Lubrication: Do's, Don't's, What's and Why's...

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Luke Warmwater
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Post Posted: Mon Jan 26, 2009 1:52 pm
Post subject: Lubrication: Do's, Don't's, What's and Why's...
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As requested, here's my rambling thoughts on lubricating our helis. I guess if you lot want it as a sticky, then one of the mods will sort that for us.

When it comes to lubricating your pride and joy, there are many ideas and thoughts on the subject, some right, some wrong…but mostly partly right. What I aim to do here is give some of the basic info on what you need to do, what you shouldn’t do and bit of the reasoning behind it, so you can make an informed decision. Shocked


A Bit...Well, A Lot Of Background
There are three principle forms of lubricating anything: oil, grease and solid lubricants. These are all often available to us, the end user, in the form of a combination of the above. If you don't want to know about this, skip straight on to What To Use And Where Rolling Eyes

Oil is essentially a liquid, refined from crude oil (usually – although there are other oils, such as vegetable oils etc. For this purpose, we are talking about engineering grade oils refined from stuff found under the ground / sea). Silicone oil can be used too, but in the scheme of things it is not a great lubricant and is normally used where you are looking at wide extremes of temperature, as silicone is very stable across a broad temperature range. It is widely used in the automotive industry (inside brake and clutch cables for example), where they want a standard car that can be used in the Arctic or the Sahara. All that said, silicone oil can be useful on R/C helicopters; I’ll come on to that later.

Grease is made from an oil which is added to a thickener. The thickener is usually some sort of soap, most commonly lithium. This can also be in the form of lithium complex, aluminium complex, aluminium soap, calcium etc. A thickener very simply holds the oil and stop sit from running away. It acts in a manner very similar to a sponge holding water. If you squeeze it, the oil will run out!! If you have a can of grease and leave it for a long period of time, it will separate into oil and thickener. You can simply stir it up to get it re-mixed and usable. Cool

Solid lubricants take many forms. Often it can be Molydenumdisulphide (MoS2 or Moly for short), graphite or even PTFE. They can appear in the form of a spray coating that dries or more often, as an additive to a grease or oil, to help it withstand high loads or pressures.

So, all this is great and of passing interest, but where does it leave us with regards to helicopters. Well, in selecting an oil or grease, we should always look at 4 elements. So, LETS look at them, No really, I mean L.E.T.S. It’s a mantra used in industry so ensure we have covered all aspects of the application. It breaks down like this:

Load
Environment
Temperature
Speed

Load refers to how much pressure we are putting the lubricant under. Will it be lubricating a four tonne gear wheel driving an 8 tonne shaft? Or will it be a small gear wheel which with centrifugal force takes a few kilos max? We categorise this fairly generally, as the spectrum is enormous; we sue simply high, medium or low loads. On a heli, we are talking about low to possibly medium low loadings.

Environment refers to just that. Is it dirty, dusty, wet? What are the surfaces it will go on? Will it get wiped off easily by water or blown off by air? On a heli, there are few areas of concern regarding the environment, although one area is critical. What will it go on? What is the substrate? This is a going to be plastic or metal (I will come to a belt later). And I’ll come to the reasons that it is important later as well!

Temperature is again defined as high, medium or low. High is usually above 160 deg C, low is below -30 or -40 deg C, and medium is in between. Now, obviously, you and I would look at something at 80 deg C and call that hot, because we would burn our hands on it. But in terms of lubrication, it is pretty average. Helis then, will run at medium, ambient temperatures.

Speed is fairly obvious. On an engineering basis, we would measure the Dn value of rotating equipment (this measures the amount of surface distance that comes into contact every minute or second etc) rather than RPM. Something that rotates at 2000 rpm will have a far larger Dn value if it is a metre across, than something that spins at the same rpm, but is only 10mm across. However, you will be please to know that for helis, it is fairly easy to state that they are a high speed application when talking about gear wheels and shafts etc.

So, lets summarize. We have 3 main areas to lubricate on a heli: Gear wheels and shafts, servos and linkages and possibly a tail belt (depending on the heli).

Gear Wheels and Shafts etc
High speed, plastic and metal substrates. No real temperature or load considerations.

Servos / Linkages
Low load, low speed, plastic and metal combinations.

Belt
I was going to write that I’ll come on to it later again, but lets deal with it now and then it’s out the way. It’s not complicated: use a silicone spray. You are not looking to lubricate so much as keep it supple. Furniture polish has a lot of silicone in it and is fine. If you want to be real technical, Rocol do belt dressing spray, which has all sorts of advertised benefits, but any silicone type spray will be fine. There we go, I won’t be mentioning the belt again. Laughing

The oil in the grease needs to get between moving parts and physically keeps them apart! That is what stops wear and allows free movement. In general, it is said that industry rides on 2 microns. That is the gap that an oil or grease will generally fill between two moving parts. So you don’t need much!!! For the technically minded, this is called hydro-dynamic lubrication and works on exactly the same principle as a car aqua-planing. This is where the forces of speed and pressure come together and the material in the middle (puddle water for a car, grease or oil in industry) doesn’t get forced out, but stays in place and keeps the surfaces apart.

