In this centenary issue of our newsletter the discussion topic is volatility – an important characteristic of lubricating oil that is often overlooked. Volatility is the tendency of a substance to evaporate since volatile matter has the capability to go into the vapor phase. In the case of oil this usually happens at elevated temperatures. Volatility has a critical impact on the performance of lubricating oils – low viscosity engine oils in particular – that are operating at high temperatures.

The base oil used to manufacture lubricating oil is a complex mixture of lighter (low viscosity) molecules and heavier (high viscosity) molecules. Low viscosity multigrade engine oil contains a larger percentage of lighter fractions. These lighter molecules are more volatile and vaporise (evaporate) first when the oil is heated, leaving the heavier molecules behind and the viscosity of the remaining oil is increased.

Many motorists do not realise that when they notice that their vehicle is “using” oil, they are often witnessing the effects of volatilisation. Motorists usually just buy extra oil to replace what they assume the engine has “used”, but is adding more oil necessarily the best solution to the problem? What they are doing by adding more oil, is burning up their money. Extra oil is not the only expense motorists face when dealing with volatilisation. As motor oil goes through the process of volatising, the chemically lighter (or more volatile) portions are always the first to “boil off”. This leaves the heavier, less pumpable portions behind. This heavier oil cannot be relied upon to flow easily and quickly to all engine components. The result is decreased fuel efficiency, premature component wear and deposit formation within the engine. The expense to the motorist can be quite substantial. It is therefore no surprise that concern about engine oil volatility has increased over the last number of years.

The following factors have a major influence on the performance of engine oil:

LOWER VISCOSITY ENGINE OILS
Vehicle manufacturers are very concerned about fuel economy. Thinner, low viscosity oils are being recommended to reduce fuel consumption. Traditionally SAE 20W-50 and SAE 15W-40 multigrade engine oils were used, but today many manufacturers recommend oils as thin as SAE 5W-30 and even SAE 0W-20 to improve fuel efficiency.

HIGHER ENGINE TEMPERATURES
Many modern internal combustion engines are turbocharged and have higher compression ratios. Oil sump capacities are reduced to minimise engine size and weight. To aggravate matters airflow around the engine is reduced due to better aerodynamics. All these lead to increased engine oil operating temperatures.

EXTENDED DRAIN INTERVALS
Engine oil volatility strongly effects drain intervals. Many manufacturers have introduced oil service intervals of 30,000 km or two years (whichever comes first) for petrol engines and 50,000 km for light duty diesel engines. Drain intervals for heavy duty diesel are typically between 80,000 and 120,000 km. These extended drain intervals lead to increased stresses on the oil.

ADDITIVE VOLATILISATION
The most widely used and effective anti-wear and anti-oxidation additives in engine oil contain phosphorus that can partially volatilize during high temperature engine operation. Volatile phosphorus in the exhaust stream degrades the function of the exhaust catalyst in reducing air pollution. Early studies have indicated that phosphorus volatility is not related to base oil volatility or to phosphorus content in the unused engine oil. More recent research has shown that phosphorus additive chemistry is the main contributor to catalyst poisoning and major additive suppliers have since come up with improved phosphorus technology to minimise additive volatilisation.

Based on the above it is evident that the volatility characteristics of the base oils used to manufacture low viscosity engine oils are key to the performance and longevity of the final product. The Noack Volatility Test is a critical measure of lubricating oil quality. There are several variants of the test, but in principle they all determine the evaporation loss of oil at high temperatures.

In the most commonly used Noack Volatility Test (CEC L-40-A-93, ASTM D5800 and DIN 51581) a 65 gram sample of oil is heated for one hour at 250°C while a controlled stream of air carries the volatile components away. Results are reported as a percentage of the mass an oil has lost during the test – the lower the percentage, the better, as this demonstrates resistance to oil volatility and breakdown. There are no formal Noack Volatility limits or specifications for lubricant base oils, but experience has confirmed that a maximum limit of 15% allow engine oils to perform satisfactorily for long periods at elevated temperatures.

