I do have a flu and fever so I do have to ask.
Are there any cases where a critical speed of a motor or in this case a motor/pump assy where stiffening the mount would result in lowering the critical speed of the motor and/or the complete assy? Olov

I don't see how stiffening anything would 'lower' it's natural frequency... won't it always increase the natural frequency?

Thank´s Rusty, that´s what I needed, I was starting to believe that the world gone crazy. Olov

There can be situations where the area to which stiffness is added has a non-flexing (no differential motion, rigid) behavior at the mode of interest. Siffening this area may do nothing more than add mass, which will lower the frequency.

Once the area has become rigid (stiff enough for practical purposes) adding addional stiffeners may actually lower the resonance.

As an example, take a rigid rotor and increase the diameter. The rotor will be stiffer but more massive also. The rotor resonance on resilent supports should decrease although the rotor would be stiffer. It is difficult to add stiffness in most cases without increasing the mass.

I, C, O, rectangle, triangle and many different type of section give you different ratio of mass and stiffness....


Regards,

Well, the current discussion here is how a electric motor tested in a testbench in motorshop, possibly on spring mount but that is unconfirmed, where it is running very well can have 10 times higher vibration when running connected on site and adding shims along the full length of the motor feet reduced vibe levels by 60-70% for a week and then it came back to hi vibe levels. A resonant behaviour is in my view confirmed by knock test, run up/down waterfall and a factor more than 3 btw. horis and vert readings. So the argument is put forward that it is normal for critical speed in electrical motors to drop as they are mounted stiffer. Never heard of that before, so that´s why I ask. I can agree to your argument William but can you figure out a similar case when you only change the mounting that in my view mainly would influence the ever illusive bearing stiffness? I agree, in this case from workshop to connected operation on site there is a lot more we change and that is also what I argue, if we have a critspeed problem, it should be tried to be confirmed by a knock test on the shaft. Olov

If you mount a motor resiliently in the workshop for test and then mount it to a solid base the resonance may raise closer to opperating speed.

The changes after installation are odd.

Thanks, that is also my opinion, to increase the complexity I can add that the mount is supported by 7 m steel columns in backfill and there are waste canals and old foundations in the floor also and there are different suppliers of the machines and foundations to make it really fun topped off by vibration levels are holding back delivery testing of the plant..... Olov

Oli, you do remember Ringhals 3 RCP´s? W stiffened the bearing and expected 24 up to 34 Hz. Result was as nicely predicted by Critspd.exe (Prof. Gunter) sank to 22 Hz. Increased span between nodes yields reduction of critical speed. I would do a calculation, it will reveal the reality well.

Yep, that was my first question before going to the site, if there was a calculation made by anybody, I still have no answer so I guess there is nothing done. But is it normal for standard horisontal blue motors to behave like the W RCP motor? Seeing is believing. From the knocktest on the motor that is spot on, ODS and runup my bet is mounting/foundation resonance but I may be wrong. Olov

If blue means Siemens, they sure behave with the same laws of physics. Have a case with driven machines on both ends, 650 kW / 2985 RPM. Spot on crit. in operation, very unstable levels. In fact they are not aware being blue and are expected to behave. You can take the approx dimensions and download crit sp soft from the web and check with different brg stiffness. Multistage pump and compressor rotors behave exactly the same. If you get to the actual designer, you will surely find calculations. I doubt anyone can afford to avoid a proper calc. today.

Nope, it is the Swedish blue make with the hollow cast feet. With those flimsy feet would fiddling with the shims really make such difference in bearing stiffness so critspeed would move? There are 2 pumps VFD up to like 3500RPM. Run up indicate different peak freq. for them, to me more correlated to floor placement one in the middle and one near the edge, than possible shaft variation. Another suggestion is that this is a typical VFD related problem, I have problems to see any connection to such known VFD problems. Olov