Bearings and Model Engines

by Paul McIntosh

There are a lot of myths and half-truths regarding bearings used in model aircraft engines. This article is intended to address the more common properties of bearings as they pertain only to model engines.

What do they do?
Both front and rear bearings hold the crankshaft centered in the crankcase bore to reduce friction.

Front bearings take up the axial (fore and aft) loads presented by the starter motor and prop. They also maintain the axial location of the crankshaft.

Rear bearings absorb the radial (centrifugal) loading of the crankshaft and help somewhat with the axial position.

Myths and half-truths
Some of you have heard that ABEC-7 bearings are the best you can get. Others say that special polymer retainers are better than steel or brass. There are even people who will tell you that the metal shields on your front bearings prevent fuel and air leakage! Let's start out with a little anatomy lesson. Below is a cut-away diagram of a typical ball bearing.

All of these parts are found in some form in all model engine bearings.

Accuracy
Lets look at bearing accuracy first since this causes a lot of controversy. Bearings generally come in the four classes shown in the table below.

If you want to know what the actual tolerances are, just do a Google search on ABEC.

For most model engines, ABEC-1, Normal Class, is completely adequate. Very few sport engines are manufactured to tolerances tighter than would be required by this class bearing. Special racing engines may benefit from ABEC-3 or ABEC-5 bearings. Regarding the statement that ABEC-7 bearings are the best you can get, that is true only in the fact that tolerances are controlled very close. They are not "better" for use in model engines because the cost of these bearings may actually exceed that of the engine and you will not see any performance increases. Most engines are supplied with ABEC-1 or ABEC-3 bearings from the factory. The inline skating world has recently got caught up in the ABEC-7 craze. You see all kinds of claims being made regarding these bearings. Suffice to say that the bearing MANUFACTURERS say that most of the claims are not supported by actual tests and that ABEC-1 bearings may actually be better for that application.

Clearance
Next on the list is clearance. Some people equate clearance with quality. They feel a loose bearing and think the quality is low. This is not the case at all! A certain amount of clearance is necessary to prevent friction and binding of the balls and races. As your engine heats up, the clearance can change due to different metal properties. Choose a bearing with too little clearance and your bearing can overheat and fail. See the table below for standard clearance ratings.

The Group number corresponds to the C-number commonly used. That means that a Group 3 bearing corresponds to C-3. Most engines can run reliably with either a Normal Group or a Group3 bearing. Some engine manufacturers designate C-3 bearings.

Retainers
Most bearings use a stamped steel or brass retainer to keep the balls evenly spaced. These retainers may be tabbed or riveted together. Quite a few people think that special high speed polymer retainers are better than steel or brass retainers. In very few cases is this actually the case. Look at the piece of a bearing makers spec sheet below.

This is for a very popular .40-.50 size rear bearing. Notice that with oil lubrication, this bearing is rated for 26,000 RPMs MINIMUM. Smaller front bearings are rated for MUCH higher speeds! This is with the standard stamped steel retainer. Does your engine run that fast? I have even read some bearing manufacturers' beliefs that the steel and brass retainers may actually be better for our applications because these retainers hold more lubricants next to the balls. There are a very few applications where the polymer retainers may be better. One is with ceramic bearings (discussed later) and certain high speed applications such as some racing engines where an excess oil film actually can increase friction and heat.

Seals and Shields
There is a primary difference between shields and seals you need to be aware of. Shields do not seal! The primary purpose of shields are to help retain grease, and to keep large pieces out. They do not prevent fuel or air leakage past the bearing. In most engines, a small amount of leakage is desirable. This is how the front bearing gets lubricated. In our engines, shields are primarily used to keep dust and dirt out of the front bearing. Nothing more. If you have an engine with excessive leakage or air draw from the front of the engine, then you MIGHT benefit from a sealed bearing. Sealed bearings are used on some engines to prevent leakage from the crankcase. Others use them because there is no built-in path for the lube in the fuel to get to them. One of these is the YS line of four strokes. They use a pressurized crankcase as part of their supercharging system and there is an O-ring separating the crankcase from the front bearing.

