Anti Seize or Grease on Bolt Shanks

[JCorsico]

Hello everyone!

We are having a debate with the mechanical people who actually have to assemble, disassemble and service our designs. They have a strong preference to apply grease or anti-seize to all bolt shanks before assembly, as they think it makes the parts easier to assemble and disassemble.

Specifically, these are MS21250 bolts used in shear to connect various linkages together. The actual pivot points are spherical bearings or rod ends. Or, occasionally, delrin bushings which pivot directly on the bolt shank.

We don't like this practice, as we suspect the grease collects dirt and other grit, which wears the cadmium plating on the bolts or gets into the spherical bearings and destroys the teflon liners. Although we don't have any hard data, we suspect this greasing actually shortens the life of the assembly and results in increased (and expensive) bearing replacements.

Has anyone seen this greasing practice before?

Thank you!
Jon

 

[RoarkS]

I have a Mooney M20J 201.
Per the manual Royco 363, Triflow, or Aeroshell 22 goes on just about everything from control linkages to landing gear. Tons of rodends. Most shops I think use a spatula to apply it from the looks of it.
I literally just bought another tube of Molykote G-n for screws on inspection panels too... That's not for assembly... that's for disassembly because getting a rusted out stripped out Philips head that hasn't been pulled in 10 years is not my idea of a good time.

EDIT: As a side note... on certain aircraft guns, a Teflon grease is used excessively in them. Our thinking that we relay to customers is: would you rather have dry dirty parts, or greasy dirty parts? I apply that logic to a lot of my life.

 

[3DDave]

In order to collect dirt and grit there have to be dirt and grit. The dirt and grit aren't repelled by the lack of grease. As long as the contamination is stuck in the grease it's not going anywhere, like further into the joint.

However, don't go too far. I've heard tell of a small plane owner concerned that the control cables needed lube. So they did. Except that the trim cable went around a multi-turn drum to generate enough friction while still being able to slip if the load was too high. He found out that he could no longer trim the plane when he got in the air and the resistance to those loads was cut to almost nothing. Worked in the hanger. Oops.

 

[3DDave]

Just a distraction:

https://imgur.com/gallery/o2HBoBY

 

[Ng2020]

Non-sealed lubricated pin joints usually need relubing at defined intervals, to eject contaminants which accumulate and are trapped in the grease, including water, abrasion debris, and corrosion products.

 

[Tmoose]

Are the inner races of the spherical bearings or rod ends clamped axially by serious torque by the MS21250 bolts used in shear ?

 

[Ng2020]

Good question
Overclamping sphericals is a real issue. In the past I have seen minimal torque specified for pin joints where overclamping was a concern. You will need to ensure self locking feature still engages at the reduced torque.

 

[JCorsico]

Are the inner races of the spherical bearings or rod ends clamped axially by serious torque by the MS21250 bolts used in shear ?

Good question, but no, the bearings should not be over-torqued. The bolts are undersized compared to the spherical bearings. There is a "top hat" bushing that fills up the extra space between the bolt shank and the bearing bore.

For example, a 1" bore bearing will use a 5/8" bolt.

 

[Sparweb]

A major commercial OEM specifies LPS on fastener threads vulnerable to galling.

https://duckduckgo.com/?q=lps+anti-seize&ia=web

For any fitting where the fastener could be subject to rotation, consider oilite bushings or something similar.

The use of delrin bushings as you describe them makes me imagine rather low forces on the fittings of the linkages, and perhaps you also mean low bolt torque - otherwise the delrin would "flow". If a delrin bushing fits, surely a bronze bushing could fit. Then, if you incorporated the bronze bushing into the joint correctly, you'd be able to fully torque the fastener. If I'm not picturing your joint the way you intended, could you please provide more detail?

 

[WKTaylor]

MS21250 is a very 'odd/over-kill' bolt for an application as described... especially since the head-to-shank fillet Radius is fairly substantial.

I would have expected an alloy steel or A286 160-TKSI/95-SKSI or 180-TKSI/108-SKSI hex shear-bolt with short or medium length threads [CAUTION: NEVER use titanium for these installs].

None-the-less...
MIL-HDBK-1599A BEARINGS, CONTROL SYSTEM COMPONENTS, AND ASSOCIATED HARDWARE USED IN THE DESIGN AND CONSTRUCTION OF AEROSPACE MECHANICAL SYSTEMS AND SUBSYSTEMS has a section dedicated to lubrication.

Many of Your concerns are well founded. Although Alloys steel parts are notorious for corrosion so I see the shops'/maintenance perspective. But grease/CPC/anti-seize compounds can destroy self lubricated bearing-liners and can cause 'slippage' or 'swelling/binding/peeling'. However for dissimilar metals-on-metals 'light butter lube' with a corrosion preventative grease or CPC can have value... for permanently assembled steel/steel bolt bearing installs that are carefully preloaded against inner races

On the other hand follow the drawing/engineering/company process specifications to establish clear design intent AND make sure the production planning procedures FAA/MIL do no leave any 'wiggle room' for mechanics to violate good engineering practices.

My head hurts.

Regards, Wil Taylor
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