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Calculated

Breaking Weight by Flexure.

F.

No. of Experiment.

Error per Cent.

1

7 8

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+19.8 + 7:1 -12-2 - 2.0 + 3.5 + 8.3

4.8

4.8 + 8.3 + 8.4

5.5 -22:1 +15.0 + 0.3 + 8.3 +0:1 + 4.9 -12.5 + 8.8

0.0
+12:7

3.4
6.3

6:6
+13.5
+ 12.3
- 13:0
-10.4
-17.2
+ 5.2

3:5 + 2.0

7.9 +10.7 + 3.4 + 7.4

3:8 + 0.2 + 2.8 - 5.7 (8)

172 2,036 9,531 23,800

558 6,613 30,960

1,812 21,480

5,812 28,250 19,410

560 6,260 31,370

1,672 21,300

5,915 66,780 17,650 87,485 58,270 197,500 1,281,000

6,340 9,386 11,610 17,800 23,090 28,090 48,710 13,970 32,030 19,750 29,230 43,580 39,330 43,070 62,120 141,060

(9)

HOLLOW CYLINDRICAL PILLARS of Cast IRON.

Breaking Weight.

By Calculation.

By Experi

ment.
Lbs.

Calculated

Crushing
Strain due to

Area.
CP.

Lbs.

143 1,902 10,861 24,291

539 6,105 32,531

1,904 19,752

5,262 22,918 15,107

487 6,238 28,962

1,662 20,310

6,764 40,250 11,255 32,007 17,468 22,867 24,616 5,585 8,357 13,311 19,855 27,883 26,707 50,477 13,693 33,763 17,810 28,353 40,569 33,679 30,383 26,729 34,037

(6)

172 2,036 9,531 23,800

558 6,613 30,960

1,812 21,390*

5,703* 21,685* 11,770*

560 6,260 31,370

1,672
21,300

5,915
43, 800*
11,250*
35,990*
16,870*
21,420*
22,996*
6,310
9,386
11,610
17,800
23,090
28,090
48,710
13,970
32,030
19,750
29, 230
43,580
32,380*
30,450*
27,480*
32,100*

(7)

21,550 81,490 199,200 331,200 21,550 84,490 199,200 21,550 84,490 21,300 51,110 21,550 22,420 85,690 209,800 21,550 87,930 22,420 86,210 21,550 52,010 21,550 23,310 23,310 148,000 175,700 173, 100 226,500 246,900 319,400 377,000 148,000 135,500 148,000 175,700 224,200 85,600 64,830 41,120 38,690 (10)

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

29 30

31 32 33 34 35 36 37 38 39 40 (1)

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reasoning as to the cause. Considering the pillar as having two functions, one to support the direct crushing weight, and the other to resist flexure; when the pressure necessary to break the pillar is very small because of its great length in proportion to its diameter, then the whole strength of the material may be considered as employed in resisting flexure. When the breaking weight is half of that required to crush the material, one half only of the strength may be considered as available for resistance to flexure, the other half being employed in resisting crushing. When, through the shortness of the pillar, the breaking weight is nearly equal to the crushing strain, we may consider that no part of the strength of the pillar is applied to resist flexure, &c. It was found by experiment, that when the load on a pillar was | only of the crushing strain, there was a sensible falling off in the strength as calculated by the rules in (151), due therefore to “Incipient” rather than absolute Crushing.

As the combined result of reasoning and experiment, Mr. Hodgkinson gives the rule :

(164.)

Po = F x Cp - (F + 1 Cp). In which F = the breaking weight by flexure as due by the

rules in (151) (156), &c. Cp = the crushing strain due to the area of the

section and the specific strength of the

material. Pc = the reduced actual breaking weight; all in

the same terms.

This rule requires some caution in its application; where F is less than 1 Cp, the effect of it would be to make the calculated strength greater than F. Now, the strength of a pillar can never be greater than is due to flexure, hence there is a limit beyond which the rule must not be applied :—when F is exactly & Cp, the effect of the rule is nil: when F is greater than į Cp, the rule is necessary, and will reduce the calculated strength of the pillar as due by flexure:-when F is less than

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Cp the rule will give the erroneous result of making Po greater than F. Thus, say Cp = 80 and F = 20, or exactly 1 Cp; then, 80 x

1 .75 = 60, and the rule (164) gives Pc = 20 X 80 = (20 + 60)

20, or the same value as F, the effect of the rule being nil. Again, say Cp = 80, F = 10, then Po = 10 x 80 - (10 + 60) = 11:43, which is greater than 10, or the value of F, and is impossible, showing that the rule has been applied in a case where it was not admissible. Again, say F 30, and Cp = 80 as before:—then the rule gives P. = 30 x 80 = (30 + 60) 26.67 tons, which is less than 30, the value of F, and is a correct result.