Ceramic bearings
In recent years, the price of ceramic bearings has come down to the point that they are economical alternatives to standard bearing steel. Are they better? In many cases, yes. The best ceramic bearings for model engines use Silicon Nitride balls and steel races. These balls are many times harder than steel and cannot rust or corrode. They are also very resistant to shock and thermal loads. Their surface finishes are generally much smoother than steel balls. The balls are also much lighter than steel. One other benefit often overlooked is they cannot conduct electricity and have no magnetic properties. What this means is they will not develop magnetic fields and attract metal particles like steel bearings can. These bearings are now so good that, in clean conditions, they can safely be run with virtually no lubricants! In properly maintained model engines, it is not unreasonable to expect them to last 2-5 times as long as steel bearings. Because their lubricant demands are so low, they benefit from polymer retainers low friction properties. Another benefit is that on larger more expensive bearings, the polymer retainer allows the balls to be salvaged from worn races and reused with new races.

Lubricants and rust
Most bearings come packed with a light, rust inhibiting grease. When replacing bearings, it is perfectly acceptable to leave this grease in the bearings to protect them until you run the engine. Your normal two stroke lube will eventually wash out the grease and replace it. If you like, you can also clean the bearings in regular fuel and apply some high quality oil such as Klotz Super Techniplate, or Sig Castor before installing. I would not use WD-40 as it can turn hard after a while.

Bearings rust for one reason: oxygen reacting with the iron in the steel. Certain chemicals can accelerate this action. Water and acids are the most common culprits. Most modern fuels have rust inhibitors either added or as a property of the oil used. Castor oil is a good rust inhibitor as are many modern synthetics. There are several ways to prevent or minimize rust in your engine. Two of the best are to run your engine dry after each flying session and store your engines in a dry location. For most of us, our engines are stored in a garage or shed. Not the ideal place! As extra insurance, an after-run oil may help reduce rust during longer periods of storage. Some fuel and aftermarket companies sell after-run oils and you can use oils made for air tools such as Marvel Air Tool oil. For this to be effective, the after-run oil has to reach all of the steel parts in your engine. A couple drops in the glow plug hole won't do that. A better approach is to open the throttle (two stroke) and place about 1/2cc (several drops) down the throat and let it run into the crankcase. Then spin the engine briefly with your starter or flip a few times by hand. Short of removing the backplate, there is no way to assure adequate coating of the steel parts. Four strokes with a breather nipple on the crankcase can have a small amount of oil injected there. Normally, this oil will reach the cam and bearings. The pushrods and rockers should be periodically cleaned and lubed separately.

Summary
Engine bearings should be chosen for their particular use, not by hype and superstition. Most standard quality bearings will perform just fine in your engine and spending a lot of money for "high precision" bearings may only make your wallet a little lighter. Ceramic bearings are becoming popular for their low friction and long life. Costs for these bearings are getting close to that of premium steel bearings and will continue to come down as more people start using them.

If you have any questions about bearings, please feel free to send me an email.

Paul McIntosh
Desert Sky Model Aviation

http://fly.mcintoshcentral.com
http://www.RC-Bearings.com

 

Electric specs

Gerry Davis raises a popular question on this month's post box when he asks for some guidance on what size electric motor he should use when converting a model designed for i/c power to electric. "Is there a conversion table or formula I need to use", he says, "especially as the wing loading and wing area must presumably come in to the equation".

As ever, there is never a simple, straightforward solution to questions of this sort. In the case of an electric power train, factors such as model weight, power pack size, type of motor, prop size, gearing, etc, all play their part in the final solution - alter one or more and you are going to get a different result.

When I asked Graham McAllister of McAllister Designs what his thoughts were on this, his immediate response was rather like the lost tourist asking directions of the local inhabitant and being told, "If I was you, I wouldn't start from here!" Graham's conviction is that IC and electric designs should be very different in structural design and so conversion is always a compromise, although accepting that this does apply more at the lower power/cost end of the electric spectrum than at the higher end. For Graham, the sensible way into electric is to choose a model that is designed for electric power and follow the manufacturer's recommendations - as long as the manufacturers does his job and makes his recommendations for motor, direct drive or gearbox, battery pack and propellor in just the same way as i/c powered models have their engine specified simply by capacity.

