4 Likely Reasons Why Your String Trimmer Head Won’t Spin

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Unlike the blade on a lawn mower, the oscillating teeth on a hedge trimmer, or the cutting chain on a chainsaw, the thin pieces of plastic line extending out of a string trimmer’s head don’t seem very threatening. In fact, there’s no need to wear protective gloves when replacing string trimmer line since the line has no sharp edges. All that changes when the string trimmer’s throttle trigger is pulled and the trimmer head begins to rotate. Suddenly, that harmless plastic line is strong enough to rip through grass, weeds, and small foliage.

String trimmer line effectively cuts through the overgrowth because the trimmer head can spin as fast as 12,000 rpm or nearly 400 miles per hour on gas-powered string trimmers, making this hand-held piece of power equipment an exceptionally versatile addition to your landscaping tool collection. However, if you have a broken string trimmer head, that trimmer line won’t have much chance of damaging a marshmallow. Repair Clinic.com has the answers for what makes a gas-powered string trimmer head spin and the most likely causes for the head not spinning.

How a gas-powered string trimmer head works

While you’ll want to make sure you’ve properly installed the string trimmer line, that line won’t be of much use unless the trimmer head can spin freely at least 2,700 rpm. How does a string trimmer head work? The crankshaft of the string trimmer engine extends out the side of the crankcase and connects to a centrifugal clutch which engages with the drive shaft attached to the trimmer head. When the engine runs at idle speed, the clutch prevents the drive shaft from rotating. When the trigger is pulled, a cable opens the throttle on the carburetor which accelerates the engine. This causes the clutch flyweights to extend outward and engage the drive shaft which spins the trimmer head.

As the trimmer head spins, the trimmer line spreads out and whips around fast enough to cleanly cut down weeds, tall grass, and light brush along the edges of fences, trees, and other hard-to-reach places. The size and thickness of trimmer line varies depending on the string trimmer model, but the most common diameters for residential use range from 65/100th of an inch to 95/100th of an inch. The trimmer line may be wound around a spool, or the line may be cut into small individual strips that are loaded into the trimmer head. Most wound string trimmer heads have a bumper, or bump knob, that can be tapped on the ground to feed additional line out of the head as the line is worn down, although newer models have an “auto-feed” feature. If too much line extends out of the trimmer head, a blade located on the guard will trim the line as the head spins.

4 reasons why the string trimmer head is not working

What prevents a string trimmer head from spinning? Repair Clinic has four likely causes you should troubleshoot:

  1. Worn clutch – Since the string trimmer clutch engages with one or more drive shafts attached to the trimmer head to rotate the head when the throttle trigger is depressed, a worn-out clutch may prevent the trimmer head from spinning. A new clutch assembly is relatively inexpensive and easy to install.
  2. Damaged drive shaft – A similar situation to a worn-out clutch occurs if a string trimmer drive shaft is damaged. A drive shaft unable to rotate will not allow a trimmer head to spin. Again, a new shaft can usually be purchased individually to repair the string trimmer.
  3. Defective trimmer head – The string trimmer head itself could be defective and unable to spin. This is another replacement part that can be purchased separately. Keep in mind, if the threads of the drive shaft are stripped, it’s a good idea to replace both the trimmer head and the drive shaft at the same time.
  4. Broken throttle cable  – If the string trimmer throttle cable is broken, the engine won’t accelerate when the throttle trigger is pulled and the clutch won’t engage the drive shaft to turn the head. As with the items listed above, a new throttle cable can be purchased separately to repair the string trimmer.

Repair your string trimmer with help from Repair Clinic

As you repair partner, Repair Clinic can assist you in fixing a broken string trimmer head, clutch, or throttle cable yourself with a free content library filled with step-by-step procedural videos, articles, and diagrams. Whether you need to replace the trimmer head on a Ryobi (Model RY253SS) string trimmer or install a new clutch on a Husqvarna (Model 128CD) trimmer, Repair Clinic has the resources to guide you to a successful repair.

Use only genuine OEM string trimmer parts

A successful repair is also dependent on installing the exact genuine manufacturer part that matches your string trimmer model. Repair Clinic keeps this aspect of the repair process simple. Enter the full model number of the string trimmer in Repair Clinic’s “Shop Parts & Get Answers” search bar to view a comprehensive list of compatible parts, genuine OEM replacement parts from all the top string trimmer brands, including Craftsman, Echo, Husqvarna, Kawasaki, MTD, Poulan Pro, Ryobi, Toro, and Troy-bilt. You can then use the “Part Category” navigation filter (examples: “Trimmer Head”, “Transmission, Brake, Clutch”) along with the “Part Title” navigation filter (examples: “Bump Knob”, “Clutch”) to narrow the list down to the specific part or parts you need for your repair.

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