Journal Bearing: tight tolerances for a large bore 1

[fairgentlemanZ]

Does anyone have experience using tight clearances for a bore around 4"?

I'll try to keep this as simple as possible. The bearing is being used to axial locate tooling during machine operation. The tool needs to rotate for indexing.

Steel bearing, steel shaft. Shaft ~ 4", l/d ~ 1/2, the current diametric clearance is .00098" - .00208" . We would like to achieve less than .001" max. So yes an RC1 fit.

The tooling rotates inside the bearing at approximately 30 rpm (Velocity = ~ ~30 ft/m, .5 ft/s), and in indexing increments less than 180 degrees (so periods of less than 1 second). The operation is several starts/stops, no prolonged operation, low static friction needed.

The load is negligible, but for the sake of arguement lets throw an exagerated value of 500 lbs, P = ~62.5.

PV < 2,000.

Every text, "bearing design woorksheet", and example fit recommend diametric clearances of at least .001d and up (.0005d high lead), and for dry bearings .002d and up. So lets ignore conventional wisdom.

To reiterate, low speed, low load, tight tolerances for a large bore, low start torque. Open to all econimical material options. No ball bearings, we want our cake and eat it too.

Thanks,
Jeremy

 

[byrdj]

Any reason you wouldnot want to use something like oillite brass or cast iron for that bushing

Are you going to provided forced lubrication, then machine balancing rings on the ID to float the shaft and then tight fit would keep lube flow to minimun

 

[fairgentlemanZ]

No forced lube, just a zerk fitting and grease.

 

[Tmoose]

Is there a thrust bearing in there to help providing some guidance? with L/D ~ 1/2 (and my imperfect understanding of the arrangement) I'm concerned the tool holder will wobble a bit just from clearance, and maybe even jam.


Making big round precisely sized, well aligned holes is not cheap, and sometimes danged near impossible. If I thought I could not live with several thousandths clearance with resulting position variation, I'd be thinking of ways to make the bearing radially flexible to allow preloading. Maybe a V-block or pivoting shoes at 0 and 120 degrees, with a preloading shoe at 270.

http://www.mbbw.com/ClassicTrail/1910_35_foot_trailer_front.jpg

Lots of plastic composite bearings would enjoy that PV range even unlubricated

 

[fairgentlemanZ]

In this application we are not able to use a horizontal surface as a thrust bearing. We do have another component that is made out of cast iron AND we are able to use it as a thrust bearing.

Every manufacturer of plastic bearings "recommends" diametric clearances of .003 - .005d. So for our application we are looking at clearances of .012 - .020".

 

[unclesyd]

Could you use a Spieth guide bushing. We used on our machines that machined high precision spinnerettes for spinning synthetic fibers.

http://www.ame.com/line.cfm?id=comp-hydguidbush

 

[Tmoose]

"Every manufacturer of plastic bearings "recommends" diametric clearances of .003 - .005d. So for our application we are looking at clearances of .012 - .020"."

That's why I shiver at the idea of a cylindrical shaft being asked to oscillate in a (too) short rigid cylindrical bore, and find a shaft preloaded into a cradle so appealing, again without really knowing your arrangement.

Automotive Steering racks use something similar to stuff the rack into the pinion and thus maintain zero lash with the pinion for 200,000 miles.
See items "rack guide" and "spring" here -

http://homepages.ihug.co.nz/~stompyz/images/steering.gif

I'd cut a tongue or flap in the least loaded area of the cylindrical bearing insert's wall, drill and tap a few holes in the housing to align with the flap, install the bearing insert, drop a stiffish coil spring in each tapped hole, and wind a set screw down on top of each spring to apply preload something greater than the anticipated radial load, or at least reduce the running clearance to the minimum.
Depending on loading and life requirements, creating 3 point contact by slightly relieving the insert opposite the preload screws might be useful in creating the "cradle" or V-block effect. Kind of like a simplified tilt pad bearing as used in many kinds rotating machinery.

http://www.arla-online.com/images/armd-tiltbr-sample1.gif

http://its.fvtc.edu/machshop3/images/mill/vblck.jpg

http://www.drillpresstips.com/Acdmy_Fig4.jpg

Note a similar method is used to pilot drills in drill sharpening machines.

http://its.fvtc.edu/machshop1/Bench/sterlingdrl/Vtrough.jpg