Modern Machine-Shop Practice Part 232

A lagged cylinder is one clothed, which is sometimes done with wood or metal strips, leaving an air s.p.a.ce around the cylinder, while in others this s.p.a.ce is filled with felt or some non-conducting material.

Experiments made by Charles E. Emery gave the following general results: The thickness of the pipes and of the non-conducting materials was kept constant.

Hair felt was the best non-conducting material of all those tested, and the value of a thickness of two inches of hair felt was taken as unity and the maximum.

The value of two inches of mineral wool as a non-conductor was 0.832 of hair felt; two inches of mineral wool and tar was 0.715. Two inches of sawdust, 0.68; two inches of a cheaper grade of mineral wool, 0.676; charcoal, 0.632; two inches of pine wood, across the grain, 0.553; two inches of loam, 0.55. This was from the Jersey flats, and almost all vegetable fibre not yet become compact. Slaked lime from the gas works, expressed decimally, with hair felt as unity, 0.48; coal ashes, 0.345; c.o.ke, only 0.277, the same as used for fuel; two inches of air s.p.a.ce, only 0.136, which dashes a great many people's hopes, and is as interesting as any part of the data; two inches of asbestos, 0.363; two inches of Western c.o.ke, about the same as the other c.o.ke; two inches of gas house charcoal, 0.47.

These are very interesting, particularly so this matter of an air s.p.a.ce.

It has been supposed that an air s.p.a.ce around a pipe is as good as anything we can have. The fact is, convection or circulation takes place; the air is cooled on one side of the s.p.a.ce, descends, and rises on the other, and it is necessary to break up the air s.p.a.ce, and that undoubtedly accounts for the efficiency of these different materials. It is the air probably that is the non-conductor; but it should be kept quiescent instead of being allowed to circulate. The air s.p.a.ce itself is of very little value until the circulation is prevented.

THE PISTON.

In calculating the power of an engine it is the piston speed that is taken into account, and not the length of the stroke, the latter being used merely in order to obtain the piston speed.

Long strokes are usually employed upon engines running at moderate piston speeds, as from 300 to 500 feet per minute, and short strokes for piston speeds from 400 to 800 feet per minute.

The Porter Allen engine has been run noiselessly at 1,100 feet per minute.

In determining the stroke of an engine the nature of the valve-operating mechanism is taken into account.

In releasing mechanisms, or those in which connection between the eccentric rod and valve spindle is broken in order to permit the valve to close quickly, too high a speed of revolution may cause the tripping mechanism to fail to act, hence a high piston speed is obtained by means of employing a comparatively long stroke.

In positive valve gears, or those in which the valve is controlled throughout the whole of its movement by the eccentric, the valve mechanism may operate quicker without danger of missing, hence the piston speed may be greater.

When the stroke equals the diameter of the cylinder bore, the cylinder presents the least amount of exposed surface in proportion to its cubical contents.

To obtain the same amount of expansion in a short as in a long stroke engine, the steam must be expanded through an equal proportion of the stroke; thus, if the steam is cut off at half stroke in both cases, the amount of this expansion will be equal.

Pistons are made an easy fit to the cylinder bore, a steam-tight fit between the two being obtained by means of the piston rings.

Solid pistons are provided with snap piston rings.

A snap piston ring is one that is larger in diameter than the cylinder bore, and is closed in to get it into the cylinder, while it depends on its own spring outwards for its fit to the cylinder bore, having no supplementary rings or springs to force it out.

Piston rings that are expanded by supplementary springs should be tapering in thickness, the thickest part being opposite to the split, and the thinnest at the split. This causes the ring to conform itself to the cylinder bore, and makes it sit more evenly around its whole circ.u.mference. These rings are made larger in diameter than the cylinder bore, in proportion of about 1/8 inch per foot of diameter, the split being closed when the ring is sprung into place in the cylinder. But if made of bra.s.s, the split must be left open enough to allow for the expansion, or otherwise the ring expanding more than the cylinder will seize and cut single.

The split of a piston ring should be placed on the bottom of the piston (in a horizontal engine), so that the piston head, in resting on the cylinder bore, will cover up the opening of the ring.

When two or more rings are employed, the splits may be placed on the lower half of the cylinder, so as to cover up their splits as much as possible.

The follower of a piston is a plate or cover that is employed to hold the piston rings in place, and the piston rings should be so fitted that the follower should be bolted firmly up, or otherwise the bolts may come loose and work out, and getting between the piston and the cylinder cover, may cause the piston to knock the cylinder cover out.

