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marineguy_gw

What makes a mower deck noisy?

marineguy
16 years ago

It's an obvious question with an obvious answer, or so I thought. I recently mowed with my Dad's JD 265, which has a 48" deck (with 700 or so hours). The deck is so quiet I couldn't even hear it engage, and once when I bounced in the seat and it disengaged, I didn't notice (until I lapped the yard and saw the uncut pass). My GT225 (2-blade 42") is still pretty much in like-new condition, with hours in the low 200s, but my deck is a heck of a lot louder, just like every other riding lawn mower I've ever used. My spindles are tight and well-maintained. It's not a bad sound, just a sound like a big fan. Why is the 48" deck fitted to the 265 so much quieter? Is it becase of the shorter blades? I haven't compared pulley sizes between the two, but I just assumed with the shorter blades they'd have to spin faster, making them more noisy.

Of course if the smaller 16" or so (48") blades spin at the same RPM as the larger 21" (42") blades, it would make sense that they would make less noise, moving more slowly through the air.

I know this is probably the least important issue ever posted by someone other than lb59; I'm just baffled at how a deck could be so quiet only the visual cues (grass shooting out the side) let you know it's operating. No, it wasn't drowned out by engine noise. His Kawasaki is probably quieter than my Kohler.

Comments (38)

  • tmajor
    16 years ago

    I'd say vibration is the biggest factor. While this could be entirely blade balance, it could be the deck fabrication, as well ("tin canning"). Three blades vs. two could also factor in, as well as the belt routing for same. ??

  • deerslayer
    16 years ago

    The blade tip speed between the two decks should be similar. The type of blades can affect the noise level. My Gator blades are noticeably noisier than my WH factory blades. Since the Gators are mulchers, more turbulence under the deck is a good thing.

    -Deerslayer

  • tmajor
    16 years ago

    I believe, it was called "tin canning", but "oil canning" might be more widely used. It's been a long time!

    As you point out, the noisy er deck, at least lets you know when you are mowing.

  • canguy
    16 years ago

    The lighter decks will not absorb as much noise. Hi lift blades will also move more air, hence will be louder.

  • duke88
    16 years ago

    marineguy , You may want to think about trading the GT225 for a LA series....LMAO !

  • rustyj14
    16 years ago

    Why, its as plain as the nose on yer face! Yer ear plugs fell out!
    RJ

  • mownie
    16 years ago

    I also think that it is probably the "tin can acoustics phenomenon" and the 3 bladed deck having more mass to attenuate "sound wave vibrations". It might also be that the shorter blades are somewhat stiffer and therefore don't "quiver" as much as the longer blades. They "quiver" because they try to bend in reaction to striking the air and the grass. The front edge area of the deck puts up the greatest resistance to the passage of the blade, causing the blade to want to dip down ever so slightly. When the blade tip reaches the trailing edge area of the deck, the load lessens a bit, causing the blade to try to rise a little. This translates into a vibration or "quiver". Longer or more flexible blades will develope more "quiver" than shorter, stiffer blades. This "quiver" or vibration is likely the dynamic behind a good deal of noise generated by any deck. Add that to the noise created by the belts, and the impact of the grass stems against the blades, spindle bearings with there own unique acoustic signature......why it's a veritable cacaphony of sound.

  • butchs_hobby
    16 years ago

    Now that gives me a chuckle, rustyj.

  • bighagrid
    16 years ago

    did ya check to see if the neighbors cat was under there? hmmmmm

  • johntommybob
    16 years ago

    I've a GT225 with a 42" deck also, but I had never thought about how loud it is until you posted this. Now that I think back, before I bought this mower, I remember trying out a used Kubota G-something that as I recall had a 60" mower deck on it. It was on consignment at a Kubota dealersip. Seems the owner had died and they were selling it for his widow. Anyway, they had a big grass area to try mowers out on and I remember thinking, "Is this thing mowing?" But I could see some grass clippings flying out the grass chute (the grass was rather short from many mowings). I never have to ask that about my GT225. It was a very nice tractor, but I decided that a 60" deck was just to wide for my mowing needs. It sure was quiet though.

  • marineguy
    Original Author
    16 years ago

    canguy,
    I think your catch is the biggest difference between the two. I have hi lift blades, and I'm pretty sure he does not, since the mower does not have a bagger. I've often wondered why you wouldn't go with hi lift blades, even if you aren't using a collection system, but if noise is a tradeoff, then I guess that's why.

    mownie,
    I did consider the noise attenuation properties of 3 vice 2 blades, but since they're all spinning at the same speed in the same direction, I didn't think it would factor in. Helicopter blades are designed to lead, lag and flap as they encounter various airflow patterns throughout their rotation. Some helos have rigid rotor heads, some semi-rigid, and some fully articulated. Whatever the design, the blade tips speed up, slow down, rise and fall as they whirl around. This is where most of the chopping noise comes from.

