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Intermediate Shaft Bearings

09/04/2010 12:00 PM

In long shaft application, we use bearings between multiple shafts. Why do we need bearings? can we just connect the shafts together by their end couplings? what good does the bearing do?

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Guru
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#1

Re: Intermediate Shaft Bearings

09/04/2010 3:01 PM

You don't provide enough information to say for certain why your company does this. I would suspect though that this has everything to do with flexing and shaft resonance though.

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#2

Re: Intermediate Shaft Bearings

09/04/2010 7:54 PM

Vanuta, google eccentricity.

Regards JD.

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Guru
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#3

Re: Intermediate Shaft Bearings

09/04/2010 8:16 PM

Have you ever seen a well-endowed braless female jogger? Some things need support, or all hell breaks loose.

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#4

Re: Intermediate Shaft Bearings

09/05/2010 12:30 AM

Any shaft which is laid horizonatal between end supports will 'bend' a little. When you rotate this shaft it will set up cyclical vibrations due to this bend. This will directly put cyclical load on the end supports. If frequency of this load happens to match (or come near) the resonance frequency of the shaft (system) it could cause severe damage to end supports. Bearing supports are used to minimize this bend. If the shafts are laid vertical bend due to gravity does not exist. However, it is still a good idea to use a few bearing supports (though less than for the first configuration).

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Guru
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#5
In reply to #4

Re: Intermediate Shaft Bearings

09/05/2010 12:56 AM

"Can we just connect the shafts together by their end couplings? What good does the bearing do?" Imagine that you discard the(se) intermediate bearing(s) and connect the shafts together by their end couplings. These intermediate shaft(s) is (are) held between the flexible coupling and has the freedom to run away from the rotating axis as far as possible. This will produce huge centrifugal force (W/g X w2 X r) and note 'r, the radius of rotation' is a dominant parameter. Other parameter being the same, greater the 'r', greater is this annoying centrifugal force leading to greater 'amplitude' of vibration. With addition of intermediate bearing(s), you would ensure the radius of rotation being as less as the available running clearance of the bearing. Of course, if the intermediate shaft is not well balanced or the drive couplings have excess flexibility (due to worn-out flexing elements like coupling bushes and like), this undesirable centrifugal leads greater wobbling motion and spells on the life of these intermediate bearings.

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#6

Re: Intermediate Shaft Bearings

09/05/2010 2:12 AM

Most of the couplings if you look are not very rigid since they have to account for

a) Slight eccentricities

b) Thermal expansions.

Due to this fact the shafts joined through the couplings will not have the same rigidity as the single shaft would have. Of course the above two features of the coupling are necessary and can not be wished away.

Since you are working with long shafts the above factors will be more prominent especially the thermal expansion gap to be provided on the coupling faces.

Due to the above factors the shafts now won't have the required centerline axis on the side of the coupling and hence will wobble even if the shafts as well as the bearings are balanced.

This wobbling is avoided by putting the aditional restraint in terms of the bearing.

There is one additional point in terms of manufacturing. Suppose you have the assembly in 3 modules. The shaft in module-1 having one side bearing (say drive side) module-2 won't have any and the module 3 will have on non drive side.

How do the manufacturer assemble the modules with the shafts hanging one one or both sides? After all a lot of checks will be made putting the shaft in actual position and the equipment modules will be tested thus.

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#7

Re: Intermediate Shaft Bearings

09/05/2010 6:50 AM

The reason for the need of bearings has been explained........to prevent shaft whip.........this will still occur if shaft droop (overhang of shaft and a coupling from a bearing) is not taken into account........this means bearing placement and alignment of bearings are critical..........as is shaft alignment.......otherwise s#*t will happen.

Just an example of what can happen due to shaft and bearing misalignment!

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#8

Re: Intermediate Shaft Bearings

09/07/2010 3:54 AM

Although you have not specified your shaft sizes the reasons that come to my mind are two.

For small diameter shafts with the torque transmission and rpms they will tend to flex along the midsection mainly and your system will be failing far too frequently.(if it can run in the first place that is)

For large diameter shafts though stiff enough the problem will come in the weight. the unfortunate end bearings that will have to carry all that load...

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