Modern Machine-Shop Practice Part 234

A griddle valve is one that has two or more ports at each end upon a seat that has two or more ports for each steam pa.s.sage.

Double ported valves are employed in some cases to increase the admission of live steam to the cylinder, and in others to increase the exhaust openings also. The effectiveness of a double ported valve is mainly valuable at the beginning of the stroke, and is especially valuable in cases when the travel of the valve is diminished to hasten the point of cut off, because in such cases the outer edges of the valve do not open the steam port to its full width, and a single port is apt to wire draw the steam. By the employment of more than one port, or several ports, a sufficient admission may be obtained with less valve travel.

[Ill.u.s.tration: Fig. 3303.]

The Allen double ported valve is one in which the second port increases the port opening for the admission only, as shown in Fig. 3303, in which the valve is moving in the direction of the arrow; the port K will receive steam through the opening at _g_, and from a port pa.s.sing through the valve, the steam entering it as shown by the arrow. The second port forms part of the lap of the valve, and enables the travel to be short enough to be cut off at early points in the stroke, without employing so much steam lap as to widely distort the points of cut off, this latter being a defect of the D valve.

Webb's patent slide valve is circular, and is so arranged as to be free to revolve in the hoop of the valve rod, the effect being that the valve moves around, or to and fro in the hoop, without any special mechanism to produce such movement, and the result is, that the valve and port facings wear smooth and even without any tendency to become grooved.

BALANCED VALVES.

A balanced valve is one in which means are employed to relieve the back of the valve of the steam pressure, and thus prevent its being forced to its seat with unnecessary pressure.

In some of the most successful balanced valves this is accomplished by providing a cover plate, which may be set up to exclude the steam from the back of the valve which works (a sliding fit) between the valve face and the face of the cover plate. Such a method of balancing is sufficiently effective for all practical purposes, if the following conditions are observed: The valve rod must be accurately guided so as to avoid side strains; the valve must fit accurately to its seat and to the cover plate, and the adjustment so made that the valve slides freely at first, being steam tight, and yet allowing room for lubrication to enter. When the travel of a valve, balanced by a cover plate, is varied to alter the point of cut off, the construction must be such that the ends of the valve at the shortest stroke pa.s.s over the ends of the seat and cover plate faces, or otherwise the middle of the seat and cover plate faces will wear hollow.

The Buckeye, Porter-Allen, and Straight-Line Engines are examples of practically balanced valves. The first of these has a balancing device that follows up the wear; the second has an adjustment whereby the cover plate may be set up to take up the wear; and in the third the wear is reduced to a minimum, by accurately fitting and guiding the parts.

[Ill.u.s.tration: Fig. 3304.]

The construction of the valve in the Straight-Line Engine is shown in Fig. 3304, in which B represents the cylinder bore; the valve _v_ rests on a parallel strip _n_, and on its top rests the parallel strip _m_, the pressure relieving plate P is set up firmly against the pieces _m_ _n_, whose thicknesses are such as to leave the valve a working fit between the faces of R R and of P.

[Ill.u.s.tration: Fig. 3304 _a_.]

Instead of the valve sliding on a flat face, it may work upon a shaft or spindle as a centre, its face moving in an arc of a circle, and its action will be the same as a flat valve having the same proportions.

Fig. 3304_a_ represents a valve V of this construction, whose shaft is at S, A being an arm fast on S, and driven by the eccentric rod R. To find the necessary amount of travel for such a valve, we draw lines, as _f_, _g_, from the inner edges of the steam ports, through the centre of the shaft S, and also draw an arc through the centre of the eye of arm A, and where lines _f_ _g_ cut the arc, as at _d_ and _e_, are the extremes of motion of A.

PISTON VALVES.

[Ill.u.s.tration: Fig. 3305.]

