Rotary Mowers

The performance characteristics of rotary mowers depend a great deal on the design of the mower’s deck.

Usually steel or aluminum is used in deck construction. Steel decks cost less, but can rust.

Fourteen gauge is the most common thickness. If the steel is thinner, regardless of the grade, the mower deck will flex easier, resulting in increased vibration.

Stiffness can be increased by properly shaping the housing and adding reinforcements. It should also be noted that the higher the number used to denote metal gauge, the thinner or lighter the product.

More suction is derived from extra deep decks; this decreases the possibility of objects being thrown out from under the mower. These mowers also permit cutting heavy and tall growth.

However, deep decks require long engine shafts, which are subject to bending, and allow cut grass to pile up against the deck’s underside.

Shallow decks have the advantage of a short engine shaft that resists bending. However, objects can be thrown out from under shallow decks easily, and these engines won’t perform well in tall or heavy growth.

Rather than hitting objects head-on, engine shafts may be bent when the blade goes over or under objects like large tree roots or rocks.

Decks also have additional safety features such as trailing plates at the rear.

All walk-behind mowers comply with federal safety standards, requiring the blade control to be held for the blade to turn and the blade to come to a complete stop within three seconds after the control is released.

To accomplish this, some mowers have a blade-brake-clutch (BBC) mounted on the engine crankshaft that will stop the blade or turn off the engine.

If engine stop is chosen, the mower will have either an electric starter to restart and battery that will be constantly charged while the engine is operating; or the engine can be manually restarted by a recoil starter if the pull handle is located within 24″ of the top of the mower handle.

An exception allows the pull handle to remain on the engine if the mower meets a 360 degrees foot-probe test.

The pull handle also can remain on the engine if the mower deck has extra guarding around the outside.

The bottom edge of the front of the deck should be lower than the cutting edge of the blade. In order to meet thrown.


Quality rotary mowers must have properly designed tempered-steel blades. If they are not tempered in the middle, blades can twist or bend, ends of long blades may flex up or down when run at high speed. Range of Rockwell hardness should be about 42 to 47. Rotary blades have a lip behind the cutting edge that creates suction and throws cut grass out the chute. A lip one or two inches long, with a moderate angle, performs both functions well. If the lip is too long or if the angle of the lip is too steep, it will have a tendency to throw the cut grass up against the deck instead of out the chute.

If a blade isn’t sturdy, it will vibrate when run at high speed; this is especially true of longer blades.

For safety, the blade tip must ride 1/8″ or so above the bottom edge and along the sides and rear of the deck and it should ride 1/4″ or so below the top edge of the rod or bar across the bottom of the grass chute.

It is necessary to take extra care to maintain precision balance when a blade is re-sharpened.


Cutting widths vary from 16″ to 22″, depending upon the blade length.

Smaller sizes cost less, can be easier to handle and may use less gas.

Some have 2-1/2-hp engines, but most have 3 hp. The 21″ and 22″ models usually have 3-1/2 hp.

Persons who choose the larger mowers because they have large lawns may be good candidates for a move up to riding mowers when they replace their old mower.

“Mini-mowers” with 16″ cutting widths are good suggestions for trimming or for cutting small lawns.


Bow shaped, T shaped or slanted T shaped with rubber or plastic grips are the basic handle designs.

A handle attached at an angle that allows the user to push forward instead of down provides easier operation.

Quality features to look for are easy attachment and removal, height adjustment and upright position locks for storage or transportation in car trunk.

Swing-over handles may be dangerous, allowing the mower to roll back on the user’s foot. Also, it encourages the user to run the mower backwards half the time, and this results in poor performance.

Wheels can be plastic or steel. Mowers with larger wheels are more easily pushed. Wide wheels are best; narrow rubber tires may leave wheel marks or tracks.

Plain wheel bearings have steel, plastic or nylon sleeves, requiring at least some lubrication. Sintered bearings require no lubrication. Because sand will destroy plain bearings, some are replaceable. Ball bearings, if shielded or sealed, require no lubrication since oil cannot get out and dirt cannot get in.

Wheels are held by a threaded stud or by a bolt-and-nut arrangement.


Three types of height-adjustment mechanisms exist. Removal of all four wheels is the first. This system is inexpensive and used on low-priced mowers.

The most widely used mechanism is a lever-and-cam arrangement at each wheel. The prime disadvantage is that it may encourage the dangerous practice of holding the deck up with one hand and shifting the lever with the other.

The third approach raises or lowers the entire deck by adjusting a lever or turning a knob. This costs more, but is convenient and safe.

Many mowers have up to five height adjustments. A height change from 1″ to 3-1/2″ is usual, with four to six heights available. In California, many prefer a mower that will cut closer than 1″.


Grass-chute location determines whether the mower is staggered wheel or an in-line model.

Rotary mower cuts are done in the 180 degree semicircle in the front half of the deck; with a staggered wheel model, grass is cut and ejected instantly, much of it in a straight line.

If the chute is in the center of the right side of the deck, little grass is ejected in a straight line, and a hard object hit by the blade has a good chance of hitting the deck before it is thrown out, thus losing much of its speed and danger.

Clogging is often the result of a thin deck edge at the rear end of the chute and/or too small a chute opening. If the engine isn’t stopped before the chute is cleaned, this can be dangerous.

