Cyclic Vibration b/t Two Motors

Some more information would be helpful.

Basically, I would suggest looking into using vibration isolation at the mounts for each motor. Without knowing the size, etc. I can't get more specific without being irrelevant.

Changing the speeds with respect to each other could help as well. You might be hitting near a resonance mode for one of the motors or the overall assembly.

Check into shaft coupling issues as well. Certain types of shaft couplers that are designed to add tolerance for shaft misalignment can really dampen vibration propagation from a motor to what it is turning and vice versa. These shaft couplers include flexible spiders, helical beam, slotted disc, bellows, etc.

An interesting problem with noise occurred a few years ago on two motors attached rigidly to a skid frame. Both motors were driving pumps, on a duty/standby basis. The duty pump had been running for some time and it was decided to stop the duty and start the standby instead. The standby pump, when on, generated much more noise than the duty pump, even though the duty pump had much longer run-hours.

The cause was determined to be the vibrations from the duty pump causing the bearings in the standby pump to hammer microscopic flat spots in the shaft of the standby pump over time. When the bearings passed over each flat, a degree of noise was generated by the skip/impact of each bearing component at each flat spot.

The moral of this story is: mechanically isolate duty and standby pumps wherever possible so that the one running does not damage the other while the other is stationary.

Great story man! I had a couple of pairs of shoes like that. I swear to God the more I wore the one pair, the less comfortable the other pair got...and they were sitting right next to each other. Is this from the same thing?

In all seriousness, that's a pretty awesome find. My guess on this dudes (or dudettes) problem is in line with the resonance issue. If running the units individually shows no issues (I'm assuming that's been done), then balance and resonance ought to lead you home.

Your analogy don't seems appropiate, I think.

Please be a little calm and benevously.

Thanks

This is a so calld "brinelling" of bearings. This happens many times during the sea transport of cars. 60 years ago when quality of bearing steels was not as high as today the jeeps send to europe presented some times this kind of failure. The liberty ships were not very well isolated with respect to vibrations.

I think that the key word here is "cyclic", which generally refers to the situation where there is a relatively loud noise/vibration which is amplitude modulated with a recognizable period (e.g., somewhere between 1/4 sec - say 5 secs) emanating from the pair of motors/machines.

This type of vibration is called beats and is caused by the interaction of 2 slightly different vibration frequencies which move through a period of reinforcement to a period of cancellation, at a beat frequency equal to the difference between the 2 individual causative frequencies. The typical place where it is commonly encountered by most people is in a twin engine (jet or piston) aircraft where the speed of the 2 motors is not perfectly synchronised. The solution is to either synchronise the frequencies "perfectly" or decrease the vibration from each source using standard techniques.

Sounds ingenious...I think I might have heard that theory somewhere before.

Oh yeah! That would be harmonic resonance! The "beats" you are referring to are the "resonant beat" of harmonic imbalance. This is what butcher and I were referring to.

It is also a method for tuning string instruments (I.E. guitar). You can play the same note at two octaves (on two strings) and listen for the "beat". Turn the tuner till it stops and the pitch is harmonized and your string is now in tune with the other.

Guys,

The two motors are on separate structures and everyone drives a fan. Both are enclosed in one room.

We ran one motor and made the other one OFF. But we found vibaration on the non-running one?

What if we tried to synchronise the two speeds? and how to synchronise?

Kind regads,

The method for synchronization will depend on the type of motors and what control equipment you use.

I highly suggest you call the motor and/or controls manufacturer and ask them for a recommendation before you try anything.

Basically, if they are DC, run them off the same source in parallel. They will split the current and run at the same speed (assuming same loads and that they are the same type of motor). If they are AC, you can run off of the same inverter, but its a little trickier. I've mainly worked with DC, so I'm not sure of details with AC.

http://www.controldesign.com/articles/2002/235.html is a decent discussion of how to sync different types, and should at least give you a place to start.

Alternatively, try adding vibration damping at the motor mounts. There are lots of ways to do this; look up 'vibration' on www.mcmaster.com and look at 'about vibration control mounts' to see what methods might be practical. Or, look into running both fans off of one motor with a belt system. You might have to get a larger motor for that; check into the torque loads.