Flywheels and keys
The second most common issue we see at Powered Agriculture after carburetor repairs has to be lawnmower flywheel key replacements.
What does a flywheel do?
The flywheel on your mower engine was designed to store the momentum from combustion to keep the crankshaft turning in between the engine’s power strokes (see “Four-stroke” or “two-stroke” engines). The flywheel also serves several other purposes. The fins help cool the engine by distributing air around the engine block as well as blow air across the air vane on a pneumatic governor, maintaining the desired engine speed. Magnets mounted in the outside surface of the flywheel are required for ignition. On engines with starter motors, lights or other devices, magnets mounted inside and outside the flywheel are at the heart of the electrical system. Because the electrical systems pertaining to the flywheel are timed with the mechanical aspects of the the flywheel, then it is important that the flywheel rotates precisely with the crankshaft. To prevent catastrophic damage to the engine manufacturers have engineered a “break-point” between the flywheel and crankshaft.
The flywheeel key
If a lawn mower or tiller blade hits a rock, curb, or toy, the flywheel key can sometimes absorb the damage, reducing the chance of crankshaft or flywheel damage. The crankshaft and flywheel are usually made of steel or iron, but the flywheel key will be made of softer materials such as aluminum or zinc. In the event of a blade strike, the soft metal will bend or shear to prevent the rotational mass of the flywheel from turning the crankshaft. Always check for damage by removing the flywheel to inspect the key and the keyway, the key’s slot on the crankshaft. Because of the previously mentioned timing, the soft metal key must eliminate play between the flywheel and crankshaft.
Removing The Flywheel
1. Disconnect the spark plug lead and secure it away from the spark plug. Then, loosen the bolts holding the shroud in place and remove the shroud.
2. The engine may be equipped with a flywheel brake, remove any cover and disconnect the outer end of the brake spring.
3. The flywheel is attached with a nut, use a flywheel holder as a brace, and remove the flywheel retaining nut with the appropriate socket.
4. With the flywheel nut threaded onto the crankshaft, install a flywheel puller so its bolts engage the holes adjacent to the flywheel’s hub. If the holes are not threaded, use a self-tapping flywheel puller or tap the holes using a 1/4 X 20 tap.
NOTE: Never strike the flywheel or crankshaft. Even a slightly damaged flywheel presents a safety hazard and must be replaced.
5. Rotate the puller nuts evenly until the flywheel pops free. Then, remove the flywheel and key.
Inspect the flywheel and key
Check for cracks on your crankshaft or broken fins on the flywheel. Replace them if you find such damage. The tapered sections must be clean and smooth, with no play between the two. Next, inspect the keyway and flywheel for damage. Slight burrs may be removed with a file. Then, make certain there is no play or wobbling when the flywheel is placed on the crankshaft.
Inspect the flywheel key. If there are any signs of shearing or if you have doubts about the condition of your flywheel key, replace it. It’s simple and inexpensive. Place the flywheel back onto the crankshaft and look through the flywheel hub to align the keyways on the flywheel and crankshaft.
With the flywheel in place, push the key in the keyway; it should fit securely. If you feel play, check to see if the key is upside down. Debris can also prevent the key from seating in the keyway. Once the key and flywheel are securely in place, reattach the flywheel nut. Consult your dealer or service manual for the torque specifications for your make and model.
If the engine suddenly begins to run rough (or not run at all) following a blade strike or other impact, chances are the flywheel key has been damaged and now the engine ingition is no longer timed with the valves. Be sure to abide by your manufacturers service and safety procedures before attempting this type of service, or just take your equipment to an Authorized Service Dealer especially if your equipment is still under warranty.
