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tragedies #5: My planer's broken bearing

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Blog entry by Gary Fixler posted 1740 days ago 3750 reads 0 times favorited 14 comments Add to Favorites Watch
« Part 4: Jimi_C nails it - my planer suffered a blown bearing Part 5 of tragedies series Part 6: Anybody figure out what I did wrong the other day? »

I got some shots of the gruesome outcome inside the machine. The replacement bearing has been ordered through Sears Parts Direct and should be here in a week or two. Meanwhile, the carnage…

broken bearing and shaft

Here are some closeups of the broken bearing from the side where it opened up:

broken bearing

Note the broken cage and missing balls. No idea where they went:

broken bearing closeup

The shaft was pitted beneath the bearing. At first I thought it was damage, and it may be, but I’m leaning a bit toward it being a deliberate roughing-up of the surface to help hold the bearing in place through its high-vibe activities. That’s the story I’m sticking with. let me know if you think otherwise.

pitted shaft

I think if the bearing itself caused pitting, it would be more ‘smeared’ around the shaft. This doesn’t look ground in. It looks like someone took a pin and a hammer and banged in some dents somewhat evenly around the shaft before sliding on the bearing. I didn’t note any ground-up metal falling out from the bearing when I pulled it free with the gear puller. Again, who knows?

pitted shaft closeup

pitted shaft

Here’s a blurry shot with my finger for scale:

pitted shaft with finger for scale

The current, full gallery of pics in this mini saga is here.

-- Gary, Los Angeles, video game animator



14 comments so far

View ajosephg's profile

ajosephg

1850 posts in 2194 days


#1 posted 1740 days ago

I think you are on the right track. Here’s a follow-on theory. The shaft was under tolerance and the bearing was sloppy when installed on the shaft. They then roughed up the shaft with a center punch and then forced the bearing on with a (hammer?). This lead to a premature failure due to:
1. Damage caused by impact damage when it was forced onto the shaft.
or
2. Normal heat generated by the bearing was not able to flow out of the bearing into the shaft leading to overheating of the bearing leading to lubrication breakdown and the ultimate failure of the bearing.

If this is true, you may find the new bearing won’t fit properly. :(

You might ought to check the other side and see if it is going down the same path.

-- Joe

View davidroberts's profile

davidroberts

1002 posts in 2118 days


#2 posted 1740 days ago

The pits are random. Do you think a previous owner milled the pits. I don’t think the manufacturer did so. Maybe those guys at OWWM have seen this before.

-- God is great, wood is good. Let us thank Him for wood......and old hand tools.

View JJohnston's profile

JJohnston

1577 posts in 1924 days


#3 posted 1740 days ago

I’m with ajosephg: that’s definitely “peening”, done to displace metal so the bearing will fit on an undersize shaft. You’d be wise to go ahead and replace the whole cutterhead while you have the planer apart.

-- "Sorry I'm late. Somebody tampered with my brakes." "You should have been early, then."

View Bob Kollman's profile

Bob Kollman

1796 posts in 1823 days


#4 posted 1722 days ago

Gary,
who ever did it very uncool. A machine shop repair would be to turn the shaft down, press a sleeve on, then press the bearing on over the sleeve. But I have never seen anyone do that. Even in china if the shaft was under sized I would think they’d throw it away. I would mike out the other side of the shaft, there will probably be a diffrence of .0015 or more. JJohnston, makes a good argument for changing the whole cutter head. Press fits the Shaft will be .001 – .002 larger then the I.D. of the bearing. Usually when I have a tolerance for a bearing surface it will be + .001 and -.000. Good luck on your rebuild. Bob

-- Bob Kenosha Wi.

View MedicKen's profile

MedicKen

1599 posts in 2095 days


#5 posted 1722 days ago

I think joe is right on track. The bearing was the wrong size and the previous owner dimpled the shaft with a punch and then forced the bearing on. If you get the proper sized bearing you may have some difficulty getting it to seat properly. If it too loose I would use loctite, the type to set engine cylinder sleeves and valve seats. The other option would be take the shaft to a machinst and have the bearing surface knurled to hold the bearing.

