Metal failure

I was vacuuming leaves the other day when the "6.5 hp" Harbor Freight-powered vac let out a terrible bam-bam-bam... and started slowing quickly. Quickly shut off and locked tight. Oh, crap! Up to my ass in leaves and a broken vac!

It had to be the engine, of course, but when I got it in the shop, I found this:

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When the blade failed, its backing plate bent and the blade tip started hitting bolt heads in the case. All the blades had real cracks - one is circled - and were not far from failure. I can think of no other reason than centrifugal force. The blades are about 3/16" thick - pretty surprising that they would crack like that.

Now, here's the best part: I had a spare impeller in the shed! Who keeps spare impellers? Impellers don't fail, why would anyone have a spare? You're looking at him, as they say.

Fixed in a flash and back to the leaves. Very satisfying.

Bob

Reply to
Bob Engelhardt
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Bob, my best guess is that the steel used was substandard (ie perhaps too much sulphur), and welded in a way that left some residual stress.

Reply to
Ignoramus30301

You'll pay for that...later.

Reply to
Tom Gardner

Ugh. FWIW, the centrifugal force is unlikely to have done it. Without a macro-level photo of the break, it's hard to tell, but it probably is fatigue, possibly on weakened areas that, as Iggy says, could have been bad welds.

When a fan or turbine spins, unless it's precision-balanced, it will vibrate. Ekven a balanced one can vibrate if the air currents are "noisy" from preceding blades. That can put a lot of stress on fan blades.

Reply to
Ed Huntress

That makes a lot of sense. More so when you consider that it's being driven by a 1-cylinder gas engine, hardly a smooth power source.

Maybe I should grind stress relief at the top of the blades where they meet the hub. Like so (engage imagination):

/ ____ / U | |

Bob

Reply to
Bob Engelhardt

I can't help you there. If it's fatigue, it's very dependent upon two things: the number of cycles it goes through, and the percentage of yield strength that the load applies.

Steel has a very long fatigue life unless it's stressed close to the yield limit. (Aluminum, for the record, has much less resistance to fatigue.) If you reduce the thickness or length to which the load is applied, by grinding in relief, the result could be worse than just leaving it alone.

That depends on whether you're starting off with microcracks at or near the surface of the welds. If there are microcracks, they're starting points for fatigue.

Unless you're making parts for an aircraft turbine, trying to calculate these things is not worth it. Just go with your sense of how metal behaves and be thankful your fan isn't made out of some expensive superalloy. d8-)

BTW, look closely at the breaks. If some part of the break area is shiny, you're dealing with fatigue. It could be a ring around the break, or some area across the break. When metal fatigues it "frets," or rubs against itself, before it fails. That usually leaves a small shiny area. As the break progresses, the area to which the load is applied gets smaller, until the metal breaks from being overstressed. That part of the break is likely to be dull or crystalline in appearance.

But it's not so cut-and-dried. Sometimes it's hard to tell.

Reply to
Ed Huntress

Bob, are you sure you didn't vacuum something other than leaves? Like a rock or piece of metal lying in the leaves?

Paul

Reply to
Paul Drahn

AKA Made in China

Reply to
clare

That would be my guess too. It doesn't take long, just a few seconds to make a mess that looks like it took many hours to create. The offending piece/part could have easily been thrown out and is many feet away.

The "hammers" used in tub grinders have been known to break and get thrown. They have gone through roofs and walls many blocks away from where the grinder was operating...

Reply to
Leon Fisk

I'm pretty sure that I have, over the years of using it. But I don't think that was a rock: there isn't a dent in the blade. And all the blades are cracked - a single rock would only be hit by a single blade before being ejected. It might be that the accumulated impacts of pebbles over the years was what caused the fatigue.

Bob

Reply to
Bob Engelhardt

When I do suck up something larger than sand, I know it. There is a single whang when a blade hits it and it's ejected.

Bob

BTW - the leaf vac is a set up of my power wagon. See page 9 here:

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Reply to
Bob Engelhardt

That's usually the problem. What got sucked up didn't get thrown out right away. I remember something similar happening years ago but I can't remember it clearly enough. Been bugging me ever since you posted this. Anyway... it happened so fast and looked so bad it was hard to believe all the damage done in just a few seconds. It was probably when I worked on a farm ~40 years ago... I do remember pulling odds & ends out of the grinder-mixer. It could bring a 50hp tractor to a stall if you pushed the input too hard. Typical hammer-mill with different sized screens for determining how coarse the output feed would be. You would have to shut down and remove the screen to find the foreign noise maker. A lot of times you really couldn't tell what the offending beat up piece of metal taken out once was...

I like it, nice write up. I have a similar version in my mind but I still need the exercise. Also agree that the DR version costs way too much. If I live long enough though, there will come a day...

Reply to
Leon Fisk

I had taken the impeller to the dump yesterday. I went back today, intending to retrieve it, cut out a sample of a crack, and examine it for signs of fatigue. It was gone! Who would want an old, really beat up, impeller?

Reply to
Bob Engelhardt

I bartered some cast iron for a rusty wagon front axle with an old man who picks up -any- scrap metal to sell to the junkyard.

I was hoping it might be wrought iron, but a little cleaning showed that the rust pattern isn't directional.

Reply to
Jim Wilkins

Hmm...somebody who wants an excuse to give to his wife for not gathering up the leaves? d8-)

Reply to
Ed Huntress

That's good! I'm going to have to stockpile a few broken parts to use that with. "Oh, honey, my squeegee is broken - I can't do the windows today."

Reply to
Bob Engelhardt

In my house, I could get multi-season use out of it. "Sorry, hon, but the lawnmower blade is broken." "Oh, sorry, babe, but the fan for the weed-wacker just took a dive." "Omigosh, the snow blower compressor just broke into pieces."

My wife is not a machinery type. d8-)

Reply to
Ed Huntress

My wife would just give me "the look" and tell me "You've got a machine shop . Either fix it or make a new one . " . Sometimes a blessing is also a curse .

Reply to
Terry Coombs

At least THAT would give you an excuse and permission to go out to the shop and work on something (else)

Reply to
clare

In Mensa we made a game of trying to fit a previously chosen odd word or phrase smoothly into a dinner conversation. I'm going to see how long it takes to use "My squeegee is broken".

Reply to
Jim Wilkins

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