Q8Oils offer a comprehensive range of multigrade engine oils blended with high quality base oils and the latest additive technology to minimise volatilisation. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.

In this final part of our Rheology series we conclude with a discussion of the rheological properties of lubricating grease. Two key characteristics of grease are consistency and viscosity. It is therefore not surprising that these rheological properties appear in all grease specifications and product literature. Another key rheological characteristic of grease that is often overlooked, is deformation (visco-elasticity). The deformation behaviour of lubricating grease under shear or stress is critical in the satisfactory performance of the grease.

The smooth and reliable operation of oil lubricated machinery depends on consistent lubrication between moving surfaces. To achieve this the lubricating oil must be of Newtonian nature to uphold a robust film to separate the moving surfaces. The viscosity of Newtonian oils does not change with shear, stress or rate of flow.

For most machines in intermittent service it is desirable to have a non-Newtonian lubricant with a thicker consistency to remain in place during shutdown, but it must, however, have the fluidity of an oil when the equipment is in operation.

The American Society for Testing and Materials (ASTM) defines lubricating grease as “A solid to semifluid product of dispersion of a thickening agent in liquid lubricant. Other ingredients imparting special properties may be included” The thickener holds the oil in place but releases it under shear or stress to give the effect of oil lubrication with similar viscosity as the base oil used in the grease. When the shear/stress is removed, the thickener ‘absorbs’ the oil again and the grease thickens to its original consistency. In this instance the rheological behaviour of the non-Newtonian grease is thixotropic. In fact, most greases available today are of thixotropic nature.

In some instances a non-Newtonian grease that hardens when subjected to stress is preferred. Typical examples are large opencast mining machines where the grease must be pumped long distances from a central grease reservoir to the actual lubrication points. The grease needs to be soft enough to be pumped and then thickens under mechanical pressure to adhere to heavily loaded components and provide superior lubrication, especially under shockload conditions. The rheological behaviour of such non-Newtonian greases is rheopectic. These greases require a gentle touch when being pumped. It is crucial to employ a special pumping system with low shear for rheopectic greases. High shear forces can accelerate the thickening process and may cause grease feed lines to get clogged. Tailor-made pumps are therefore required that deliver a smooth, controlled flow to preserve the rheopectic nature of the grease.

Q8Oils offer a comprehensive range of high-quality greases for a wide variety of automotive, commercial, construction, mining, agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.

In OilChat 97 we discussed rheology in general. In this issue we will address the rheological behaviour of lubricating oil.

The most important rheological parameter of liquid lubricants is viscosity, hence viscosity is reported on all oil specifications and product data sheets. It also affects the tribological properties like friction and wear between moving surfaces in contact with each other.

The smooth and reliable operation of machinery and mechanical components depends on consistent lubrication at the contact area between moving surfaces. To achieve this, the lubricating oil must be of Newtonian nature to maintain a robust film to separate the moving surfaces. The viscosity of Newtonian oils does not change with shear/stress or rate of flow. Most monograde lubricating oils (mineral and synthetic) can be classified as Newtonian.

Having said that, monograde oils may become non-Newtonian in service. A typical example is diesel engine oil that is loaded with soot (partially burned fuel). When the soot concentration in engine oil reaches a level that can no longer be dissolved by the dispersant additive, the soot particles clump together to increase the viscosity of the oil. High viscosity may result in cold-start problems and risk of oil starvation. Once the oil is circulated by the oil pump the soot particles are dispersed in the oil and the viscosity will decrease, but when the engine is shut down the oil will thicken up again. In this instance the rheological behaviour of the oil is thixotropic.

Multigrade oils may also exhibit non-Newtonian behaviour when subjected to stress. Multigrade oils are formulated with Viscosity Index Improvers, also referred to as Viscosity Modifiers. These are normally long chain polymers that expand with increase in temperature to improve the “thickening characteristics” of the oil at elevated temperatures. The rheological behaviour of these polymers is described below:

 

 

 

 

Fig 1: When the oil temperature is low, the polymers curl up into tight balls that flow readily with the oil molecules and with no effect on the oil viscosity.

