Friction: The good, the bad and the greasy

Know your fundamentals and you’ll be on firm ground when things get slippery.

Friction is a good and bad thing. Without friction we couldn’t walk, light a match or stop our automobiles. However, friction and wear cost billions of manufacturing dollars in machine and process failures. In fact, in his study of manufacturing in the United States, Dr. E. Rabinowicz of MIT estimated annual losses to be $194 billion. Simply put, lubrication is the way to minimize friction.

Friction
Friction is the resistive force caused by one body sliding over another. If the bodies are rigid, it’s called solid friction. Solid friction can be static or kinetic. Static is when movement is initiated with a body at rest; kinetic is when the bodies are already in motion. This is important since it is estimated that 70 percent of wear and damage occurs during machine start up.

There are two causes of friction. The first is welds, the micro bonds between two flat surfaces that occur at high loads and high temperatures. Friction is the consequence of breaking these micro welds, resulting in the generation of heat, abrasion and wear to the surfaces in contact.

Surface profile or roughness is the other source. No matter how well a surface is machined or polished, no surface is perfectly smooth. The irregularities that protrude from the surface are called asperities. Asperities between two surfaces collide when the surfaces are moving, causing friction.

Unlike solid friction, fluid friction is caused by molecule collision of the fluid in motion. The resistive force in fluid friction is much lower than solid friction so, in lubrication, you are substituting low fluid friction for the high solid-to-solid friction.

Lubrication
Two types of lubrication will account for almost all of your maintenance needs: boundary and hydrodynamic.

Boundary lubrication addresses the solid-to-solid contact that occurs when machinery is started or when heavy loads are introduced to equipment. Lubricants contain additives that form a film on the solid surface to prevent metal-to-metal contact. This chemical film is sacrificial and prevents metal wear. This property is formulated into lubricants with additives known as anti-wear and extreme pressure (EP).

A key point to remember is that base oils of lubricants seldom wear out; however, the required additives to make a lubricant function well are used up over time and therefore require replacement.

You can also address boundary lubrication by using higher viscosity (thicker) oils if the application permits. However, we have to be careful that the viscosity doesn’t create a secondary problem — fluid friction. Too much fluid friction will cause higher operating temperatures, which in turn cause the lube to thin to a lower viscosity and not offer the protection required.

Hydrodynamic lubrication occurs when a full film of oil separates two surfaces in motion. An example would be when a turning shaft has a full oil film between it and its support bearings. In this case, for all practical purposes, friction is eliminated. The only friction present is fluid friction described earlier. In theory, if temperature, operating speed, and load were kept constant, this hydrodynamic lubrication would last indefinitely.

Once you have a basic idea of lubrication functions, you need to understand conditions that impact their performance, mainly temperature and load. The lubricant property most impacted by temperature directly is viscosity.

Viscosity is the resistance of a fluid to flow. It is the measurement of a fluid’s thickness. For example, water has a very low viscosity and molasses has a very high viscosity. A fluid’s viscosity changes with temperature — lower temperatures cause higher (thicker) viscosity and higher temperatures cause lower (thinner) viscosity. Additives can be used to change a lubricant’s viscosity index, since lubricants with a higher viscosity index thin less at high temperatures and thicken less at low temperatures.

Load also plays a major role in lubricant selection. A low viscosity oil will simply be squeezed out by high load, causing direct surface contact. Conversely, extremely high viscosity lubes could cause excessive fluid friction and ruin the lube’s performance.

Lubricant selection
In light of the prior discussion, you probably realize that there is no easy answer to selecting the proper lubricant for your application; however, there are ways to make a good selection. Start by asking a series of questions to narrow your choices.

1. What is the application? Is it low constant speed, high speed, or continuous?

2. What are the temperatures involved? Is it a freezer or a furnace? Indoor or outside?

3. Is there any water or harsh environment involved? Does it involve processing strong chemicals or solvents?

4. Are there extreme loads such as gears or heavy metal parts on a conveyor?

5. Are there frequent starts and stops?

6. Does it have to be NSF compliant? If so, is there a chance of incidental food contact (H1) or no chance of contact (H2)?

7. Does the application require conducting a current, such as plating operations?

8. Does your application require a non-flammable lube?

Once you have identified the application requirements, your second step will be to choose a wet or dry lubricant.

Dry lubricants
Dry lubricants are boundary lubes; they will reduce friction by forming a film on the opposing surfaces.

Molybdenum, commonly known as “moly” has excellent extreme pressure properties. It will withstand up to 500,000 psi, and is effective up to 650 degrees F constant and 750 degrees F intermittent. Moly is non-conductive.

Graphite is effective up to 850 degrees F constant and 1,000 degrees F intermittent. Graphite has excellent conductivity.

PTFE has the lowest coefficient of friction known, works from minus 40 degrees F to 572 degrees F and meets military ammunitions specifications. Common trade names for PTFE are Teflon and Krytox.

Wet lubricants
Wet lubricants are hydrodynamic and reduce friction by building a liquid wedge between opposing surfaces.

Silicones work up to 550 degrees F, are available in multi-viscosity grades (low grades are NSF approved) and are suitable for a wide variety of applications.

Oils are the most common lubes for closed systems and come in a variety of viscosity grades. Oils generally work in limited temperature ranges, meet many military specifications and are normally the lowest cost choice.

Synthetic oils offer superior durability (longer life) and are available in multi-viscosity grades. While synthetic oils have a higher price per unit, the cost is often comparatively lower based on service life.

Greases are simply thickened oils in a semi-solid state. Greases are available in different viscosity grades and serve very well in open systems where oil would simply run out.

You may also use advanced technology to assist in your lubricant selection. Laboratory analysis of spent oil, vibration analysis with sensory instrumentation and heat analysis with sensor instruments are just some of the tests that help pinpoint your exact needs.

If these techniques aren’t available for you, however, use your senses. Our ears will hear chatter and friction if we listen closely, and our hands will sense heat and tell us if the machine or motor is running hotter than normal.

You should always start with the equipment manufacturer’s recommendation for a lubricant. Remember though that most recommendations are made for a specific set of operating conditions, and yours may vary in speed and temperature, requiring a change to find the best lubricant.

Russ Mays is senior product manager, Krylon Products Group, a division of Diversified Brands. He is a 36-year veteran of the chemical and coatings industry with 21 years of R&D and 17 years of sales and product management. He has won corporate awards for new technology development, product innovation, technical excellence and marketing excellence. He can be reached at 216-515-7796; e-mail: ramays@sherwin.com.

This article appeared in the February/March 2006 issue of MRO Today magazine. Copyright 2006.

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