Grease selection techniques

INTRODUCTION TO GREASES

Prepared By Md Aminul Islam

GREASE SELECTION TECHNIQUES

”The Art” of chosing the right grease lies mainly in proper ranking of the properties required from the base fluid, thickener and additives as a result of the critical operating conditions.

LIFE EXPECTANCY

TEMPERATURE LOAD & SPEED

ENVIRONMENT GREASING METHOD

Critical operating conditions can f.ex. be; 1. High, Low or varying operating temperature 2. Shock and impacting loads 3. Water washout, flushing or other wet housing 4. Severely vibrating housing 5. Low starting temperature 6. Under-sized or inadequate central system 7. High centripetal acceleration 8. Long re-lubrication or exchange intervals 9. Agressive environment - chemicals, radiation, dust etc. 10. High, Low or wide speed range 11. High, Low or great variation in axial and radial load

Greases – Selection techniques

Choosing the best lubricant for an application is driven by several equipment considerations.

1. OEM recommendation

2. Operating speed

3. Operating temperature

4. Operating load

5. Shock load

6. Operating environment (heat, dust, water, chemicals, others)

Greases – Selection techniques

Greases – Selection techniques - OEM recommendation

Particularly for OEM equipment that is still under the warranty period, never recommend a lubricant that does not meet the OEM recommendation. If you do, you may assume financial liability for the machine, should it fail due to your recommendation. . . .

If and when a piece of equipment is not performing to levels expected by the OEM or the customer, consult with the OEM to determine if you may improve perfromance through lubrication. If successful, you may result in a change to the OEM specification & listing of Shell products in the approval list

Sometimes the OEM will not recommend products, but require Shell to meet their standards. This transfers the liability to the lubricant vendor

Be aware of OEM standards when rationlising products from many to fewer, ensure you do not compromise an application for the sake of convenience.

Greases – Selection techniques - operating speed

Operating speed is the single most important factor in choosing a grease. It will determine the viscosity required.

There are “standard methods” of determining viscosity, from very basic, using DN value of a bearing, to very sophisticated, using a23 bearing life.

We will discuss several methods that will allow you to chose the right viscosity for operating conditions.

Greases Selection techniques - operating speed/viscosity

DN Value - for friction bearings (bushes, plain bearings, journal bearings)

Dynamic Number value is determined by multiplying the average bearing circumference (in mm) by the revolutions per minute of the bearing. This number is the DN value, sometimes also referred to as “speed factor”.

Remember that for bushings (friction bearings), the diameter is the inside diameter of the bearing or the outside diameter of the shaft.

So for a shaft size of 175 mm (0.55 metres) x revolutions per minute (500 RPM) = surface speed of 275 metres per minute, the DN value is 275

Once DN value is determined, we refer to standard charts for lubricating bearings & select the viscosity range (according to temperature of the bearing).

Now refer to chart. In this example we have a plain bearing with a shaft diameter of 175 mm rotating at 500 RPM. The DN Value or speed factor is 275

You can recalculate on this slide by double clicking the embedded excel sheet & entering new RPM


Shaft diameter

(mm)

Shaft diameter

(metres)

Shaft

circumfrence

(metres)

RPM DN (speed

factor)

75 0.08 0.24 500 118

100 0.10 0.31 500 157

125 0.13 0.39 500 196

150 0.15 0.47 500 236

175 0.18 0.55 500 275

200 0.20 0.63 500 314

225 0.23 0.71 500 353

250 0.25 0.79 500 393

275 0.28 0.86 500 432

300 0.30 0.94 500 471

325 0.33 1.02 500 510

350 0.35 1.10 500 550

375 0.38 1.18 500 589

400 0.40 1.26 500 628

425 0.43 1.33 500 667

450 0.45 1.41 500 707

475 0.48 1.49 500 746

500 0.50 1.57 500 785

Now compare DN value to operating temperature on the chart. In this example we have a plain bearing with a DN Value or speed factor is 275 @ operating temperature between 50 oC and 65 oC, so the viscosity requirement is ISO 100.