Fig 2: As the temperature increases, the polymers expand into large stringy coils that restrict the normal oil flow, which has a thickening effect on the oil. When the oil cools down, the polymers return to their original shape. The result is that when these polymer based additives are blended in the correct proportion with for example SAE 15W base oil, the oil flows like a SAE 15W at low temperatures and similar to a SAE 40 oil at high temperatures. The outcome is a SAE 15W40 multigrade oil that will provide adequate protection over a wide temperature range. It should be noted that there is actually no SAE 40 base oil in a SAE 15W40 formulation.

Fig 3: The viscosity of most multigrade engine oils decreases with increase in the rate of oil flow or shear. This is because the ‘viscous grip’ of the oil on the expanded polymers is reduced with increased stress which causes distortion and rotation of the polymers. The decrease in viscosity of these non-Newtonian multigrade oils is recovered when the shear rate is reduced and the phenomenon is called Temporary Viscosity Loss or TVL.

Fig 4: When the oil is violently sheared, the polymers may be stretched beyond their ability to accommodate the motion of the oil and one or more of the polymers may be broken. Consequently, the viscosity of the oil-polymer solution decreases permanently because the viscous contributions of the broken, shortened polymers are less than that of the original long chain polymers before degradation. In contrast to TVL, this polymer degradation phenomenon is called Permanent Viscosity Loss or PVL since, in this case, the viscosity lost is not recovered and the rheological behaviour of the oil is non-thixotropic. In the early days of multigrade oils, PVL was a factual problem but with modern, shear-stable polymer technology it is unlikely to occur if the oil is used for the intended application. If, however, a modern multigrade oil is used incorrectly, PVL may still occur. A typical example is when a multigrade engine oil is used in a heavily loaded gear system.

Q8Oils offer a comprehensive range of shear stable multigrade oils for a wide variety of automotive, commercial, construction, mining, agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.

Rheology is the science of flow and deformation of matter and describes the interrelation between force, deformation and time. Rheology is applicable to all materials, from gases to solids. Fluid rheology is used to describe the consistency of liquid matter by referring to viscosity and elasticity. Viscosity is the resistance to flow or thickness and elasticity relates to stickiness or structure.

Newtonian behaviour is a phenomenon that is firmly embedded in the science of Rheology and is also a key property that affects the performance of lubricants.

NEWTONIAN substances are matter of which the viscosity does not change with shear/stress or rate of flow. Typical examples of Newtonian fluids are water, alcohol and oil.

NON-NEWTONIAN materials are liquids or gels of which the viscosity does change when subjected to stress. Non-Newtonian fluids may thin down or thicken up when sheared or stressed as discussed below:

Thixotropy is the property of certain fluids and gels to become thinner when a constant force is applied, and after removal of the force the viscosity recovers fully to the initial state in a finite period of time. The higher the force that is applied, the lower the viscosity becomes. Thixotropy is a time-dependent phenomenon, as the viscosity of the substance must recover within a certain timeframe when the applied force is removed. Tomato sauce is a typical example. It is usually quite thick, leaving you frustrated waiting for it to run out of the bottle. In response you shake it, giving you that red shower that drowns the fries on your plate. Reason is that tomato sauce is thixotropic. Its thickness and viscosity decrease depending on for how long and how fast you shake it. When you allow the tomato sauce to settle for some time it “gels” again.

In some materials the structural strength/viscosity decreases while shearing but the viscosity does not fully recover after an appropriate rest period. It remains thinner than the initial state which indicates that the structure does not recover completely. A typical example of this behaviour is yogurt. After stirring, the viscosity of yogurt remains thinner than it was initially. Such substances may be classified as non-thixotropic.

