Traction (engineering)
From Wikipedia, the free encyclopedia
refers to the friction between a drive member and the surface it moves upon, where the friction is used to provide motion.
For the purposes of driving a wheeled vehicle, high friction is generally desired, as it provides a more positive connection between the driving and driven members. In contrast, motion in a geared mechanism is provided by interference, and friction is usually detrimental because the gear mechanism has intrinsic sliding, and sliding under friction causes heating losses.
Traction between two surfaces usually depends on several factors including
- Material properties of each surface.
- Macroscopic and microscopic shape or "roughness".
- Force of contact.
- Area of contact.
- Contaminants at the material boundary including lubricants and adhesives.
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[edit] Formula for friction
A common approximation is F=μFN. Here, μ summarizes material properties and roughness and is called "the coefficient of friction". FN is the normal force, which is applied perpendicular to the contact. In a simple system in equilibrium, such as a mass sitting on a surface, the normal force is equal to mass multiplied by gravitational acceleration (FN = mg). The statement of this expression is that friction is directly proportional to the intrinsic friction caused by the materials and contaminants; and that friction is also directly proportional to the force. In practice, this is an approximation but in many situations other factors, e.g., the area of contact, play a role.
[edit] Friction trade-offs
In most applications, there is a complicated set of trade-offs in choosing materials. For example, soft rubbers often provide better traction but also wear faster and have higher losses when flexed -- thus hurting efficiency and sometimes causing early failure due to heat build-up. Subtle choices in material selection may have a dramatic effect. For example, tyres used for track racing cars may have a life of 200 km, while those used on heavy trucks may have a life approaching 100,000 km. The truck tyres have worse traction and also thicker rubber, but the race car tires cannot simply use thick rubber without compromising weight, heat build-up, and so on.
Traction also varies with contaminants. A layer of water in the contact patch can cause a substantial loss of traction. This is one reason for grooves and siping of automotive tyres: most water must be displaced from the contact, but inertial effects limit the speed with which it can be displaced. Grooves hurt dry traction but reduce the distance the water travels to escape. Note there are applications where the distances are already short, for example bicycle tires have a narrow and pointed contact and so even slick tyres give good traction on a wet pavement. Where the roadway surface is substantially flexible or malleable, tread can also form divots in the road, leading to interference-type traction (as in gears) rather than friction.
Traction applies across a wide variety of materials and scales. For example, railroad locomotives use steel wheels on steel rails to provide traction; slot cars use rubber on plastic; and so on.
[edit] Traction boundary condition
Particularly in the context of the finite element method, a traction boundary condition is a portion of the boundary of a body for which forces—tangential, normal, or both—is prescribed. See also Navier-Stokes equations.
[edit] Traction forces in a system
The traction force is given by:
- Traction Force = Driving Torque/Radius of Wheel.
Using conservation of energy, we are aware that F=ma and hence P=Fv or rate of work done. In order to calculate power:
- PE = dTF / dt + dPL / dt
where Pe = Efficient Power, PL = Power Loss during mechanical conversion, and TF = Traction Force.
[edit] Maximizing multi-wheeled vehicle traction
It is important due to broad application to point out the specific case of multi-wheeled vehicles or vehicles with multiple contact patches between the tyre and the road surface. The constant coefficient of friction approximation is not adequate to describe real world maximum traction situations. If the normal force is increased, per given area of contact patch, the coefficient of friction decreases and as the normal force decreases, the coefficient of friction increases. If this were not true, then increasing tyre width, lowering tyre air pressure or increasing tyre diameter (all of which increase the area of the contact patch) would have little effect.
The importance of having a coefficient of friction with the above properties has significant implications in multi-wheeled vehicle handling. The case of two wheels sharing a given normal force is particularly important in vehicle design. Two identical tyres sharing a common load achieve maximum traction when they share the load equally. Likewise, an unequally loaded pair of tyres sharing a common load will not be able to achieve the same maximum traction. Consider the "traction pair". The less laden tyre’s coefficient of friction has increased but it’s load has decreased resulting in a modest drop in traction. Conversely, the heavier laden tyre’s coefficient of friction has decreased and even though it’s traction has increased, it is not enough to make up for the drop in traction of the less laden tyre. Put another way, when unbalanced, the heavier laden tyre’s traction increases less than the less laden tyre’s decrease in traction.
A vehicle has balanced or neutral handling when the front and rear pairs of tyres achieve maximum traction proportional to the normal force on each pair of tires. Example: If 60% of a vehicle's total normal force is at the front of the vehicle, then 60% of the traction should also need be in the front for balanced handling. This can be achieved by a number of means. Achieving balanced handling is non-trivial due to the dynamic forces involved such as changing corner radius, bank, braking, acceleration, aerodynamic loading and coefficient of friction changing factors such as road surface debris, moisture, temperature etc. Automotive engineers attempt to minimize the effect of non-linear forces as much as possible in order to simplify design considerations.
