Stihl BR 400 Blower keeps breaking woodruff key

[trooper1954]

Hi all,
Resurrecting a thread from 2 year ago about a Stihl 400 blower that keeps shearing the woodruff key.
Changed 2 myself but broke them straight away....finally broke down and took it to a dealer who fixed it...got it home....yes....broke straight away.
The shop mechanic with 30 years experience said he wont look at it again!!
When I made the original post a fellow called M6erfan said the following;

If the flywheel is a little loose on the crank you can use some lapping compound and rotate the flywheel back and forth on the crankshaft. Check for an even dull finish and repeat until you have a lapped joint.

Could he, if he reads this, or someone explain this procedure a little more in detail, and tell me when I know I've reached a "lapped joint"?

I've just pulled it out of the back of the shop, bought some valve grinding permatex and want to give this another chance. It has been a great blower and I just don't throw it away...will try and find a used flywheel if it's deemed necessary.

Thanks in advance for any and all help.

 

[The Cobbler]

take the flywheel off, take the key out. apply valve grinding compound to the inside of the flywheel. set it on the crankshaft without the key/nut etc. slowly turn it back & forth on the shaft .
you will be able to tell where the 2 mating surfaces are touching or not touching.
you can also use blue sharpie to do the same thing, where it touches will wear off the sharpie .
but, is the flywheel loose when the key breaks or still tight?

 

[ericg]

Check out this article. Guy had the same issues with broken flywheel keys on a chainsaw

I keep shearing flywheel keys

I have a Jonsered 451EV that for some reason all of a sudden it shears flywheel keys after every 5-10 minutes of running. I never had any problems with this saw before. Does anybody have a clue what could be going on?

www.arboristsite.com

 

[ericm]

The key is just supposed to locate the flywheel. The taper is what keeps it from turning.

Make sure there's no raised edges on the shaft along the keyway and that the taper in the flywheel is clean. Then lap them together.

 

[PCustoms]

Usually when you resurrect a thread you do it in that thread...

Problem with Stihl BR 400 blower

Hi all, I have a 30 year old blower that has served me well and given me no problems until now. The problem is it keeps breaking woodruff keys....I have replaced three myself, and each time the key breaks after a very short while, and the blower is a bear to pull start. I finally took it to a...

www.garagejournal.com

Have you done anything since 2022?

Before you lap, use dye or a heavy marker to mark the shaft, then fit the flywheel. A perfect fit would wear off all the die when the flywheel is fit to the shaft (skip the key here), my guess is you have a few points of contact.

Apply lapping compound, spin the parts together for a bit (again, no key) clean and repeat the dye check to see if there's an improvement in the fit.

 

[trooper1954]

Apologies PCustoms...copy that.

 

[purplezr2]

I would use something like dykem or Permatex Prussian blue to see what the taper fit is like.

I tried to find a youtube video with no luck.

Basically paint the taper with bluing compound, and install the flywheel, then remove flywheel it will show you what type of contact you are getting, which should lead you to answer some the the questions above about contact and lapping.

 

[Wamsutta]

So what's the underlying issue here? Is it the area of the crankshaft not making enough contact with the flywheel? Is it because the crankshaft is tapered and the flywheel isn't? What does the lapping compound do; increase the area of contact?

 

[PCustoms]

So what's the underlying issue here? Is it the area of the crankshaft not making enough contact with the flywheel?

That's the theory. A proper taper fit "locks" and transmits torque. Right now the key is taking all the load and failing.

Is it because the crankshaft is tapered and the flywheel isn't?

It's because the taper are not matched.

What does the lapping compound do; increase the area of contact?

Lapping compound will, well, lap the 2 parts into a perfect match. It's a light abrasive in grease, and will basically sand the 2 pieces to each other.

 

[Wamsutta]

That's the theory. A proper taper fit "locks" and transmits torque. Right now the key is taking all the load and failing.



It's because the taper are not matched.



Lapping compound will, well, lap the 2 parts into a perfect match. It's a light abrasive in grease, and will basically sand the 2 pieces to each other.

OK that explains it right there. For some reason my mind could not fathom the idea of them being two different tapers. That would definitely be an issue. I'm very surprised a manufacturer like STIHL would make that error.

 

[LopezBart]

All it takes is a tiny burr somewhere to cause this to happen. If it's a mismatched taper, it's a factory defect and not likely a one-off problem.

Question about Stihl BR 400 blower [Archive] - Shop Floor Talk

[Archive] Question about Stihl BR 400 blower Mechanical & Electrical

www.shopfloortalk.com

This video show someone lapping in the flywheel on a ATV:

 

[rust in the eye]

If the interference fit is satisfactory the flywheel will be "stuck" a bit once it's been tightened. If you can easily remove the flywheel by hand the fit is too loose.

 

[LopezBart]

If the interference fit is satisfactory the flywheel will be "stuck" a bit once it's been tightened. If you can easily remove the flywheel by hand the fit is too loose.

