Disclaimer: The information on this page has not been checked by an independent person.   Use this information at your own risk.
ROYMECH

Click arrows to page adverts

Home
Tribology_Index

Viscosity

More Details on on Viscosity are to be found in the links below the table

Viscosity

A plate of area "A" moving under the action of a force "F" with a resulting velocity "V" over a fixed plate separated by a film of fluid "h" thick.   The shear stress on the fluid is "F / A".    For a Newtonian fluid the shear stress is directly proportional to the velocity gradient in the fluid- "dV /dh" .

P / A = m . dV /dy

The proportionality constant " m " is the viscosity and has units of ( N/m 2) .( m ). ( m / s ) = N . s / m 2...

Dynamic Viscosity

The coherent SI unit of dynamic viscosity is the pascal second (Pa s).   This equates to a Newton second per square metre (N.s / m 2 ).  In the cgs unit the poise is the corresponding unit which has dimensions of dyne seconds per cm 2.   The centipoise (cP = 10 -2 poise) is the most common unit used in tables and handbooks.... 1 Pas = 1000. cP.   The viscosity of water at room temperature is approximately 1 cP

Kinematic Viscosity

This is the ratio of the dynamic viscosity and the fluid density r ( kg /cubic m )  The SI unit of kinematic viscosity is the m 2 / s.   The equivalent cgs unit is the stoke (St) which has the unit of cm 2 / s.  The unit commonly used is the centistoke(cSt)....... 1 m 2 / s = 10 6 cSt


Absolute Kinematic Viscosity (m 2 / s ) = m ( Pa s ) / r (kg / m3 )


Kinematic viscosity (centiStokes) = m (centiPoise) / r ( grammes /cubic cm )
For water with a density of about 1000kg/m3 the conversion is simply 1 cST = 1 cP

Viscosity conversion table Viscosity conversion table


Saybolt Universal Viscosity

A method of determining a lubrication fluids viscosity is to measure the rate of flow of the fluid through a test device.  The rate is measured in seconds the greater the number of seconds the more viscous the fluid.  This viscosity is seconds t = Saybolt universal Viscosity - seconds. The dynamic viscosity relates to the Saybolt universal viscosity (t) by the following formula

Viscosity m (Poise) = (0.00022 .t — 0.18 / t )




SAE Viscosity grading

SAE (Society of automobile engineers ) has allocated numbers for specifying the viscosity of Engine Oils..

The SAE grades 0W through 25W, where W stands for Winter, have a maximum viscosity specified at low temperatures (—5 through —35C), to ensure easy starting under low temperature conditions, and a minimum viscosity requirement at 100C to ensure satisfactory lubrication at the final operating temperature.   The SAE grades 20 through 60 only have limits set at 100C as these grades are not intended for use under low temperature conditions.

For marine applications, monograde oils of SAE 30 or SAE 40 are used because of the steady operating conditions in a ship's engine room.

On the other hand, automotive oils are normally formulated with Viscosity Index Improvers (VI Improvers) to provide multigrade performance.    VI Improvers are very large molecules, which are chemically made by linking together smaller molecules in a process called polymerization.   The resulting products, called polymers, may have molecular weights 1000 times or more greater than the base stock molecules.    The use of these special polymers makes it possible to meet both the low temperature viscosity requirements of the W grades as well as the high temperature requirements of the non-W grades.  Thus a SAE 20W 40 has the low temperature viscosity value of a SAE 20W oil at low temperatures and the high temperature viscosity of a SAE 40 oil at the higher temperature.

Different tests are completed to establish the viscosities for different operating regimes.   Tests are completed for low temperature

  • Pumping in engines starting at low temperature;
  • Lubricating the crankshaft bearings at low temperature
  • Lubricating the engine at normal operating temperatures
SAE
Viscosity
Grade
Viscosity (cP)* at Temperature
(oC), Max
Viscosity mm2/s (cSt)
at 100o C
Cranking Pumpability Min Max
0W 3250 at—30 30,000 at —35 3.8
5W 3500 at —25 30,000 at— 30 3.8
10W 3500 at —20 30,000 at —25 4.1
15W 3500 at —15 30,000 at —20 5.6 --
20W 4500 at —10 30,000 at —15 5.6 --
25W 6000 at —5 30,000 at —10 9.3 --
20 5.6 9.3
30 9.3 12.5
40 12.5 16.3
50 16.3 21.9
60 21.9 26.1


ISO Viscosity grading

The ISO 3448 grading system for Industrial Engine Oils simply relates to the midpoint viscosity of oil in cStokes ( at 40odeg ) as shown in the table below.

