Adjusting Rotary Cutters

Richard L. Parish  |  12/3/2004 9:43:07 PM

Figure 1. Rotary cutter.

Many people use a rotary cutter (often referred to by the copyrighted name “Bush Hog”) to cut grass and weeds, but, in too many cases, the implement is not correctly adjusted (Figure 1). This can cause premature wear, poor quality of cut and an excessive power requirement.

Height of Cut
The most common error with the use of rotary cutters is cutting too low. Rotary cutters are not designed to be finish mowers. If you want to cut turfgrass 2 inches high, you should use a lawn mower or a finishing mower (also called a “grooming mower”). Since a rotary cutter typically has a wide deck with only one large blade (or multiple large blades on wide commercial and agricultural units) and only one gage wheel, it cannot follow ground contours and is prone to scalping if run too low. The lowest recommended cutting height for a rotary cutter is 3 inches.
To achieve the desired cutting height on a typical 3-point hitch mounted rotary cutter, you must do two things. First, you must adjust the rear gage wheel to the desired height, and then you must set the 3-point hitch control to hold the front of the mower deck at the correct height. You should make these adjustments with the tractor and mower parked on solid, level ground – preferably pavement. The gage wheel may have a slide adjustment, a series of holes for adjustment or a stack of bushings that are moved above or below the collar. Set the deck approximately level with the 3-point hitch, measure the height from the ground to the cutting edge of the blade at the rear (with the tractor turned off), and then adjust as needed to obtain a cutting height of at least 3 inches.

Deck Angle
It may come as a surprise that the deck should not be run level. With any rotary mower, it is desirable to set the front slightly lower (¼ to ½ inch on a 4- to 6-foot rotary cutter) so that the blade cuts at the front and the rear of the blade does not recut the grass or drag on the grass. This will reduce the power requirement and provide a cleaner cut. After you have set the cutting height at the rear using the gage wheel, you will need to adjust the 3-point hitch so that the cutting edge of the blade at the front is ¼ to ½ inch lower than at the rear.
If your tractor has an adjustable stop on the 3-point hitch control, you should set it at this point. If the tractor has a marked scale on the 3-point hitch control, note and remember the setting. If your tractor has neither, just take a pen or marker and make a small mark at the correct setting so that you can come back to it again after you raise or lower the mower.

Sway Bars/Chains
The sway bars or chains on the 3-point hitch of the tractor should be reasonably tight to assure that the mower follows straight behind the tractor and doesn’t sway. This is especially important when you have to back up with the mower.

Freedom to Pivot Vertically
A rotary cutter must be able to pivot vertically about the lower link pins of the 3-point hitch to allow the mower deck to flex up when the rear of the tractor goes into a depression and flex down when the rear of the tractor goes over a bump, yet it must also have a limit on this flexing motion to allow the mower to be raised for transport. All rotary cutters employ some type of moving linkage on the upper hitch point to allow this, but improper adjustment can negate it. You must adjust the upper link of the 3-point hitch on your tractor to allow some flex of the mower deck, but still be able to lift the mower.
This adjustment is somewhat subjective, but you should adjust the length of the upper link out enough that when you lift the 3-point hitch above the normal operating point, the gage wheel remains on the ground for a while as the front of the mower lifts, but the gage wheel eventually lifts at least a foot off the ground when the 3-point hitch is completely raised.

Side to Side Level
A rotary cutter must also be level from side to side. The easiest way to check this is to bend down behind the deck (with the PTO disengaged) and sight over the deck at the tractor rear axle. If the deck is not parallel with the tractor axle, you will need to adjust one of the lower 3-point hitch lift arms until it is parallel.

All of these steps are simple to do and take only a few minutes, but they contribute significantly to both the quality of cut and the longevity of your mower.

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