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Anyone know the part numbers for the bearings on a Thunder Tiger .36? I have a swap meet rescue that needs new ones. Thx

Anyone know the part numbers for the bearings on a Thunder Tiger .36? I have a swap meet rescue that needs new ones. Thx

   When I worked as a maintenance technician at a large commercial printing plant, I worked with and had to identify bearings that ran the range of what some call "sewing machine bearings" to the main blanket cylinder bearings on some presses that are about 10 inches in outside diameter and weigh about 25 pounds! We had a book in the maintenance department from Nachi Bearing, and it listed every bearing you could think of by dimension, style and number. I have  never looked for it on line but I bet they have that listed somewhere. The standard numbering system will be helpful in finding out what mystery bearings are. If we would have had more than one of those books, one might have come home with me!
   Type at you later,
  Dan McEntee

Mike,

What I'm not sure of is if TT had different models of 36's that used different size bearings. The Motorman makes a good point: if you have to take out the bearings anyway, then measuring them with a micrometer (or accurate caliper) will prevent you from buying something that won't actually fit. You generally don't need to measure to tenths of thousandths. Bearings come in English and Metric series, with well established sizes. (The leadtime for truly custom bearings can be 3 to 5 years, and the cost will make your eyes fall out of your head....And to get those, you don't take to a distributor, you go beg each of the manufacturers until someone puts you on their production schedule. The long lead item is grinding the rings, generally. Unless you need a seal. That can be slow, too.)  The engine manufacturer takes care of the fits with the sizes machined into the case and on the crank.

So the question is whether your bearings really need removal and replacement. If they are rusted, then even when cleaned their life may be fairly short and they will be noisy. However, if the engine was put away gunked up with castor you are in luck. Cook away in your crockpot with antifreeze for a day or two. Flush with brake cleaner. Spin the shaft (piston, liner, rod removed) and listen. If not noisy or does not feel gritty, I'd oil them up and leave them in. I've done this many times with engines that were just not worth putting a new set of bearings in. And some of them have run a long time after that.

The only engines I have had to watch the bearings really carefully on were racing engines, and of those, really only the F2C diesels. They just pound the daylights out of the main bearings--which tended to be undersize to achieve absolute minimum weight and case size.

The Divot

Ceramics are interesting. Here are a few thoughts.

If you have very high cyclic accelerations and the ball mass is a significant percentage of your total inertia, then there is something there to be had in terms of performance improvement. But we have little need to optimize that, considering where we are operating our engines.

I would use slightly different terminology about bearing failures. From long use, there will be millions of stress cycles and the metal will fatigue. The balls actually have higher contact stresses than the races due to the difference in the radius of curvature of the ball as compared to the race groove. So the balls will generally fatigue first in an all-metal bearing. This results in subsurface damage, and "spalling" which I suppose looks like a chip. Some people will look at a bearing and say the ball is “flaking.” To reduce these failures, you need to reduce either the stress levels, or the number of cycles. Since we run the the engine at the speed we need to run, it just means that you have to replace the bearings more often. As we did on the F2C engines. As you point out, replacing them right after they spall is too late—it generally trashes the inside of the engine. If you skid a ball on one of the pieces that comes off another ball, then things can go bad quickly.

If you are having issues with bearing loads and life, be sure you are getting races made from 52100 steel. (High carbon chrome steel.) Definitely do not get stainless, like 440C which has a lower allowable stress value. While the greater immunity to rusting sounds attractive, proper maintenance will prevent rusting and not leave you with even more life problems.

Before we all rush out to buy hybrids (ceramic balls in steel races), know that when a ceramic bearing lets go, it is at least as catastrophic. I recall sitting in the pit area of the speed circle at the Nats and watching Joey disassemble an OS 65 engine. When he got the backplate off, he poured out a handful of sand. That was all that was left of the balls. It had sandblasted the insides of the engine.

Because silicon nitride balls have a higher elastic modulus (ie. they are stiffer than steel) the ball will not deform as much as a steel one would at the point of contact therefore not distributing the load over a greater area, and the race actually sees higher stresses. There is a cross-over point somewhere that means that your failure mode changes (now races, not balls) and life is not improved further.