Viscosity Grade Midpoint Viscosity at 40 o C Kinematic Viscosity Limits( cStoke )
Min Max
IS VG 2 2,2 1,98 2,42
IS VG 3 3,2 2,88 3,52
IS VG 5 4,6 4,14 5,06
IS VG 7 6,8 6,12 7,48
IS VG 10 10 9,00 11,00
IS VG 15 15 13,5 16,5
IS VG 22 22 19,8 24,2
IS VG 32 32 28,8 35,2
IS VG 46 46 41,4 50,6
IS VG 68 68 61,2 74,8
IS VG 100 100 90,0 110
IS VG 150 150 135 165
IS VG 220 220 198 242
IS VG 320 320 288 352
IS VG 460 460 414 506
IS VG 680 680 612 748
IS VG 1000 1000 900 1100
IS VG 1500 1500 1300 1650



Viscosity conversion Chart

Kinematic viscosity (cST) = Dynamic Viscosity (cP)) / [1000.Density (kg/m3) ]

Table below is based on fluid with a sg = 1 (1000kg/m3)


Centipoise (CPS) Poise Centistokes Stokes Saybolt Seconds Universal
Millipascal seconds (mPas) (P) (cSt) (S) (SSU)
1 0.01 1 0.01 31
2 0.02 2 0.02 34
4 0.04 4 0.04 38
7 0.07 7 0.07 47
10 0.1 10 0.1 60
15 0.15 15 0.15 80
20 0.2 20 0.2 100
25 0.24 25 0.24 130
30 0.3 30 0.3 160
40 0.4 40 0.4 210
50 0.5 50 0.5 260
60 0.6 60 0.6 320
70 0.7 70 0.7 370
80 0.8 80 0.8 430
90 0.9 90 0.9 480
100 1 100 1 530
120 1.2 120 1.2 580
140 1.4 140 1.4 690
160 1.6 160 1.6 790
180 1.8 180 1.8 900
200 2 200 2 1000
220 2.2 220 2.2 1100
240 2.4 240 2.4 1200
260 2.6 260 2.6 1280
280 2.8 280 2.8 1380
300 3 300 3 1475
320 3.2 320 3.2 1530
340 3.4 340 3.4 1630
360 3.6 360 3.6 1730
380 3.8 380 3.8 1850
400 4 400 4 1950
420 4.2 420 4.2 2050
440 4.4 440 4.4 2160
460 4.6 460 4.6 2270
480 4.8 480 4.8 2380
500 5 500 5 2480
550 5.5 550 5.5 2660
600 6 600 6 2900
700 7 700 7 3380
800 8 800 8 3880
900 9 900 9 4300
1000 10 1000 10 4600
1100 11 1100 11 5200
Centipoise (CPS) Poise Centistokes Stokes Saybolt Seconds Universal
Millipascal seconds (mPas) (P) (cSt) (S) (SSU)
1200 12 1200 12 5620
1300 13 1300 13 6100
1400 14 1400 14 6480
1500 15 1500 15 7000
1600 16 1600 16 7500
1700 17 1700 17 8000
1800 18 1800 18 8500
1900 19 1900 19 9000
2000 20 2000 20 9400
2100 21 2100 21 9850
2200 22 2200 22 10300
2300 23 2300 23 10750
2400 24 2400 24 11200
2500 25 2500 25 11600
3000 30 3000 30 14500
3500 35 3500 35 16500
4000 40 4000 40 18500
4500 45 4500 45 21000
5000 50 5000 50 23500
5500 55 5500 55 26000
6000 60 6000 60 28000
6500 65 6500 65 30000
7000 70 7000 70 32500
7500 75 7500 75 35000
8000 80 8000 80 37000
8500 85 8500 85 39500
9000 90 9000 90 41080
9500 95 9500 95 43000
15000 150 15000 150 69400
20000 200 20000 200 92500
30000 300 30000 300 138500
40000 400 40000 400 185000
50000 500 50000 500 231000
60000 600 60000 600 277500
70000 700 70000 700 323500
80000 800 80000 800 370000
90000 900 90000 900 415500
100000 1000 100000 1000 462000
125000 1250 125000 1250 578000
150000 1500 150000 1500 694000
175000 1750 175000 1750 810000
200000 2000 200000 2000 925000


Links to Liquid Lubrication
  1. Engine Oil Bible ...Very Useful for motorists
  2. Texaco ...Product Data and information
  3. Exxon ...Product Data and information
  4. Schaeffer oil includes Technical Data Sheets ...Lubrication data sheets
  5. Faculty of Technology Plymouth ... Lubrication Course Notes- Some Useful Calcs
  6. QTC Gears ... A very useful article with lots of information on gear lubrication, Oil Viscosity etc

This Page is being developed

Home
Tribology_Index

Send Comments to Roy Beardmore

Last Updated 17/01/2013