1

Thick Walled Cylinders

Consider a thick walled cylinder having an inner radius = a; outer radius = b. Let the cylinder is subjected to internal pressure pi and outer pressure po. For the purpose of analysis, thick walled cylinder can be considered to consist of a series of thin rings (Figure 1a.). Consider a typical ring located at a radius r having a thickness dr * As the result of internal and external pressure loading, a radial stress r would develop at the

interface between rings located at a radial distance r. * A slightly different radial stress (r + dr) would develop at a radial position (r + dr) * These stresses would be uniformly distributed over the inner and outer surface of the ring. * Shear stress would not develop on the surfaces, since the pressure loading do not tend to force

the rings to rotate with respect to one another. * A tangential or hoop stress develops when a pressure difference exists between the inner and

outer surface. * The planes on which these tangential stresses act can be exposed by considering only a small

part of a ring * The tangential stress is assumed to be uniformly distributed through the thickness of the ring,

since the thickness of the ring is very very small. * A relationship between r and t can be derived by considering the equilibrium of a small

element (Fig. 1b) * Axial stress a, that may be present is omitted, since it does not contribute to equilibrium of the

element in radial and tangential directions.

Fig 1a

Fig 1b

2

Considering the equilibrium in radial direction i.e.. the sum of all the force in radial direction is zero 0rF =

( )( ) ( )2 sin / 2 0r r r td r dr d dl rd dl drdl d + + = (1) By neglecting higher order terms and noting ( )sin / 2 / 2d d = The equation (a) can be reduced to

0r r tdrdr

+ = (2)

The equation (2) can be integrated since r and t are functions of radial position r. For the case of thick cylinders, the axial strain a at any point in the wall can be expressed as

( )r tE

aa

+= (3)

This means plane transverse section before remain plane after loading. So far as the axial stress a is concerned, two cases are of interest in a wide variety of design applications. i) Axial load induced by pressure not carried by the walls of the cylinders (a = 0) eg. Gun barrels hydraulic cylinders. ii) Walls of the cylinder carry the loads for example, pressure vessels with closed ends. Regions of the cylinders away from the ends, axial stress are uniformly distributed. Hence a, a,

E are constant, therefore

( )r t 12a aE C

+ = = (4)

Where C1 is a constant Substituting t into differential equation, we obtain

12 2r rdr Cdr

+ = (5)

3

( )2 12rd r C rdr = (6) Integrating this equation further yields

2 21 2rr C r C = + (7) Where C2 is a constant of integration. Thus

21 2rCCr

= +

(8)

221 2tCC rr

=

Values of the constants C1 and C2 can be obtained from the known values of internal and external pressures.

at r = a at r = b

r i

r o

pp

=

=

(- sign indicates that the pressure produces compressive radial stresses). Using these boundary conditions, we can solve for C1 and C2 as

2 2

1 2 2i oa p b pC

b a

=

(9)

( )2 2

2 2 2i oa b p pC

b a

=

Then the radial and tangential stresses can be obtained as

( )2 22 22 2 2 2 2

1i oi or

a b p pa p b pb a b a r

= (10)

( )2 22 2

2 2 2 2 2

1i oi ot

a b p pa p b pb a b a r

= + (11)

4

Radial and circumferential deformations play important roles in press-fit and shrink-fit problems. Change in circumference c of the thin ring when internal and external pressures are applied can be expressed in terms of radial displacement of a point in the ring as ( ){ }2 2 2c r dr r r = + = ; where r is change in radius of the ring Circumferential deformation can also be expressed in terms of circumferential strain 2c t r =

r tr = (12) For most applications a = 0. Tangential strain t can be expressed in terms of radial and tangential stresses by Hookes law as:

( )t rt E

= (13)

Thus the radial displacement (or change in radius) at any radius r is given by

( )t r rrE

= (14)

Where the radial and tangential stresses are calculated at radius r. This change in radius can be expressed in terms of internal and external pressures as

( )2 22 22 2 2 2

1 1 1i oi o a b p pa p b pr rE b a E b a r

+ = + (15)

The maximum numerical value of r is found at r = a to be pi, provided that pi exceeds po. If po > pi, the maximum r occurs at r = b and equals po. On the other hand, the maximum t occurs at either inner or outer radius according to the pressure ratio. The maximum shearing stress at any point equals one-half the algebraic difference between the maximum and minimum principal stresses. At any point in the cylinder, we may therefore state that

5

( ) ( )2 2

max 2 2 2

12

t r i oa b p pb a r

= =

(16)

The largest value of max is found at r = a, the inner surface. The effect of reducing po is clearly to increase max. Consequently, the greatest max corresponds to r= a and po =0 is given by

( )

2

max 2 2ip b

b a =

(17)

Since r and t are principal stresses, max occurs on planes making an angle of 45o with the plane on which r and t act. Special cases Internal Pressure only: If only internal pressure acts equations for stresses and change in radius reduce to

r = a2 pi

b2 - a2

1 -

b2

r2 (18)

t = a2 pi

b2 - a2

1 +

b2

r2 (19)

r = a2 pi r

E (b2 - a2)

(1 - ) + (1 + )

b2

r2 (20)

Since b2

r2 1, r is negative (compressive) for all r except r = b,

in which case r = 0. The maximum stress occurs at r = a. t, it is positive (tensile) for all radii, and also has maximum at r = a. This is illustrated in the figure 3.

