Brake rotors are made from castings. Any casting that has surfaces that are not machined, such as the cooling vanes inside of a vented rotor, are subject to imbalance. An imbalance is when one side of a rotor is heavier than the other side.
Balancing is the process of reducing the unbalanced force (mass) by removal or addition of weight to a brake rotor so that this effective mass centerline approaches the true axis. You may have noticed a machined notch on the rotor surface.
Rotor balance reduces noise, vibrations, pedal pulsation, and allows a more linear work. The first signs of malfunctioning are when the center of the rotor at speed moves out from its rotational axis, causing an imbalance.
Types of rotors
There are two groups of rotors.
The first one is rigid, where the rotor doesn’t deflect up to and including the operating speed. Rigid rotors can be balanced at a low speed, just enough to produce a centrifugal force to register the imbalance.
The second is flexible rotors that “bow” up to the operating speed. This type of rotor is balanced at a low speed where the rotor does not flex. Correction for the imbalance is made, then the speed is gradually increased, and the imbalance is corrected in stages until the rotor’s operating speed is reached.
Types of unbalance
There are three types of unbalance:
1. Static unbalance. This type of unbalance, also known as force unbalance or kinetic unbalance, occurs when the principal axis is moved out of position to the shaft axis. The unbalance is corrected only in one axial plane.
2. Couple unbalance. It occurs when the principal axis intersects the shaft axis at the rotor’s center of gravity. The imbalance is usually corrected in two planes.
3. Dynamic unbalance. This type of unbalance occurs when the central principal axis and shaft do not intersect or align parallel to one another. This usually unbalances a combination of static and couple imbalance and is corrected in two planes.
Balancing is done using the horizontal and vertical balancing machines. The rotor is mounted on the machine bearings, and the machine spins the rotor. Soft bearing machines measure the displacement of the ends of the rotor and bearings. The machine measures this displacement and the phase angle, then computes the unbalance present. A balancing machine pinpoints the exact location and mass of any unbalance, so that excess material can be removed by drilling, milling, etc. from a “heavy” position on the component, or added to a “light” position by bolting or welding on balance weights. These machines are so sensitive that they can easily and accurately identify any mass axis 0.001mm off the running axis.
Some types of these machines can only identify static unbalance, for example, for balancing disc-shaped parts. Some of them will locate unbalances in two axial planes, for example, or balancing a rotor whose length is proportionally higher than its diameter.
Balancing also has limits, where the unbalance is acceptable. International and national standards are quoted for rotors, for example, car wheels are balanced to a limit of grade 40, and small electrical armatures are balanced to grade 2.5. According to ISO 1940 standards, the grades are converted to unbalance units, depending on the rotational speed of the rotor.
Due to advanced technologies, balancing is an easy task. It is applicable in every machine shop. Remember, balancing is essential for a safe drive!