Lawn Mower Knives: Requirements, Varieties, Features of Attachment And Selection

A knife is no less popular than a fishing line or cord, a working tool for a mower. If in the trimmer you can do with fishing line or cord, then the knife is basically a cutter for motokos, the power of which is noticeably greater than that of the trimmers.

Purpose and requirements

The purpose of the cutter for the mower is to cut the grass evenly and accurately in large quantities. In one session, when mowing one hundred square meters, you can collect tens of kilograms of grass. If the lawn mower has the function of shredding (mulching), such a knife should also grind the grass. The lawnmower cutter is designed so that its blades create upward traction for the freshly cut grass. She carries this grass to the grass catcher.

The best alloy for a mowing knife is stainless steel. over, it should have a hardness of not less than 55 units on the Rockwell scale. A lower hardness value will quickly blunt the cutter. especially when cutting weeds and cutting bushes. Since the grass being cut contains not only moisture, but also other active components, ordinary steel would quickly become brown after a first mowing session. Stainless steel, creating chromium oxide on its surface that resists corrosion (chromium is included in the list of alloying additives), does not allow the knife to rust.

Important! In addition to hardness, the cutter must be strong. If during work he stumbles upon a stone. then he should not scatter into pieces.

Varieties and sizes

Too large a knife will “plant” the engine speed, which will lead to greater wear of the latter.

Finally, crucial in terms of safety is the material of the torch. Most knives are made of stainless steel, but plastic knives are also found. The fact is that when the cutter bumps into a large stone, the debris of the steel can injure you with a rebound. The impact of a plastic debris is far more harmless. unless it gets into the eye. Plastic knife. the choice of owners of mowers with a curved flexible shaft. The universal knife for lawn mowers not only cuts grass and shrubs, but also crushes them. due to the larger number of blades (up to 8). Finely chopped grass will fit as top dressing (mulch) for the lawn.

Shapes of blades and adapter

If it is a simple blade cutter, then there can be several blades. but not more than 8. The shape of the blades of such knives can be as follows:

  • simple (in the form of rectangular ends);
  • rounded (extreme edges have smooth curvature);
  • pointed (in the form of a dihedral wide bayonet).
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Bladeless knives. a milling cutter resembling a circular saw on a tree. Such knives are also called circular knives. The number of teeth is up to tens. They remotely resemble a bicycle sprocket, the teeth of which sharpened sharply and changed the direction of the cut. There is a special type of cutter, in which the cutting part is not star-shaped, but stepped. It resembles rounded and curved steps, also spaced evenly. Their steep parts are sharpened sharply. The height of the teeth or "steps" reaches several tens of millimeters.

The adapter is a two-blade adapter for a knife worn on the motor shaft. On the knife side, it has holding rods threaded into the holes of the torch itself. The whole structure is tightened with a central bolt. Simply put, an adapter is a knife holder. On the reverse side, it has serrated grooves matching the teeth of the shaft. It is selected according to the inner diameter of the central hole of the knife (for example, 22 mm).

What should be the angle of sharpening?

Sharpening cutters for a lawn mower is always performed at an angle of 30 degrees. A smaller angle will make the blade too thin, like a razor foil. it will serrate quickly, bend, they can only mow low grass. Weeds and bushes are guaranteed to “hack” the tip. Too a large angle will cause the knife to beat and break the bushes more, to tear the stems of the grass. than to cut both. In addition, the tip should be sharpened on one side. and only slightly, barely noticeably sharpened on the opposite.

Video: Lawn Mower Knives: Requirements, Varieties, Features of Attachment And Selection


How to fix and replace?