Fig. 2

6

External Pressure only: In this case, pi = 0, and the equations are given as

r = - b2 po

b2 - a2

1 -

a2

r2 (21)

t = - b2 po

b2 - a2

1 +

a2

r2 (22)

r = b2 po r

E (b2 - a2)

(1 - ) + (1 + )

a2

r2 (23)

Figure 4

The maximum radial stress occurs at r = b and is compressive for all r. The maximum t is found at r = a, and is likewise compressive. And this is illustrated in the figure 4. Closed End cylinder: In the case of closed-end cylinder subjected to internal and external pressures, longitudinal stress exists in addition to the radial and tangential stresses. For a transverse section some distance from the ends, this stress may be assumed uniformly distributed over the wall thickness. The magnitude of l is then determined by equating the net force acting on one end attributable to pressure loading, to the internal directed force in the cylinder wall:

( )2 2 2 2i o lp a p b b a =

Fig. 4

Fig. 3

7

The resulting expression for longitudinal stress, applicable only away from the ends, is

l = pi a2 - po b2

b2 - a2 (23)

Now consider a thick cylinder subjected to inner pressure only. Then radial and tangential stresses are given by

r = a2 pi

b2 - a2

1 -

b2

r2

t = a2 pi

b2 - a2

1 +

b2

r2

Both tangential and radial stresses are maximum, at inner surface

max

2 2

2 2t ia bpb a

+

=

Let K be the ratio of outer radius to inner radius, K = b/a.

Then max

2

2

11t i

Kpk

+

=

min 2

21t i

pk

= or

mint =

maxt ip

The ratio of

maxt to average tangential stress or tangential stress obtained by membrane equation is

shown in the table below

K = b/a = 1 + h/a

1.1 1.2 1.4 1.6 1.8 2.0

max min/t t 1.05 1.1 1.23 1.37 1.51 1.67

8

We can observe that for small ratios of thickness to inner radius, there is a little difference in tangential stress. For instance wall thickness 20% of inner radius, maximum stress is only 10% larger. From membrane equation for cylindrical shell t is given by

t = pi ah =

pi ab - a =

piK - 1

A modification for this equation for cylindrical shell appears in Sections I and VIII of the ASME Code, for thickness range, h 0.5 Ri (inner radius).

9

Compound Cylinders (Laminated Cylinders) From the sketch of the stress distribution (Figure 3) it is clear that there is a large variation in tangential (hoop) stress across the wall of a cylinder subjected to internal pressure. The material of the cylinder is not therefore used to its best advantage. To obtain a more uniform tangential stress distribution, cylinders are often built up by

shrinking one tube on to the outside of another. When the outer tube contracts on cooling, the inner tube is brought into state of compression

and the outer tube will conversely be brought into a state of tension. If this compound cylinder is now subjected to internal pressure the resultant tangential stress

will be algebraic sum of those resulting from shrinkage and those resulting from internal pressure.

The compound cylinder increases the load carrying capacity. The outer cylinder is usually

called hoop, jacket or shell. The inner cylinder is called cylinder or tube. The compound cylinders are designed such that The jacket has inside diameter slightly smaller than the outside diameter of the tube. The difference in diameter at the common surface is normally termed the shrinkage or

interference allowance or simply interference. Normally, the outer cylinder or the jacket is heated until it will freely slide over the tube thus

exerting a shrinkage pressure (ps ) on cooling

Fig. 5

10

Compound cylinder The method of solution for compound cylinders constructed from similar materials is to break the problem down into three separate effects: a) Shrinkage pressure only on the inside cylinder;

b) Shrinkage pressure only on the outer cylinder; c) Internal pressure only on the compound cylinder

For each of the resulting load conditions there are two known values of radial stress from which the stresses can be determined in each case. Condition (a): Consider inner cylinder subjected to shrinkage pressure only

r = 0 at r= a r = - ps at r = c (Note: the internal cylinder is subjected to an external pressure of ps)

The tangential and radial stresses for this condition can be obtained from the equations 21 and 22

ria = - c2 ps

c2 - a2

1 - a2r2 (24)

tia = - c2 ps

c2 - a2

1 + a2r2 (25)

