Most of the country has pulled out a lawnmower or slasher to use for the last of the summer season, but when last did you balance your blades? There is no quicker way, short of mowing through a field of rocks, to destroy any kind of rotary cutting device such as a tractor-drawn slasher or mower, than to run it with unbalanced blades.

That’s because the vibration caused by the blades of different weights rotating about the shaft will soon destroy any shaft bearings, not to mention wearing the shaft itself. In the process, of course, the vibration will cause other components to wear and break, including cracking welds, loosening bolts and causing unnecessary engine wear through vibration.

It is imperative, therefore, that the two, or four bolt-on blades of whatever system one has weigh exactly the same.

For small bolt-on blades this can be achieved very accurately after sharpening by weighing them on a small kitchen scale ~ the kind one uses to measure out very small quantities of flour for baking.

Simply weigh each blade in turn, and dress the heavier blades until their weights correspond to that of the lightest.

For slasher blades and heavier mower blades a small, a kitchen scale will be inadequate, while a bathroom scale will be too inaccurate to achieve the desired result. In such a case one must make an old-fashioned balancing scale.

Balance Your Large Blades

Place a length of threaded rod or steel fence dropper, or very sturdy straight wire on a blade held firm in a vertical position, for example in a vice, so that approximately half of the rod is on either side of the blade. If there is a small nick in the blade it will prevent the rod from rolling up and down the blade.

Move the rod to the left or right until it balances absolutely horizontally, and using a pencil or a koki pen mark the exact spot on the rod where it lies on the blade. Hang two of the sharpened blades off the each end of the rod approximately equal lengths from the centre. One side will likely dip downward. Move the blade on the downward side a little towards the centre and check again. Continue until the rod hangs horizontally.

Now mark where the lighter blade is hanging on the rod and carefully measure the distance between the centre and the lighter blade. Remove the blades and the rod and mark off this distance on the side of the heavier blade. File/sharpen (AKA dress) the heavier blade and repeat the hanging process.

As the heavier blade loses weight through the dressing process, its position on the balancing rod will gradually move outwards until it finally reaches the exact same distance from centre point that the lighter blade has at the other end, while the rod is perfectly horizontal.

At that stage you are assured that the two blades weigh the same.

If your mower has four such blades you will need to establish the weight of the lightest one first and dress the other three in turn until they match it. If your mower blade is a single unit you can ensure both sides are in balance by suspending it through its centre hole off a nail or length of sturdy wire held horizontally in a vice. The side that dips downward is the heavier.

Because of the large diameter of the centre hole and the fact that a blade may hang a bit off-centre, and thus give you an inaccurate indication of weight, it is advisable to repeat the exercise a couple of times, turning the blade over each time, to gain an “average” idea of the amount of dressing needed.

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