Long Shaft 2.2m Concentricity and Bearing Alignment

[cckkckc]

Dear all:

I'm coming across a design problem today. We need to support a Rigid structural frame for rotation. The frame is supported by two pillow block spaced 2100mm apart, a 100mm Dia. Rod/Shaft(2200mm in length ) is connecting through them and clamped on both sides, So the center rigid frame is rotating (Somewhat like a rotating bbq chicken frame(

https://www.amazon.com/Alritz-Automatic-Rotating-Marshmallow-Stainless/dp/B07JRBM8L5

) except it is more sophisticated and gigantic.) I don't have good access to some good fab shop for high precision fabrication for this bearing shaft, and longevity of this part is required.

Problem:
1) Concentricity is very critical
2) Align Rotating Axis


Configuration:
-P-RRRRRRRRRRRR-P-
|--- L=2200 mm-------|

*Rigid Frame connecting the two pillow blocks

My Take:
1) Frame will be spaced to just next to the pillow block for direct shear and minimizing the bending
2) Speed of rotation is very low, like 10rpm or so didn't mind to consider resonance.
3) Spherical roller bearings will be used to minimized Angular mis-alignment, but not Offsets.

Any recommendation to the design? If I'm sending to a fab shop, I don't have the confidence they will get the offset and alignment right. Is there anyways to ensure this alignment/offset to be taken care of? As this shaft will take heavy direct shear, like 8,000-9,000kg
or Am I worrying too much and the two Spherical Roller Bearings will take it up without any problems?

 

[hydtools]

I suspect you will need to perform on site shimming and adjustment to achieve final required alignment. Provide for that in your structure design.

Ted

 

[Tmoose]

"Spherical roller bearings will be used to minimized Angular mis-alignment, but not Offsets."

A straight, stiff shaft will attempt turn offset misalignment into angular at each of the two bearing locations.

Are you using spherical roller bearings for the load capacity? Mechanically (seals etc) they limited to some relatively small static misalignment, like a degree or two as I recall.
Regular old ball bearing pillow block accommodate greater static misalignment nicely with their spherical ODs.

Bumping and shimming before tightening the pillow blocks will make a more workmanlike looking installation.

Nobody likes dynamic misalignment as would result from a "bent" shaft ( shaft bearing seats not co-linear) or excessive bearing seat> bearing ID clearance with cheesy-*ss set screw or eccentric locking devices.
I'm thinking the co-linearity of the shaft bearing seats //may// be what you are asking about.

I'm imagining ( tragically, no pictures or drawings have been provided ) that this "rigid structural frame" has short pieces of the Ø100 shaft grafted on the ends, so the co-linearity of the bearing seats is best attained by machining them as the last step in RSF manufacturing. Even starting with a gorgeous fully qualified continuous 2200 mm long shaft will not assure good bearing seat co-linearity, especially if any welding is involved.

 

[cckkckc]

Thank you guys! Yes, I believe shimming and onsite adjustment is the key.

"I'm thinking the co-linearity of the shaft bearing seats //may// be what you are asking about." Yes, the bearing seat co-linearity is the most important part here. Yes a whole gorgeous 300lb continuous 2200mm long shaft is the design there. To ensure no welding distortion, I will use bolts and nuts for clamping the Rigid frame onto the shaft.

 

[Strong]

Each bearing housing support should be level and at same elevation, based on precision measurements. This should take care of vertical angle and offset, although you can provide some shims stock in-place to facilitate changes. Horizontal angle and offset are controlled by rotating and sliding the bearing housing on its base. A dial indicator mounted on shaft and indicating on machined housing/bearing race surface in axial/thrust direction can guide the position for correct alignment. Use spherical roller bearing with self-aligning capability to compensate for any bearing misalignment issues.

Walt

 

[Tmoose]

Typical ball and roller bearings in commercial pillow ( plummer) blocks over a huge range of sizes have major self aligning capabilities for static misalignment.

Single row deep groove ball bearings themselves will jam internally if misaligned more than a tiny fraction of a degree.
But commercial assemblies have engineered features to eliminate needing fully machined bearing pedestals and precision machine tool grade alignment at assembly. The trick is the have spherical ODs to pivot in the pillow block bore with a spherical ID.

https://www.youtube.com/watch?v=EZQ3rgv2ccU

https://www.skf.com/us/products/bearings-units-housings/bearing-units/ball-bearing-units/index.html

https://www.ggbearings.com/sites/de...ign-pb-pedestal-bearing-1_0.jpg?itok=Mgv5sUND

Low capacity "self aligning" ball bearings and extremely rugged spherical roller bearings can pivot internally and thus accommodate static misalignment, up to a couple of degrees, when the seals jam.