The knife should be changed once a quarter or in the season of grass growth, although sometimes it should be changed earlier. The installation of the knife is as follows:

  1. if the mower is in operation. pause it; with an electric braid, just turn off the power (cord from the outlet, disconnect the battery); gas mowers are switched off with the key, unspent fuel is drained back into the canister;
  2. turn the mower on its side and remove the grass catcher;
  3. clean the earth and grass adhering to the working parts from the knife and the protective visor;
  4. remove the visor (protective cover) of the knife; it delays the grass scattered in all directions (when mowing);
  5. unscrew the fastening bolt using a wrench or head, the knife should not turn, it is attached to the adapter; Before removing the knife, it is recommended to wear work gloves so as not to cut yourself;
  6. insert a new (preferably exactly the same) knife and tighten the bolt more tightly, when it loosens, the machine vibrates, or the blade itself may fly off.
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If there are vibrations, but there is no replaceable knife, it is necessary to balance the old one. Remove the knife and check its balance by hanging it on a screwdriver or nail with a large hat. To do this, the working part of the screwdriver is threaded through the central hole of the torch itself. If there is an imbalance, one specific point will bend, in this case, grind a little metal. With repeated sharpening, the mass of the tip outweighing the others will decrease, and the center of mass will shift to its place. When you are finished balancing, re-attach the adapter and tighten the cutter that is modified in this way. If the adapter has a key that holds it on the shaft of the engine (gearbox), do not forget to press it into place.

If a spare torch purchased previously was found, immediately install it. In some cases, the holder wears out. it needs to be replaced.

Popular manufacturers

Like lawn mowers and accessories for them, for example, knives, produced by popular manufacturers, the best of which are the following:

Selection tips

First of all, the knife must be strong and durable. The best material for knives is stainless steel of the required thickness, and not, for example, black steel with stainless steel spraying or galvanized iron. Sometimes manufacturers indicate a stainless steel brand, for example, AISI 304. A knife is not a part to save on.

For portable lawnmowers, a two-knife blade is considered the minimum solution. In other words, you should pick up a knife with two cutting edges. He will cut grass. but not chop it. To mulch the grass. buy a cutter with eight points. The optimum sharpening angle is approximately 30 degrees. You will quickly learn to recognize such knives.

The thread of the rotor pins, on which the cutter is mounted with its mounting holes, does not matter. Usually it is left, if the knife rotates to the right. and vice versa. This helps to avoid loosening the nuts and flying off the knife at full speed.

If your choice nevertheless fell on plastic knives. look for a fragile and rather hard plastic that retains the factory sharpness of the cutting edge for longer. This does not fly apart from the slightest blow when carrying or rolling a lawn mower to the place where grass mowing begins.

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Common breakdowns

Most often, the malfunctions are as follows: the mower does not mow the grass or does not turn on at all.

Mower does not mow grass

There are two possible options.

  • The knife is dull or broken. Sharpen it with a file, grinder, on a grinding machine (you need a flat-cylindrical whetstone) or on the machine (or drill) with emery wheels. Check torch balancing.
  • The cutter itself is not set correctly. Set the clearance in the cylindrical mower between the rotating cylinder and the blades of the knife equal to the thickness of the sheet of paper.

A rotor with a knife does not spin in the mower

There are several options.

  • The knife itself is jammed. Pull grass and ground out from under it and scroll the torch itself. It should rotate freely, without scratching or touching the protective cover.
  • The drive belt is broken. Check belt integrity. A worn-out, “crumbled” belt must be replaced.
  • Break on nutrition. Repair the tattered wire in the cord. Replace cord if insulation is broken.
  • The switch does not work. Check the operation of the switch, try turning the mower off and on. If the switch is unusable (melted, burned contacts). replace it.
  • The power supply voltage has disappeared. Check the possible operation of the circuit breaker on the switchboard.
  • The plug is not plugged in. Plug in the plug (if it is pulled out).
  • The fuse in the device has blown. Check it with a tester (multimeter). Replace the defective fuse if necessary.
  • The heat protection of the mower turned off the motor. Wait for the motor to cool after a mowing session.

After checking all the parts and components, finding a malfunction, start the mower. Most likely, your problem will be solved. For fuel lawnmowers, either the carburetor (or drive) or the fuel has run out. Often unstable operation of the motor requires searching and eliminating the breakdown of a number of parts before it is possible to find and replace the faulty one.

How to replace the knife on a lawn mower, see the video.