Trimmer: Shaft Vibrates Excessively

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 gears can wear down over time from heavy use and a lack of lubrication. The teeth may break off the gears, throwing their balance off and causing your trimmer to vibrate. A grinding noise from the gear box is a good indicator of this problem. To remove the gear box, remove any covers on the head that stop debris from flying if they are in the way. 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 housing for the flex shaft secures and supports the shaft as well keeps debris away from it that could impact its operation. 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. You may even hear a noise in the damaged spot when in operation, further indicating you have a problem. 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 have a flexible drive shaft installed in their 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. Trimmers can also vibrate when the connection between the flex shaft and the trimmer head is loose or worn. To inspect the drive shaft, remove the engine assembly from the shaft and look to see if the shaft is in its proper place. Re-install the shaft if it has slipped from its place. If the shaft is damaged or broken, it must be replaced. Separate the drive shaft housing from the engine housing. You may have to disconnect the ignition wires and throttle cable on some models. On other models, you may have 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 drive shaft with a pair of needle-nose pliers. Lubricate the new shaft from top to bottom with some grease and re-install the shaft into the 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 and re-attach the ignition wires and throttle cables.

The clutch transmits power from the engine to the drive shaft. When the clutch is worn and does not fully engage when throttling up the engine, it can cause a vibration. You will need a new clutch if this happens. To replace the clutch, separate the drive shaft from the engine housing by simply removing the screws holding the drive shaft housing to the engine housing. Remove the engine covers, allowing you to access the clutch and the spark plug. Disconnect the ignition wires and throttle cable if needed. Remove the spark plug boot and spark plug, and insert a small piece of rope inside the cylinder to prevent the piston from moving as you remove the clutch. Remember to keep a small portion of the rope out so you can remove it later. Use a clutch removal tool to un-thread the old clutch. Install the new clutch by threading it onto the shaft and tighten it down. Remove the rope from the cylinder and re-install the spark plug and boot. Re-install the engine covers that you removed and re-attach the drive shaft to the engine housing.

The trimmer head houses the spool of wire, cap, and spring, and takes a lot of abuse in the process. The first thing you should check if you have a bad vibration is that the trimmer head is tight. A loose trimmer head will cause a serious vibration and will eventually work its way off the trimmer. It can fly off which could result in an injury to you. If you cannot tighten the trimmer head (keeping in mind they-are reversed thread) there could be some damaged threads and you will require a new trimmer head. 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 in place to prevent movement as you tighten the head down.