You can recalculate on this slide by double clicking the embedded excel sheet & entering new temperature


Plain Journal bearings Plain Journal bearings

Shaft surface speed 0 oC - 30 oC 30 oC - 60 oC > 60 oC Shaft surface speed

= Shaft circumference

(meter) X RPMR&O ISO Grade R&O ISO Grade R&O ISO Grade


(meter) X RPM

< 60 68 220 1,000 < 60

60 - 150 46 150 460 60 - 150

150 - 300 32 - 46 100 220 150 - 300

300 - 760 22 46 - 68 150 - 220 300 - 760

> 760 10 46 150 > 760


(meter) X RPMR&O ISO Grade R&O ISO Grade R&O ISO Grade


(meter) X RPM

Shaft surface speed 0 oC - 30 oC 30 oC - 60 oC > 60 oC Shaft surface speed

Plain Journal bearings Plain Journal bearings

Approximate operating temperature range


DN

Va

lue (

Sp

eed

fa

ctor)

DN

Va

lue (

Sp

eed

fa

ctor)


DN Value - for anti friction bearings

Dynamic Number value is determined by multiplying the average bearing diameter (in mm) by the revolutions per minute of the bearing. This number is the DN value, sometimes also referred to as “speed factor”.

Remember that for bushings (friction bearings), the diameter is the inside diameter of the bearing or the outside diameter of the shaft.

So for a shaft size of 100 mm x 1,000 RPM, the DN value is 100*1,000 = 100,000

Once DN value is determined, we refer to standard charts for lubricating bearings & select the viscosity range (according to temperature of the bearing). For example the bearing is operating at between 80 oC and 90 oC.

Look up the DN value for the bearing you are considering on the chart below.

You can refer to all by double clicking the embedded excel sheet. Note that fields marked in gold are beyond the scope of the tool


50 100 150 200 250 500 750

25 1,250 2,500 3,750 5,000 6,250 12,500 18,750

50 2,500 5,000 7,500 10,000 12,500 25,000 37,500

75 3,750 7,500 11,250 15,000 18,750 37,500 56,250

100 5,000 10,000 15,000 20,000 25,000 50,000 75,000

125 6,250 12,500 18,750 25,000 31,250 62,500 93,750

150 7,500 15,000 22,500 30,000 37,500 75,000 112,500

175 8,750 17,500 26,250 35,000 43,750 87,500 131,250

200 10,000 20,000 30,000 40,000 50,000 100,000 150,000

225 11,250 22,500 33,750 45,000 56,250 112,500 168,750

250 12,500 25,000 37,500 50,000 62,500 125,000 187,500

275 13,750 27,500 41,250 55,000 68,750 137,500 206,250

300 15,000 30,000 45,000 60,000 75,000 150,000 225,000

325 16,250 32,500 48,750 65,000 81,250 162,500 243,750

350 17,500 35,000 52,500 70,000 87,500 175,000 262,500

375 18,750 37,500 56,250 75,000 93,750 187,500 281,250

400 20,000 40,000 60,000 80,000 100,000 200,000 300,000

425 21,250 42,500 63,750 85,000 106,250 212,500 318,750

450 22,500 45,000 67,500 90,000 112,500 225,000 337,500

475 23,750 47,500 71,250 95,000 118,750 237,500 356,250

500 25,000 50,000 75,000 100,000 125,000 250,000 375,000

50 100 150 200 250 500 750

Bea

ring

dia

mete

r (m

m)

