![]() ![]() Separate the drive shaft housing from the engine housing. If the shaft is damaged or broken, it must be replaced. Re-install the shaft if it has slipped from its place. To inspect the drive shaft, remove the engine assembly from the shaft and look to see if the shaft is in its proper place. ![]() Trimmers can also vibrate when the connection between the flex shaft and the trimmer head is loose or worn. These flex shafts can start to vibrate when they wear out, or when they aren't properly lubricated. Most string trimmers have a flexible drive shaft installed in their shaft housing, even if the shaft is straight. Install the new drive shaft housing assembly by re-attaching it to both ends, tightening down the bolts and screws, and re-attaching the ignition wires and throttle cable. Transfer any handles, guards or anything else off the old drive shaft housing to the new drive shaft housing. Remove the shaft from the gear box housing by unbolting it and pulling it apart. If needed, detach the throttle cable and ignition cable and pull apart. To replace the drive shaft unit, separate the drive shaft from the engine assembly by removing the screws attaching the shaft housing to the engine. You may even hear a noise in the damaged spot when in operation, further indicating you have a problem. If you notice any damage impacting the operation of the flex shaft, it needs to be replaced. Inspect the flex shaft housing for damage. If the housing has become damaged and is rubbing and/or applying pressure to the flex shaft, it can cause a vibration. The housing for the flex shaft secures and supports the shaft as well keeps debris away from it that could impact its operation. Re-install the flange and the trimmer head, keeping in mind that the threads are reversed. Install the new gear box and rotate the spindle until it lines up with the inner shaft, then tighten down the bolts. Remove the trimmer head by rotating it counter clockwise trimmer heads are reversed-thread. ![]() On some models, you might have to lock the screwdriver in place against the washer, after it is locked inside the hole. Rotate the trimmer head until the screwdriver locks into place. Insert a screwdriver into the hole on the gear box located just above the trimmer head. To remove the gear box, remove any covers on the head that stop debris from flying if they are in the way. A grinding noise from the gear box is a good indicator of this problem. The teeth may break off the gears, throwing their balance off and causing your trimmer to vibrate. These gears can wear down over time from heavy use and a lack of lubrication. These gearheads were very successful when introduced to the field, so our design team made the decision to mount a similar gearhead to smaller trimmers in the homeowner and entry level professional applications, such as the FS56/56R and the FS70R, in an effort to give the same capability to the customers operating trimmers in these particular applications.The gear case or gear box is located at the bottom of the drive shaft and transfers power from the drive shaft to the trimmer head. These gearheads have a higher grade of seals, bearings, and pinions that have the capability to withstand a good deal of use and punishment. The purpose in this was to offer a professional user (such as clearers of fire trails and road crew workers) a product that was as maintenance free as possible to allow them more time to complete their task and less time maintaining the components, such as the gearhead. Several years ago, we began offering our three largest clearing saws (FS360C, FS460C, and FS560C) with these same types of gear heads mounted to them. This is not a new innovation for STIHL products. I just received this reply from a Stihl Tech Service Rep after asking for an explanation of why two different gear head designs are being used with their trimmers and here is his reply: ![]()
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