A piston valve acts the same as a flat or plain (D) valve, having the same amount of lap lead and travel. In Fig. 3305 we have a cylinder with a flat valve on one side and a piston valve on the other, the head end ports being about to take steam, and it is seen that the eccentrics occupy the same positions for the two valves. The steam ports are, for the piston valve, annular grooves provided in the bore in which the valve fits. The piston valve is balanced because it receives its steam pressure on the ends, but it will not follow up its wear as the flat valve does, hence it is liable to leak.

SEPARATE CUT OFF VALVES.

[Ill.u.s.tration: Fig. 3306.]

Meyer's cut off valve is constructed as shown in Fig. 3306, M being the main valve, and _v_ _v_ the two cut off valves, whose sole duty is to cut off the steam at an earlier point than the main valve would do. If the engine is to have a fixed point of cut off, or, in other words, if the cut off is always to occur at some one particular point in the stroke, the valves may be set to do so, and equalize the points of cut off.

Variable points of cut off with the Meyer's valve may be obtained by shifting the position of the eccentric that operates the cut off valve, but it is usually done by means of moving the valve by a right and left hand screw, such as shown in Fig. 3306. The cut off eccentric is set ahead of the main eccentric, so that the cut off valve will close the ports before the main valve would do so; thus, in the figure the cut off valve is shown to have effected the cut off for port _a_ by the time the main valve has fully opened port _a_, and is reversing its motion. If the engine requires to reverse its motion, the cut off eccentric is set exactly opposite to the crank, but otherwise, it may be set 8 or 10 degrees either ahead of or behind the crank, but if set too little ahead of the crank, the port may reopen after the cut off has been effected.

[Ill.u.s.tration: Fig. 3307.]

Gonzenback's cut off valve is constructed as in Fig. 3307, the steam chest having two compartments. A, A are the cylinder steam ports, C the main valve, and E the cut off valve, whose ports (as G) are made wider than the ports F.

Reducing the travel delays the point of cut off in the Gonzenback valve, whereas in the common slide valve it gives an earlier cut off.

THE ECCENTRIC.

When a single eccentric is used, it is simply termed _the_ eccentric. If a cut off valve (or two cut off valves) are used upon the engine, then the eccentric that works the main valve is called the main eccentric, while that which works the cut off valve or valves is called the cut off eccentric. The main valve is that which works on the cylinder face; the cut off valve is that which effects the cut off.

A shifting eccentric is one that is _moved across_ the shaft so as to alter its amount of throw, and, therefore, the amount of valve travel, the effect being to vary the point of cut off.

In engines where a constant amount of lead is given, or in other words, when the eccentric position is intended to be fixed, the eccentric should be secured to the crank shaft by a feather or key sunk into the crank shaft so as to prevent the eccentric from moving, while enabling it to be taken off and replaced without requiring any operations to adjust its position with relation to the crank.

The feather should fit tight on the sides, as well as on the top and bottom, and may have a slight taper on the sides, which will make it easier to fit the featherway or keyway to the feather, and easier to put the eccentric on or take it off.

By this means the eccentric cannot shift, and may be replaced after being taken off without having to set the whole valve motion over again.

When the amount of valve lead or of compression is varied to suit the speed at which the engine is to run, or to aid the counterbalancing of the engine, a feather cannot be used because it will not permit the eccentric to be moved to effect the adjustment.

Set screws possess disadvantages, inasmuch as that the point of the set screw may leave an indentation, which, if the eccentric is moved a trifle, may cause the set screw point to fall back into the old indentation, thus rendering it difficult to make a small adjustment of eccentric position.

[Ill.u.s.tration: Fig. 3308.]

An eccentric is the exact equivalent of a crank having the same amount of throw, as may be seen from Fig. 3308, in which the outer dotted circle represents the path of the crank and the inner one the path of the centre of the eccentric. A small crank is marked in, having the same throw as the eccentric has, and the motion given by this small crank is precisely the same as that given by the eccentric whose outer circ.u.mference is denoted by the full circle.

