Modern Machine-Shop Practice Part 225

On account therefore of the excessive strength of the sh.e.l.l plates in boilers of small diameter, b.u.t.t straps are rarely used in stationary boilers, while punching the rivet holes and other inferior modes of construction are employed. We may now consider the circ.u.mferential seams of the boilers for stationary engines, such boilers sometimes being of great length in proportion to the diameter.

In proportion as the length of a boiler (in proportion to its diameter) is increased, the construction of the circ.u.mferential or transverse seams, as they are sometimes called, becomes of more importance.

The strength of the circ.u.mferential seams is so much greater than that of the longitudinal seams that it is often taken for granted that they are sufficiently strong if made with a lap joint and single riveted, but that such is not always the case will be shown presently.

[Ill.u.s.tration: Fig. 3255.]

In Fig. 3255 is represented a boiler composed of three strakes (_i. e._, three rings or sections), and it is clear that as the thickness of the sh.e.l.l is doubled at the circ.u.mferential seams where the ends of the middle strake pa.s.s within the end strakes, therefore the strength of the lapped joint of the sh.e.l.l to resist rupture in a transverse direction, as denoted by the arrows A, B, is actually increased by reason of the lap of the riveted joint. But suppose this boiler to be supported at the ends only, and the weight of the sh.e.l.l and of the water within it will be in a direction to cause the middle of the boiler to sag down, and therefore places a shearing strain on the rivets of the circ.u.mferential seams.

Moreover, the temperature of the outside of the boiler cannot be made or maintained uniform, because the fire pa.s.sing beneath the bottom of the boiler first will keep it hotter, causing it to expand more, and this expansion acts to shear the rivets of the circ.u.mferential seams. In proportion as the heat of the fire varies in intensity, the amount of the expansion will vary, and the consequence is that the circ.u.mferential seams may get leaky or the joint may work, especially in boilers that are long in proportion to their diameters. It is clear, therefore, that for the very best construction at least a double riveted circ.u.mferential joint should be employed.

Leaving these considerations out of the question, however, we may find the amount of stress on the circ.u.mferential seams by multiplying the area of the end of the boiler by the working pressure, and dividing by the cross-sectional area of all the rivets in one circ.u.mferential seam.

Suppose, for example, that the diameter of the boiler is 36 inches, the working pressure 60 lbs. per square inch, and that there are in each circ.u.mferential seam 50 rivets, each 3/4 inch in diameter, and we proceed as follows:

The area of a circle 36 inches in diameter = 1017.87 square inches.

The area of a rivet 3/4 inches in diameter = .4417 square inch.

Then

Area of Working boiler end. pressure.

1017.87 60 ------------------ = 2765 lbs. per cross-sectional square inch of rivet.

50 .4417

Number Area of of rivets. each rivet.

By multiplying the area of the boiler end by the working pressure, we get the total steam pressure acting to shear the rivets, and by multiplying the number of rivets by the area of one rivet, we get the total area resisting the steam pressure, and then by dividing the one quant.i.ty into the other, we get the shearing stress per square inch of rivet section.

In the case of longitudinal seams, we have as follows, the pitch being say 2-1/8 and the rivets 3/4.

Diameter Steam Pitch.

of boiler pressure.

in inches.

36 60 2.125 ------------------------------ = 5196 lbs. per square inch of rivet 2 .4417 area.

Rivets in Area of one pitch. rivet.

It is seen, therefore, that the stress placed by the steam pressure on the transverse seam is about one-half of that it places on the longitudinal seam. But, as before remarked, the transverse seam is subject to racking strains, from which the longitudinal seams are exempt; thus, for example, the expansion of the boiler diameter, whether uniform or not, does not strain the longitudinal seam, whereas it may severely strain the transverse seam.

The English Board of Trade rules, in a.s.signing values to the various constructions and qualities of workmanship, a.s.sign a certain value, in the form of an addition to the factor of safety, which takes into account the difference in the stress upon the transverse and longitudinal seams, the quant.i.ties in each case having been determined both from experiment and from experience. A comparison of the different values may be made as follows:

The rules take a boiler sh.e.l.l made of the best material, with all the rivet holes drilled after the strakes are rolled into shape and put together, with all the seams (both longitudinal and transverse) fitted with double b.u.t.t straps each at least five-eighths of the thickness of the sh.e.l.l plates they cover, and with all the seams at least double riveted, with rivets having an allowance of not more than 75 per cent.

over the single shear, and provided that the boilers have been open to the inspection of their surveyors during the whole period of construction, and say that such a boiler sh.e.l.l shall be allowed a factor of safety (divisor of seam strength) of 5.

But for every departure from this, which they deem the best mode of construction, a penalty in the shape of an addition to the factor of safety is made. These additions to the factors of safety with reference to the longitudinal as compared to the transverse seams, are given in the following table:

-------------------------------------+----------------+--------------- |Addition to the |Addition to the |factor 5 if the |factor 5 if the Nature of the deviation in the |deviation is in |deviation is in construction or workmanship. |the longitudinal|the transverse | seam. | seam.

-------------------------------------+----------------+--------------- The holes not fair and good | .75 | .2 Holes drilled out of place after | | bending | .15 | .1 Holes drilled before bending | .3 | .15 Holes punched after bending | .3 | .15 Holes punched before bending | .5 | .2 Joints lapped and double riveted | | instead of having double b.u.t.t straps | .2 | .1 Joints double riveted but have single| | b.u.t.t straps | .3 | .1 Joints single riveted and have a | | single b.u.t.t strap | 1.0 | .2 Joints lapped and single riveted | 1.0 | .2 -------------------------------------+----------------+---------------

[Ill.u.s.tration: Fig. 3256.]