A baffle at the rear of a side-chuted mower may help to prevent clogging, depending on the design.

A rod or a bar across the bottom of a side chute is necessary for safety to prevent the blade hitting the ground.

Staggered and in-line wheel mowers trim on the left side only, since chutes must be extended to meet foot-probe requirements.

Rear-chute mowers that bag at the back offer several design advantages. They allow the user to trim closely with either side of the mower, since there is no bag or chute protruding from the side, and clogging is less likely with their large discharge chute.

Also, the larger rear-bag size reduces the frequency of emptying.


Removing and reattaching grass catchers should be both quick and easy for the user.

There are two types of discharge for grass bagging. One utilizes the standard chute opening; the other blocks the normal chute and opens up direct entry into the bag, tending to fill it from back to front.

Leaf mulching does not require closing the chute if the blade is properly designed, and if there is a baffle in back of the chute.


Mulching mowers eliminate the need for bags altogether since they discharge clippings into the lawn. Thatch buildup is eliminated too, because the enclosed deck and cutting action converts grass clippings into tiny pieces that filter into the lawn without clumping.

Ecology-minded consumers like the idea of the mulch produced by these mowers because it decomposes rapidly, usually in about two weeks, and returns valuable nitrogen to the soil. In addition, some states are beginning to ban community trash disposal of grass clippings and leaves.

The closed deck or cutting chamber produces a powerful vacuum action that straightens grass when cut and re-circulates the clippings until they are re-cut into the fine mulch.

In addition to the benefits that the mulch contributes to the soil, mulching mowers also mean less work since there are no bags to empty.

It should be pointed out that mulching mowers may be less effective in heavy, wet grass, which may “ball up” and drop onto the lawn in clumps. And in extremely tall grass, the mulching action is less efficient.

Some manufacturers offer a mower that can be converted from a mulcher to a standard rotary by opening a plate in the side of the deck for discharge and changing to a standard cutting blade.


Based on different engineering approaches, a variety of driving mechanisms has taken the push out of hand-propelled mowers.

This may involve front or rear-wheel drive, but most are driven by two pinions. Some are driven by pinions that press against comparatively smooth tread rear tires; but under some conditions, these pinions slip and the wheels don’t turn. This can cause the mower to become semi-self-propelled.

Cogged pinions which fit into grooved notches on rear or front tires power other models. This approach works well until the pinion teeth become worn. If the pinions are easily replaced, this presents no problem.

Some models are driven by metal pinions with teeth that mesh into the metal cog wheels attached to the inner sides of the mower’s wheels.

A different approach uses pinions like those on a reel mower. These pinions are on a shaft driven from a transmission. This type of design usually costs more but is very satisfactory.

Another approach is to use a variable-speed transmission with a differential gear box located on the rear axle.

Putting some self-propelled mowers into gear may require raising the handle, while pulling back on the handle disengages the gear. This works well with rear-wheel drive, which is usually less expensive.

Others are put into gear with a lever and wire, or lever and rod, like a gas-throttle control. This is preferred with front-wheel drive. When such a mower is standing with the engine running, it cannot be bumped and put in gear.

An engine-kill/electric restart “deadman” control is available on self-propelled mowers. This control bypasses the starter cord and allows the operator to start mower at the flick of a switch.


Often known as bicycle-wheel mowers, these units have 14″ to 24″ rear wheels, 6″ to 8″ front wheels and 21″ to 24″ cutting widths. They are designed to cut heavy or rough growth and on uneven or rough terrain.

When seeking quality characteristics, consumers should look for sturdy rear wheels with strong spokes and rims, pneumatic tires and shielded or sealed ball bearings. Some engines are 3-1/2 hp but 4 or 5 hp are preferred, and engines are sometimes mounted on marine plywood to reduce vibration.

Swivel front wheels that can be locked if desired are also offered. Most have blade clutch, permitting the belt-driven blade to be idle while the engine is running; rear-wheel drive is used if self-propelled Another feature to consider is extra sturdiness in blades longer than 21″.

These mowers are used extensively in the South.

There are also lighter units available with 14″-to-16″ rear wheels. These come with pneumatic or semi-pneumatic tires and with a regular rotary as the cutting unit. These cost less and are applicable for fine lawns as well as for rough cutting. Widths are 21″ to 22″.


These models run at full speed and require no gas, oil or starting mechanisms. However, cords can present a problem in handling, especially around trees, shrubs and other obstacles. Also, some people will not use them in wet grass.

Many have swing-over handles to make it easier to reverse directions without tangling the cord. Such mowers should have front and rear baffles for operator foot protection.

Electric mowers meeting the Consumer Product Safety Commission’s mandatory safety standard have controls similar to those required for gasoline-powered units. The blade must stop when control is released and this control must require two movements before it can activate the blade. The two movements are required to prevent accidental blade startups.

Get Ready for Next Year
When the mowing season ends, plan ahead and get your lawn mower in shape for next season by:
1. Disconnecting the spark plug wire from the lawn mower.
2. Cleaning out old grass and dirt from the blade and mower surfaces by brushing them with kerosene.
3. Draining gas and oil from the tank.
4. Removing and cleaning the spark plugs.
5. Removing and sharpening the rotary blades.
6. Tightening all bolts.
7. Checking and replacing the muffler if it has rusted.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.

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