The importance of keeping a sharp edge on rotary blades

An example of a worn and nicked blade.
Photos by Danielle Broadbent.
Examples of rotary blades.

A rotary blade is a thin, rectangular piece of metal, usually between 18 and 22 inches long. On the outer edge of the blade, one side is sharpened and the other side is bent upward at an angle. This is called the sail, and it is designed to pull the grass blades up toward the sharpened cutting edge. The blade turns like a fan in a circular motion, driven by the engine or some other attachment, in a chamber. As the blade rotates, the sharpened edge cuts the grass as it is sucked upward by the action of the sail.

The design of the chamber, which is sometimes called a deck or deck shell, does a lot to affect how the grass is cut and discharged. Most rotary mowers discharge the grass clippings through a cut out in the chamber, either from the side or the rear. Some mowers discharge the grass clippings out of the chamber and into a collection system, such as a bagger. Lastly, there are also chambers that are designed, in conjunction with the design of the blade, to keep the grass clippings above the blade, continuing to get chopped up as they fall into the blade path until they are cut into small particles that fall to the turf as mulch. This is a recycler or mulching mower.

Conventional wisdom suggests that the rotary mower can be up to 90 percent efficient, meaning that it cuts 90 percent of the grass blades over which it passes. That makes it an effective tool in lawn and landscape care, much more so than a reel-type mower. Rotary mowers are also much easier to maintain than a reel-type mower, as there are far less adjustments needed to operate the mower. A rotary blade is also easier to sharpen or replace than a reel blade. This is what makes the rotary mower the obvious choice for landscapers.

Sharp versus dull

A blade of grass has a horrible life in this modern age. It is never allowed to grow to its full height. It is constantly getting hacked down, week after week. Although this continuous cycle of growing and cutting is not exactly ideal for the plant, there are ways of making it healthy. Ample irrigation, fertilization and other treatments are needed for proper turf care, but maintaining sharp rotary blades is just as important.

When a blade of grass is cut with a dull blade, it is not actually cut, it is torn. The top of the grass blade is tugged upward by the suction force created by the sail of the rotating blade. A sharp blade will cut the grass blade cleanly, but a dull blade will cause the top of the blade to tear off. In other words, cutting grass with a dull blade is like grabbing a handful of grass and pulling on it until it breaks off.

This may not be effective, but why is it unhealthy? Grass, like all plants, converts sunlight to fuel during photosynthesis. This fuel is carbohydrates, just like in humans. Grass that is torn rather than cut burns more carbohydrates to repair the damage. This takes away from carbohydrates that can be used for normal, healthy growth. Repeated cutting with dull blades takes its toll on the plant, leading to dormant growth or even death. It also leaves the plant more susceptible to turf diseases.

All of this translates into disaster for your clients. A healthy lawn equals high profits; an unhealthy lawn equals unhappy customers.

Checking for sharp blades and keeping them sharp

Now that you understand what a dull rotary blade does to grass, take a look at your clippings. Are the edges that were cut off frayed and dry? How about the cut edges of the grass blades on the lawn, are they frayed with split ends? Does the top of the blade appear fragile? If so, you are probably cutting with dull blades.

A wall-mounted blade balancer.

The obvious way to check for dull blades is to look at the blades themselves. Does the blade area have a lot of nicks and dimples in it? If you press your finger against the blade, does it make a crease in your skin? If not, it’s time to sharpen it.

Any lawn mower repair facility can sharpen a rotary blade quickly and efficiently, and usually at a reasonable price. It can be done with a vise and a file. Put the blade in the vise vertically and file the bottom side of the blade at a 45-degree angle until you feel a sharp edge develop. The same effect can be achieved with a grinding stone, which is faster and requires less effort. Try to shave off the same amount of metal on each side or the blade will be out of balance. After sharpening the blade, check the balance using a blade balancer. If a blade balancer is not available, insert the blade bolt hose into a screwdriver shaft. If the blade stays horizontal, it is balanced. If it falls to either side, then more metal needs to be taken off from whatever side to which the blade falls. An unbalanced blade installed on a mower will cause excessive vibration.

Sharp rotary blades really make a great impression. They ensure a healthy lawn and happy clients. It’s an easy thing to check, and just as easy to maintain. So check those blades while you have a chance!

The author is a service writer for a commercial turf equipment distributor in the greater Philadelphia area.