Cut quality 42" and 48" decks

Discussion in 'Lawn Mowing' started by jonathanone, Mar 9, 2011.

  1. jonathanone

    jonathanone LawnSite Member
    from Ct
    Posts: 122

    The 42 has 2 blades spinning at about 18,800 the 48 has three blades spinning at about 16,700 both decks are the Bob-Cat 'Dura' deck would anyone venture a guess as to what differences might be observed??
     
  2. GMLC

    GMLC LawnSite Platinum Member
    Posts: 4,353

    Good question and I dont know the answer. I do know a 42" deck is not that common, they are out there but kinda an odd ball.
     
  3. davidcalhoun

    davidcalhoun LawnSite Bronze Member
    Posts: 1,022

    I think you are referring to blade tip speed not RPM.

    Blade tip speed is: engine speed X blade length X 0.262

    The 42" deck uses 2 - 21" blades where the 48" deck uses 3 - 16" blades
     
    Last edited: Mar 9, 2011
  4. dishboy

    dishboy LawnSite Platinum Member
    from zone 6
    Posts: 4,014

    If the blades are not timed the two blade design will mandate a longer wheelbase and different offset between wheels on corresponding passes. Otherwise, if the ground is not level you should get a flatter cut with the three blade design if the decks are level. IMO a TIMED two blade design will cut better than both mowers in question but I don't that is what you are referring to?
     
  5. jonathanone

    jonathanone LawnSite Member
    from Ct
    Posts: 122

    yes, tip speed , from searching tip speed here the consensus is that faster is better, some say much better ???? But could the two blade deck also turn out to be a screamer ?

    gm, we are thinking along the same lines-if it doesn't work out the deck size may bite us trying to get rid of it..-IF we could get rid of it. Probably would not be much interest for the pro market, unless we gave it away ditto the home owner market also.

    This is one of those times that if we buy it we really would own it.
     
  6. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    My experience tells me the 2 blade 42" should be a better all around deck than the 3 blade 48".

    The first thing you'll notice when you engage the deck will be the 42" deck will probably be quieter. Despite the higher blade tip speed of the 42", the 48" blades are actual turning more RPM's than the 42".

    3300 (approx) RPM - 42" Deck
    3800 (approx) RPM - 48" Deck

    48" 3 blade decks sound like a buzz saw. You'll probably feel more vibration out of the 48" than the 42" (more rotating components and at higher speeds) due to the higher blade RPM. The spindle bearings will probably last longer on the 42" than the 48" because of the lower rotational speed. that translates to fewer rotation of the bearing for every hour the machine is used. Not to mention lower bearing temps.

    Everything else being equal, the 42" should cut better than the 48"

    The blades on the 42" (21.5" approx) will be longer than the blades on the blades on the 48" (16.5" approx)

    Generally speaking, longer blades cut better than shorter blades as ground speed increases.

    I'm not going to to go into the math (unless it would help make the point) but...

    The higher the ground speed, the more grass is cut per cutting cycle of the blade. (2 cutting cycles per revolution) Think forward movement here, not vertical height or grass density. The closer to the rotational center of the blade you get, the closer the cutting edge speed gets to zero.

    The 42" deck also has one less blade overlap to leave stripes when processing large amounts of material.

    The 42" will probably be cheaper to maintain. One less blade to buy, one less spindle to replace, etc.

    Manufacturers like the 3 blade 48" because it shorter from front to back and a allows the machine to be shortened. The 42" will physically take up more space under the machine.

    That's about all I can think of at the moment. Let me know if anything needs clarification.

    Q
     
  7. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    I agree with you. When the blade tips pass directly by each other it disturbs air flow and creates noise.

    Not sure if you've ever noticed it but at times, you can actually hear the blades going in and out of phase. It's that "drone" you can hear withe blades enagaged but not cutting grass ... loud LOUDER loud LOUDER loud LOUDER loud..... Ha! :laugh: I'm sure you get what I mean.

    That is another reason the 2 blade will probably be quieter than the 3 blade. One less set of blade tips passing each other and creating noise.

    Q
     
  8. kilgoja

    kilgoja LawnSite Senior Member
    Posts: 944

    personally i've never noticed a difference in the cut between a 42", 48" or a 54"...only difference is the width lol...just keep the blades sharp and all is good :)

    you could get a 54" as the blade tip speed is high as a 42" and it has 3 blades
     
  9. jonathanone

    jonathanone LawnSite Member
    from Ct
    Posts: 122

    thanks all;

    dish/Jim, I found more info stating that the rpm for both are the same, 3550, both .250 thick high lift offset; and Jim you were real close on the rpm and blade size, 2 blade deck uses 21", 3 blade uses 16" as David stated-does this indicate anything as to what might change?
     
  10. JimQ

    JimQ LawnSite Bronze Member
    Posts: 1,139

    Don't confuse engine RPM and blade spindle RPM.

    You can always change the clutch pulley and/or spindle pulley to put the blade RPM where you want it. It doesn't have to be a 1:1 drive ratio. It's generally not.

    I would bet the blades are longer than 21" and 16". If they weren't, you'd have no overlap and probably have stripes between the blades. ....or maybe there is overlap and they are not true 42" and 48" decks.

    Q
     

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