One of the most important principles students must learn before they can begin drivetrain design is that of Friction.
FRICTION is the force that opposes motion when two surfaces rub together. It is a reaction force only. It occurs when two surfaces are in contact and a force is applied such they slide along one another. If an object has no forces causing it to try to move, there can be no friction. No applied force, no reaction force.
There are two types of friction: Static Friction and Kinetic Friction.
Static Friction is the frictional force between two objects that are NOT moving relative to each other. It is the initial force that must be overcome in order for things to move. If the force trying to move the object is less than the force of static friction, the object cannot move.
Kinetic Friction is the frictional force between two surfaces that ARE moving relative to each other (sliding along each other).
Once an object has overcome static friction and has started moving, it has kinetic friction acting on it, resisting its motion.
In the above diagram one can see the opposing relationship between applied force and 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. Static friction is greater than kinetic friction, so once the mass begins sliding it takes less force to keep it sliding.
It is easy to duplicate both types of friction by simply pushing the palm of one hand against the palm of another and trying to move them in a sliding motion. This motion will be resisted by the texture of the skin and the magnitude of the applied force. The harder the hands are pushed together, the harder it is to move them. This is static friction.
As the sliding force increases, the hands begin to slide and they are moving relative to each other; now kinetic friction is present. One can note how after the hands break loose from static friction, it takes less force to keep them sliding.
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), and 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)
Ff = Cf x N
COEFFICIENT OF FRICTION:
As stated above, a coefficient of friction is a constant that describes the “grippyness” of two surfaces sliding against one another. Note, that 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.
One shouldn’t confuse pure friction with actual sticky surfaces like tape or high friction coatings that bind to the other surface. These surfaces almost need to be looked at as being joined as one. For instance, tape resists sliding even when there is no normal force, or even when there is a negative normal force.
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 diagram, 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.
As covered in Unit 7, and as seen in the diagram above, when a torque is applied to a wheel, it applies a force along the ground. However, one can imagine that if 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.
Note that 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.
Increasing Traction:
Since traction is dependent on the friction between the wheel and the surface, to increase traction one must maximize this friction. As seen above, the friction between two objects is dependent on the coefficient of friction between them (in this case between the wheel and the surface it drives on) and the normal force (the weight of the robot pressing the wheel to the surface). 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).
Building a Pushing Robot:
In order to build a robot capable of pushing or pulling with great force, the robot requires two things. It requires high traction wheels, and a significant amount of torque driving those wheels. Friction is a reaction force; if there is no applied force there will be no traction. To maximize traction, the torque applied to the wheels needs to be enough to reach the maximum static friction of the wheels.
A car can have all the traction in the world, but if it has a small engine it won’t be able to push or pull anything. This is why small cars can’t tow big trailers or boats.
Friction in VEX:
There are a variety of components in the VEX Robotics Design System which can be used to gain traction, including several types of wheels. Each of these has different characteristics on different surfaces. It is important for designers to experiment and determine which wheel is best for a given application.
Friction between the wheels and the floor is not the only friction relevant on VEX Robots. Friction also acts as a brake on the rotating components of the robot and reduces the amount of power which gets from a motor to its output. The VEX Robotics Design System has several parts designed to reduce friction in a robot design. Metal against metal contact is not desirable in moving systems. The plastic parts such as the bearing blocks, spacers, and washers allow for lower friction contact points.