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potential efficiency saving, all moving componentry on a typical automobile is now being scrutinized for friction and energy losses. On a typical passenger car or light truck, there may be as many as 50 different greases used to lubricate components. The majority of these have no influence on the efficiency of the vehicle. An example of this would be the grease to lubricate the driver’s seat rail. However greases used in the wheel and accessory equipment bearings, steering, and transmission components all have been shown to influence efficiency. Transmission components include support bearings, universal joints (UJs) and constant velocity joints (CVJs), which transmit power and motion from the gearbox to the wheels.
At the 2013 NLGI annual meeting there were three presentations (2), (3), and (4) focussed on the efficiency of industrial and automotive lubricants. The first paper (2) focussed on methods of testing and the development of low friction greases. The second paper (3) looked at methods for determining the performance of energy efficient industrial gear oils. The third paper (4) investigated the influences on the efficiency of rolling element bearing greases. Many of the same measurement and assessment techniques were employed in all three papers. Both of the two papers focussed on grease (2) and (4) examined different aspects of the role that grease plays in respect to energy efficiency.
Yamamoto and Imai (4), as part of their development of energy efficient greases, reported that aside from the losses due to the sealing system, there are two main contributors to energy losses in deep groove ball bearings: churning through the grease; and viscous rolling resistance. In this type of ball bearing, the centre of the ball on raceway contact sees primarily a rolling motion but is a mixture of sliding and rolling on the edges of the contact patch. The ball slides against the cage pocket wall, but in these contacts the stresses are comparatively low and have little measurable effect on efficiency. Deep groove ball bearings are the primary design used in electric motors, driveshaft support bearings and accessory drive components.
Wheel bearings are a different case. Because that they have to support both radial and axial loads, deep groove ball bearings cannot be used. In many North American vehicle applications, a pair of opposed tapered roller bearings are used as wheel bearings. In other markets such as Europe, an opposed pair of angular contact bearings are used. In both types of bearing set ups, there are friction losses due to sliding contacts of their rolling elements against the cage, and the cage against the raceways. The sliding speeds, lubrication, and contact conditions are different, which leads to varying energy losses and efficiency.
According to Bowden and Tabor (5) the coefficient of friction in a sliding contact is an order of magnitude greater than for a rolling contact. If the amount of sliding in a contact could be reduced, it would contribute to lower frictional losses. Along with the current pressures of better durability, reduced size and weight, improving frictional efficiency of components is another significant challenge to designers. The other way to reduce sliding frictional losses is to reduce the coefficient of friction of the lubricant. In the 1920s, friction modifiers were first investigated (6) as a means of lowering friction under boundary lubrication conditions in liquid lubricants. In 1963, the first organic molybdenum friction modifiers were developed (7) but not widely used until the 1980s for low friction greases (8). These low friction greases were initially developed to reduce the noise, vibration and harshness (NVH) profile of plunging CVJs in front engine, front wheel drive (FF) transmissions (8). Similar organic molybdenum additives were later applied to improve the fuel efficiency of crankcase oils but now more focus is being given to their application to improve energy efficiency of automotive and industrial greases (2).
Friction
Friction is one of the most important forces in nature, but it is not fully understood by scientists. It is the reaction force that opposes the motion of two bodies in contact (5). Friction itself has been unfairly labelled as a negative force as it reduces the useable energy of machines and converts it to heat or vibrations. There are many influences on the energy efficiency of componentry, friction losses being only one source. Friction also has its benefits. Frictional forces keep people from falling over when they walk by preventing slip between the soles of their shoes and the ground. It has been widely reported that friction is the enemy of energy efficiency but this is an over-simplification. Friction losses in the driving mechanism of automobiles account for approximately 25% of the energy losses, but without friction the wheels would slip and the vehicle would be undriveable. Neither clutch mechanisms nor brake systems would function.
Leonardo Da Vinci is known to have studied friction 500 years ago. The first laws of friction were devised by G. Amontons (1699) later modified by C-A de Coulomb (1785) and L. Euler. Amontons’ first law of friction states that “The force of friction is directly proportional to the applied load” and his second states that “The force of friction is independent of the apparent area of contact”, (6). One of the basic assumptions made is that the contacting materials are perfectly rigid and inelastic, which is clearly not true for real world applications. A good example being automobile tires, where increasing the width provides increasing traction. Friction is dependent on the real area of contact which is different from the apparent area of contact. There are essentially two types of friction: static and dynamic (3). Static friction, also called adhesive friction, is the component of resistance to motion as the contacting bodies try to move from rest. It is not the same as inertia.
Dynamic friction, also called kinetic friction in many texts, is the component of resistance to motion of the contacting bodies as they move relative to one another. Both need to be considered and understood in order to reduce the friction between the surfaces. Both play a significant role in friction measurements determined using the SRV test. Static friction is caused by the interaction of the surfaces trying to start moving across one another and has two components: interfacial adhesion and roughness interaction. Dynamic friction is caused by the interaction of the surfaces moving across one another and is made up of three components: interfacial adhesion, roughness interaction, and the shearing of the lubricant or surface layer separating the contacting loaded surfaces. Coulomb’s law of friction states that “Dynamic friction is independent of the sliding velocity.” For dry sliding without interfacial transfer this can be true but for most lubricated contacts this is not true.
The friction coefficient is typically defined as the ratio of the normal force to the tangential force and is dependent on the local tribological system. In the case of an object being pulled along, the normal force is the weight of the object.
Figure 1. Tribological system Continued on page 16
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LUBE MAGAZINE NO.129 OCTOBER 2015
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