Rheopexy is the rare property of some Non-Newtonian fluids to show a time-dependent increase in viscosity; the longer the fluid undergoes shearing forces, the higher its viscosity becomes. Rheopectic fluids, such as cream, thicken or solidify when stressed. Fresh cream usually flows readily, but when you shake or whip it long enough, its consistency changes, it stops to flow and eventually becomes solid. In summary, Rheopectic fluids thicken when subjected to shear forces.

In the next issue of OilChat we will discuss the significance of rheology in lubrication. If you have any questions about rheology (or any other lubricant related issues) in the interim, you are welcome to email us at info@bcl.co.za.

Demulsibility is a term commonly used in the language of lubrication. Demulsibility is the ability of oil to release or separate from water. In other words, it is a measure of how well the lubricant can resist emulsification. A high demulsibility rating means that the lubricant will resist forming an emulsion with water, while a low number indicates that it will not.

Oil and water separate readily because similar molecules attract each other, i.e. oil sticks with oil, water sticks with water. However, when a mixture of oil and water is agitated, an emulsion is formed. Three oils with various degrees of demulsification and emulsification are shown on the right:

In the sample on the left all the oil and water are demulsified.
There is an interface (emulsion) of oil and water in the middle sample,In the sample on the right all the oil and water are emulsified.

In most instances the oil and water will separate completely when left to settle for an adequate period of time. The water drops to the bottom since water is denser than oil.

In lubrication the demulsification property of oil is a benefit since water shedding is important for systems that have the potential to become contaminated with water. Water that enters a circulating system and emulsify can increase wear and corrosion, reduce load-carrying capacity, promote oil oxidation, deplete additives and plug filters. In hydraulic systems it can also adversely affect the operation of valves, servos and pumps. Although highly refined oils permit water to separate readily, demulsibility can be affected negatively by the presence of impurities and contaminants in the oil. Some oil additives such as rust inhibiters and dispersants can actually promote emulsion formation.

The impact of demulsibility depends on the level of contamination and the residence time (the time the oil spends ‘resting’ in the reservoir) of the system. When the resident time is sufficient the demulsified water can be removed by engineering solutions such as drain valves, suction, etc.

Demulsibility testing can show failure in the lab, but with sufficient residence time, the oil may shed water at an acceptable rate that does not impact oil performance. Small oil reservoirs with lower residence times require better demulsibility performance than larger sumps. It is recommended that testing for demulsibility should be conducted on a regular basis if the oil system is exposed to water or if demulsibility performance is suspicious.

The ASTM D1401 demulsibility test method is commonly used to determine the ability of oil to separate from water. A 40 ml sample of the test specimen and 40 ml of distilled water are stirred for 5 minutes in a graduated cylinder at a specified temperature. The time required for the separation of the emulsion is recorded after every 5 minutes. If complete separation or emulsion reduction to 3 ml or less does not occur after standing for 30 minutes, the volumes of oil, water, and emulsion remaining are reported.

Regardless of what is said above, a few equipment manufacturers do not recommend hydraulic oils that shed water. Caterpillar hydraulic oil is for instance formulated to hold water in dispersion.

The oil contains emulsifiers specifically designed to disperse water. Caterpillar does not recommend oils that “separate,” “shed,” or “release” water. They claim that separated water drawn through the hydraulic system can damage pumps and other components. If the water freezes, it can also cause serious damage to hydraulic systems. Notwithstanding this many Caterpillar machines in mixed fleet operations are operating satisfactorily with lubricants that do demulsify or shed water.

Q8Oils offer a comprehensive range of demulsifiable lubricants for a wide variety of automotive, construction, industrial, mining, agricultural and other applications. For more information phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.

Finally we would like to wish all our loyal followers a prosperous and rewarding 2025.

The FZG gear test evaluates anti-wear characteristics and load carrying capacities of lubricants. The test rig was developed by the Technical Institute for the Study of Gears and Drive Mechanisms (Forschungsstelle für Zahnräder und Getriebebau) at the Technical University of Munich.

