Thursday, November 17, 2016

Numerical Problems on Rankine’s formula.

1. The external and internal diameters of the hollow cast iron column are 5cm and 4cm respectively. If the length of this column is 3m and both of its ends are fixed, determine the crippling load using Rankine’s formula. Take σc= 550N/mm2 and a=1/1600.

Step 1: Data:
D = external diameter = 50mm
d= internal diameter = 40mm
Rankine's constant=a=1/1600.
length of column= 3m = 3000mm
condition=both of its ends are fixed
crippling load =??
critical stress σc= 550N/mm2

Step 2: Calculation of the area of the column
A= π/4(D2-d2)
A= π/4(502-402)
A = 706.85 mm2

Step 3 :Calculation of the moment of inertia  of the column
I = π/64(D4-d4)
I = π/64(504-404)
I = 181.04X10mm4



Step 4:Calculation of Effective length
condition=both of its ends are fixed
therefore,
Le=l / 2
Le = 3000/ 2
Le = 1500mm

Step 5:Calculation of radius of gyration
K= √(I/A)
K = √(181.04X10/706.85)
K = 16.00 mm

Step 6:Calculation of crippling load
P=  (σc A)/(1+a(Le/k)2)
P = ((550) (706.85))/(1+(1/1600)(1500/16)2)
P = 60.33KN

2. The hollow cylindrical cast iron column is 4m long with both ends fixed. Determine the minimum diameter of the column if it has to carry a safe load of 250KN with a FOS of 5 take internal diameter as 0.8 times the external diameter. Take σc= 550N/mm2 and a=1/1600.

Step 1: Data:
D = external diameter
d= internal diameter = 0.8 D
Rankine's constant=a=1/1600.
length of column=4m = 4000mm
condition=both of its ends are fixed
critical stress =  σc= 550N/mm2
safe load =  250KN
Factor of safety = 5

Step 2: Calculation of the area of the column
A= π/4(D2-d2)
A= π/4(D2-(0.8D)2)
A = 0.2826 D2 mm2

Step 3 :Calculation of the moment of inertia  of the column
I = π/64(D4-d4)
I = π/64(D4-(0.8D)4)
I = 0.029 Dmm4

Step 4:Calculation of Effective length
condition=both of its ends are fixed
therefore,
Le=l / 2
Le =4000/ 2


Le =2000mm

Step 5:Calculation of radius of gyration
K= √(I/A)
K = √(0.029 D4/0.2826 D2)


K = 0.3203 D mm

Step 6: Calculation of crippling load 

FOS = Crippling load / safe load

5 = Crippling load / 250
Crippling load= 250X5 = 1250KN


Step 7:Calculation of diameter
P=  (σA)/(1+a(Le/k)2)
1250 = ((550) (0.2826 D2))/(1+(1/1600)(2000/0.3203 D)2)
D = 136.33 mm
d = 0.8 D = 109.06 mm

3. A hollow cast iron column of external diameter 250mm and internal diameter 200mm is 10m long with both ends fixed. Find the safe axial load with FOS of 4.Take σc= 550N/mm2 and a=1/1600.


Step 1: Data:
D = external diameter = 250mm
d= internal diameter = 200mm
Rankine's constant=a=1/1600.
length of column= 10m = 10000mm
condition=both of its ends are fixed
safe load =??
critical stress =  σc= 550N/mm2
Factor of safety = 4

Step 2: Calculation of the area of the column
A= π/4(D2-d2)
A= π/4(2502-2002)
A = 17671.45 mm2

Step 3 :Calculation of the moment of inertia  of the column
I = π/64(D4-d4)
I = π/64(2504-2004)
I = 113.20X10mm4

Step 4:Calculation of Effective length
condition=both of its ends are fixed
therefore,
Le=l / 2
Le =10000/ 2


Le =5000mm

Step 5:Calculation of radius of gyration
K= √(I/A)
K = √(113.20X106/17671.45)
K = 80.03 mm


Step 6:Calculation of crippling load
P=  (σA)/(1+a(Le/k)2)
P = ((550) (17671.45))/(1+(1/1600)(5000/80)2)


P = 33.44MN

Step 7: Calculation of crippling load 

FOS = Crippling load / safe load

4= 33.44 / safe load

safe load= 8.36MN


4. Find the Euler’s crippling load for a hollow cylindrical steel column of 40mm external diameter and 4mm thick. The length of the column is 2.5m and is hinged at both the ends.  Also compute the Rankine’s crippling load using constants 350MPa and 1/7500.Take E=205GPa.