Sometimes the engine designer tries to make the engine more compact. And picks a bearing with a small cross section. Which therefore has smaller balls. Which go thru more rotations per crank revolution. Which cuts into the operating hours per million fatigue cycles.

And, as usual, engineering is a compromise. While the ceramic balls have a higher allowable dynamic stress level, they do not like impact loads. For Joe Bellcranks like me, if I hit the ground, I would assume that a hybrid bearing is gone unless I was flying over grass. Soft grass.


The Divot

There are multiple effects on bearings from heat, and all play a part:

1. Higher temps reduce the strength of the materials.  No real effect on silicon nitride, and small effects on the steels at our temps.
2. The change in bearing preload due to bulk temperature changes and gradients. Potentially a big effect on our engines.
3. The lubrication changes with temperature and operating state.

There are probably others, as well.


It is hard to know the separate temps of each of the components, so it's hard to know exactly how your fits are changing. The easy thing to do is to run the engine on a stand as hard as you expect it to run, then stop it and immediately check endplay again. Under no circumstances should you be able to "feel the balls." Don't know how to say it better than that. (Well, I do, but as I have been "counseled" by my peers, Stunt Hanger is not the appropriate forum for technical matters. Flat earth society, you know?) You of course could fill in some of the temps by external measurement (infrared thermometer) of the case. The case spacing will grow in length more per degree than the crank due to the difference in materials. You might get an idea of the crank bulk temp by just looking at the end of it. Most of the other effects should actually reduce bearing preload during operation, although the contact angle will also affected by the axial thrust to a small extent. All model engines I know of have DB preload geometry.

Bearings self-heat as they run due to friction. More so when run at higher surface velocities. Smaller balls equates to higher surface velocities. A primary function of lubrication is to cool the bearings. Easy to see in the main, less so in the front bearing. But the heat has to migrate further to get to the front, so it’s not as bad as you might expect. And with a front rotor engine, you are dumping coolant thru the crank on every revolution.

I will point out that some engines I have had were more sensitive to bearing internal clearances. In my case, on Nelson .15s, they wanted a bit more clearance than the usual C3 fit coming from Boca. I talked to Henry about this and he agreed, saying that he had success using parts that were a bit more controlled than the C3 spec required. He stated a preference for a specific European supplier that had just what he needed. Unfortunately, he did not have any (many?) left.

You can get an idea of how hot an engine maker thinks his engines will be run by how much of a shrink fit he uses. You don't want the fit between the outer race of the main bearing and the case to become loose at max operating temperature or you will pound the bearing and fret the case. Think black junk coming out the exhaust. I would think this is less of a concern on the front bearing, and yet, many times the front is harder to remove. Perhaps in part because the main often comes out with the crank and you need a tool to simulate the same “ease” when doing the front. Perhaps because the clearance from temp soaking the smaller bearing is less.

When guys go to a catalog to buy bearings, there are often logical questions. "Should I get seals, not shields?"  "I can't see how the front bearing gets enough lubrication. If I get double seals and a grease packed bearing will that be better?" And so on. Interestingly, in many cases you can get a variety of things to work, and seem to work well enough for that engine and their duty cycle to get acceptable life expectancy, that they conclude it is preferred. In fact, perfect. Well, it may be good enough, and still not be perfect. For example, grease packed bearings self-heat more. You are plowing the grease. Work is being done. There is no heat transfer due to flow-thru of lubricant. And bearings are often notoriously over-packed, not grease-plated. My preference is for a shingle shield on the front bearing (outside) and no other shields or seals. If the engine has a close fit between the shaft and the case between the bearings, the total leakage is controlled and limited. Check to be sure there is acceptable end play. And go have fun. I’ll say what every other Joe Bellcrank says---“Well, it’s working for me!”

I suspect that some of the control line speed guys are very knowledgeable about bearing selection and setup for model engines. We could probably learn from them.

Again, just a few more thoughts, for what they are worth. Forgive me for stating the obvious as I rambled.

The Divot

Lauri,

Why would you mix ball materials in a single bearing?

Dave

I have the  bearings for the  Thunder Tiger 36 in stock, and  YES both  front sizes

Randy