The test rig simulates a misaligned gear set operating in the test lubricant. The gears are loaded in 12 increasing “stages”. Tooth-wear is examined after each stage. The performance of the fluids is determined by the load stage at which excessive wear occurs. A high pass load stage indicates better resistance to wear. If failure does not occur at load stage 12, it is reported as >12.

 

 

There are a number of different FZG load tests with different speeds, oil sump temperatures, gear types and direction of rotation. Following are the most commonly used test procedures:

FZG Gear Wear (ASTM D5182) evaluates gear scuffing resistance of fluids using A profile gears. The rig is operated at 1450 rpm up to 12 progressive load stages at 15-minute intervals. Standard tests are run at a fluid temperature of 90oC. Gear teeth are inspected for scuffing after each load stage. In addition to a visual evaluation of the gear teeth condition, gear weight loss is also measured.

FZG Gear Wear (ASTM D4998) assesses gear wear resistance of fluids using A profile gears. The rig is operated at 100 rpm under constant load for 20 hours. A visual tooth surface rating and gear weight loss are measured.

FZG Pitting Type C Gears evaluates gear pitting resistance of fluids using C profile gears. Tests are run up to 300 hours under constant load, temperature, and speed. Inspections are conducted at predetermined intervals for pitting damage on the gear tooth faces.

FZG A10/16.6R/120 is a more severe version of the ASTM D5182 Load Stage Wear test. This test requires A10 gears (half the tooth width of the A gears) and is run at 2880 rpm in reverse mode.

These tests are also defined by the German Institute for Standardization (DIN), the International Organization for Standardization (ISO) and others. In addition to the standard FZG gear tests, some test facilities may offer customised FZG tests. The gear type, oil temperature, load, test duration, direction of rotation and speed can all be altered to meet specific customer requirements.

The FZG gear test is primarily used to assess the resistance to scuffing of mild additive treated oils, such as industrial gear oils, transmission fluids, and hydraulic oils. High EP type oils, for example, oils meeting the requirements of API GL-4 and GL-5, exceed the capacity of the test rig and therefore cannot be evaluated with FZG gear tests.

Q8Oils offer a comprehensive range of wear inhibiting lubricants for a wide variety of automotive, construction, industrial, mining, agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.

There are several test methods to evaluate the wear preventing properties of lubricants and each test is used for a different purpose. The Four-Ball Test method is used to determine the relative wear-preventing properties of lubricants in sliding steel-on-steel applications

 

In the Four-Ball Test, three ½” (12.7 mm) diameter steel balls are clamped together and covered with the lubricant to be evaluated. A fourth steel ball, held by a chuck, is rotated against  the  three lower clamped balls as shown on the right. The red dots denote the three points of contact between the balls. By adjusting the speed of the rotating ball and the load applied to it, several lubrication regimes can be simulated. There are two Four-Ball Tests that determine the tribological characteristics of  lubricating oils and greases.

Four-Ball Weld Load Test. This procedure (ASTM D2783 for lubricating oils and ASTM D2596 for greases) evaluates the load-carrying (Extreme Pressure) properties of lubricants. During the test, the top ball rotates at 1760 rpm against the three stationary balls and the load is gradually increased until the lubricant fails. This happens when welding between the balls is detected (as depicted on the left). The weld point is the lowest applied load in kilograms at which the rotating ball welds to the three stationary balls.

 

Four-Ball Wear Scar Test. This test measures the wear preventing properties of lubricants, using ASTM method D2266 for greases and D4172 for lubricating oils. The rotational speed of the top ball is 1200 rpm and is pressed with a force of 40 kg onto the three clamped balls. The temperature of the test lubricant is regulated at 75°C and the duration of the test  is 60 minutes. Lubricants are compared by using the average size of the scar diameters worn on the three lower clamped balls. An enlarged 1,87 mm diameter wear scar is shown on the right.