Considering the motion of both the crank and the eccentric, therefore, we may treat them precisely the same as two levers, placed a certain distance apart, revolving upon the same centre (the centre of the crank shaft), and represented by their throw-lines.

[Ill.u.s.tration: Fig. 3309.]

In Fig. 3309, let the full circle E E represent an eccentric upon a shaft whose centre is at C, and let the centre of the eccentric be at _e_. The path of revolution of the eccentric centre will be that of the dotted circle whose diameter is B, D. As the eccentric is in mid-position (_e_ being equidistant from B and D), the valve will be in mid-position as denoted by the full lines at the bottom of the figure.

Now suppose the eccentric to be revolved on the centre C, until its centre moves from _e_ to V, its circ.u.mference being denoted by the dotted circle A A, and if we draw from V a vertical line cutting the line B, D at _f_, then from C to _f_ will be the distance the eccentric would move the valve, which would then be in the position denoted by the dotted lines at the bottom of the figure. It becomes clear then that if we suppose the eccentric to have moved from mid-position to any other position, we may find how much it will have moved the valve by first drawing a circle representing the path of the centre of the eccentric, next drawing a line (as B D) through its centre, and then drawing a vertical line as (C _e_) through its mid-position and also a vertical line from the eccentric centre in its new position, the distance between these two vertical lines (as distance C _f_ in the figure) being the amount the eccentric will have moved the valve.

It may have been noticed that the diameter of the eccentric does not affect the case, the distance B D, or the diameter of the circle described by the centre of the eccentric, being that which determines the amount of valve motion in all cases. This being the case, we may use the circle representing the path of the eccentric centre for tracing out the valve movement without drawing the full eccentric, and the diameter of that circle will of course equal the full travel of the valve.

[Ill.u.s.tration: Fig. 3310.]

The position of an eccentric upon a shaft is often given in degrees of angle, which is very convenient in some cases. If a valve has no lap or lead, the eccentric will stand at a right angle or angle of 90 degrees when the crank is on the dead centre.

The division of a circle into degrees may be explained as follows:

Suppose we take a circle of any diameter whatever and divide its circ.u.mference into 360 equal divisions, then each of these divisions will be one degree. The number 360 has been taken as the standard, and this being the case, there are 360 degrees in a circle, in a quarter of a circle there will therefore be 90 degrees, because 90 is one quarter of 360. By means of dividing a circle in degrees therefore we have a means of measuring or defining any required portion of it.

In Fig. 3310 the degrees of a circle are applied for defining the relative positions of a crank and an eccentric. As the zero position of the crank is on a dead centre, it is so placed in the figure, while as the zero position of the eccentric (which is for a valve having no steam lap) is at 90 degrees from the crank, therefore the dotted circle representing the path of the eccentric centre has its O or zero point at 90 degrees from the crank. Now suppose the eccentric centre stood at _v_ and the eccentric throw line at _c_ _v_, and it will stand at 30 degrees from O, hence the angular advance of the eccentric is in this case 30 degrees, or in other words, it is 30 degrees in advance of its zero position, or the position it would occupy when the crank is on the dead centre and the valve has no lap and no lead.

If we measure the distance apart of the crank and the eccentric in degrees, we find it is 120 degrees, hence place the crank where we may, we can find the corresponding eccentric position because it is 120 degrees ahead of the crank. The sign for degrees is a small placed at the right hand of the figures and slightly above them; thus, thirty degrees would be written 30.

FINDING THE WORKING RESULTS GIVEN BY A D SLIDE VALVE.

Although not strictly within the line of duty of an engineer or engine driver, he is nevertheless sometimes called upon to find out how a valve of given proportions will dispose of the steam, or what proportions to give to a valve to accomplish certain results.

This is easy enough when either the travel of the valve or the amount of the lap and the width of the port are given, but if the width of the port alone is given, and all the other elements are to be found, it becomes a more difficult problem.

An engineer, however, is rarely called upon to solve the question from this stand-point, which properly belongs to the draughtsman or engine designer.