-- My job is to give my kids things to discuss with their therapist....medic20447@gmail.com

View alanealane's profile

alanealane

365 posts in 2523 days


#6 posted 1641 days ago

I’m assuming you may have already resolved this problem since the last post (80 days ago). That’s a long time to have any shop tool out of order.

Just in case your planer is still down:

I second JJohnston’s comment about replacing the cutterhead. The only other thing you might be able to do is have someone weld more steel onto the area where the peening was done, then have a machinist re-grind the shaft to the proper size. This is risky because the heat during welding could cause the cutterhead to slightly warp, thus ruining its concentric tolerance (it might wobble).

Bob Kollman’s idea above might work well…have a machinist turn the shaft until the peen holes were gone, press on a sleeve, and press the bearing over that. But machining the shaft too small could also potentially ruin the keyseat and the threads on the end of your cutterhead

I’d just find out from the manufacturer what a new cutterhead would cost. I’d guess around $100.

I know all too well the perils of planer maintenance. I just overhauled my DeWalt 733 with a new motor housing, gearbox housing, and feed roller driveshaft. It’s easier to just pay the manufacturer for new parts, rather than pay a machinist (unless you have a good machinist friend who will do you a favor). And a new planer is certainly out of my budget right now, but I HAD TO HAVE A WORKING PLANER, so I ordered the parts and spent the time to fix it.

Let us know if everything’s fixed. I hope it went well!!

-- Lane Custom Guitars and Basses

View sdlawrence's profile

sdlawrence

4 posts in 1422 days


#7 posted 1422 days ago

Hi Gary, sorry for dredging up an old post, but this one had me intrigued enough to create an account just to comment on this one thread of yours.

I don’t think the pitting on the shaft was a result of “peening”, or anything artificial on the part of the manufacturer or the previous owner, as was suggested. The surrounding areas of many of those pits reveal little eccentric circular regions, ‘teardrops’ and other shapes where the steel was actually tempered—locally. Those are hot spots – localized areas that reached extremely high temperatures relative to the adjacent regions, with tiny, once-molten areas evacuating those regions, spreading out radially, creating small pits as a result. Peening a shaft could create the pits, but that’s all. It could not have produced that tempered effect by itself.

It looks like whatever happened was caused in situ, and that the pits are mere symptoms, or clues, rather than the actual cause. Your catastrophic failure appears to me to have resulted from a phenomenon known as either the “Morton” or “Newkirk” effect (depending on whether or not a ‘rub’ was involved); a synchronous rotor instability (vibrational, thermal) from an unbalanced journal bearing.

http://turbolab.tamu.edu/uploads/files/papers/t37/T37-TUT02.pdf (small .pdf file)

Your photos are great. I wish I could have had an accurate flat “map” of the entire circumference of the shaft for further analyses, but on the surface that’s what it looks like to me. As a test, you might see steel-tempering evidence of the hot spot effect transferred onto the bearing itself, once disassembled and cleaned.

View ajosephg's profile

ajosephg

1850 posts in 2194 days


#8 posted 1422 days ago

sdlawrence:

Very interesting paper – but really technical. I didn’t spend a lot of time with it, but have a major question.

The paper says the phenomenon occurs in “flluid film” bearings. The affected area on the planer shaft is not a fluid film bearing. If the roller bearing assembly is properly fitted to the shaft there is very little relative motion between the bearing and there is only a bit of assembly lube there.

So, can you explain further?

-- Joe

View JJohnston's profile

JJohnston

1577 posts in 1924 days


#9 posted 1422 days ago

Hmmm. I have to agree. The bearing in question is not a fluid film bearing, so the phenomenon described in the paper isn’t going to happen (unless the bearing were so loose on the shaft that the shaft acted like the bearing journal and “orbited” inside the inner bearing ring – but if it had been that loose, and never peened, the bearing would have just fallen off). Those teardrop shaped areas could have been smeared as the bearing turned on the shaft when it shouldn’t have.

