6 FRICTION
Friction: blessing and curse
When two objects are in contact and one object is moved past the other, a force is created which resists the movement of the objects: it is called friction. Friction occurs in all machines and is usually considered detrimental to the machine’s efficiency, but there are times when it is required for the correct functioning of a machine. Although the effect that friction has on an engineering system depends on the nature of the materials that are in contact, certain facts are known to be consistent for all materials.
In many mechanical design specifications, engineers must bear in mind several principles of friction. These are, specifically:
n Friction is a force that opposes movement between two objects.
n There is always some friction when two touching objects move relative to one another.
n To initiate movement from a standstill, more force is needed to overcome friction than the force required to maintain a constant speed when the objects are already in motion.
n Generally speaking, the smoother the surfaces, the lower the friction.
n Friction can be reduced by lubrication and by good design.
If friction is not taken into account by engineers beginning work on a design, the results are both costly and damaging. Any machine with bearings loses a certain amount of power in overcoming the friction in the bearings, but the friction also causes wear and heat. As surfaces wear, one of two things can happen. Either greater friction is caused by the worn parts, requiring more power to overcome it, or good contact is lost between the surfaces, causing machine failure. Where corollary heat is generated, the components can reach very high temperatures and their material properties can change.
In order to reduce friction, the surfaces that are in contact need to be made smoother. In addition to machining (or polishing), this can be done in two ways. Firstly, a lubricant can be added to reduce the coefficient of friction and so minimize wear and heat. There are many types of lubricants: thick or thin oil;
powders, like talc; solids, such as graphite, and even acoustic lubrication, i.e., sound. Secondly, the components can be designed so that the friction is minimal. A ball bearing, consisting of many balls rolling freely in a groove, is an example of a part designed to overcome friction: the friction of the rolling balls is lower than the sliding friction between a shaft and its housing.
Many examples of components that are affected by friction can be found inside an internal combustion engine. The pistons require lubrication because they slide up and down inside the cylinders. Without lubrication, this sliding would quickly cause overheating and the engine would seize – the pistons might even weld themselves to the inside of the cylinders. Gears and cams too require careful lubrication: components are often subject to pitting as a result of erosive wear, which is a common cause of car engine inefficiency.
The lubrication system also provides oil to the moving parts of the engine. Before being pumped to the relevant parts of the engine, however, oil must be passed through a filter in order to remove dirt which would otherwise create friction and cause wear to components. For the same reason, frequent oil changes with the machine at rest are necessary to prevent dirt causing damage.
Not all friction is harmful to the performance of a car, however. Some parts of cars are deliberately designed to dissipate energy through friction – brake pads and tyres, for example. In fact, most engineering systems usually contain a mixture of parts that require lubrication to reduce friction and parts that deliberately wear out, using friction ‘usefully’.
49
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134