As with all bench tests, the Four-Ball Test attempts to create a reliably repeatable condition that can be performed relatively inexpensively and in much less time than would be required for field trials. In this respect the test passes with flying colours and Four-Ball Test rigs are commonly used in lubricant test labs worldwide.

However, the conditions in the test rig bear little resemblance to those seen in machinery. There are two components to this – the size of the interaction between the balls and the interaction type.

With regards to the size of the interaction, the intersection (contact area) between two spheres is only a tiny point or spot. The result is that the test load is being concentrated on a small area that increases the surface pressure drastically.

In addition, the contact between the balls is a sliding interaction. This combination is rarely found in machinery, where the most severe combinations are line contact with sliding (as in gears or journal bearings as illustrated on the left) or point contact with rolling (as found in ball bearings). This confirms that the test rig does not accurately simulate real-world contacts.

 

Regardless of this the Four-Ball Test carries considerable weight as a development tool. It is inexpensive and repeatable and can give tribologists and lubricant developers an idea whether their formulation is directionally correct. However, as a tool for lubricant selection, one should be cautious and rather look at other test procedures to evaluate the likely performance of candidate products. A possible alternative is the FZG Test which we will discuss in the next issue of OilChat.

If you have any questions about the Four-Ball Test (or any other lubricant related issues) in the interim, you are welcome  to phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.

 

 

 

 

 

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Timken OK Load is a measurement that indicates the possible performance of extreme pressure (EP) additives in lubricants. In the Timken EP Test a standardized bearing race is rotated against a steel test block, as shown on the left. The contact area is flooded with the grease or oil that is being tested. Increasing loads are placed on the test block until a score mark is made on the test block. The load immediately before the score mark is made, is reported as the Timken OK load.  The units of measurement are pounds-force or kilograms-force.

When reviewing the technical data sheet for a lubricant or the lubricant specifications for a piece of equipment, you will often see reference to the Timken OK Load Test. So what does it mean when one lubricant has a 35 pound Timken OK Load rating and another has a 40 kg Timken OK Load rating? The truth is … it may have very little relevance to the amount of EP protection you are getting.

There are basically three reasons for this:

1. The Timken Test Machine was designed by the Timken Bearing Company in 1935 and manufactured until 1972. Timken used the machine to confirm that extreme pressure chemistry in the cutting oils they were using (to manufacture bearings) was still working. It was never designed to quantify performance in any terms other than pass or fail with a 35 pound load applied. Large wear scars or scoring represented a fail and smooth scars were a pass. The test was later adopted by the lubricants industry when Timken began selling the machines. The American Society for Testing and Materials published the ASTM D-2509 procedure for testing greases and ASTM D-2782 for testing oils. From that point on manufacturers started competing for bragging rights to the highest Timken OK Load rating. The Timken test became a marketing tool rather than a simple indicator of the presence of EP additives. The Timken Bearing Company attempted to clarify the misunderstanding about the test with the following statement,“It was generally assumed that the higher the O.K. value, the more load the lube could hold without the film strength being compromised. However, this is not necessarily the case, and the primary purpose of the test is to determine whether or not the lube has an EP additive. Values higher than 35 lbs. indicate the presence of an EP additive.”
2. There are certain EP additives that can be added to lubricants to give very high Timken results. However, no correlation has ever been established between these extraordinarily high Timken OK Loads and actual field performance. In fact, some of the additives that produce such high Timken OK Loads can be corrosive and/or hazardous and may damage the equipment they are formulated to protect.
3. In terms of scientific testing, the Timken OK Load test has very poor repeatability and thus a wide margin of error. In fact, the official ASTM D-2509 Timken test procedure states that the results from one test to the next of the same lubricant on the same Timken machine can vary as much as plus or minus 23%. This means a lubricant that gets a pass rating at 60 pounds could actually be anywhere from a 46 pound load to a 74 pound load. If the same product is tested on a different machine, the rating can be off by plus or minus 59%, which means it could actually be anywhere from a 25 pound load to a 95 pound load. The poor repeatability of the test makes it possible for a lubricant manufacturer to repeat the test until the highest result is found and claimed. There are also many modified versions of the Timken machine used by some marketers to “demonstrate” load carrying properties. These are not accepted tests for determining EP properties as the results can easily be manipulated.

