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Posts: 1,810
Reply with quote  #1 
At one of my sites I monitor a bunch (23)of  two- and three-stage IR Centac air compressors.  These (to me) are fairly large units, and all motors are C-face mount to the compressor.  But they all have at least some support of the outboard end of the motor -- not a lot of support by any stretch of the imagination, but at least some.

Except for the last 2 installed that is.  They have no support for the motor.  One of the 2 has a strong, dominant vibration at 120 Hz which I have found to usually be frame distortion.  These are 600 hp motors.  My thinking has always been that even if this 120 Hz vibration is "stable" it can't be good for the windings to be exposed to 4 billion cycles per year at 0.1 in/sec

Is this mounting arrangement common?  Did they just decide to save some money and forgo the option (?) of the external motor support?  Haven't discussed it with them yet as I wanted to know what you guys usually see.


"The trend is your friend"
Dave Reynolds

Posts: 53
Reply with quote  #2 

Monitored 900HP for 8 years, same type of c face motor arrangement, vibration was more at 1x 60hz not 120hz. Is the 120hz in one axis or all axis of measurement?

Couple things catch my eye when I zoom in on your is it the floor, concrete and majority of the grout be spotless but yet the steel base is a mess? I also see some flat strap that is welded at 45 degree angle in at least two spots like someone added them as by means to make a change? They don't look like they are factory installed. I also see some rust coming out from under steel base in bottom left corner, this would tell me the base may not be making good contact with grout.

I do not see a bunch of anchor bolts holding base to concrete either, was machine leveled with carpenter square or machinist level? SS shims at each base anchor to level the machine?

Have you taken any vibe data on the base next to anchors/floor and see if same 120 hz is there also? Tapped the base for hollow spots?

Walt Strong

Sr. Member
Posts: 889
Reply with quote  #3 
I would be more concerned that there is no outboard motor base bolts or any support under that location! If the motor and compressor is a C-face arrangement, then was the design intended for the compressor to support the motor, or the motor to support the compressor?

Dan Timberlake

Sr. Member
Posts: 116
Reply with quote  #4 
Well, several days back Walmart had bunch of XMas stuff on display, so maybe it is not too early for this reference to one of my favorite preliminary tests.

Substitute "every machine base/ foundation interface" for "nose".

In my book, Any detectable motion, especially differential motion, strongly suggests the installation ain't right.

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Posts: 48
Reply with quote  #5 
I have seen this before Rusty, 600HP 2Pole REB with D flange mounting, motor feet are about 40 inches above the floor...

We have had this units motor rotor rebarred (just shy of 2 years ago) due to a suspected rotor bar issue (before our shop had the tools to properly ID a true rotor bar integrity issue)

From time to time the customer calls and complains of a significant audible beat, I did convince them to install some legs/stilts under the ODE feet as a band-aid and typically when the legs are tight up to the feet the beat disappears, but it has returned recently and I suspect the ODE housing is loose again (bored and sleeved during the rebar event) and we now have a significant eccentricity issue more than it is a rotor integrity issue...It is marked for removal so we will see when it comes in to the shop what we really find...


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