NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Why you really don't want to attempt to ..  
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Why you really don't want to attempt to move an immovable object

The rotating blades and mass of the internal engine parts pack quite a punch. Speeding along merrily mowing away one doesn't think about this. However, if the blade should hit an obstruction, you may have no choice.

There are various safeguards to protect the mower from damage should a blade tip hit something but these don't always work. Why?

There is protection for the upper and lower parts of the crankshaft after all:

In many cases, both of these will break free at the same time.

However, if the shock is severe enough, much more serious damage can result. Here is why: When one end of the blade hits a curb, for example, the inertia of the mass of the blade alone (rotating at high speed) will attempt to push the shaft sideways. This is pretty much independent of the rest of the engine.

In the diagram below, the blade is rotating clockwise. When the left-hand tip hits the curb, the right-hand side due to the inertia of the entire right-hand half of the blade wants to continue to move (down in this diagram) with the 'X' as the fulcrum. The entire left-hand half section of the blade contributes relatively little. This results in a net significant sideways (downward in the diagram) bending force on the crankshaft. The unavoidable arrangement of the fulcrum at one end and the shaft in the middle makes the situation even worse as the force resulting from the blade tip (the right hand one in this example) is amplified by the up to 2:1 mechanical advantage of the lever arm (the tip is twice as far from X and the shaft).

    Blade tip   /
  hits curb.  X-------____________________________________
    Ouch!   / |                                           |   Inertia of this
          /   |                   - O -                   | | side of blade
        /     |____________________________________       | | attempts to 
                                    |   |   |   |  -------  | continue in same
                                    V   v   v   v   v   v   v direction.
                            Net sideways force
                              on crankshaft

While the rotating mass of the engine is attempting to shear the blade lock key, the inertia of the blade is trying to push the crankshaft sideways. The net result could be a severely bent crankshaft - a very expensive repair. An 8 to 10 degree bend is not unusual for a typical Craftsman-class mower running at full power. Any detectable bend in the crankshaft requires replacement - it is not safe to attempt to straighten it. A bend resulting in the blade tips wobbling by more than a fraction of an inch, there will be unacceptable and dangerous vibration when the mower is run. In addition, the original trauma (as well as attempting to run with a bent crankshaft) can damage other parts like the main bearings and connecting rod. The blade lock and flywheel keys will likely be broken as well but these are insignificant in comparison to the cost of major replacement parts and the labor involved in their installation.

The time and effort needed to disassemble the engine is significant and the crankshaft is probably the single most expensive part of the engine. In fact, purchasing a new crankshaft may be more expensive than an entire new lawn mower! It is quite possible that unless you have access to low cost replacement parts from a salvage yard and have the free time to do the work, repair may not make sense.

Therefore, don't let this happen to you. Your curbs and rocks don't grow that quickly and do not generally require mowing!

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