Can anyone see now, why we might want to look at a different grease for lower speed applications? Idea Lower speed gives no “aqua-planing” or hydro-dynamic effect….? Well, the good news is, that on our heli, where the speed is lower, so is the load, so the grease still doesn’t get forced out, and so one grease should suffice. Smile

For those that are interested in the principles here, this is where you would look at a grease with solids in it (to help physically keep things apart), or even a dry film lubricant, where the solids would effectively burnish into the surfaces making them ultra-smooth. In this instance, the surfaces may be coming into contact, but friction is reduced by this burnishing effect.

So, you’ve read this far, and you still don’t know what to use. So in the next section, I’ll tell you just that! Confused Laughing


What To Use and Where
OK, for those of you that skipped the first boring blurb, welcome aboard. Laughing For those of you that sat through it….you’re nearly there!! Laughing

Lithium grease is what I hear (or read) nearly everyone recommending when it comes to lubrication for an R/C Heli. But what does it mean? Well, nothing special! As I pointed out in the first section, it is just a thickener, and probably the most common one available at that! All greases have thickeners and it is usually lithium. The thickener does NOTHING other than hold the grease in suspension (but if you’d read the first section, you’d know that! Surprised ). The bit that does the lubricating is the oil in the grease, and this is what is critical.

The most common oil is mineral oil. This is an oil that is partially refined and comes from crude oil. It is widely used and widely available. Like all these oils, it is made of hydrocarbons. Can you think of anything else made from hydrocarbons?? PLASTIC!! Idea Exclamation Hydrocarbons like hydrocarbons. They like to mix and mingle, which sounds nice, but is NOT GOOD from our point of view. Mixing and mingling hydrocarbons make things go soft. More specifically, they make plastic go soft!! So, we don’t want a mineral oil in our grease. And many, many lithium based greases will contain mineral oil.

Fortunately, there is a way to resolve this. When oil is refined, it is refined by passing it up a big column, The further up the column you take the oil out, the more refined it is. And we all know that refined people don’t mingle on a mad, frantic basis. And they certainly only like to mingle with people like them. Hydrocarbons are the same Laughing , so take a refined hydrocarbon (ie a synthetic oil) and it doesn’t like to mingle with the hydrocarbons in plastic! Can you see where this is going?

So, if you have plastic and metal, as we have, you want a grease with a synthetic oil in it. Don’t worry too much about the thickener. It is not totally irrelevant, but it is not important at the moment for us. The most common synthetic oil is a polyalphaolefine. This is usually known as a POA for short. And this IS compatible with plastics and metals. There are others, but the more refined the oil, the more expensive the grease.

So, generally speaking, on a heli, we need a grease that is compatible with plastics and metal, and that grease will be a PAO based grease, where the thickener is not overly important.

I personally use a grease called Molykote PG-65 Plastislip. Molykote and Rocol greases are just about the best out there, and are available through many bearing suppliers, as well as places like RS Components.

Now, there are two big different areas on a heli that we need to lubricate, the high speed spinning bits and the slow speed linkages etc. High speed for us really means we don’t want an grease that will whip up into a froth and we want it to stay on the moving parts when they spin. So a grease with good cling and low fling is ideal. Not all greases however, are marked up like this. The best thing to do is to inspect visually; if the grease is gone, re-lubricate.

I think that this will just about do it for grease selection, but if anyone has queries, please feel free to pm me. Next, I’ll move on to how to lubricate!


How to Lubricate
This really isn’t too complicated. Use sparingly is the key. You want grease in moving parts. I use a small brush and apply with that. You will know if you have put too much on, as it will fling out. Gear wheels need to look “wet”. If they look like that, they have their 2 microns and should be fine. Wipe off any excess; it gets messy and will attract and hold dirt.

Which brings me on to my next point, dirt. All grease attracts dirt. Dirt will stop the grease from working and get in between gear teeth and cause wear etc etc. So you will need to clean it off fairly frequently. It is ALWAYS good practice to wipe away old grease or oil before applying new. You may top the oil up in your car every now and then, but we all have to change to oil completely eventually. This is the same. Adding fresh grease to old grease is only a stop gap and over time will just cause a “cake” to build up. This cake is effectively a grinding paste and will cause more wear!

Which brings me on to another point…(the cake bit, that is) Laughing . Not all greases can be freely mixed Exclamation . Many greases are incompatible and if you use one type, and then simply change to another, this mixing can very (I said very, but I meant VERY Exclamation Exclamation ), quickly cause a hard cake to build up that can seize things. If you are changing oil or grease types / brands, you MUST clean the old stuff off!!


Well, I hope this helps a bit. I know it rambles on a bit aimlessly sometimes, but any probs or questions, please feel free to post or pm me (along with any other advice or info). I’ll finish with a simple Do and Don’t list….

Don’t use a mineral oil based product (such as 3 in 1 or WD40)
Do use a synthetic oil based grease (PAO is ideal…or plastic compatible at least)
Don’t worry too much about “lithium” (although that is OK)
Don’t put too much on
Do check and re-lubricate frequently
Do clean old grease of first, especially before changing grease types



MOD (Friskle) Nice Post Luke.
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