This depends on the angle of the taper... steeper tapers are always self-releasing, and require more clamping force to generate the friction needed to resist inertial torques... an R8 collet has a steep taper... shallow tapers such as those used for drill press chucks are self-holding, as are B&S tapers used for drills. Mechanical engineering nerd detail: if I remember right, if the arc tangent of the taper's angle is less than the coefficient of friction, the taper is self-holding...

 

[Firebrick43]

I have found normal valve lapping compounds such as clover to coarse for flywheel fit ups and use diamond compounds in grease. It cuts faster and smoother, I start with red(30 micron) and switch to blue(15 micron). Very little diamond compound is necessary

While lapping the surface will frost and you can see your work, it will have a different look as you progress through the grits. . To check you can color in the surfaces with a black sharpie after you clean them well with alcohol and spin one time around and check to see that it was evenly and completely rubbed off on the areas where the flywheel and crank mate.

 

[PCustoms]

I have found normal valve lapping compounds such as clover to coarse for flywheel fit ups and use diamond compounds in grease. It cuts faster and smoother, I start with red(30 micron) and switch to blue(15 micron). Very little diamond compound is necessary

While lapping the surface will frost and you can see your work, it will have a different look as you progress through the grits. . To check you can color in the surfaces with a black sharpie after you clean them well with alcohol and spin one time around and check to see that it was evenly and completely rubbed off on the areas where the flywheel and crank mate.

You do know that there's like four different grits of clover, correct?

I'm pretty sure it is just diamond compound in Grease anyway

 

[captain14]

M6erfan is still active on GJ. Maybe send him a PM with a link to the two threads so he reply back for all to see?

 

[PCustoms]

M6erfan is still active on GJ. Maybe send him a PM with a link to the two threads so he reply back for all to see?

Or use the @ symbol to tag him

@M6erfan

 

[Firebrick43]

You do know that there's like four different grits of clover, correct?

I'm pretty sure it is just diamond compound in Grease anyway

There is 8 and the finest (800) is still coarser and slower cutting than the the the 15 micron (1200 equivalent) paste.

Even the 30 micron (600 equivalent) paste doesn’t feel gritty compared to the clover.

Clover is NOT diamond. It’s silicone carbide.

 

[finn]

OK that explains it right there. For some reason my mind could not fathom the idea of them being two different tapers. That would definitely be an issue. I'm very surprised a manufacturer like STIHL would make that error.

The tapers are supposed to provide an interference fit when the nut on the end of the shaft is torqued to spec. If the nut is insufficiently torqued, the tapers will develop fretting, or fretting corrosion, which is manifested as wear on the tapers. Once the tapers are worn, the motion accelerates, which leads to more wear.

If you can’t lap in the joint, the machine is essentially worn out junk.

As others have stated, it’s clamp force from tightening the nut that transmits torque. The key is strictly for indexing the flywheel for the ignition timing to be correct. The sheared key would be considered a victim part failure, not the root cause.

 

[trooper1954]

Thanks to everyone for the comprehensive help.Off to the hardware store to get Prussian blue, then down to the shop to see what damage I can do! I'll update with any progress.

 

[Wamsutta]

The tapers are supposed to provide an interference fit when the nut on the end of the shaft is torqued to spec. If the nut is insufficiently torqued, the tapers will develop fretting, or fretting corrosion, which is manifested as wear on the tapers. Once the tapers are worn, the motion accelerates, which leads to more wear.

If you can’t lap in the joint, the machine is essentially worn out junk.

As others have stated, it’s clamp force from tightening the nut that transmits torque. The key is strictly for indexing the flywheel for the ignition timing to be correct. The sheared key would be considered a victim part failure, not the root cause.

Thanks for the info Finn. Very useful.

 

[The Cobbler]

M6erfan is still active on GJ.

Actually, he was shown the permanent exit door . He's no longer active.

 

[hoston23]

What serial number do you have??? Some of the stihl backpack blowers are having issues with the rist pin failing, causing them to seize up to the piston. You could have an issue with that causing the key to shear.

 

[rd65]

What serial number do you have??? Some of the stihl backpack blowers are having issues with the rist pin failing, causing them to seize up to the piston. You could have an issue with that causing the key to sh

The piston pin should have nothing to do with any load on the flywheel.

 

[rd65]

Is the fanwheel bolted to the flywheel? I cant tell from looking at the parts breakdown. If so, check to make sure that the fan isnt causing any load to the flywheel. Stihl does state to clean the flywheel/crankshaft mating surfaces with degreaser before assy. The service manual is not listed on Stihl site, but the flywheel and crankshaft from that model is also found on a BR340/420 that does list a service manual. The listed torque for the flywheel nut is 25 Nm. I put a length of rope (old recoil starter rope) down the spark plug hole to act as a piston stop.

 

[Fixr]

Another thought: I have run into aluminum small engine flywheels with the taper "stretched" by overtightening the nut. This can lead to the flywheel bottoming on a step on the crankshaft without seating on the taper.