# Using Multiple Motors to Increase Vibration Strength

There are several ways to increase a motor’s vibration strength, such as increasing the motor speed, the eccentric mass, or the mass’ distance from the motor shaft. Unfortunately, most of these are unavailable to our customers as they are factory defined.

However, it is possible to use multiple motors to increase overall vibration strength.

When two motors of the same speed are aligned so their shafts are in the same direction, the rotation of the masses will eventually synchronise. This has the same effect as combining the masses into one motor, and the result is the sum of the two motor’s vibration amplitudes.

Two aligned motors with their shafts facing the same direction so the rotation of the masses will eventually synchronise

This is possible because the two motors, even if they are the same type and batch number, will have slightly different speeds. Eventually, the faster motor will catch up with the slower one. Once their masses synchronise it requires an extra bit of energy for the faster motor to ‘jump’ out of sync. This energy is unavailable as the motors are driven at the same (rough) speed, and continue to turn together.

If two motors of the same model are used, it is equivalent to doubling the mass. The formula for vibration strength (for more info see AB-004: Understanding ERM Vibration Characteristics) is given as:

$$F_{0} = mr\omega ^{2}$$

We can see that doubling the mass doubles the force produced $$F_{0}$$.

This behaviour works from cylindrical ERMscoin vibration motors, and linear resonant actuators (although based on the same principle, the formula is different). Remember for coin motors the motor shaft is vertical!