Drive train Make your robot move!

Friction and Traction Friction is the force that opposes motion when two surfaces rub together Friction is a reaction force If an object has no forces causing it to try to move, there can be no friction No applied force, no reaction force! Friction comes in two forms: Static and Kinetic

Friction As the applied force increases, the opposing frictional force also increases Up until the mass starts sliding, the frictional force is static friction Once the applied force exceeds the maximum static friction the mass will begin to move After the mass begins moving kinetic friction acts upon it

Try it! It is easy to duplicate both types of friction Push the palm of one hand against the palm of another and trying to move them in a sliding motion Before they start moving, friction caused by the texture of the skin resists the motion. This is static friction. As the sliding force increases, the hands begin to slide and move relative to each other. This is kinetic friction Note how after the hands break loose from static friction, it takes less force to keep them sliding.

Calculating Friction There are two factors which determine the maximum frictional force which can occur between two surfaces: How “grippy” the surfaces are (known as the Coefficient of Friction of the surfaces) How hard the two surfaces are being pushed together (known as Normal Force). The maximum Force of Friction (Ff) between two surfaces is equal to the Coefficient of Friction (Cf) of those two surfaces multiplied by the Normal Force (N) holding those surfaces together. Maximum Force of Friction = (Coefficient of Friction) x (Normal Force)

Coefficient of Friction A coefficient of friction is a constant that describes the “grippyness” of two surfaces sliding against one another This is not function of a single surface, but of two surfaces.  For example, a tire on its own has NO coefficient of friction, but a tire sliding on pavement DOES have a coefficient of friction. Slippery objects have a very low coefficient of friction while sticky objects have a very high coefficient of friction This constant is determined for a pair of surfaces (not a single surface.)  Each pair of materials will have a coefficient of static friction, and a coefficient of kinetic friction.

Normal Force The force which presses two sliding surfaces together is referred to as NORMAL FORCE.  This normal force is always perpendicular to the two surfaces (if an applied force is not perpendicular to the two surfaces, only a portion of it will act as normal force.)  Often the normal force acting on two surfaces is simply the weight of one object resting on the other; in this case the normal force is caused by gravity.  As seen in the above below, when an object is on an inclined plane, gravity is not acting perpendicular to the sliding surfaces. In this case only a portion of the object’s weight would act as normal force.

Traction TRACTION can be defined as the friction between a drive wheel and the surface it moves upon. It is the amount of force a wheel can apply to a surface before it slips A wheel will have different traction on different surfaces; as described above, the coefficient of friction is based on pairs of surfaces.  The tractive force is equal to the frictional force between the wheel and the ground If the wheel is rolling along and not slipping, it is equal to the static friction If the applied force exceeds the maximum static friction then the wheel will start to slip, and now the tractive force is equal to the maximum kinetic friction.

Traction As seen in the diagram below, when a torque is applied to a wheel, it applies a force along the ground Imagine that the wheel was spinning on ice. The wheel would slip and would not move forward The friction between the wheel and the ground is necessary to make it move forward, this is the tractive force.

Increasing Traction Since traction is dependent on the friction between the wheel and the surface, to increase traction one must maximize this friction. How can we do this? To increase traction, one must either increase the coefficient of friction (grippier wheels) or increase the normal force acting on the wheel (heavier robot, or more weight on drive wheels).