However, given the interest and desire of many modellers to convert i/c designs to electric, Graham advises that you should start by looking at how you want the model to perform and apply the Watts per Pound rule to your requirements. The Watts per Pound rule is a way of looking at the likely power required to fly a particular model at various levels of performance. It is only really valid when applied to a sports, scale or aerobatic type of model. Very clean glider type designs are more efficient aerodynamically and can manage with much less power. The following is a guide to the watts per pound you require for 'normal' electric flight:

  • 25 - 30 watts per pound - level flight.
  • 40 - 50 watts per pound - take off from sealed surfaces, reasonable climb.
  • 50 - 60 watts per pound - take off from grass, sports aerobatics.
  • 70 - 100 watts per pound - pattern aerobatics, long vertical climbs - the higher value being required for a more 'draggy' aircraft shape.

Combined with that, Graham says, you need to consider wing loading too. Graham would suggest 16 oz/sq. ft as a good place to be - his Electro Tutor design flew very well at that, with easy take offs, good climb and loops.

Choice of motor will follow depending on how many watts you require. Graham would take as a limit 25 amp full throttle, some are happy with 30 amp using it for takeoff and aerobatics only. Using that limit it is then possible to come up with the number of cells required. Use 1 volt per cell in the equation Volts x Amps = Watts.

All of this, though, depends on you having a reasonable understanding of Volts, Amps and Watts and probably the ability to use an Ammeter and already have a few motors and battery packs to test it all out on. A very good place to begin to get that understanding is Graham's excellent web-based publication Basic Guide to Successful Electric Flight which is available FREE on the McAllister Design website. This is where you will find the Watts per Pound rule and a wealth of other information,

If, like me, you still struggle with all this technology, then I reckon the following information on the two principle types of motors is the simplest, most straightforward advice we are likely to see to get us started - again, taken from Graham's book.

"SPEED 400. (Case length 37mm, dia. 27.5mm, shaft 2.3mm) Used as a direct drive motor (i.e. without a gearbox) the SPEED 400 will provide sufficient thrust for a streamlined sports or fighter type up to around 36" span. Likewise it will power a self-launching glider of around 60" to 65" wingspan. The motor being driven by 7 x 700mAh cells in both cases. Propeller size near to 6 x 4.

SPEED 600. (Case length 57mm, dia. 37mm, shaft 3.17mm) Used as a direct drive motor will power a clean aerobatic model or fighter type of around 48" span on a battery of 7 or 8 x 1200mAh to 2000mAh cells, driving an 8 x 4 or 8 x 6 propeller. Alternatively a self-launching glider up to around 78" wingspan, on 7 cells using an 8 x 4 or 9 x 4 propeller."

But there is one further way of finding out the ideal power train for your model, whether it's an own-design, an electric conversion or just your desire to beef up an existing model - use the power of your PC! Our 'flying doctor', Paul Alexander Cook, tells me that there is actually software available into which you can enter model details and the program will come back with motor/prop/batteries, etc., or you can enter motor/prop/batteries to calculate performance of the model! Two such programs are MotoCalc and ElectriCalc. I have linked their websites to the program names so you can take a look at them both, if they interest you.

MotoCalc offers a downloadable, 30-day trial and I gave it a try. For the novice in electric flight the program has a rather nice MotoWizard that prompts you for various parameters such as model weight, style of flying desired, type of wing (chosen from pictorial guides), and other simple-to-answer questions. Once done, the program itself will then produce a complete list of various options of motors/props/cell packs etc., from which you could make a choice. The experienced electric flyer can input proposed power train details to 'test' entire ranges of props, cells and gear ratios, filter out undesirable combinations by one or more of current limit, power loss, efficiency, thrust, pitch speed, or run-time - and much, much more (far beyond my comprehension, I must admit!). Take a look - it might be just up your street!

 

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