How to tell if a Lawnmower Crankshaft is Bent?


When you mow your lawn, it is always possible that you hit a rock, a toy, or another hidden obstacle while. Such accidents can cause physical damages to the mower, and even the flywheel can break. If the mower hits or runs over something while running at high speed, both the chances and severity of physical damages can increase. One such physical damage that is not uncommon to occur is a bent crankshaft. The crankshaft connects the mower blade to the motor and rotates at a very high rpm. If the mower hits a hard obstacle, the crankshaft can slightly bend, causing problems. A bent or partly broken crankshaft has some clear symptoms that are not difficult to observe. In this blog, we will go through diagnosing a bent crankshaft and how to fix one.

How to tell if a Lawnmower Crankshaft is Bent:

  • Symptom 1 is excessive vibrations. Abnormally rigorous vibrations and grinding noise are signs of a bent or broken crankshaft. Look out for excessive vibrations when using your mower that was not there before.
  • Symptom 2 are issues with the transmission. A bent or broken crankshaft can hit other parts causing problems in the transmission system. When this happens, your mower will have trouble starting up and running smoothly.

You can also conduct a physical inspection to check for a bent or broken crankshaft in addition to these symptoms. In this regard, you can check the following things:

  • If the gap between the crankshaft and other parts is what it should be.
  • If the bolt that holds the blade shows any wobbles.
  • You can also position a dial indicator on the shaft’s various locations to check if it is bent.
  • You can also look for blade wobbling, which could be an indicator of a bent crankshaft.

A bent or broken crankshaft can be very dangerous. If you have determined that there is some issue with the crankshaft, don’t use your mower unless the issue is fixed. The crankshaft rotates at very high RPMs- around 3000 rpm or even more. If the blade rotating at this speed comes off because of a bent crankshaft, it can cut both your feet. So, you cannot be too careful when it comes to crankshaft issues.

Let’s get into the details of known symptoms of a bent crankshaft and the physical inspection methods. After that, we’ll also provide you some information on how a bent crankshaft can be fixed.

Symptoms of a Bent Lawnmower Crankshaft:

There are two common symptoms associated with a bent crankshaft. You don’t need to be an expert mechanic to judge these symptoms. If the crankshaft on your mower is indeed bent, you’ll observe these two symptoms for sure. Just keep your eyes and ears open.

● Excessive Vibrations:

For moving objects, vibrations are inevitable. All moving objects are vibrating at some frequency that is characteristic or specific to that object and an amplitude specific to the motion it is going through. When an object is deformed, or its motion characteristics are changed, its vibration amplitude will change. For example, when the force exerted on a moving object changes, it results in a change in its amplitude of vibration-an increase in force can cause an increase or decrease in the amplitude. An increase in amplitude means that the vibration becomes more rigorous and observable.

Excessive vibrations are the most common symptom associated with a bent crankshaft because of its high rotational speed. The vibrations can be felt for some seconds after intervals of smooth-running, but they are vigorous enough to know that there must be an issue. Excessive vibrations are also accompanied by grinding noise. If you observed these, immediately stop mowing and turn the engine off. Inspect the crankshaft and get it fixed before mowing again.

● Transmission Problems:

Transmission problems are also commonly associated with a bent crankshaft. This is because of two reasons. The bent crankshaft can hit other parts messing the ignition system up, or the impact that bent the crankshaft could also break the flywheel and mess the ignition system. In any case, the mower will have trouble starting and will also not run smoothly.

Physical Inspection for a Bent Crankshaft:

You can also conduct a physical examination of the crankshaft to determine if it is in the correct shape or not. A physical exam must be done if you have observed signs associated with a bent crankshaft or as a maintenance routine. Doing a physical exam now and then is recommended because this way, you’ll always stay updated and will know of any issues before they become too serious.

Multiple methods could be used to examine the crankshaft of your lawnmower physically. Some of these methods are listed below.

● The Gap between the Crankshaft and the Deck:

In this method, you check if the gap between the crankshaft and the deck changes throughout one complete rotation of the crankshaft. First of all, disconnect the spark plug. The clamp a ruler to the deck using duct tape so that its other edge is touching or almost touching the crankshaft. Then rotate the shaft manually, closely observing the gap between the ruler and the shaft. If the gap changes as you rotate the shaft, it means that it is bent and needs to be fixed.

Remember to wear safety gloves during this procedure to avoid an injury to your hands. Once again, the spark plug should be disconnected when you perform this exam.

● Look for Bolt Wobble:

First of all, disconnect the spark plug. Then ask a friend of yours to pull the mower’s starter cord. As they do so, carefully observe the bolt that holds the blade to the crankshaft. Any wobble (sideways motion) of the bolt as the starter cord is pulled indicates that the crankshaft is bent and needs repair.

● Use a Dial Indicator:

A dial indicator is an instrument with a dial that measures or detects minimal changes in distance and amplifies them to be visible to the observer. Professionals use dial indicators to measure small displacements in all types of structures, e.g., beams, flat plates, shafts, etc.

You can also use a dial indicator to check if your crankshaft is bent. For this purpose, the dial indicator should be placed at specific points on the crankshaft. The dial’s placement depends on the make and model of the shaft, and you can look for more info in this regard, the manufacturer’s manual or can take help from a professional.

● Look for Blade Wobble:

Just like a wobbling bolt, a wobbling blade can also be an indicator of a bent crankshaft. To observe the blade, park your mower on an open flat area and keep the engine running. While the engine runs, carefully observe the blade for wobbling and shaking. Blade wobbling can be caused if the blade is broken, but it can also be an indicator of a bent crankshaft.

Fixing a Bent Shaft:

A couple of techniques are commonly used by professionals for straightening bent crankshafts. These include:

● Shaft Straighteners:

Professionals use shaft straighteners to straighten bent crankshafts. In this method, a mechanical press force is applied at the shaft at the required locations. The shaft may be hot or cold while the force is applied. This is a very efficient technique for straightening the shaft and can make your bent shaft suitable for use once again.

● Sledgehammers:

The second method is the use of a sledgehammer to straighten a bent shaft. The difference between this method and using a shaft straightener is that the sledgehammer applied impact force, whereas the straightener applies a pressing force on the shaft.

These methods are not very difficult, and you can perform them yourself at home. Tutorial videos can help in this regard.

Replacing a Bent Shaft:

Sometimes a shaft is too bent and can’t be repaired enough to make it suitable for use. Also, trying to straighten a shaft using the above methods can result in the warranty getting canceled. In any of these cases, you’ll have to replace the bent shaft with a new one. Ensure that the new shaft matches the shaft being replaced and is compliant with your lawnmower engine.

Related Questions:

● What causes a bent crankshaft?

The blade of your lawnmower rotates at very high rpm. An rpm of around or over 3000 is not uncommon. This means that the blade and crankshaft possess a lot of kinetic energy, which gets dissipated when the mower hits an immovable obstacle. It can result in a bent crankshaft, hitting a hard rock or a tree stump, or a fence.

Final Remarks:

When your lawnmower hits a hard obstacle when running at high speed, its crankshaft can bend. A bent crankshaft causes many problems, such as excessive vibrations and transmission issues. Furthermore, a bent crankshaft can result in serious injury if you keep using your mower. Using the guidelines mentioned in this article, you can determine if your mower’s crankshaft is bent. Once you determine that it is bent, you must get it fixed or replaced as soon as possible. Remember never to use your mower with a bent crankshaft.

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