In order to balance the first critical speed two equal masses, which are in phase on the rotor can be used. Without destroying the balance quality, which was reached on the first critical speed, when the second critical speed is crossed, the rotor can be balanced by means of two equal masses which are opposite phase of the rotor.
This is a concept related to the modal balancing of the rotor and a mixed procedure can be used: influence coefficient and modal analysis of the rotor.
