Practise questions

Answers later

Qs. 1 on VI

The coefficient of dynamic viscosity values for a high grade oil and a low grade oil at 40 oC are 15 cp and 60 cp respectively. It is required to determine the viscosity index (VI) of an oil having a viscosity value of 25 cp (centipoise) at 40 oC. The test and reference oils have the same viscosity values at 100 oC. Determine the VI of the test oil using the Dean Davis method.

Step 1. Check whether the viscosity of the test oil at 40 oC (U) is in between those of the high grade oil (H) and low grade oil (L) or not.We see that L = 60 cp, H = 15 cp, and U = 25 cp, H < U < L

Therefore we can use the formula,

100.HL

ULVI

Practise qs. 2 on tilting pad brg.The following figure shows a bearing undergoing hydrodynamic lubrication. Reynold’s equation in 1 dimension gives the pressure (p) drop in the x-direction as

Find the pressure drop in the x direction when the height of separation “h” is 0.5 m. Calculate the flow rate in the x direction given its formula per unit width as

Given: U = 20 cm/s, coefficient of dynamic viscosity = 40 cp. The distance of separation at maximum pressure ho is 0.4 m. There is negligible flow in other directions.

3

o

h

hhU6

dx

dp

2

hU

x

p.

12

hq

3

x

Upper surface is stationary

hBottom surface moves with velocity U

x

Oil wedge

• Width of bearing B = 50 mm• L/B = 1• Sliding velocity = 5 m/s• Min. film thickness = 0.02 mm• Viscosity = 0.02 Pa.s• Attitude K = 2.0Find load, friction and coefficient of friction

KK

K

KW e

2

2)1(log1*

2

22* .

6

/

B

h

U

LWW

B

WKh

K

KLB

h

UF eh 2

)1(log. 2

20,

Prob. Temperature rise in tilting pad brg.

-What is the temperature rise in the oil for the previous problem. Density of the oil = 880 kg/m3, specific heat capacity = 1900 J/kg.KMechanical equivalent of heat = 1 J heat per 1 joule of friction energy since we are using SI units throughout

)1(

)2(.

1

2 K

K

h

F

cLJt

C06.7)21(

)22(x

10x02.0

861.8

10x50x1900x880

1 o33

•Journal dia. = 100 mm•L/D = 1•Radial clearance = 0.025 mm•Journal speed = 3000 rpm•Eccentricity ratio = 0.6•Viscosity = 0.02 Pa.s

Find attitude angle, load, friction and coefficient of friction for Sommerfeld’s half condition and narrow brg. theory

Sommerfelds half condition

2/120,h )1(c

URL2sin

R2

WcF

2/12 )1(

.4

tan

)1)(2(

)]4([

c

RUL6W

22

2/12222

21 12

Tan

Narrow brg.

1.116

)1(.

4.

c

LUW 2

2222

3

Prob. Viscosity and friction for tilting pad brg.A tilting pad slider bearing has the following operation parameters•Angle of inclination = 0.4o

•Leading edge height = 0.75 mm•Trailing edge ht. = 0.25 mm•Load to be carried = 6000 N•Sliding speed = 2.5 m/s•Bearing length L = 8 cm•Bearing width B = 7.5 cmCalculate the viscosity of the oil required to operate under these conditions and friction at the two surfaces

1

0

**2

22

6

.dxp

LBU

hW

K2

2

K

)K1(log

K

1W e* =

)16...(2

)1(log. 2

20, B

WKh

K

KLB

h

UF eh