Note: These are the stresses in the inside cylinder due to shrinkage pressure only

11

Condition (b): Consider outer cylinder (jacket) due to shrinkage pressure only

r = 0 at r= b r = - ps at r = c (Note: the external cylinder is subjected to an internal pressure of ps)

The tangential and radial stresses for this condition can be obtained from the equations 18 and 19

rob = c2 ps

b2 - c2

1 -

b2

r2 (26)

tob = c2 ps

b2 - c2

1 +

b2

r2 (27)

Note: These are the stresses in the outer cylinder due to shrinkage pressure only. Condition (c): internal pressure acting on compound cylinder

r = 0 at r= b r = - pi at r = a

the tangential and radial stresses for this condition can be obtained from the equations 18 and 19

rc = a2 pi

b2 - a2

1 -

b2

r2 (28)

tc = a2 pi

b2 - a2

1 +

b2

r2 (29)

12

Combining all these three cases the stresses in inner or outer cylinder can be obtained. Stresses in inner cylinder (Total) due to internal pressure

ri = - c2 ps

c2 - a2

1 -

a2

r2 + a2 pi

b2 - a2

1 -

b2

r2 (30)

ti = - c2 ps

c2 - a2

1 +

a2

r2 + a2 pi

b2 - a2

1 +

b2

r2 (31)

Stresses in outer cylinder (total) due to internal pressure

ro = c2 ps

b2 - c2

1 -

b2

r2 + a2 pi

b2 - a2

1 -

b2

r2 (32)

to = c2 ps

b2 - c2

1 +

b2

r2 + a2 pi

b2 - a2

1 +

b2

r2 (33)

Shrinkage or interference allowance Consider a compound cylinder made up of two different materials. Let the pressure set up at the junction of the cylinder owing to the shrink fit be pressure ps. Let the tangential stresses set up on the inner and outer tubes resulting from the pressure ps be ti (compressive) and to (tensile) respectively at the common radius of these tubes c. Let o = radial shift of outer cylinder and i = radial shift of inner cylinder at radius c Since, circumferential strain = diametrical strain Circumferential strain at radius c on outer cylinder = o/c = to Circumferential strain at radius c on inner cylinder = i/c = - ti (negative since it is a decrease in diameter) Total interference or shrinkage (I) = o + i = c(to - ti) (34) Now assuming open ends, i.e. a = 0,

to = toE1 -

1E1 (-ps) since ro = - ps (35)

13

ti = tiE2 -

2E2 (-ps) since ri = - ps (36)

where E1, and 1, E2 and 2 are the elastic modulus and poissons ratio of the outer and inner cylinders respectively.

Therefore total interference or shrinkage allowance (based on radius)

( ) ( )1 21 2

1 1r to s ti sI p p cE E

= + +

(37)

Where c is the initial nominal radius of the mating surfaces.

Note: to and ti are evaluated at radius c of outer and inner cylinders respectively. ti being compressive will change the negative sign to a positive one when its value is substituted.

Shrinkage allowance based on diameter will be twice this value.

Generally, however, if the tubes are of the same material

E1 = E2 = E and 1 = 2 =

The values of to and ti may be determined from the equations in terms of shrinkage or interference allowance.

14

15

16

Press Fits

In a press fit, the shaft is compressed and the hub is expanded. Press fits, or interference fits, are similar to pressurized cylinders in that the placement of an oversized shaft in an undersized hub results in a radial pressure at the interface.

Characteristics of Press Fits 1) The shaft is compressed and the hub is expanded. 2) There are equal and opposite pressures at the mating surfaces. 3) The relative amount of compression and expansion depends on the stiffness (elasticity

and geometry) of the two pieces. 4) The sum of the compression and the expansion equals the interference introduced. 5) The critical stress location is usually the inner diameter of the hub, where maximum tensile hoop stress occurs.

( )( )( )

2 2 2 2

2 2 22r

s

b c c aEpc c b a

=

Where r is the radial interference for hub and shaft of the same material, with modulus of elasticity, E. Where r is the radial interference for hub and shaft of the same material, with modulus of elasticity, E. If the shaft is solid, a = 0 and ps is given by

17

2

212r

sE cp

c b

=

If the shaft and hub are of different materials

2 2 2 2

2 2 2 2

rs

o io i

pc b c c a cE b c E c a

=

+ ++ +

Tangential stress at the internal diameter of the hub is given by

2 2

2 2to sb cpb c

+=

Tangential stress at the outer diameter of the shaft is given by

2 2

2 2io sa cpc a

+=

if shaft is soilidio sp =

Fig 1bSpecial casesFig. 3Fig. 5