    Duke
    No way; I don't have time to run it to the dealership every time I need to grease the spindles!

  • wally2q
    16 years ago

    There are a multitude of noise generating, and noise deadning properties built into the decks.

    However, the #1 noise causing design aspect is the lift of your blades. High lift blades move a ton of air - they are like little propellers. You'll notice the sound your deck makes is in fact very much like an aircraft prop.

    Go to zero lift blades, and all you'll have left is the other noises like belts, vibrations etc.

  • metal
    16 years ago

    Things that affect noise IMHO:
    Hi-Lift Blades
    Number of Blades
    What you are mowing over (drive your mower onto a concrete driveway and notice the difference).
    Height of mowing (higher mowing leaves more space below deck for noise to emit)
    Deck build

  • ervie
    16 years ago

    I guess a three blade deck would be quieter than a two blade
    deck of the same width. I base this on fan area. For a 48"
    cut, you might get by with two 24" blades. They would fan 904
    square inches. Or you could use three 18" blades for good overlap and only fan 762 square inches. Less fan area should give less air flow and less noise.

  • deerslayer
    16 years ago

    That's an interesting calculation. Since a mower blade isn't shaped like a fan blade, I think the area of the lift wings should be compared. In the case of a flat blade with no lift wings, air noise should be close to zero.

    -Deerslayer

  • duke88
    16 years ago

    OH BOY , I just ordered a set of Gators for my LA120 ...should I get ear plugs ?. marineguy says the LA series are light weight so I hope it stays on the ground with Hi lift blades.....HEY maybe I could do a fly over along side marineguy in his copter. lol

  • marineguy
    Original Author
    16 years ago

    Duke, that won't work because the blades lift grass, not push it. At best you should be able to mow up the side of your house as the high vacuum holds your LA to the siding. But if you're going to try hovering, you could mount the belt in a figure-8 so the blades spin backwards.
    And make sure your wife takes pictures!

  • machiem
    16 years ago

    My Kubota deck is relatively quiet until I start mowing and also when I get a build-up of grass under there. Then it starts sounding like I'm going 55 mph with baseball cards stuck in my spokes.

  • rcmoser
    16 years ago

    I don't know about the JD265, but if its like the JD210 the deck has cover plates over the spindle drive pulleys and it has one common pulley sticking out driving the blade spindles pulleys.

    After I got the 210 running this was the first thing I noticed how quite the deck was. I chalked it up to older model with lower blade speed and the covers over the spindle and idler pulleys. IMO on most decks the noisest pulley is the tension pulley. Just spinning them by hand they make noise.

  • wally2q
    16 years ago

    Deerslayer is right...

    if you want to see what happens with the air-movement from the blades eliminated from the equation, take the blades off, and reattach your deck, turn it on, and be amazed....

  • marineguy
    Original Author
    16 years ago

    rcmoser,
    That's another factor; his does have the cover plates, whereas mine only had one on the left side and I removed it in the interest of not trapping moist grass clippings in the nooks and crannies of my deck, where they would do nothing but corrode the sheetmetal. So now I have two strikes: hi lift blades and exposed pulleys. All I need now is a discharge chute tapered like a bullhorn.

    Walley,
    I never doubted the blade was the main source of noise. I was just wondering if the blade length was the main factor in the very large discrepancy between the 48" and 42" decks, but I think we've pretty much got it nailed down that the amount of air moved by the deck determines how loud it is.

    Speaking of blade speeds, I recall someone mentioning that OSHA or some government entity started regulating blade RPMs a few years back, and newer tractors actually have slower blades than old tractors.
    Question 1) What's so much safer about slower blades?
    Question 2) Can you actually cut faster with faster blades? I'd expect that they'd be more susceptible to bogging down under heavy load. It seems to me that if you mow slow (like 3mph or less), fast blades would benefit you. But if you mow fast (3-7mph), you'd want a lower-geared blade speed which would be less likely to slow down in the thick stuff--at the expense of a rougher cut.
    Any thoughts?

  • deerslayer
    16 years ago

    Question 1) What's so much safer about slower blades?
    Slower blades are less dangerous during a failure. Also, they throw hit objects at slower speeds.

    Question 2) Can you actually cut faster with faster blades?
    Yes, most commercial ZTRs have faster blade tip speeds than consumer grade equipment.

