Can some one explain the definition of rigid- and flexible mounting.
E.g.: a vertical mounted unit on board of a ship, consist out of an electrical motor, shaft bearings and pump, mounted direct on a platform which is welded to the tanktop.-- rigid or flexible and why

Thank you in advance

In my view, there are no or very few rigid mounted machines anywhere. Olov

It appears you have a skid welded atop a tank.
On the skid you have mounted a motor which in turn drives a pump via a shaft or torque tube; correct?

The pump is mounted fixed, level, free of anchor bolt stress and pipe stress. Now for the drive shaft - as you describe it appears to be captured in its own set of bearings - correct?
If so, what type of coupling? It should be flexible and between the motor and drive shaft should also be a flexible coupling.

If the drive shaft has no bearings of its own, then it is treated like a cooling tower setup or long spacer coupling arrangement which is also flexible but with coupling types of U-joint construction or Thomas disc-type configuration to allow some flexibility and accomodate some degree of misalignment.

Thank you, the configuration is like the attached pic. The pump is bolded down to supporting base consist out of vertical orientated steel plates welded to the tank top.
Shaft bearing has ist's own support and the drive shaft is connected via a flexible rubber coupling
How is the unit mounted, flexible or rigid ?

Regards

Sven

Sven,

That motor-pump is probably a flexible foundation. An impact test would prove this. I have worked on a lot of ship engine room machinery like this and found that to be the case. My simple rule of thumb is that a flexible structure has at least one natural frequency below service speed, and a rigid foundation has natural frequencies above service speed. This rule is separate from, but similar to, defining a stiff/flexible rotor. A machine on a seismic base (springs) is intentionally designed to be a flexible foundation.

Walt