Trimmer: Vibrates Excessively

The gear case is located at the bottom of the drive shaft and transfers power from the drive shaft to the trimmer head. These gears can wear down over time from heavy use and a lack of lubrication. The gears can suffer damage from abuse, or just wear out over time, throwing the balance off and causing a vibration. The teeth may not be catching each other from too much wear, which can also result in a vibration. A grinding noise from the gear box is a good indicator if you have vibration as well. To remove the gear box, remove any covers on the head that stop debris from flying. Insert a screwdriver into the hole on the gear box, located just above the trimmer head. Rotate the trimmer head until the screwdriver locks into place. On some models, you might have to lock the screwdriver in place against the washer, after it is locked inside the hole. Remove the trimmer head by rotating it counter clockwise; trimmer heads are reversed thread. Unbolt the gear box and remove it. Install the new gear box and rotate the spindle until it lines up with the inner shaft, then tighten down the bolts. Re-install the flange and the trimmer head, keeping in mind that the threads are reversed.

The shaft housing secures and supports the flex shaft. If the housing has become damaged and is rubbing and/or applying pressure to the flex shaft, it can cause a vibration. Inspect the flex shaft housing for damage. If you notice any damage impacting the operation of the flex shaft, it needs to be replaced. 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. If needed, detach the throttle cable and ignition cable and pull apart. Remove the shaft from the gear box housing by unbolting it and pulling it apart. Transfer any handles, guards, or anything else off the old drive shaft housing to the new drive shaft housing. 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.

Most string trimmers use a flexible drive shaft inside the shaft housing even if the shaft is straight. These flex shafts can start to vibrate when they wear out, or when they aren't properly lubricated. Vibration can also occur when the connection between the flex shaft and the trimmer head is loose or worn. To inspect the shaft, separate the engine assembly from the shaft assembly by removing the screws fastening the two together, and look to see if the shaft is in its proper place. If the shaft has slipped from its place, simply re-adjust it back in place. If the shaft is damaged or broken, it will need to be replaced. With the drive shaft housing separated from the engine housing, disconnect any ignition wires and the throttle cable if needed. Some models may require you to remove the trimmer head and gear box to pull out the shaft instead of separating the engine from the drive shaft. With the shaft housing separated from the engine, remove the flex shaft with a pair of needle-nose pliers. Lubricate the new shaft from top to bottom with some grease, then re-install the shaft into the shaft housing. Rotate the shaft around till you feel it lock into place in the trimmer head coupler. This is a trial and error process and can take some time. Once the shaft locks into place, the end of the shaft should be flush with the shaft housing. Re-assemble the engine assembly and drive shaft assembly back together. Re-attach the ignition wires and throttle cables.

Covers will not cause a vibration but if the cover itself is vibrating from operation, check the bolts or screws that fasten them down for tightness, and tighten if needed.

The trimmer head houses the spool of wire, cap, and spring. It takes a lot of abuse as it is in constant contact with the ground. The first thing to check if you have a bad vibration is that the trimmer head is tight. If you have a loose trimmer head, it will cause a serious vibration and if left unattended, could eventually fly off which could result in an injury. If you cannot tighten the trimmer head, and the trimmer head is loose, there could be some damaged threads on the trimmer head and you will require a new one. To remove the trimmer head, insert a small screwdriver or Allen wrench into the hole on the gear box, located just above the trimmer head. Rotate the trimmer head until the screwdriver locks into place. On some models, you might have to lock the screwdriver in place against the washer after it is locked inside the hole. Remove the trimmer head by rotating it counter clockwise. Install the new trimmer head with the screwdriver or Allen wrench to prevent movement as you tighten the head down.