    -Deerslayer

  • wally2q
    16 years ago

    blade speed does play a role, but only in relation to the lift the blade creates. You could spin a zero lift blade 2x as fast as a high lift blade, and the high lift blade will still be louder.

  • deerslayer
    16 years ago

    The reason you can cut faster with a faster blade tip speed is because the faster the blade rotates, the less grass it cuts with each half revolution. Normally, approximately the first inch of the blade cuts grass. Here's the math for 4mph ground speed and 3,000 blade rpm.

    4 x 5280 x 12 / 60 / (2 x 3000) = .704 inches

    In other words, at a ground speed of 4 mph the tractor will advance .704 inches with each half rotation of the blade. You can see that if you increase the blade rpm, less grass will be cut with each half rotation.

    Since the force on the blade is proportional to the amount of grass cut with each half rotation, faster blade rotation allows a faster ground speed.

    -Deerslayer

  • rcmoser
    16 years ago

    IMO there is a difference in sound vs blades and spindles, idler, and tension pulleys. The air swish vs. the high pitch noise from the pulleys it two different sounds to me.

    It makes sence to me to have shorter blades. I had a 30" single blade rider along time ago it was cheap and noisey (MMMM does these two have anything in common????) Cut like a push mower but it would bogg down very easily(probably underpowered also) Hence the one big blade vs two or three. IMO we get back to the two arms or legs is better than one and three is better than two????!!??? argument. I know 8 is better than two. For those of us that think V8 makes anything better. Better is of course subjective in someones mind....

  • marineguy
    Original Author
    16 years ago

    The fact that the three-blade deck didn't bog down in the least as I mowed his two-weeks uncut lawn surprised me as well. I attributed it to his 17hp Kawasaki having a little bit more torque than my 15hp Kohler, but I guess the shorter blades are the reason for that too.
    It's pretty obvious that the 48" deck is an all-around better choice for any yard big enough to justify a tractor. The only advantage to the 42" is that it seems to move more air and therefore doesn't need a powerflow blower to use a collection system. But the 3-blade is quieter, trims better, and cuts faster (it just might not fit through your shed door).

  • mownie
    16 years ago

    ..........which would make a fine excuse (reason) for getting that bigger shed with wider doors, right?

  • john_wa
    16 years ago

    Marineguy, this has been a really interesting thread! I noticed that when a mulch kit was put onto my GX335 54" deck, it got a LOT quieter. I guess the chute opening was eliminated and much of the underside of the deck got covered up in thick plastic.

  • duke88
    16 years ago

    I just put Gator blades on my LA120 and as far as noise they didn't seem any louder than the JD mulch blades that were on. The leaves are starting to fall after a couple nights in the 30s (frost in some areas) and I gave the Gators a test run yesterday after installing and was very happy with the results ... noise not bad & mulched leaves very well. Looks like it won't be long till we will be talking about snow blowers....

  • rico567
    16 years ago

    I didn't notice any increase in noise when I switched from regular to Gator blades on my Dixon 4521. It's a 42" deck, using 3 x 16" blades. This is a commercial mower, using what seems to me very heavy deck metal, and the blades are at least 3/16" thick. Of course, I wouldn't notice the noise as much in any case, because I always wear earmuffs while mowing. Unless your mower is electric, you probably should wear ear protection, too.

  • oldtiredbob
    16 years ago

    I'm always impressed with Deerslayers knowledge or better intelligence. I'm an old dumb ass and all I worry about is keeping the conquest clean and how high to tee my golf ball. Another important issue is what club to use for the amount of wind. Never new what it was like to be smart. I'm impressed.

  • marineguy
    Original Author
    16 years ago

    Either Deerslayer is one of the most cerebral, well-read, thought-provoking, mechanically-minded, authoritative, and downright helpful tractor historians alive, or he stayed at a Holiday Inn Express last night. I can't decide which...

  • mownie
    16 years ago

    Concur! Though I doubt HIE had any bearing on said member's profile.

  • deerslayer
    16 years ago

    Nope, MarineGuy is right. It's the Holiday Inn Express.

    -Deerslayer

  • wheely_boy
    16 years ago

    I kind of figured it was the thousands of blades of grass breaking the sound barrier simultaneously.

  • deerslayer
    16 years ago

    Wheely_Boy, that's an interesting hypothesis. Let me think on that for awhile.

    -Deerslayer

  • deerslayer
    16 years ago

    Wheely_Boy, I thought about it for awhile. I think your comment is BS. Please don't take this personally. I like your Photoshopped pictures. 8^)

    -Deerslayer

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