Rounded off driveshaft STIHL trimmer

[Merkava_4]

STIHL FS 91 R

Yesterday while cutting a customer's grass in the backyard, the trimmer head all of a sudden stopped turning and I could hear this ZING sound coming from the clutch drum. My first thought was I either fried the clutch or broke a clutch spring. Come to find out after getting the trimmer home and upon disassembly, the driveshaft that goes into the clutch drum was rounded off. It's supposed to be a perfect 5mm square. The other end of the driveshaft that goes into the gearbox was perfectly fine.

My suspicion of why the driveshaft got rounded off was due to running the trimmer line too far out. The blade on the guard will cut the line somewhere between five and six inches. At times I've had the trimmer line out as far as eight inches especially on low load situations such as edging a sidewalk.

It's running the line too far out while cutting wet tall grass is what I suspect rounded off the driveshaft end.

You may wonder why I took the guard off. The short answer is I feel more coordinated with the trimmer with the guard off, especially while edging.

If all I had to do it cut grass and never have to edge, I'd probably have the guard on.

One thing for sure is that the guard cuts down on a lot of green splatter that's constantly spraying me.

What I used to have is a little piece of 1/4 inch plastic tubing cut to a length of six inches.

I would slide that tubing over the trimmer line up against the eyelet and cut the portion of line off that extended past the end of the tube.

But I lost that little piece of tubing.

Ace Hardware had a new driveshaft in stock last night. I couldn't believe it.

Lastly, I wonder if a splined driveshaft would've held up better.

 

[EFSM]

STIHL FS 91 R

Yesterday while cutting a customer's grass in the backyard, the trimmer head all of a sudden stopped turning and I could hear this ZING sound coming from the clutch drum. My first thought was I either fried the clutch or broke a clutch spring. Come to find out after getting the trimmer home and upon disassembly, the driveshaft that goes into the clutch drum was rounded off. It's supposed to be a perfect 5mm square. The other end of the driveshaft that goes into the gearbox was perfectly fine.

My suspicion of why the driveshaft got rounded off was due to running the trimmer line too far out. The blade on the guard will cut the line somewhere between five and six inches. At times I've had the trimmer line out as far as eight inches especially on low load situations such as edging a sidewalk.

It's running the line too far out while cutting wet tall grass is what I suspect rounded off the driveshaft end.

You may wonder why I took the guard off. The short answer is I feel more coordinated with the trimmer with the guard off, especially while edging.

If all I had to do it cut grass and never have to edge, I'd probably have the guard on.

One thing for sure is that the guard cuts down on a lot of green splatter that's constantly spraying me.

What I used to have is a little piece of 1/4 inch plastic tubing cut to a length of six inches.

I would slide that tubing over the trimmer line up against the eyelet and cut the portion of line off that extended past the end of the tube.

But I lost that little piece of tubing.

Ace Hardware had a new driveshaft in stock last night. I couldn't believe it.

Lastly, I wonder if a splined driveshaft would've held up better.

I’ve seen this many times. Splined driveshafts work way better. I don’t think I’ve ever seen one stripped out. The Stihl flex cables are actual more reliable, in my experience, since they absorb the shock and don’t wear as fast. You can flip the driveshaft if you’re in a real bind (put the stripped out end down in the gearbox, since it’s always the clutch end that strips).

 

[Merkava_4]

I’ve seen this many times. Splined driveshafts work way better. I don’t think I’ve ever seen one stripped out. The Stihl flex cables are actual more reliable, in my experience, since they absorb the shock and don’t wear as fast. You can flip the driveshaft if you’re in a real bind (put the stripped out end down in the gearbox, since it’s always the clutch end that strips).

I need to start paying attention to my line length. Either that or go back to the guard.

 

[EFSM]

I need to start paying attention to my line length. Either that or go back to the guard.

Actually, it’s a problem with those trimmers no matter what you do. High hours will wear the shafts out, especially if the anti-vibe bushing is worn and letting the engine droop. That’s what wears the upper end. Replace the rubber or add a shim to the top side to realign the engine to driveshaft.

 

[Merkava_4]

Actually, it’s a problem with those trimmers no matter what you do. High hours will wear the shafts out, especially if the anti-vibe bushing is worn and letting the engine droop. That’s what wears the upper end. Replace the rubber or add a shim to the top side to realign the engine to driveshaft.

Are you talking about the rubber bushing that's in the fan housing? #10?

I've got two of those fan housings and rotate them out because the bearings inside them get dry.

 

[EFSM]

Are you talking about the rubber bushing that's in the fan housing? #10?

I've got two of those fan housings and rotate them out because the bearings inside them get dry.

View attachment 480624

Yes. I’ve already jammed a shim of inner tube rubber between the top of #11 and #10, wrapping it over the top of the aluminum clamp. That helps to align things if everything is a bit worn.
The clutch bearings are molded into the housing. IIRC they are shielded and not sealed, so they draw in moisture eventually. If you’re hard up enough, you could remove the clutch drum, trim away part of the lip of plastic that retains the bearing, and drive it out. The plastic deforms enough that you can drive the bearing out crooked without removing all the lip. Then install a new actually sealed 6201 bearing and it should be fine for the rest of the trimmer’s life. If everything is good with the alignment, there should be no side stress on the clutch drum, so any amount of lip will hold the bearing fine. I’ve done this before because I couldn’t stand the thought of installing a new housing only to have the bearing fail again in a couple years due to the insanity of using a shielded bearing. I think they’re even NTN’s, but it doesn’t matter good they are if they can’t stay water-free.
Or buy yourself an Echo 2620 or Redmax EXZ260–NOT A RELABELED HUSQVARNA, and be done with all your aforementioned problems