Most lubrication engineers and industry professionals agree that the Timken Test should only be used to indicate whether EP agents are present without implying anything about relative performance levels. Most equipment manufacturers now specify more relevant tests that have proven direct correlation to actual field experience (such as the Four Ball EP Weld Test, the  FZG Gear Tests and others) to indicate precisely how much EP and anti-wear performance they require of a lubricant.

So, how much EP is enough? What if grease “A” quotes a 100 pound Timken OK Load pass rating and a 400 kg 4-Ball Weld Load and Grease “B” quotes a 60 pound Timken OK Load pass rating and a 800 kg 4-Ball Weld Load. Which one would perform better in extreme pressure applications? The Timken OK Load ratings tell you that both greases have EP chemistry, but the 4-Ball Weld tells you grease “B” (even with the lower Timken OK Load rating) has the more effective EP chemistry.

In summary, do not let extreme Timken OK Load ratings over-influence the selection of lubricants. Actual field performance, other EP and performance tests (such as corrosion protection and oxidation resistance) may have far greater relevance in many applications.

Q8 offers a comprehensive range of extreme pressure lubricants for a wide variety of automotive, construction, industrial, mining, agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.

Couplings are used to join two shafts to transmit rotation and power from one shaft to the other. A typical example is the coupling between an electric motor and water pump as shown on the right. In an ideal world components can be installed in perfect alignment. Rigid couplings are generally used in applications with precise alignment.

In the real world, however, alignment is seldom perfect. Misalignments can be one of several fundamental types, including lateral, axial, angular or skewed/angled. The greater the misalignment the less efficient the motor is in generating speed and torque. Misalignment also contributes to premature wear, broken shafts, bearing failure and excessive vibration.

Flexible couplings are used extensively to compensate for misalignment between components. These couplings range from simple designs (e.g. an elastic element between two shafts) to intricate couplings such as constant velocity (CV) joints. The following metal-to-metal flexible couplings are commonly used in industrial applications:

GEAR COUPLINGS compensate for misalignment via the clearance between gear teeth. Shaft-mounted external gear teeth on both shafts mate with internal gear teeth on a housing that contains a lubricant. Another design mesh external teeth on one shaft with internal teeth mounted on the other shaft.

 

CHAIN COUPLINGS operate similarly to gear couplings. Sprockets on each shaft end are connected by a roller chain. The clearance between the components, as well as the clearance in mating the chain to the sprockets, compensate for the misalignment. Loading is similar to that of geared couplings.

 

 

GRID COUPLINGS use a corrugated steel grid that bends to compensate for loading induced by misalignment. Grooved discs attached to the ends of each shaft house the grid, which transmits torque between them. Low amplitude sliding motion develops between the grid and grooves as the grid deforms under load, widening in some locations and narrowing in others over each revolution.

Unless specifically mentioned by the manufacturer, metal-to-metal couplings are generally grease lubricated. Coupling components are protected primarily by an oil film (which is released from the grease) and seeps into the loading zone to lubricate the metal contact surfaces. Greases formulated with high-viscosity base oils, anti-scuff additives and metal-wetting agents are recommended to overcome the boundary lubrication conditions that often exist in flexible couplings. High oil viscosity also reduces leakage rates.

Centrifugal forces in flexible couplings can be severe, becoming even more extreme as the rotational speed and the diameter of the coupling is increased. Even moderately sized couplings can generate centrifugal forces thousands of times greater than gravity (commonly referred to as G-force).  Centrifugal forces have a centrifuge effect on the grease inside flexible couplings. If a general purpose grease (in which the thickener is of higher density than the oil) is used, the thickener and the oil may separate – similar to a cream separator that splits the heavier milk from the lighter cream. One problem with separation of the oil and thickener is that the oil will tend to leak out of the coupling. A much greater problem, however, is that the thickener which is separated out, is moved by centrifugal force to the outer part of the coupling and against the torque transmission elements (e.g. the gear teeth in a geared flexible coupling). The thickener coats the transmission elements and keeps the oil component of the grease from lubricating them.

