A string trimmer is an essential tool for keeping your yard looking neat, but few things are more frustrating than an engine that roars to life only for the cutting head to refuse to spin. This is one of the most common and perplexing issues you’ll encounter on a small engine repair blog.
Don’t assume your trimmer is dead! The problem is often isolated to the drive system. Here is a detailed guide to the primary culprits when your trimmer head won’t spin.
1. Worn or Failed Centrifugal Clutch
The clutch assembly is the crucial link between your engine and the drive shaft. When the engine reaches a high enough speed (RPM), the centrifugal clutch shoes fling outward to grab the clutch drum, which then spins the drive shaft.
The Failure
- Worn Shoes: Over time, the clutch shoes can wear down from normal friction and use. If they are too worn, they can’t effectively grab the drum to transfer power, causing the engine to rev high while the head remains motionless.
- Broken Springs: The small springs holding the clutch shoes may break or lose tension. This can cause the head to either not spin at all or, sometimes, spin continuously even when the engine is idling.
The Fix: A worn clutch assembly is a relatively simple and inexpensive part to replace, generally requiring the removal of the engine housing and flywheel.
2. Damaged or Seized Drive Shaft/Cable
The drive shaft (or drive cable, depending on your trimmer model) is the long, flexible rod that runs inside the main shaft tube, connecting the clutch to the gear head at the trimmer end.
The Failure
- Broken or Stripped: The most common issue is a broken drive shaft that has snapped in two or has had its square/splined ends stripped where it connects to the clutch or gearhead.
- Melted to Drive Tube: This is a severe, less common failure but indicates a major binding issue. If the inner drive cable spins but the outer tube is bent or has debris, friction can cause intense heat, literally melting the flexible drive cable to the inside of the outer drive tube and causing it to seize solid.
The Fix: The drive shaft cannot be repaired. If it is broken, stripped, or melted, you must replace the entire cable or shaft assembly.
3. Gearhead (Gearbox) Damage and Lubrication
For trimmers that have a solid shaft (non-cable) drive or for multi-tool systems, the gearhead at the base is essential for power transfer. It changes the rotation direction from the shaft to the cutting head.
The Failure
- Lack of Grease: Gearheads require regular lubrication. Neglecting this maintenance allows the internal metal gears to grind against each other without a protective layer of grease. This causes premature wear and tear, chipping the gear teeth or causing the gears to seize entirely.
- Impact Damage: Hitting a solid object, such as a rock or tree stump, can put excessive strain on the gear teeth, causing them to crack or shear off.
The Fix: Once the gear teeth are damaged, simply adding grease won’t fix it—the damage is already done. The entire gearhead assembly must be disassembled and the damaged gears replaced, or more commonly, the entire gearhead assembly is swapped out for a new one.
4. Bent Drive Tube (Shaft Housing)
The drive tube is the long, rigid metal or plastic housing that protects the inner drive shaft/cable.
The Failure
- Binding: If the outer tube is severely bent (often from being dropped or crushed), it will pinch and bind the inner drive shaft, preventing it from rotating freely, even if the engine and clutch are working perfectly. This bending is often what leads to the extreme heat and melting of the drive cable mentioned earlier.
The Fix: A severely bent tube cannot be straightened reliably. The only solution is to replace the entire shaft/tube assembly.
5. Trimmer Head Itself is Defective or Jammed
Before tearing into your engine components, always check the simplest cause first.
The Failure
- Line Entanglement: Grass, weeds, or vine material can become tightly wrapped around the spool or the mounting bolt, creating resistance that the engine cannot overcome.
- Internal Seizure: The internal parts of the trimmer head (spool, eyelets, spring) can be seized due to dirt, debris, or a failed bearing, preventing the head from turning even when the shaft is spinning.
The Fix: Remove the head and thoroughly inspect it. Clean out all debris, disassemble the head, ensure the spool rotates freely, and check the mounting threads for any stripping. A simple cleanup often solves the problem!
Knowing the key components—the clutch, the drive shaft/cable, and the gearhead—will save you time and money on your next repair. Tackle the simple fixes first, and work your way back to the power source.
For a visual guide to troubleshooting, including inspecting the gearbox head and checking for a damaged throttle cable or clutch, check out the video, Why Your Weed-Eater Gearhead Won’t Rotate: