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You know how car wheel bearings last for decades?

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What’s with the special greases I’m supposed to use for my Bushcat wheels? Why not just normal bearing grease?

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14 minutes ago, danny_galaga said:

What’s with the special greases I’m supposed to use for my Bushcat wheels? Why not just normal bearing grease?

I don’t know but it’s usually based on the design engineer doing something unusual,   like getting into a tight space by using a bearing smaller than usual, or running a bearing at a speed which would cause it to overheat, then resolving the problem with a special grease. A dozer, for example requires several different types of grease all doing different jobs. I’d ask the manufacturer.

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29 minutes ago, danny_galaga said:

What’s with the special greases I’m supposed to use for my Bushcat wheels? Why not just normal bearing grease?

May be sealed if build manual doesn't specify. Best ask the supplier. Cheers

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special greases for aviation are due I think to the fact that the wheels have to withstand temperatures from way below zero at altitude, up to several hundred degrees on a fast landing. For our low slow planes I really cannot see any problem with high temp bearing grease, but I am no expert.

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Greases are basically lubricating oil with additives, held in suspension in metallic soap compounds (thickeners), or silica gel, modified urea, or clay-based compounds.

Every grease type is designed for a specific use, temperature range and specific lubrication abilities.

 

Greases must pass a minimum of 9 basic tests - more, if it is a grease developed for a specialised application.

Aviation applications are particularly severe, with extreme temperature ranges, and aviation greases must be highly resistant to water, which is often sprayed up at high speed, and therefore high pressures.

 

Shell explain it better than I ever need to do, in the link below.

There are two major important factors associated with poor grease performance - oil separation, whereby the oil drains out of the thickener base - and mixing of greases that contain incompatible bases, which leads to chemical reactions which reduce performance.

 

https://www.shell.com/business-customers/aviation/aeroshell/knowledge-centre/the-aeroshell-book/_jcr_content/par/textimage_1433441235.stream/1445042875796/1d024cf49b16b7091e0368a866e9ca6b0ef6f275ac75de066f2004ed372bbef1/aeroshell-book-5greases.pdf

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I hadn’t thought of the temperature extremes, although it certainly doesn’t seem like there would be that much temperature ‘stress’ in an ultralight...

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3 hours ago, danny_galaga said:

What’s with the special greases I’m supposed to use for my Bushcat wheels? 

So someone can make money....

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7 hours ago, danny_galaga said:

I hadn’t thought of the temperature extremes, although it certainly doesn’t seem like there would be that much temperature ‘stress’ in an ultralight...

People say that with dozers too, after all they're just crawling along the ground; what could go wrong, and there's only one grease gun.

A Cat D7 drive shaft replacement costs about $45,000.00

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  What airline wheel bearing greases have to put up with has no real relevance to an ultralight. The heat of the brakes can melt fuse plugs and deflate tires and at altitude they get exposed to temps  around minus 60 C  OAT. OK they aren't extended at that level normally but there's cold soaking and they can be extended for a slow speed emergency descent to give more drag.

  Most heat an ultralight brake would get is from a good set of brakes, and being used severely. The other issue is water ingress and lack of grease. (not being serviced or seal failure.)  A good marine high temp grease should cover it.. As said,  a grease is just oil in a metallic stearate ( usually lithium soap) to stop it running out  of where you have put it. Some greases are OK with rubber and used in conventional brake systems (not mineral oil).  That's SPECIAL rubber grease in a tube. and NOT for bearings. Some aero brake systems use mineral oil. and have special seal material to cope with it.  I have to mention this so there's no confusion .  The most important thing with your wheel bearings is the adjustment  unless it's sealed bearing with fixed spacers. .Nev

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In light aircraft operated mostly in Day VMC conditions below 10,000 ft, the biggest destroyers of wheel bearings are water and grit which are thrown up from the ground surface when the wheels are in contact with it. 

 

To extend the life of the bearings, it is advisable to remove them for servicing every 100 hours or at a minimum, annually. Service the bearings more often if the aircraft is operated from unsealed runways. Servicing involves washing all the grease and muck from them in an solvent bath.  After drying with a blast of air they must be packed with grease. A word of CAUTION. While it is fun to put the bearing on your thumb and get it to spin by blowing pressurized air across the outer shell, the friction generated if the bearing is dry can cause damage. 

 

Here's some advice on choosing a grease suitable for light aircraft wheel bearings:  https://www.skylinkintl.com/blog/wheel-bearing-grease

 

This is how to properly pack grease into a bearing:

 

https://www.google.com/search?q=how+to+pack+a+bearing+with+grease&rlz=1C1GGRV_enAU749AU749&oq=how+to+pack+a+bearing&aqs=chrome.1.69i57j0l5.8460j0j7&sourceid=chrome&ie=UTF-8#kpvalbx=1

 

 

 

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Grease seals really are the weak point in wheel bearings. I always use a minimum of double-lip seals, and triple-lip seals, if they're available.

Once a small amount of water gets inside a bearing housing, it will create corrosion on the exposed surfaces of the rollers and races, when the bearing is sitting still for an extended period, and when it goes through regular temperature and humidity changes.

I've always had a great deal of success with protecting trailer wheel bearings by fabricating a thick felt washer that is then coated with grease on one side, and slid (with a tight fit) onto the axle, prior to hub assembly.

The thick felt washer is positioned with the greased side against the inner bearing seal, and it thus acts as a simple additional labryinthine face seal, to further protect and enhance the regular lip seals performance.

This is particularly effective on trailers used on unsealed roads as the stones flung up by the towing vehicle really hammer the regular lip seals. I'm not sure if it's possible to do the same for light aircraft wheel bearings, lack of room might be a problem there.

Edited by onetrack
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Car wheel bearings these days are sealed bearings and they do indeed last for decades and thousands of hours.  On gliders ( the first glass gliders are approaching 50 years old now ) there are several sealed bearings which are difficult to remove. In the worst case, you need to cut a hole in the structure to get at the bearing. Other unsealed bearings need tricks like super-long tubes attached to rods to lubricate them. Felt discs like onetrack uses have been used and a hypodermic needle might be the only way to inject some oil in there. 

 

 

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 Grease can go hard and not get into the bearings where it's needed. A really good chain oil can reinvigorate wheel bearings if you can get it into it. You need to lift each wheel  in turn and check for play and roughness/noise A tapered roller is eminently suitable for wheel bearings (Load and speed) and will indeed last a long time if water is kept out of it. They are sometimes lightly pre loaded so adjust with care and lock pin or stake the nut. A dust cover on the exposed inner sides is a good idea as you have a cap on the outer side.  Nev

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My MATCO wheel bearings are non-adjustable, factory sealed.

About $6.00 each from the local brg shop..... punch them out, punch them in.... too easy maintenance.

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39 minutes ago, Downunder said:

My MATCO wheel bearings are non-adjustable, factory sealed.

About $6.00 each from the local brg shop..... punch them out, punch them in.... too easy maintenance.

Interestingly enough, these are Matco wheels. But these don’t have sealed bearings 😞

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Remember that sealed bearings come in a variety of types - just with steel shields, with steel-encased felt seals, with moulded rubber seals - in contact and non-contact style - and seals made from molded Viton, glass-reinforced PTFE and Teflon.

Each type is designed for a specific use or position, or temperature range, or specific contaminants that it needs to cope with. The tighter the seal lip, the more drag on the rotating component.

Some small bearings designed for high speeds need a non-contact seal to keep drag to a minimum.

When replacing sealed bearings, always ensure you acquire the correct type of sealed bearing for your application.

 

Wheel bearings can also suffer from "brinelling" - the indentation of bearing rollers or balls, or races, caused by heavy or repeated impacts when the bearing sits in the same position without moving.

Further to that, you can have "true" brinelling, and "false" brinelling.

If you drop a bearing from chest height onto a concrete floor, you have probably incurred "true" brinelling of the bearing.

This is shown as indentation marks in the rollers, balls or races, where they were severely impacted in the one spot.

 

New vehicles transported by rail or truck used to commonly suffer from wheel bearing "false" brinelling, particularly in the 1930's and 1940's as transport speeds increased.

The "false" brinelling was caused by rough road surfaces, heavy springs with poor compliancy, and transported vehicles rocking back and forth slightly whilst in transit. Heavy vibrations from rail tracks and train diesel engines caused the problems in rail transport.

The "false" brinelling led to noisy bearings in new vehicles, with accompanying shorter lifespan. It caused consternation amongst vehicle manufacturers before WW2, and GM even commissioned a study into the problem.

 

The problem was reduced by improved greases with anti-brinelling properties, tighter tie-downs of equipment and vehicles being shipped, better (more compliant) springing in transport equipment, and better road and rail surfaces.

Continuously-welded rails were a big improvement in reducing "false" brinelling in rail transport - and smoother roads with improved paving systems played their part in road transport.

Regardless, the problem of "false" brinelling still appears occasionally, usually due to poor transportation techniques, extremely rough remote roads, and wheel bearing grease becoming degraded with age and high kms.

 

A brinelled bearing exhibits excessive noise and roughness when in operation, and is warning you it needs replacement, sooner rather than later.

Edited by onetrack
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