Highly loaded plain flanged bearings [limited shaft rotation, very high loads] 1

[WKTaylor]

guys...

I am have a problem with grease lubricated plain-flanged bearings. Flanged bearings are inserted in both ends of a 300-KSI steel MLG hole. The bearing ends are spaced ~0.160 apart on installation, forming a grease channel around the circumference; and a lube hole was drilled to this gap thru the fitting. The CuBe bearings are ~3.875 OD in ~0.003--0.005 interference; while the ID is line-bored [all bushing bores] to 3.625 ID, 32-microinch RA

Problem: one customer has noted all of the bushings in their installations slowly rotate over time. Our judgment is that the potential for corrosion damage to the steel housing bore is high and the bushings must be removed/replaced when ANY rotation is noted [restoring corrosion protection]. Needless to say the customer is living with rotation... and now wants "limits" to allow some rotation.

We have altered the installation design in numerous small but significant ways [bearing alloy hardness, interference, etc]; and the customer has added grease grooves [better sread-around the grease... but the problem persists.

Do certain customers tend to have unique circumstances of operation, such that these unique issues can arise? Or do You think other customers are not noting/reporting these small rotations?

Also I am looking for ways to reduce shaft rotational friction loading and/or increasing bearing OD to housing ID friction [for bearing retention].

NOTE. Years ago I had a situation, where the finish pattern on the bushing IDs apparently met drawing requirements [64-microinch RA final]; however, wear-out and eventual high friction [bearings finally spinning] were troubling. Finally the OEM engineers stepped in and indicated that a distinct cross-hatched surface finish had been developed and applied to the drawing; but was hard for vendors to attain without ball-honing of the bearing IDs starting at 32-RA [after bearing IDs had been line-bored]. The cross-hatching helped retain/distribute grease, and was well within the 64-RA-microinch limit noted on the drawing. The solution was fairly easy: install bushings in interference; line-bore the IDs to same dimension; then quickly ball-brush hone the bearing IDs to break sharp edges and attain the cross-hatched finish. After this was instituted, the premature wear issues went away [unless the ball-brush honing was omitted].

Anyone have advice??? I have looked in our design manuals... only a brief discussion of machined surface finish for bushing IDs: however, it is assumed that a 32-RA finish was the cure-all. After this there was very little discussion of plain bearing problems/resolutions.

Regards, Wil Taylor

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[rb1957]

hi will, i've just started working with a client who has manufactured replacement bushings out of a proprietry plastic material, which is self-lubricating. they haven't looked at really high loadings yet, so maybe it's not applicable to your application.

 

[WKTaylor]

RB'57 [WT'52]

REF SAE AIR1594 Table 1 for Journal Bearings, CuBe AMS543*... we are running ~90-KSI for static Joints... that encounter slight rotation [a few degrees] as LDG truck pitches slightly up/down.

Still trying to get our customer to quantify "rotation".

What confuses me is that the pitch-up rotation should be balanced by pitch-down actions... but doesn't seem to be.

Does Your [all-?] plastic bearing material have equal to or better mechanical properties than the SL materials listed on AIR1594 Table 3?

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[rb1957]

i will see if we have a copy of that SAE spec

 

[tbuelna]

wktaylor-

Just one comment and one question.

My comment is that with your minimum 0.003" interference fit at RT on a 3.875" OD BeCu bushing in an alloy steel hole, if this MLG was cold-soaked during flight down to -65degF or less, you would lose much of the bushing OD interference fit. I might suggest adjusting the tolerances of your bushing OD and MLG bore, and/or increasing the interference fit as much as stress limits will permit. You can also increase the coefficient of friction at the bushing OD interface by using a friction enhancing compound like Loctite.

My question is whether there is any appreciable thrust load applied to the bearing flange during the shaft rotation? High friction and high thrust force at the flange face may be helping to produce the bushing rotation.

Lastly, when you install grease lube fittings it is good practice to also make sure there are provisions for the old grease to be discharged. When adding grease the idea is for the new grease to completely displace the old grease, along with any trapped debris or moisture.

Hope that helps.
Terry

 

[WKTaylor]

Thublena... Thanks for Your insight.

We have been using company design charts for this interference. HOWEVER, Your comments RE Thermal expansion/contraction are important, especially since the original [design] static thermal limits for the aircraft were -65F to +160F; but the real thermal limits appears to be closer to -100F to +200F [for a variety of military reasons].

SAE AIR1594 and SAE AIR809 have thermal expansion/contraction [metal dimensional change] data which I need to review and reanalyze very carefully.

NOTE. We use AMS-S-8802 sealant for highest possible environmental sealing/retention. I think I will also suggest using polysulfide sealant adhesion promoter [AMS3100] on the cadmium plating post-chromate treatment surfaces, just prior to bushing Installation.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

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