KM 94 vibrations

[Mark Bolam]

I use a KM94 combi head with a hedge cutting attachment.

 

The last few outings the vibrations have been savage.

 

The AV is never the best on these things, but it’s noticeably worse than last season.

 

I can’t see anything physical wrong, and the head is greased, with sharp blades.

 

Any ideas?

[Will C]

Check the white plastic bushes in the tube each side of the join, the drive shaft rattles around at the joint when there worn. 

[Tictac98]

I had to replace the engine pan on my km90, the AV mounts snapped

[spudulike]

One little tip on the hedge cutter attachment...more for the older model - if you take the head off, stick a suitable tight fit - flat head screwdriver in the hole the shaft usually fits, you can move the screw driver clockwise and anti clockwise and the blades should react to the change of direction without too much backlash. 

If this isn't the case and there is appreciable backlash, it shows the rods, gears or bearings may be worn or damaged. The later model with the white alloy blade support is a bit more reliable.

The retaining bungs are worth checking as previously mentioned. Some Stihl shafts have a plastic sleeve in them, I have seen the shaft melt some of the sleeve and this makes the shaft REALLY shake and vibrate but I don't think your shafts have them unless using the carbon fibre extension. Worth giving the shafts a good greasing.

It is worth revving the machine and trying to determine the source of the vibration shaft, head or engine.

[doobin]

On 23/06/2022 at 19:24, spudulike said:

One little tip on the hedge cutter attachment...more for the older model - if you take the head off, stick a suitable tight fit - flat head screwdriver in the hole the shaft usually fits, you can move the screw driver clockwise and anti clockwise and the blades should react to the change of direction without too much backlash. 

If this isn't the case and there is appreciable backlash, it shows the rods, gears or bearings may be worn or damaged. The later model with the white alloy blade support is a bit more reliable.

The retaining bungs are worth checking as previously mentioned. Some Stihl shafts have a plastic sleeve in them, I have seen the shaft melt some of the sleeve and this makes the shaft REALLY shake and vibrate but I don't think your shafts have them unless using the carbon fibre extension. Worth giving the shafts a good greasing.

It is worth revving the machine and trying to determine the source of the vibration shaft, head or engine.

I've had the melted shaft (ooh matron!) issue too. A slight bend in the tube can be all it takes- pretty sure mine was on a strimmer. Pulled the driveshaft out and it was wrapped around like a candy cane, couldn't scrape it off either.