Grease Fundamentals and Analysis
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Grease is a thickened lubricant that remains in contact with moving surfaces. It is formulated from a base oil thickened with a soap, clay, or polymer. Grease properties depend on the base oil viscosity and consistency grade determined by penetration testing. Additives provide oxidation resistance, extreme pressure properties, anti-wear performance, and other characteristics. Standard tests evaluate grease properties like dropping point, oil separation, load carrying capacity, and resistance to water and corrosion. Proper grease selection depends on application parameters like temperature, speed, and load.
Grease Fundamentals and Analysis
- 3. Simple Definition • Grease: – A lubricant that is a solid to semi‐fluid dispersion of a thickening agent (thickener) in a liquid. A lubricating grease may be formulated with additives that impart special properties such as resistance to oxidation or wear. – Origin: from Old French graisse, based on Latin crassus ‘thick, fat.’ Source: NLGI
- 4. Stronger Definition • Grease: – A grease is a lubricant which has been thickened in order that it remain in contact with the moving surfaces and not leak out under gravity or centrifugal action, or be squeezed out under pressure. Thus, a major practical problem is the provision of a structure that will stand up under shear and at all temperatures to which it may be subjected during use. At the same time the grease must be able to flow into the bearing through grease guns and from spot to spot in the lubricated machine as needed, and must not of itself add significantly to the power required to operate the machine, particularly at the start. This is an exacting set of rheological requirements. Ref: Vold, J. Marjorie, and Vold, Robert D., "Lubrication and Lubricants", J. Inst. Petroleum Technology, Vol. 38, 1952, P155‐163.
- 5. Grease Rheology • Rheology is the science of deformation and flow of materials – Most solids are considered elastic, a material that when stressed will store deformation energy and recoil to its original shape – Most fluids are considered viscous, a material that when stressed does not store deformation energy and will start to flow • Greases are viscoelastic materials, a little of both
- 8. Base Oils • Mineral • Synthetic – Polyalphaolefins (PAO) – Esters – Polyglycols – Polyethers – Dialkylbenzenes • Silicone
- 9. Base Oil Viscosity VISCOSITY APPLICATION ISO 15 ‐ 32 Used for high speeds (>3600 rpm), lower loads, good at low temperatures ISO 68 ‐ 100 Used for high speeds (>3600 rpm), lower loads, higher temperatures ISO 100 ‐ 220 Moderate speeds (<3600 rpm), good load carrying, typical multi‐purpose grease’s oil ISO 460 Medium speeds, high load carrying ISO 1000 ‐ 1500 Slow speeds (<100 rpm), excellent load carrying
- 10. Thickeners • Thickeners are fibrous, like a sponge, and are used to affect grease properties: – Texture, the appearance and feel of a grease which affects its adhesiveness – Dropping Point, the temperature at which the grease releases a drop of oil – Shear Stability, the ability to resist permanent changes in consistency due to work – Water Resistance, the ability to withstand water without adverse effects – Pumpability, the ability of a grease to flow under pressure
- 11. Soaps • Lithium • Calcium • Sodium • Aluminum • Barium Non‐Soaps • Clays (Bentonite) • Polyurea Thickeners
- 12. Thickeners
- 13. Simple • Prepared by reacting a single organic acid with one or more inorganic bases Complex • Prepared from two or more organic acids – Primary soap (metallic stearate) – Complexing agent (metallic salt) • The complexing agent modifies grease characteristics and usually increases the dropping point. Soap Thickeners Source: NLGI
- 15. Grease Properties by Thickener Shear Stability Dropping Point (°F / °C) Water Resistance Maximum Temperature Lithium Good 375 / 190 Yes 250 / 121 Lithium – Complex Excellent 500 / 260 Moderate 300 / 149 Calcium – Hydrated Good 190 / 88 Yes 150 / 65 Calcium – Anhydrous Poor – Good 290 / 143 Yes ‐‐‐ Calcium – Complex Good 500+ / 260+ Yes 300 / 149 Sodium Poor – Good 360 / 182 No 250 / 121 Aluminum – Normal Poor 180 / 87 Yes 150 / 65 Aluminum – Complex Good 480 / 249 Yes 300 / 149 Barium Good 400 / 204 Yes 250 / 121 Clay Good 500+ / 260+ Yes 300 / 149 Polyurea Med – Good 470 / 243 Yes 300 / 149 Ref: Tool and Manufacturing Engineers Handbook, Volume 1, p. 4‐44, Library of Congress, 1983
- 16. Penetration (ASTM D217) • Cone penetration test results provide one measure of the consistency of a grease – Worked penetration results are required to determine to which NLGI consistency grade a grease belongs – Undisturbed penetration results provide a means of evaluating the effect of storage conditions on grease consistency Ref: ASTM D217‐10, Standard Test Methods for Cone Penetration of Lubricating Grease, ASTM International, West Conshohocken, PA, 2010.
- 17. NLGI Consistency Number NLGI Number ASTM worked (60 strokes) penetration at 25 °C tenths of a millimeter Appearance Consistency food analogy 000 445‐475 fluid cooking oil 00 400‐430 semi‐fluid apple sauce 0 355‐385 very soft brown mustard 1 310‐340 soft tomato paste 2 265‐295 "normal" grease peanut butter 3 220‐250 firm vegetable shortening 4 175‐205 very firm frozen yogurt 5 130‐160 hard smooth pate 6 85‐115 very hard cheddar cheese Ref: Rudnick, Leslie R. (2005). Synthetics, Mineral Oils, and Bio‐Based Lubricants: Chemistry and Technology (Chemical Industries). CRC. p. 468.
