stunthanger.com
Engine basics => Engine set up tips => Topic started by: Steve Helmick on July 18, 2010, 05:54:38 PM
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Ball bearings, that is. How do you know when the bearings are kaput, and need replacement? Don't tell me "when they feel like there's sand in 'em." How do you recognize the fact from engine run characteristics? Noise? Fuel consumption? Exhaust color? Inconsistencies? What?
On second thought, how do you know when a plain bearing engine needs to be fitted with a new bushing, case, or scrapped? ??? Steve
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Hi Steve ,
The best way to tell bearing wear is by the amount of clearance between the race(s) and rolling elements .
If you can get a bearing chart , that coresponds to your bearings , it should give you a clearance spec for that particular bearing (unmounted) . Th chart might give you a spec of how much interference the bearing will take .
The problem with the small bearings , is that it is very difficult to get a "feeler gauge" in them , to measure them .
You could try , maybe , a dial indicator to measure the clearances ?
Hope this helps ...
Rob K.
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Steve,
I know I can hold the nose up and just move the prop back and forth. That usually tells me if there is a potential problem. But historically, if it seems flaky, I pull the engine apart and use a feeler gauge on the bearing. If it's out of spec, I replace it. Of course, this is if I can find the chart for the particular bearing to find out what the spec is. If not, I just replace it if it in anyway seems questionable. Usually, I got to the Bearing Shop in Tacoma. They can match about anything.
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The engine starts to show really weird symptoms like shutting off, losing power during the run, and things like that were what happened the first time I wore out a set of bearings. I don't have access to charts. ;D AND! If they *feel* funny when you slowly turn the prop, you can bet on it. Easy swap to do and cheap so I just do it whenever I suspect the bearings going south.
Big Bear
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Ball bearings, that is. How do you know when the bearings are kaput, and need replacement? Don't tell me "when they feel like there's sand in 'em." How do you recognize the fact from engine run characteristics? Noise? Fuel consumption? Exhaust color? Inconsistencies? What?
On the very rare occasions I had bearing issues, it was the sound it made. The moderately famous example was when the rear bearing went over the hill on a practice flight in the first round of qualifying at the 2003 NATs. It had a low-frequency sound like a 60-cycle hum in a hi-fi system. Very loud. It still ran fine and I actually flew an official flight with it like that and wound up a very close second on the circle after David. Actually, the runs that particular day were the first where I got the engine fully sorted-out, I run it exactly the same way since then. Ted's did the same thing after the W/C the next year, pretty familiar sound and also still ran fine.
Brett
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One thing to watch, first seen (by us) in Nelson T/R engines, is the lumpy feeling in the bearings when cold. This is due to incorrect tolerances for the bearing installation at room temp, BUT the engine does not work at room temp. Run the engine and when warm, you find an absolutely perfect silky bearing set-up.
If you're getting that lumpy feeling, check if you have a stunt engine set up with (for want of a better word) - Nelson tolerances. If you have, it will feel like this from new. If it develops later on you could have worn bearings or got some grit in them.