Step 1: Data:
D = external diameter = 40mm
Thickness = 4mm
d= internal diameter = 40-2(4)=32mm
Rankine's constant=a=1/7500.
length of column= 2.5m = 2500mm
condition=hinged at both the ends
crippling load =??
critical stress =  σc=350N/mm2
E=205GPa.

Step 2: Calculation of the area of the column
A= π/4(D2-d2)
A= π/4(402-322)
A =452.38mm2

Step 3 :Calculation of the moment of inertia  of the column
I = π/64(D4-d4)
I = π/64(404-324)
I = 0.074X10mm4

Step 4:Calculation of Effective length
condition=both of its ends are fixed
therefore,
Le=l
Le = 2500mm
Le = 2500mm

 Step 5:Calculation of radius of gyration
K= √(I/A)
K = √(0.074X106 /452.38)
K = 12.789 mm



Step 6:Calculation of crippling load by Rankine's formula
P=  (σA)/(1+a(Le/k)2)
P = ((350) (452.38))/(1+(1/7500)(2500/12.789)2)
p= 965.04KN 

Step 7: Calculation of crippling load by Euler's formula
P = Π 2E I/ L2
P = Π 2 (2.05X105 )0.074X106 / (2500)2
p=23.95KN

5. Design the section of circular cast iron column that can safely carry a load of 1000KN. The length of the column is 6m. Rankine’s constant is 1/1600, FOS 3.One end is fixed and other is free. Critical stress is 560Mpa.

Step 1: Data:
safe load= 1000KN
length= 6m= 6000mm
Rankine's constant =a = 1/1600
σc = 560Mpa
FOS =  3

Step 2: Calculation of crippling load 

FOS = Crippling load / safe load

3= Crippling load / 1000

Crippling load = 3000KN

Step 3:  Calculation of radius of gyration
K= √(I/A)
K = √(πd4/64 /πd2/4)
K = d/4

Step 4:Calculation of Effective length
condition=.One end is fixed and other is free
therefore,
Le=l / 2
Le =6000/ 2
Le =4242.64 mm

Step 5:Calculation of diameter  by Rankine's formula
P=  (σA)/(1+a(Le/k)2)
3000X1000 = ((560) (πd2/4))/(1+(1/1600)(4242.64/(d/4))2)
solving the above 
d = 89.69mm

6. A column with circular section of 20mm diameter is hinged at its both ends. The column with two different lengths is tested under buckling load resulting

Length in mm
Buckling load (KN)
300
60
400
47

Determine the Rankine’s constant and crushing stress for the material of the column.

Step 1: Calculation of Area of cross section 
A= πd2/4
   =  π(20)2/4

   = 314 mm2

Step 2: Calculation of  Moment of inertia
I= πd4/64
   =π(20)4/64
   = 7853.98 mm4

Step 5:Calculation of radius of gyration
K= √(I/A)
K = √(7853.98 /314)
K = 5 mm

Step 6:Calculation of crippling load by Rankine's formula
case1
P=  (σA)/(1+a(Le/k)2)
60X1000((σc) (314))/(1+(a)(300/5)2)
60000+ (216X106 ) a = 314 σc
Case 2 :
P=  (σA)/(1+a(Le/k)2)
47000 = ((σc) (314))/(1+(a)(300/5)2)
47000+ (300.8X106 ) a = 314 σc
a= 1/5000=0.0002
σc = 286.538 N/mm 2

7. A hollow cast iron circular section column is 7.5mm long and pinned at both the end. The inner diameter of the column is 160mm and thickness of the wall is 20mm. Find the safe load by Rankine’s formula, using FOS of 5.Also find the slenderness ratio and ratio of the Euler’s and Rankine’s critical loads. For cast iron take σc= 550N/mm2 and a=1/1600 and E=8X104N/mm2.