Shaft SPEED RPM

Shaft SPEED RPM

1,000 1,250 1,500 1,750 2,000 2,250

25,000 31,250 37,500 43,750 50,000 56,250 25

50,000 62,500 75,000 87,500 100,000 112,500 50

75,000 93,750 112,500 131,250 150,000 168,750 75

100,000 125,000 150,000 175,000 200,000 225,000 100

125,000 156,250 187,500 218,750 250,000 281,250 125

150,000 187,500 225,000 262,500 300,000 337,500 150

175,000 218,750 262,500 306,250 350,000 393,750 175

200,000 250,000 300,000 350,000 400,000 450,000 200

225,000 281,250 337,500 393,750 450,000 506,250 225

250,000 312,500 375,000 437,500 500,000 562,500 250

275,000 343,750 412,500 481,250 550,000 618,750 275

300,000 375,000 450,000 525,000 600,000 675,000 300

325,000 406,250 487,500 568,750 650,000 731,250 325

350,000 437,500 525,000 612,500 700,000 787,500 350

375,000 468,750 562,500 656,250 750,000 843,750 375

400,000 500,000 600,000 700,000 800,000 900,000 400

425,000 531,250 637,500 743,750 850,000 956,250 425

450,000 562,500 675,000 787,500 900,000 1,012,500 450

475,000 593,750 712,500 831,250 950,000 1,068,750 475

500,000 625,000 750,000 875,000 1,000,000 1,125,000 500

1,000 1,250 1,500 1,750 2,000 2,250

Bea

ring

dia

mete

r (m

m)

Shaft SPEED RPM

Shaft SPEED RPM

Now refer to chart. In this example we have an anti friction bearing with 100,000 DN @ between 80 oC and 90 oC, so the viscosity requirement is between ISO 150 and ISO 320.

You can recalculate on this slide by double clicking the embedded excel sheet & entering new temperature


A/ F bearings A/ F bearings

Speed factor (DN) 0 oC - 30 oC 30 oC - 60 oC 60 oC - 80 oC 90 oC - 120 oC Speed factor (DN)

= Bearing bore (mm) X

RPMR&O ISO Grade R&O ISO Grade R&O ISO Grade R&O ISO Grade


RPM

10,000 32-68 100 320 1000 10,000

25,000 32 100 320 460 25,000

50,000 22-32 68 220 460 50,000

75,000 22 46 220 320 75,000

100,000 22 46 150 320 100,000

250,000 10 32 100 220 250,000

> 250,000 10 32 100 220 > 250,000


RPMR&O ISO Grade R&O ISO Grade R&O ISO Grade R&O ISO Grade


RPM

Speed factor (DN) 0 oC - 30 oC 30 oC - 60 oC 60 oC - 80 oC 90 oC - 120 oC Speed factor (DN)

A/ F bearings A/ F bearings



OK, that’s the simple method, now lets explore SKF’s more sophisticated method

SKFs adjusted bearing life formula … Lna = a1 a2 a3 (C/P)p

where

C = Dynamic load rating

P = Dynamic bearing load

p = Exponent depending of bearing type

a1 = Life adjustment factor for reliability

a2 = Life adjustment factor for material

a3 = Life adjustment factor for operating conditions


Greases – Selection techniques - operating load

When loads are heavy & speed is low to medium, it can be an advantage to increase base fluid viscosity & select a lubricant with AW & EP additives

It may also be an advantage to select an adhesive thickener & additives to prevent the lubricant being squeezed out of the bearing

When shock loads are present & speed is low to medium, it can be an advantage to increase base fluid viscosity, select a lubricant with AW & EP additives & utilise solid film addiitves to protect against breaking through the fluid film

It will also be an advantage to select an adhesive thickener (e.g lithium calcium) & tackiness additives to prevent the lubricant being squeezed out of the bearing

Greases – Selection techniques – shock load

Greases – Selection techniques - operating environment

Where environmental contamination is present (heat, dust, water, chemicals, others) the primary selection criteria is the thickener, that the thickener may resist these contaminants.

For wet applications – use a thickener that will resist water wash-out

For hot applications applications – use a thickener & base fluid combination that will resist radiant or inherent heat

For dusty applications – use a thickener that is very adhesive & that will form a seal to prevent contaminant entry past the seals

For very dusty & applications where corrosives, scales & water are present– use an application system that will ensure that grease flows past the seals to prevent ingress of contaminants (e.g, increase grease application volume or utilise single point lubricators (Tactic)

Greases – Selection techniques – process logic

When we have many products to choose from, the easiest and most accurate method is to eliminate those which are not perfectly suitable.

To quote from Conan Doyle’s Sherlock Holmes “If you eliminate the impossible, then whatever is left, no matter how improbable, must be the truth”

For our purposes (with some artistic license taken)

“If you eliminate the unsuitable greases, then whatever is left, no matter how improbable, must be fit for purpose greases”

Engineering

Grease selection techniques