Blue Chip HSC 350 High Speed Coupling Grease is specifically formulated to resist separation of the oil, even under the high centrifugal forces encountered in couplings. This ensures reliable coupling lubrication over extended periods, even during high speed operation. For more information about HSC 350 High Speed Coupling Grease and the complete range of Blue Chip greases, phone 011 462 1829, email us at info@bcl.co.za  or visit www.bcl.q8oils.co.za.

Worm gears have been used for many centuries. In fact, their existence was described by Archimedes around 250 BC. Worm gear drives are different from all other types of gears.  They consist of a spirally grooved screw that engages with and drives a toothed wheel (the worm wheel or gear). By nature of their design worm drives can achieve large reductions in speed in a compact space. It changes the rotational movement by 90 degrees, and the plane of movement also changes due to the position of the worm on the worm wheel as shown on the right.

The input power (usually from an electric motor) is applied to the worm gear. The rotation of the spiral ‘screw’ on the worm pushes the teeth of the wheel forward and rotates it as depicted by the  animation on the left. A worm gear set can have a massive reduction ratio with little effort.  Worm drives normally consist of a brass or bronze wheel and a steel worm. The wheel is designed to be sacrificial because it is normally cheaper and easier to replace than the worm itself.

Worm gear drives have several advantages over other gear types. The two primary ones are:

HIGH REDUCTION RATIO. Worm gears can achieve reduction ratios ranging from 20:1 to 300:1. You can use it to reduce speed significantly and at the same time increase torque considerably. It will take multiple reductions of a conventional gearset to achieve the same reduction level of a single worm gear set. Worm gears therefore have fewer moving parts and less chances of failure.

SELF-LOCKING. With standard gears the output shaft can also turn the input shaft. This necessitates adding a backstop to a traditional gearbox (to prevent reverse rotation) which increases the complexity of the gear set. With worm drives it is practically impossible to reverse the direction of power since it is unlikely that the worm wheel will rotate the worm. The compact size and inability of worm gears to reverse the direction of power (self-locking) render them particularly suitable for many lift/elevator and escalator drive applications. The least complicated form of worm gears is those that are used to tune stringed musical instruments.

There is one particularly obvious reason why one would not choose a worm gear drive over a standard gear set and that is sliding friction. With standard gear types (spur, bevel, spiral bevel, helical and hypoid) the gear teeth slide and roll on each other when they mesh. The rolling action helps to distribute the lubricant on the teeth of the gears. With worm gears the contact is predominantly of a sliding nature. As the worm slides across the teeth of the wheel, it rubs most of the lubricant off. The result is that the worm is in contact with the wheel in a boundary lubrication regime (see OilChat 22).

The high rate of sliding in worm gears results in significant frictional heat build-up. This demands the use of high viscosity lubricants, typically ISO 460 or 680 viscosity grade and even ISO 1000 in severe applications. Traditionally compounded gear oils have been used extensively in worm gears with great success in a wide variety of applications. These are mineral oils with rust and oxidation inhibitors and tallow or synthetic fatty acid (the compounding agent), giving excellent lubricity to minimize sliding wear.

In days gone by gear oils that contain active extreme pressure (EP) additives were not recommended for worm gears. There was a concern that the sulphur-phosphorous EP additives would react with the brass or bronze gear wheels. Modern non-active sulphur EP additive technology, however, has eliminated corrosive attack of the worm wheel. EP gear oils work particularly well when shock loading occurs and also protect steel gears better than compounded gear oils.

Q8Oils offers a comprehensive range of high-quality gear lubricants for a wide variety of automotive, construction, industrial, mining,  agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za  or visit www.bcl.q8oils.co.za.