An addition of .25 is also made to the factor of safety, when the strakes are not entirely under or over. In Fig. 3256 for example, strake _b_ is within or under strake _a_ at one end and strake _c_ at the other end, hence _b_ is entirely under; strake _c_ is over _b_ and _d_, and therefore entirely over; while strake _d_ is under _c_, and over _e_, and therefore not entirely under nor entirely over.

When the rivet holes are punched they do not match properly, and unless the holes are punched somewhat smaller than the required size and reamed out afterwards, some rivets receive more stress than others, and may consequently shear in detail. It is customary, however, to punch the holes for ordinary stationary boilers, and it is with seams having punched holes therefore that we have at present to deal.

In the United States the rivet diameter and plate percentages are, in the boilers of stationary engines, usually made equal, and the reasons advanced both for and against this are as follows:

First, in favor of a greater plate percentage than rivet section, it is advanced that the plate gets thinner by wear, whereas the rivet does not, hence the wear reduces the plate section; that the plate is weakened by the punching process, and requires a greater percentage to make up its strength as compared to the rivet; that the rivets are usually of better material than the plates.

In favor of a greater rivet section than plate section, it is advanced that the shearing strength of iron is but about four-fifths of the tensile strength, and that with equal plate and rivet sections the rivet is therefore the weakest; that with punched holes the rivets may be sheared in detail, and that the rivets may be sheared gradually by the working of the joint from varying expansion and contraction.

From these premises the a.s.sumption is drawn that the weakening of the plate from being punched and from corrosion about offsets the excess of the tensile over the shearing strength, and that it is best therefore to employ such a pitch that the area of the rivet and of the metal left between the rivet holes shall be equal.

In order to do this the diameter of the rivet must be determined, and the following are the proportions given by the various authorities named:

TABLE OF THE DIAMETERS OF RIVETS FOR VARIOUS THICKNESSES OF PLATES WITH SINGLE RIVETED LAP JOINT.

---------+------------------------------------------------------- | DIAMETER OF RIVETS.

+-------+---------+----------+----------+-------+------- Thickness|Lloyds'|Liverpool| English |Fairbairn.| Unwin.|Wilson.

of Plate.|Rules. | Rules. |Dockyards.| | | ---------+-------+---------+----------+----------+-------+------- in. | in. | in. | in. | in. | in. | in.

5/16 | 5/8 | 5/8 | 1/2 | 5/8 | 11/16 | 5/8 3/8 | 5/8 | 5/8 | 5/8 | 3/4 | 3/4 | 11/16 7/16 | 5/8 | 3/4 | 3/4 | 21/32 | 13/16 | 3/4 1/2 | 3/4 | 13/16 | 3/4 | 3/4 | 7/8 | 3/4 ---------+-------+---------+----------+----------+-------+------- 9/16 | 3/4 | 13/16 | 7/8 | 27/32 | 7/8 | 7/8 5/8 | 3/4 | 7/8 | 7/8 | 15/16 | 15/16 | 7/8 11/16 | 7/8 | 7/8 | 7/8 | 1-1/32 |1 | 7/8 3/4 | 7/8 | 15/16 | 1 | 1-1/8 |1-1/16 | 1 ---------+-------+---------+----------+----------+-------+------- 13/16 | 7/8 | 1 | 1 | 1-7/32 |1-3/32 | 1 7/8 |1 | 1-1/8 | 1-1/8 | ... |1-1/8 | 1 15/16 |1 | 1-3/16 | 1-1/8 | ... |1-3/16 | 1-1/8 1 |1 | 1-1/4 | 1-1/8 | ... |1-1/4 | 1-1/8 ---------+-------+---------+----------+----------+-------+-------

From the above it is seen that with thin plates the diameter of rivet employed is about twice the thickness of the plate, whereas as the thickness of plate increases the proportion of rivet diameter decreases, and the reasons for this are, first, that with rivets twice the thickness of thick plates and pitched so as to equalize the rivet and plate sections the pitch would be too great to permit of the seams being caulked steam tight.

The diameter of the rivet having been determined, the rivet area and area of plate left between the rivet holes may be made equal by determining the pitch by the following rule:

_Rule._--To the area of the rivet divided by the plate thickness add the diameter of the rivet, and the sum so obtained is the pitch. The correctness of this rule may be shown as follows:

Suppose the rivet diameter to be 7/8 inch = decimal equivalent .875, and its area will be .6013 square inch. Suppose the thickness of the plate to be 9/16 = decimal equivalent .5625, then by the rule:

Rivet area.

Plate thickness = .5625 ) .6013 ( 1.0689 5625 ---- 38800 33750 ----- 50500 45000 ----- 55000 50625

To this 1.0689 we are to add the rivet diameter, thus:

1.0689 .8750 = rivet diameter.

------ 1.9439 = pitch of the rivets.

We have thus found the required pitch to be 1.9439 inches, and as the joint is single riveted there are two half rivets or one whole one to one pitch, and if we subtract the diameter of the rivet from the pitch we shall get the width of the metal or plate left between the rivets, thus:

1.9439 = pitch of rivets.

.8750 = diameter of rivet.

------ 1.0689 = distance in inches between the rivets.

If now we multiply this distance between the rivets by the thickness of the plate, we shall get the area of the plate that is left between the rivet holes, thus: