thread on outside diameter C. For the

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FIGs. 115, 116

worm set off the pitch of thread (14.13) on a horizontal line A B (fig. 113), and on a perpendicular to A B set off A C equal to the

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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 DE equal to A C, and D F equal to width of worm and wheel (in this case 2% inches), and having divided DE 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 a y (fig. 110) on each side of these centre lines, and fasten these two

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

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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

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