(165.) We may now search for the lengths of pillars, with which the correction given by this rule becomes nil, which will happen when the length is such that F or the breaking weight by flexure is Cp.

The mean crushing strain of cast iron is 43 tons per square inch as given in (132); a pillar 1 inch diameter will be crushed with 43 x .7854 = 33.73 tons, and the required length of pillar will be that which breaks by flexure with 33.77 - 4: 8.44 tons. For pillars with both ends pointed, the value of Me as given by Table 34, is 14.73 tons, and the rule (154), namely, L 17! (Me X D3-4 ; W), becomes in our case (14.73 x 1; 8.44) 11 = 1.387 feet, or 16.64 inches. The length with

, which the correction becomes nil is therefore in this case 16.64 times the diameter. Similarly, with one end flat, and the other pointed, the value of Me being in that case 29.46 tons by Table 34; we have (29.46 x1= 8.44) '= 2.086 feet, or

: 7 25 inches, the length being thus 25 times the diameter. With both ends Alat Mp = 44.19 tons, and the length comes out (44:19 x1= 8:44) ') = 2.648 feet, or 31.78 inches, the length being thus 31-78 times the diameter.

(166.) Mr. Hodgkinson, adopting 49 tons per square inch as the crushing strain of the particular iron used in his experiments, gives the length at 15 times the diameter for pillars with both ends pointed, and 30 times the diameter in those with both ends flat.

These ratios are, however, not constant for all diameters, as is

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shown by Table 39, which has been calculated for cast-iron pillars by the rule

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(167.) Lp = '7 (Mp * D3-6 = Cp) x 12 = D. In which LD the length in terms of the diameter with which correction for incipient crushing is nil; Cp = the crushing strain due to the area of section and the specific crushing strength of cast iron, or 43 tons; Mp = the multiplier for pillars, given by Table 34; and D = the diameter in inches. This table shows that the ratio of length to the diameter is reduced as the diameter is increased, in the case of cast iron considerably, and still more so with wrought-iron pillars (202).

TABLE 39.–Of the LENGTH OF CYLINDRICAL PILLARS, in terms of

Diameter with which correction for “Incipient Crushing” becomes nil.

Diameter.

2 Ends
Pointed.

1 Flat.
1 Pointed.

2 Ends Flat.

CAST IRON.

1 2 3 6

16.64
16:08
15.60
14.98

25:04
24:01
23:48
22.55

31.78
30:47
29.78
28.60

WROCGHT IRON.

1
2
3
6

40.64
35.35
32.62
28.40

58.37
50.75
46.87
40.79

71.85 62.50 57.68 50.22

STEEL.

1
2
3
6

26:14
22.73
20.98
18.27

36.96
32:15
29.68
27:04

45.26 38.87 36.34 33.12

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(168.) As an example of the application of the rule (164) we may take the pillar 6 inches diameter, i inch thick, and 14 feet long, which we found in (160) to break by flexure with 151 tons. By a table of areas, 6 inches 28.3, and 5 inch 19•6, hence the area of the annulus = 28.3 19.6 = 8.7 square inches, and the mean crushing strength of cast iron being 43 tons per square inch (132), we obtain 8.7 x 43 374 tons for the value of Cp, and 374 x 1 = 280 for i Cp. Then the rule (164) becomes P. 151 x 374 • (151 + 280) = 131 tons, the reduced breaking weight.

(169.) The fact (164) that the correction for incipient crushing is necessary for those cases only where F is greater than ; Cp, supplies an easy method of finding beforehand where it is required, so as to save the labour of going through the whole calculation. Thus, taking No. 34 in Table 38, col. 10 gives 148,000 lbs. for the value of Cp, then Cp becomes 148000 = 4 = 37,000; by col. 9, F 19750 lbs., which is less than £ C, therefore the correction for incipient crushing is not necessary, and the breaking load of the pillar is simply that due by flexure. Again, in No. 40, col. 10 gives Cp = 38690 lbs., 1 Cp becomes 38690 -4 = 9672 lbs., but F by col. 9 is 141,060 lbs., which being greater than Cp, requires correction by the rule in (164) by which we obtain 32,100 lbs. for the reduced breaking load, as in col. 7.

The effect of the application of this rule is in some cases remarkably great: for instance in Table 55, No. 9, col. 9 gives 10,750,000 lbs. for the value of F, but Pe = 79380 lbs. only by

. Pc col. 7, or oth of F.

The rule for incipient crushing applies not only to cast-iron pillars, but equally to all other materials. It is used for wrought iron, steel, and timber in Tables 44, 57, and its correctness is proved by the general agreement of the calculations with the experimental results as shown by (959) and Table 150, that agreement being to a great extent due to the use of the rule.

(170.) “Square Pillars of Cast Iron.”—It is shown in (359) that the theoretical ratio of the strengths of square to round bars, either as pillars or beams, is 1.7 to 1.0, but the experimental

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