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Why your maintenance company should GREASE bearings and not just oil them

Hi Peter,

most likely the bronce bushes are a press fit and would be easy to replace when clapped out. But then also the pins would have to go. If it was my aircraft I´d drill the pins and fit grease nipples so you will service the bushes like in civilised countries. Lubricating by brush is really prehistoric as this requires disassembly . Great for companies to produce nice profits. In case you don´t have easy access there I´d fit SKF Permaglide bushes and forget them. Imagine, I run a pre-war Moto Guzzi with girder forks modified for Permaglides running on common stainless steel bolts, unhardened, never lubricated (that´s why). It has done more than 30 000km and believe me the girder fork does a lot more motion than the trim bits. I also fitted these bushes into the complicated Jag E-type rear suspension instead of easy corroding needle bearings. We did more than 80 000km with the Jag – no complaints from anybody. Yeah – dead illegal ……. But who will find out ?
For those who are in need to cheat with spray can oil : Do not use WD 40 , this forms a sort of sticky resin when dry and can block sliding actions. Just try this somewhere and wait two weeks. Instead clear chain grease in spray cans will do better, having a solvent to creep in gaps and leaving a durable coat of grease. Bikers don´t disassemble their chains for lubrication either.


P.S. Seems they no longer supply imperial sizes ……

Last Edited by vic at 26 Jan 03:21

I looked up Permaglide and it is fascinating. Then I realised I got to the same URL which you posted later on

This would be illegal to use, of course…

Is the relevant part just the cylindrical bush i.e.

The other components they offer are not obvious as to the purpose. Or maybe one uses all three

with the flanged part going in one end and the washer going at the opposite end. That would help keep the dirt out.

I wonder what is special about these which keeps the dirt out long-term.

WD40 is rubbish. But there are other spray lubes in use in GA whose freezing point is undefined. Nobody complains because 99% of the customers never go high enough to see low temps. I had the elevator trim freeze solid at -14C and the company told me they have been using the stuff for 20 years and never had a complaint…

Last Edited by Peter at 26 Jan 07:54
Shoreham EGKA, United Kingdom
Hi Peter,

the dimpled type of bush is the POM variant that requires initial greasing. In your picture the top hat bush is for axial/radial loads, a plain bush and an axial bearing – not necessarily to be combined. I´d go for the dry bearing with PTFE/bronce/steel composite and owner made stainless bolts, see the 4.9MB docs in the SKF link below . I don´t think you will have big troubles with dirt in your application for a long time – unless you do many Saint Ex remembrance trips to Africa. You might fit small o-rings on both sides of the bush for protection but I suspect there is no space for them. Remember the P 10 bush is dry so does not attract dirt as grease does. And the bushes are only 2 or 3 €€ so no big deal for trying. Next time you have to service the bits I´d drill and ream the bronce bushes and press fit the Permaglides – or similar types that you find via Google – and get you some bolts copied in stainless. You may do a zero clearance fit for the bolts as there is no risk of seizure . The composites have only 1mm wall thickness so maybe they don´t replace the bronce type in one go ? What size would you need, I got a selection on stock ?
Freezing lubrication seems unbelievable to me, I suspect this may have been a matter of heat/frost expansion/shrinkage of tight fitting components like aluminium material gripping a steel bolt or so. Grease may well get a bit stiffer at freezing temps but not to an extent as to block anything. You could put some (lithium) grease in the deep freeze fridge to see. Except the grease was not water tolerant and in effect the water percentage freezed. But that choice of grease cannot happen in aviation – can it ?

Cheers Vic

Vic – the trim freezing incident was on this flight.

It was definitely caused by something being put in there. The firm uses an aerosol lube from a local firm. I phoned them up and they said it is very popular for motorbikes. They didn’t know the temperature range. They formulate it in their factory.

There are many stories of trim freezing up. I know of one in a Citation – post #6 here. I got that straight from the pilot who I know personally.

I think it happens specifically on the trim is because the jackscrew runs in a long “nut” – maybe 20 threads – and if the lube inside that freezes, it does so with great effect. Whereas with other control linkages, the contact area is much smaller.

I don’t think there is any legal way to change the type of bushing on a certified aircraft – tempting as it may be. But I have other applications for this and will definitely have a play with them, and thanks for the lead.

The trim linkage is important to keep in a top condition because any play in it translates to a wobble in the elevator trim tab. I would speculate there may be possibly flutter on that tab, because the rate of wear on the four rather short piano hinges is some 100x greater than it would be on other aircraft – according to a structural chap I know.

The other curious thing is that two of the bearings in the part I originally posted (the short ones) have brass or bronze bushes on the mating component (i.e. same material on both surfaces of the bearing) and the 3rd (the long one) has a steel bush, but it isn’t stainless because it goes rusty. It is a very hard steel, with a rough ground surface which may or may not be intentional.

Last Edited by Peter at 27 Jan 08:11
Shoreham EGKA, United Kingdom
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