-- "Sorry I'm late. Somebody tampered with my brakes." "You should have been early, then."

View sdlawrence's profile

sdlawrence

4 posts in 1422 days


#10 posted 1421 days ago

I’m actually not certain of anything, and it’s all speculation on my part, of course, but, in a nutshell, all the Morton and Newkirk effects really established was that unbalances could produce localized hotspots (and eventual catastrophic failure) and vice versa. Also, there is no way to know, of course, if the bearing was properly fitted to the shaft, or even if the bearing itself was defective in some other way that might eventually produce destructive vibrations.

JJohnston: “Those teardrop shaped areas could have been smeared as the bearing turned on the shaft when it shouldn’t have.”

I thought about that, but if you look closely, some of the rings that are eccentric to the pits are in the opposite direction, on a shaft that presumably turns only in one direction (braking?).

It was more food for thought than anything. The localized tempering of the pitted areas fascinated me, and caused me to tend to discount peening for a moment and consider other possible causes.

Either way, how fun is it to put on the forensics hat and puzzle stuff like this through!

View ajosephg's profile

ajosephg

1850 posts in 2194 days


#11 posted 1421 days ago

Yep – Failure analysis is fun stuff.

Do you think some sort of electrical discharge could have made those pits?

LOL – You don’t happen to have an SEM in your bag of trips?

-- Joe

View sdlawrence's profile

sdlawrence

4 posts in 1422 days


#12 posted 1421 days ago

I wish! SEM imaging of a few of those pits would be great, wouldn’t it? Two things I wish I owned, as toys: an SEM and an ion implanter (make pistons that last forever, etc.,).

I certainly wouldn’t discount electrical charges, and you may be onto something there. My first thought was in the plasma sense, as evaporated molten metal could superheat to a tiny ball of plasma (oxidized and highly charged, electrically), which then etches into the surrounding area. But there could be an even simpler explanation for its cause. A grounding problem? Parasitic capacitance? Does this planer have VFD motor?

”The VFD waveform contains high-frequency components that are capacitively coupled to the motor shaft and discharge through the bearings. They are non-sinusoidal and contain high-frequency currents and voltages called harmonics. And even when the motor is designed for inverters, it is vulnerable to bearing failure from VFD-induced currents.

Without some form of mitigation, these shaft currents or eddy currents discharge to ground through bearings, causing pitting, fusion craters, and “fluting.” This leads to excessive bearing noise, premature bearing failure, and subsequent motor failure.” (emphasis mine)

View JJohnston's profile

JJohnston

1577 posts in 1924 days


#13 posted 1421 days ago

You guys are getting too esoteric for me, but just remember the machine we’re talking about, and the circumstances under which it was built. It’s an inexpensive, hobbyist-level machine, built as one of thousands in a mass-produced, get-it-out-the-door environment. My bet is still on the simple explanation.

-- "Sorry I'm late. Somebody tampered with my brakes." "You should have been early, then."

View ajosephg's profile

ajosephg

1850 posts in 2194 days


#14 posted 1421 days ago

No VFD motors in these planers. They use bucks down series wound AC motors. Additionally, the planer shaft is chain driven, i.e. it is not the motor shaft.

I would guess if it’s due to an electrical discharge the source would have to be a static discharge or some sort of leakage in the motor that finds its ground via the motor shaft, to the chain drive, to the planer shaft through the bearing and finally to the chassis ground. Conversely a static build up could conceivably build up as a result of the knives scraping the wood and discharging through the bearings.

Hey Gary – if you’re still around, how are the replacement bearings working? If there is some electrical issue you should be looking forward to another bearing failure, LOL :(

-- Joe

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