Step 1: Data:
Thickness = 20mm
d= internal diameter = 160mm
D = external diameter =D=d+2t=160+2(20)=200mm
Rankine's constant=a=1/1600
length of column= 7.5m = 7500mm
condition=hinged at both the ends
crippling load =??
FOS=5
critical stress =  σc=550N/mm2
E=8X104N/mm2

Step 2: Calculation of the area of the column
A= π/4(D2-d2)
A= π/4(2002-1602)
A =11309.73mm2

Step 3 :Calculation of the moment of inertia  of the column
I = π/64(D4-d4)
I = π/64(2004-1604)
I = 46.36X10mm4


Step 4:Calculation of Effective length
condition=both of its ends are fixed
therefore,
Le=l
Le = 7500mm
Le = 7500mm


 Step 5:Calculation of radius of gyration
K= √(I/A)
K = √( 46.36X10/11309.73)
K = 64.03 mm

Step 6:Calculation of crippling load by Rankine's formula
P=  (σA)/(1+a(Le/k)2)
P = ((550) (11309.73))/(1+(1/1600)(7500/64.03)2)
pr = 649 KN


Step 7: Calculation of crippling load 

FOS = Crippling load / safe load
5 = 649  / safe load
safe load=  129.8 KN


Step 8: Calculation of slenderness ratio
slenderness ratio = l/k
                            = 7500/ 64.03
                            = 117.10

Step 9: Calculation of crippling load by Euler's formula
P = Π 2E I/ L2
P = Π 2 (8X104 )46.36X106(7500)2
pe=650.085KN

step10 : Calculation of ratio of euler's load to bucking load

Pe/Pr = 650.085/649
          = 1.0017


8. A hollow circular section 2.8m long is fixed at one end and hinged at other end. External diameter is 150mm and thickness of the wall is 15mm.Rankine’s constant=1/1600 and σc= 550Mpa.Comparwe the buckling loads obtained by using Euler formula and Rankine’s formula. Also find the length of the column for which both formulas gives the same load. Take E=80GPa.

9. A column as shown in the figure below consists of three plates, each of thickness “t” welded together. It carries a axial load of 400 KN over an effective length of 4m.Taking σc= 320MPa, a=1/7500 and FOS= 2.5, determine the value of t.

10. The following particulars are given below
a) Diameter of the cylinder =400mm
b) Steam pressure in cylinder =0.6N/mm2
c) Distance between the piston and cross head = 1.25m.
Find the diameter of the piston rod allowing a FOS of 4.Assume that the piston is firmly fixed to the piston and the cross head. Take σc= 330N/mm2 and a=1/7500

16 comments:

  1. It's easy to understand the concept

    ReplyDelete
  2. It is very easy to understand this types of problems

    ReplyDelete
  3. Please solve this que.
    A) a hollow circular of cast iron column with both end fixed support on axial load of 1000 KN of its external diameter is 25 CM and length 4.5 M. Determine the internal diameter of section. Stress 100MPa and a = 1/6400

    ReplyDelete
  4. Please solve this que.
    A) a hollow circular of cast iron column with both end fixed support on axial load of 1000 KN of its external diameter is 25 CM and length 4.5 M. Determine the internal diameter of section. Stress 100MPa and a = 1/6400

    ReplyDelete
  5. I don't understand the fact that you have solved the easy questions but not the tough ones i,e the last ones. BTW it helped a lot to understand the basics. Thank you

    ReplyDelete
  6. Sir. I am sorry to say ....There is one correction in 5th example in the formula used for effective length

    ReplyDelete
  7. Sir ....Another correction in problem 6
    About length 7.5 mm or m

    ReplyDelete
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  10. I am not using any prescribed text book

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