OT: Using carbon fiber to repair a fan blade.

I've got a Cyclone Rake leaf vac which works great up until the plastic, four-bladed impeller had one of the blades crack. The CR web site says the part is back ordered and I can't get anyone to answer the phone or respond to email. Nice. I could epoxy the crack in the blade but doubt that will hold. Could I layer some carbon fiber sheets onto this thing to give enough strength to be usable?

Thanks.

SteveF said:

I've got a Cyclone Rake leaf vac which works great up until the plastic, four-bladed impeller had one of the blades crack. The CR web site says the part is back ordered and I can't get anyone to answer the phone or respond to email. Nice. I could epoxy the crack in the blade but doubt that will hold. Could I layer some carbon fiber sheets onto this thing to give enough strength to be usable?

Thanks.

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I'm not an engineer... But unbalanced things rotating at high RPMs tend to make me... skeptical.

I can be very anal about rebalancing things, not worried about that part. Know a guy with a small foundry who could probably use the part to make a mold and make me a cast aluminum one. Getting a casting defect that might cause a aluminum blade to hit me in the back of the head, that I would worry about.

Apply the same carbon fiber sheets to all the blades and it should be good enough. Probably. You'll have to be pretty anal about keeping the resin squeeze consistent too, a fat sheet on one side would be as bad as not applying it.

Edit to add: do both sides if possible. Otherwise make sure the repaired side is always under tension during operation.

PPS: Lost of plastics don't bond to adhesives or epoxies all that well. Be careful. I would not do it, but if you want to try anyway, wrapping all the way round is your best bet. Performance will possibly suffer a bit due to the weight, thickness, and geometry changes.

i would try melting the plastic back together before gluing it together. use a temperature controlled soldering iron if you have one.

What's the impeller diameter and rpm?

I've found this works amazingly well on most plastics:

Plastic Bonder Syringe - Black | J-B Weld

Laying some CF on top of the crack won't do anything IMHO, regular epoxy probably won't bond to the plastic and unless you vacuum bag it CF cloth isn't all that strong.

one interesting problem with epoxy and plastic is, the epoxy is often stiffer than the plastic, which creates stress risers at the interface.

sometimes a flexible material like for example, urethane windshield adhesive, will result in a stronger repair than epoxy will, even though the urethane only has a 1200 psi tensile strength compared to epoxy at 3000 psi tensile (if you can ever get anything near that when it comes to the adhesion to the material. )

Strostkovy said:

What's the impeller diameter and rpm?

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https://youtu.be/6rxGQrXCrts

About 12" and 3600 rpm from a Briggs 6.5 HP

If one blade cracked then remove all of the weakened part.

Not remove material from blades opposite it.

If even number just one, if odd number 2.

Make a mandrel with ball bearings to support it so it can freely rotate and remove material until balanced.

It will do for now.

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SteveF said:

I can be very anal about rebalancing things, not worried about that part. Know a guy with a small foundry who could probably use the part to make a mold and make me a cast aluminum one. Getting a casting defect that might cause a aluminum blade to hit me in the back of the head, that I would worry about.

Click to expand...

You can take that same idea and instead of aluminum use epoxy for casting. There are some good videos on youtube on how to make silicone molds and use them for casting parts with epoxy. You could probably reinforce the epoxy with carbon or glass fiber bits too. Here is one vid to get some inspiration from :

Resin Casting Headphone Parts - YouTube

I checked out the CR video and a few additional ones out of curiosity, and my take is that you'll have a very tough time trying to repair the unit you have with any adhesive. The combindation of vibration and extra flexing that occurs with rock impacts would likely destroy a bond fairly quickly.

What I'd do (as a crazed machinist), is make an Al hub about 4" in diameter with four notches it to receive flat plastic fan blades cut from a robust polyurethane about 3/8" thick. Each notch would be a taper shape, with the narrow end towards the OD, then have a wedge block that fits the slot and bears against the side of the plastic blade.

The idea is that centrifugal force would tighten the wedge as the assembly rotates, retaining the blades but allowing replacement by pushing the wedge back towards center.

Actual design should include some locking fasteners to hold everything from loosening, as well as maybe a fancier root of the blade to help with initial retention (i.e. it's wider at the root).

I'd not make any blade from metal, I figure if the housing isn't designed for blast containment a metal blade could burst it and cause some serious injury or death.

My advice comes with no warranties/guarantees/manatees, for entertainment purposes only. May cause warts and/or bad breath, consult your doctor if symptoms last more than four decades.

Yes ^^^ forget the bonding to plastic aspect of the job, make all new from CF
and with the aforementioned aluminum hub.

Silicone mold casting is great if you can stand the expense for the materials and have access to a vacuum pump to degas the stuff. You can also buy fillers for the resins, carbon fiber or even chopped Kevlar fiber. Maybe use something like one of the Task series like Task 9. TASK™ 9 Product Information | Smooth-On, Inc.

Melt some Stainless mesh into the blade, both sides and opposite blade too.