How to balance your rotary lawn mower blade when sharpening

Sharpen_blade If you are like me you try and do as much of your own machinery maintenance as you can.

I do concede and get in an expert if the job is a major one because it is usually my luck to put something back together and find that I have five screws left without five holes visible  meaning they were supposed to be inside somewhere.

I cannot remember exactly how many rotary mowers we had in the workshop or on the maintenance fleet but hazarding a guess it was probably about 18 at the peak.

Having a sharp blade is essential for two reasons. One it gives you a crisp clean cut without bruising the grass and leaving a horrid brown tip to the blade and also it  stops the engine from labouring.

As contractors we changed our blades at the start of every season but sharpened them during the season with an ordinary angle grinder. It is an easy job to do but there is one very important and essential thing to get right - balance.

If the blade is not balanced the operator will experience vibration which is not only uncomfortable after a days solid mowing but it can also damage nerve endings and lead to a long term problem.

Vibration on the machine is transmitted through the handles and causes the operator to grip excessively. There is a condition called white finger caused by excessive and continuous exposure to vibration in hand held machinery. Coal Miners and road workers became common casualties after spending all day on jack hammers  or road rollers.

Vibration can seriously damage the engine too. Long term vibration will shake important components loose or cause bearings to fail leading to an expensive problem.

When sharpening your blade you first of all need to ensure there is enough meat on the blade. If you have had the blade for a couple of years and maybe it has hit a few stones or you have sharpened a couple of times before then it is worth replacing for your safety and the longevity of the engine.

If the blade becomes too light then the engine will work too hard because the blade also doubles as regulatory flywheel.

Once you have established it is safe to sharpen the blade then secure it in a vice and run the grinder along the leading/cutting edge - usually the last 3-4 inches. Don't be too aggressive because the blade will disappear.

Stop every so often and place the blade on a nail (previously banged into an appropriate post) and let it settle. If it is balanced it will remain in the position you place it. If it is heavy on one side then the heavy end will naturally fall to the bottom. The speed at which it falls will help to determine how out of balance it is.

Gently grind the heavy end until you achieve the balance and replace the blade.



One of the reasons that you might have considered sharpening your blade is because of vibration in the handles has led you to think that the blades are out of balance after hitting a stone, tree stump or solid object resting in the grass?

If, after going through the balancing procedure and finding that the machine continues to vibrate then it is possible that damage has been caused to the drive shaft.

Imagine, if the blade is rotating at several thousand revolutions per minute and the passage of the blade is interrupted at this high speed by hitting a solid object? This immense power has to be transmitted somewhere as the momentum continues and in many cases that transmission leads to bending of the drive shaft.

Unfortunately, if this occurs then the drive shaft will have to be replaced and continued use will eventually destroy the machine through vibration.

If you suspect that hitting something in the grass has done more damage than just throwing your blades out of balance then seek help from your lawn mower mechanic.

Happy mowing!

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