Modern Machine-Shop Practice Part 224

[Ill.u.s.tration: Fig. 3249.]

[Ill.u.s.tration: Fig. 3250.]

[Ill.u.s.tration: Fig. 3251.]

[Ill.u.s.tration: Fig. 3252.]

"A double riveted b.u.t.t joint with double straps is shown in Fig. 3250, and a treble with double straps in Figs. 3251 and 3252.

"Third. By various arrangements of the rivets in conjunction with b.u.t.t joints and double straps, with which it is not necessary, at this point, to deal.

"One of the great advantages obtained by the use of the double strap is that of bringing the rivet into double shear (or in other words, the rivet must shear on each side of the plate, or in two places, instead of between the plates only, before the joint can give way by shearing), and thus obtaining an increased calculated strength of 1-3/4 times the ordinary or single shear, the rule being to find the rivet strength in the ordinary way (as before explained), and then multiply the result by 1.75.

"The Board of Trade rules for s.p.a.cing the rivets of these joints are as follows:

"Dimension E is the distance from the edge of the plate to the centre of the rivet hole. Dimension V is the distance between the rows of rivets, dimension _p_ is the pitch of the rivets, which is always measured from centre to centre of the rivets, and dimension _pd_ is the diagonal pitch of the rivets.

"The rule for finding dimension E, whether the plates and rivets are either of steel or iron, is as follows:

"Multiply the diameter of the rivet by 3 and divide by 2, the formula being as follows:

3 _d_ ------- = E.

2

"To find the distance V between the rows of rivets in chain riveted joints. This distance must not be less than twice the rivet diameter, and a more desirable rule is four times the rivet diameter plus 1 divided by 2, thus:

4_d_ + 1 -------- = V.

2

"To find the distance between the rows of zigzag riveted joints:

_____________________________ /(11_p_ + 4_d_) (_p_ + 4_d_) ------------------------------- = V, 10

that is, multiply 11 times the pitch plus 4 times the rivet diameter, by the pitch plus 4 times the rivet diameter, then extract the square root and divide by 10.

"To find diagonal pitch _pd_, multiply the pitch _p_ by 6, then add 4 and divide by 10, thus:

6_p_ + 4 -------- = _pd_."

10

[Ill.u.s.tration: Fig. 3253.]

Fig. 3253 represents a form of high percentage joint, used upon marine boilers of 10 to 14 feet diameter, and carrying from 100 to 190 lbs.

pressure of steam. The rivets are what are termed unevenly pitched, or, that is to say, on each side of the joint, there are three rows of rivets, of which the inner and outer rows are wider pitched than the middle row.

[54]"The advantage gained by this s.p.a.cing is that the shear of the outer row of rivets is added to the plate section at the narrow pitch, that is to say, if the plate section broke through the line of rivet holes at the narrow pitch, it has yet to shear the outer row of rivets before the plate can separate."

[54] From "_Steam Boilers_."

[Ill.u.s.tration: Fig. 3254.]

Fig. 3254 represents a second example of joint with rivets unevenly pitched, this form finding much favor in recent practice. The four inner rows of rivets are s.p.a.ced at narrow pitch and the two outer rows are wide pitched.

[55]"The strength percentage of this joint is calculated from three points of view, as follows:

[55] From "_Steam Boilers_."

"First. The plate section at the wide pitched rivets.

"Second. The rivet section in one pitch.

"Third. The plate section at the narrow pitch plus half the double shear of the outer or wide pitched rivet."

The steam pressures generally employed in the boilers of stationary engines range from about 60 to 100 lbs. per square inch, and as a result of these comparatively low pressures less perfect forms of construction are employed than would be permissible if higher pressures were used.

The strength of the sh.e.l.l plate of boilers of small diameter is always largely in excess of the requirements, and as a result the strength of the joints may bear a very low percentage to that of the solid plate, and yet give a sufficient factor of safety for the working pressure.

Take, for example, a boiler sh.e.l.l of 36 inches internal diameter with a sh.e.l.l plate 1/4 inch thick, and allowing the strength of the material to be 48,000 lbs. per inch of section, and with a factor of safety of 4, the working pressure will be 166 lbs. per square inch, thus:

Strength Plate thickness of the material. 2.

48000 (.25 2) --------------------------------- = 666-2/3 lbs. = bursting pressure.

36

Diameter of boiler.

By dividing this 666 by the factor of safety 4 we get 166-2/3 lbs. as the working pressure of the sh.e.l.l plate independent of the riveted joint. Usually, however, such a boiler would not be used for a pressure above about 60 lbs. per inch, and this leaves a wide margin for the reduction of strength caused by the riveted joints.

Suppose, for example, that a single riveted lap joint is used, and the strength of this joint is but 50 per cent. of that of the solid plate, and we have as follows:

Strength of % strength Twice material. of riveted the plate joint. thickness.

48000 .50 (.25 2) ---------------------------------- = 83-1/3 lbs. = W.P.

36 4

Internal diam. Factor of of boiler. safety.

Here then we find that the working pressure of the solid plate is double that of the riveted joint, and that the working pressure of the boiler is 83 lbs. per square inch, notwithstanding that the strength of the riveted joints is but 50 per cent. of that of the solid plate. Such a boiler would not, however, be used for a pressure of over 60 lbs. per square inch.

If the above-named boiler was double riveted so as to bring the percentage of joint strength up to say 70 per cent, of that of the solid plate, its working pressure would be 116 lbs. per square inch, thus:

Strength of % strength Twice material of riveted the plate joint. thickness.

48000 .70 (.25 2) ---------------------------------- = 116-2/3 lbs. = W.P.

36 4

Internal diam. Factor of of boiler. safety.

But in practice such a boiler would not be used for pressures above about 75 lbs. per square inch, hence the sh.e.l.l plate thickness is still largely in excess of the requirements, and it may be remarked that plates less than 1/4 inch thick are not used on account of the difficulty of caulking them and keeping them steam tight.