Piston followers are necessary when the rings are set out by springs or other parts adjustable within the piston head. Snap piston rings, however, permit the use of a solid piston, dispensing with the need for a follower.

The effectiveness of a piston ring may be tested, when the construction of the engine will permit it, by disconnecting the valve for the head end, setting it so that it covers the port, and then taking off the cylinder cover at the head end and admitting steam through the crank end steam port, when any leak in the piston rings will be seen by the escape of the steam.

THE PISTON ROD.

Piston rods should be of slightly diminishing diameter at the ends, so that the wear shall not leave a shoulder at each end of the rod.

In determining the diameter of the piston rod, allowance is made for turning it occasionally in the lathe to restore its parallelism, the wear reducing its diameter more in the middle than at the ends. The diameter of a piston rod is found in practice to range between one-sixth and one-tenth the diameter of the cylinder bore.

Steel piston rods wear better than those of wrought iron, being free from scaly seams which are apt to cut the packing and cause the rod to wear in grooves.

The best method of securing a piston rod to a piston head and to the cross head is by a taper seat and a key, so that no nut is needed, and the cylinder cover need not have a recess to receive the nut when the piston is at the end of the stroke, and the amount of clearance is correspondingly reduced.

Piston head key ways are sometimes given so little clearance that the key completely fills the keyway when driven fully home. This prevents the edges of the keys from bulging into the clearance s.p.a.ce in the keyway, which action is apt to cause the key to loosen in time. The key should have a safety pin at its small end.

When piston rods are threaded into the cross head, or into the piston, the threads are made an easy fit, and taper seats or split hubs secured by clamping screws are relied upon to keep the rod true to the cross head or piston, it being found that the screw alone cannot be relied upon for this purpose.

PISTON ROD PACKING.

Piston rod packing, of fibrous or similar material, should be cut in rings that will not quite fully envelop the piston rod, and the first ring should be placed with its split upwards. After two or three rings have been inserted, each having its split at a different part of the bore, so as to "break joints," the gland should be screwed up enough so as to carry the packing home to the back of the stuffing box. This process should be continued until the stuffing box is filled for about two-thirds of its depth, when the gland may be screwed home.

The gland should be screwed up quite evenly, so that the packing in the stuffing box shall be compressed equally all around the rod, and will not cause the gland to bind on the rod or in the stuffing box bore.

The wrench should be applied first to one nut, giving it a turn or two, and then to the other, and after the gland is firmly home the nuts should be eased back about two turns.

When a gland requires packing, it is proper to take out all the old packing that has become hard and set.

A leak in piston rod packing may sometimes be remedied by taking out three or four rings of the packing and reversing it.

If the packing is tightened up while the engine is running, it should be done very gently and evenly, as a very little s.c.r.e.w.i.n.g up may stop the leak, while excessive s.c.r.e.w.i.n.g produces undue friction.

Piston rods are in some of the most advanced practice packed with metallic packing, or packing composed of soft metal. In some forms of metallic packing the construction is such that the gland and packing do not attempt to restrain the line of motion of the piston rod, this duty being left to the guide blocks and guide bars, where it properly belongs.

THE CROSS HEAD.

In engines having Corliss frames, the cross head is provided with shoes and adjusting screws, to take up the wear.

When guide bars are shaped thus __| the cross head is provided with gibs (usually of bra.s.s composition) to take up the wear.

In either case care must be taken to make the adjustment correct, and thus keep the piston rod in line. The shoes or gibs should not bear hard upon the guides, but be an easy sliding fit without lost motion.

Cross head pins should be kept eased away on the two parts of their circ.u.mference which are within the connecting rod bra.s.ses or boxes and near the joint faces of the same. This is necessary because the wear is greatest on the crowns of the boxes, and the pins are apt to wear oval.

In some engines, the surface of the pin is cut away, but if it is not, and the pin can be revolved in the cross head, it is a good plan to give it half a turn occasionally, which will keep it round.

THE GUIDE BARS.

The guide bars of an engine require to be set exactly in line with the axis of the cylinder bore, so that they may guide the piston to travel in a straight line. They should be an easy sliding fit to the cross-head guide.

The top bar is more difficult to lubricate than the bottom one, especially when it receives the most pressure, as is the case when the top of the fly-wheel runs towards the cylinder.