I'm trying to understand how do gyroscopes and cmgs used for navigating keep on spinning even though there's friction. Any engine/pushing mechanism I can think of will restrict the gyro's rotor to a fixed direction and defeat the purpose... how is it done without disturbing it?
I remember reading somewhere (don't remember where) the many design intricacies of the navigational gyroscopes for the appollo missions.
The gimbals where fitted with sensors and delicate electric motors. The slightest motion would trigger these sensors. The electric motors then moved in such a way that a torque arising from friction was nullified as much as possible. Equally important, the torque motors made the gimbals so agile and responsive that they moved as if they had no inertia That way the rate of error buildup could be kept sufficiently low to enable the mission.
As you point out in your question, since the angular velocity of the rotor must be sustained the gimbal axes must somehow provide a reliable electric contact, which surely must give a non-negligable friction. These problems made the navigational gyroscopes very challenging to build.
For the Apollo missions to keep buildup of error under control only 3 gimbals were used, instead of 4. If you use 4 gimbals you can design a configuration that can always be steered clear of a state of gimbal lock The Appollo mission gyroscopes were susceptible to gimbal lock, when setting up a change of spacecraft orientation the crew had to plan ahead to avoid gimbal lock.
Mechanical gyroscopes are obsolete now for navigational purposes. The successors to mechanical gyroscopse are Sagnac interferometers (also called ring interferometers).
Actually, the purpose of a CMGS is to reorient the space-station as a whole.
That is, the purpose of a CMGS is the same that of a distribution of mini-thrusters for adjusting the attitude of the space-station; the CMGS is for muscling the spacecraft around, it's not a navigational sensor.
The advantage of a CMGS over the array of mini-thrusters is that the thrusters will deplete their supply of propellent, whereas the station's solar panels generate the electric energy that the CMGS flywheels need.
Probably an electric motor mounted in the wheel and power passed in through a commutator