Could any one tell me the maximum bearings can be spaced on a 2 3/16" shaft. Or where I may find this information. Thanks in advance.

It depends on the load. You can find the calculaions in the engineering sections of many power transmission manufacturers catalog-Falk, Martin. Dodge, etc.

Hi Friend,

As Danny wrote, it depends on the load.

However, are you suggesting that you intend to place 2 bearings adjacent to each other and you are asking about the space that should be allowed between them ?

I am not catching the full extent of your message, I am afraid. Tell us a little bit more !

Marko Leo

I have a shaft that is aproximetly 30 ft long that is driving a 6ft diameter aluminum propeller fan they have 4 bearings on them now and the overall vibration is .45 in/sec I have 16 of them. I have 2 with five bearings on them that the overall vibration is .10 In/sec. My question is would it be possible to space the 4 bearings more equally and get the same effect.

quote:

My question is would it be possible to space the 4 bearings more equally and get the same effect.

How are the 4 bearings spaced in comparision to the one with 5 bearings? ie: distance from the propeller, coupling, center?

Does the 14 with the 4 bearings all have a .45 in/sec vibration? Is this vibration on all the bearings or what?

How is the shaft being driven? Direct or belt? Are the bearings closest to the fan vibrating more than the others? How fast is the fan shaft turning? Is the overall number mainly 1X of the shaft? I'm assuming the numbers you gave are from a radial plane. How do the other planes look? Lots more questions, huh?

To answer one of your questions, though, I have a little program called Engineering Power Tools. It will figure the critical speed of an overhung load on a shaft. But a 30' shaft with just four bearings sounds like they're about 6' apart. That sounds like it could be a problem when it comes to the shaft whipping around.

Is it possible to do modal analysis on these shafts?
It will help when compare the results gained for 4bearing and 5bearing shafts.

This shaft is belt driven and turns 1060 Rpm. The vibration is on the bearing next to the motor the Horizontal is .45 In/Sec the vertical is .27 In/Sec and the axial is .28 In/Sec. I do not have the set up to do a modal analysis. The bearing spacing now is akward it has been this way for 10+ years. The bearings are spaced from the sheave 1ft 4ft 14ft and the last one is 6 inches from the propeller. These fans were balanced and the vibration averaged overall .20 In/sec. The operators re pitched the fans to get maximum air flow and the vibration went back up. They say they have every blade pitched the same but they will be down Monday so I will do some more investigating on this issue myself. Thanks

How big is the sheave? Was it balanced? Sounds like the problem is on that end. Do you have a dual-channel collector or a phototach to get some phase info? Has this been a problem for 10+ years? Good luck with the investigation.

Repeat of an earlier question,

You say

quote:

I have 16 of them. I have 2 with five bearings on them that the overall vibration is .10 In/sec.

Was the pitch changed on all the fans and are all the fans vibrating in the same way on the same bearing or are we talking about 1 fan only?
With the high vibration being next to the sheave and there being 2 bearing 3 feet apart at this location, what is the phase between the bearing?

Seems like if there was a location problem with the bearings, the amplitude would be greater somewhere closer to the 14 foot spacing, unless of course, as an ole expert puts it, "the tail is shaking the dog".

There is a reason why this is not a common configuration. When you add the third bearing on one shaft the alignment has to be perfect, otherwise the load will not be evenly distributed and the bearings will wear out quickly and probably run hot. For this unit I would probably cut the shaft in the middle and install a flexible coupling, then install 2 bearings on each shaft near the shaft ends. This would have the added benefit of raising the critical. Of course, one bearing on each shaft will need to be fixed and the one on the fan shaft will need to be capable of carrying the thrust load.

This message has been edited. Last edited by: Rick Little,

Each shaft section will have a series of critical speeds and the overhung fan will also have critical speeds dependent upon the position of at least the first two adjacent bearings. Those are some things to check before you modify the system.

If the system has run well in existing configurationt in the past I would agree with lawrence it's worthwhile to first look carefuly at sheave balance, sheave alignment and what changed.

When the system is in operation the only bearing that can be monitored is the one next to the sheave the others are inside a kiln,but when the fans were balanced I used the 2 bearings closest to the fan and some were much higher than the outside bearing then. Could the shaft be flexing when the system is in operation? The pitch was changed on all of the fans. During my investigation I discovered that all the blades on each fan were not set the same. Could this make a difference? The sheaves were supposed to be balanced when they were installed the alignment was the first thing I had checked, and the fan pitch is all that has changed.

I noticed that you said production changed the pitch. Did the ones doing it know what they were doing?
Have you said where the highest amplitude is (frequency) rather than just the overall amplitude?

The frequency is at 1XTS of the Fan Shaft. Sorry I forgot that part. The guys that changed the pitch have been on this job for 8 years and should have known. They used the degree marks on the fan hub to adjust the blades but the angle finder that I used does show some variances in the indvidual blades.

Ok I now have some phase data on the outside bearing. Vertical to Horizontal 20 degrees. Axial to vertical 0. Axial to horizontal 0. Axial to axial on the face of the bearing on each side of the shaft is 180.

Pitch angle should be no greater than +/- .2 degrees between blades on any one wheel. Anything greater than and an increase in amplitude at BPF and 1x will be seen. Attempting to trim balance a pitch issue usually doesn't work so well. I have worked on a number of kiln fans like this and usually find the problem to be within the alignment of the bearings.
Hope this helps,
Pat

When you say the bearings are in the kiln, how hot is it in there? Are all the kilns the same temp? Are any kind of allowances made for the thermal growth? With a 14' span, you may be bowing the shaft--along with bearing alignment.

275 Degrees. There is allowance made for thermal growth.

180 degree phase change on the face of the same bearing? If so, with the spacing as said, wouldn't that lean toward shaft whip or bow? Shaft bow between the bearings will present while running as runout outside of the bearings. In the ten years of service, how long have you been collecting data on the fans?