- 22. Additives • Structure modifiers – modify the grease structure & properties • Anti‐oxidants / Oxidation inhibitors – rust and corrosion protection particularly for applications involving high temperatures and/or water contamination • Extreme pressure additives – enable greases and equipment to resist extreme heat and extreme pressure, particularly in boundary lubrication conditionsRef: lubimax.com
- 23. Additives • Anti‐wear agents – reduce metal wear by binding to metal surfaces, forming a lubricious sacrificial coating • Anti‐rust / Metal deactivators – reduce metal reactivity to protect surfaces from degradation • Viscosity modifiers – modify base fluid properties to enhance performance in highly variable temperature applications Ref: lubimax.com
- 24. Additives • Application dependent – Pour point depressants – Antifoam agents – Emulsifiers – Demulsifiers – Tackiness agents – Solid additives – Friction Modifiers Ref: lubimax.com
- 25. Oil Separation (ASTM D1742) • Results of this test correlate directly with the oil separation that occurs in 35‐lb pails of grease during storage – Reports % wt of oil separated after 24 hours at 25 °C Ref: ASTM D1742‐06(2013), Standard Test Method for Oil Separation from Lubricating Grease During Storage, ASTM International, West Conshohocken, PA, 2013.
- 26. EP Properties (ASTM D2596) • Used to differentiate between greases having low, medium, or high levels of extreme pressure characteristics – Reports maximum load (OK value) under which no welding occurs – a.k.a. Four‐Ball Method Ref: ASTM D2596‐15, Standard Test Method for Measurement of Extreme‐Pressure Properties of Lubricating Grease (Four‐Ball Method), ASTM International, West Conshohocken, PA, 2015.
- 27. AW Properties (ASTM D2266) • Used to determine the relative wear‐ preventing properties of greases under the test conditions – Reports the wear scars on the lower three balls – a.k.a. Four‐Ball Method Ref: ASTM D2266‐01(2015), Standard Test Method for Wear Preventive Characteristics of Lubricating Grease (Four‐Ball Method), ASTM International, West Conshohocken, PA, 2015.
- 28. Load‐Carrying Capacity (ASTM D2509) • Used to differentiate between greases having low, medium, or high levels of extreme pressure characteristics – Reports maximum load (OK value) under which no scoring occurs – a.k.a. Timken Method Ref: ASTM D2509‐14, Standard Test Method for Measurement of Load‐Carrying Capacity of Lubricating Grease (Timken Method), ASTM International, West Conshohocken, PA, 2014.
- 30. Water Washout (ASTM D1264) • Estimates the resistance of greases to water washout from ball bearings under conditions of the test – Reports % wt of grease washed out after spraying for 60 minutes at 38 or 79 °C Ref: ASTM D1264‐16, Standard Test Method for Determining the Water Washout Characteristics of Lubricating Greases, ASTM International, West Conshohocken, PA, 2016.
- 31. Water Spray Off (ASTM D4049) • Evaluates the ability of a grease to adhere to a metal surface when subjected to direct water spray – Reports % wt of grease removed after spraying for 5 minutes at 38 °C Ref: ASTM D4049‐06(2011), Standard Test Method for Determining the Resistance of Lubricating Grease to Water Spray, ASTM International, West Conshohocken, PA, 2011.
- 32. Mobility (US Steel DM 43) • Measures pumpability of grease at lower temperatures – Reports g/min at 150 psi – A factor in centralized grease system’s lines, nozzles and fittings Source: SKF
- 34. Grease Compatibility • Most grease manufacturers produce compatibility charts that do not necessarily agree with one another (often meant only for their own products) • ASTM D6185 evaluates mixtures to confirm: – No significant decrease in dropping point – Mechanical stability remains in range – Consistency remains in range after heating Ref: ASTM D6185‐11, Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases, ASTM International, West Conshohocken, PA, 2011.
- 40. Water Contamination • Water can soften or displace grease, leading to a lack of lubrication • Water can cause corrosion or pitting of parts • Water content may correlate with wear metals or ferrous wear concentration – Trending is important, since a significant increase may occur before a corresponding increase in wear
- 41. Ferrous Wear Concentration • May or may not correlate with spectroscopic iron – If ferrous wear is lower than iron, wear is considered normal, i.e. <10 microns – If ferrous wear increases without an increase in iron, wear is considered severe, i.e. >10 microns – If both increase, wear is abnormal and should be followed up with Analytical Ferrography
- 42. Analytical Ferrography • A portion of the sample is passed over a slide on top of a magnetic plate to attract ferrous particles • The prepared slide is then placed under a microscope for examination Image courtesy AZO Materials Ferrous Particles Near Exit are Submicroscopic Non Ferrous & Weakly Magnetic Debris Deposit Randomly Non-Wetting Barrier Entry Region Oil Flow
- 43. Particle Classification • The particles are then classified by: – Shape – Composition – Size – Surface condition • As a result of this classification, determination of an abnormal wear mode can be made
- 46. Data Interpretation Tips • Grease samples are inherently difficult to ensure that they are truly representative, therefore: – Trending is the most valuable tool, try not to dwell on absolute values or limits – High levels of contamination without corresponding wear often suggest poor sampling – Sampling consistently (location, method, interval) provide the best chance for early detection of faults