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  1. #1
    Platinum Member
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    Default Single vs Double acting hydraulic cylinder.

    I purchased a cheap snow plow for my tractor. It doesn't come with the hydraulic cylinders. Anyway I was wondering why do snow plows come with two single acting cylinders vs one double acting? The reasons I came up with are: price, weight (balance), cylinders are stronger pushing than pulling (are they?). I am a novice when it comes to hydraulics.

  2. #2
    Veteran Member Spiffy's Avatar
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    Default Re: Single vs Double acting hydraulic cylinder.

    I'm a novice on snowplows, but can answer your hydralics.

    Force from a cylinder = piston area X pressure; since you need to subtract the rod's area from the retraction [pull] side of a cylinder the extention [push] can produce more force. The irony is that [especially on small rod sizes and/or long travels], you're subject to collumn buckling [bent rods], before tensile fracture [pulling it apart]. I think another thread had some great pix of that effect on a BH cylinder. Of course considering the travel you're looking at on a plow blade, it really negates any concern for rod failure.

    Since it's often cheaper to add size than number of components, I don't think the cost of the cylinder itself is your answer, but for the way it is integrated into the plow some size restriction may be important and may have a cost benefit that way as well.

    When I think of blades, it's one at each end [single or double acting] for angle adjustment. But it sure sounds like you're referring to two singles used as a double, which, barring some dimensional restriction, is something I've never seen. [img]/forums/images/graemlins/crazy.gif[/img]

  3. #3
    Platinum Member DMF's Avatar
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    Default Re: Single vs Double acting hydraulic cylinder.

    I'm a hydraulic novice as well. That said, the Fisher snowplow I used to run on my 87 Chevy had two, single acting cylinders that "pushed" the blade. I plan on building a plow frame I can use on my FEL this fall and I planned on using the same set-up unless someone more knowledgeable on this forum convinced me otherwise...

  4. #4
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    Default Re: Single vs Double acting hydraulic cylinder.

    I have no idea about snowplows. But, I can tell you there is a little force called "gravity". Single acting cylinders provide force to operate in one direction and depend on gravity (or a spring) to retract.
    So, don't use single-acting cylinders on your home-made space shuttle... it won't work!

  5. #5
    Epic Contributor Soundguy's Avatar
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    Default Re: Single vs Double acting hydraulic cylinder.

    I've seen 3 SA cyl setups for side to side and up/down.. but at that point you have a spool valve.. way better to use 3 DA cyls and get side/side , tilt, and up/down.

    In the old days, a single SA cyl worked fine for up down, and even a cable pully setup worked with a small fram on the front of the tractor. Heck.. I saw a guy use a SA cyl setup between his toplink and drawbar.. when he raised his 3pt lift.. it compressed the cyl, and forced the trapped oil thru the hose to another hyd cyl on his front snow blade, raising it. Using 1 hyd line.. 2 SA cyls, and some oil, he had made a closed loop hydro system. compressing 1 cyl extende dthe other.. made his 8n look pretty spiffy, using the 3pt to lift his snow blade..

    Soundguy

  6. #6
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    Default Re: Single vs Double acting hydraulic cylinder.

    </font><font color="blue" class="small">( I saw a guy use a SA cyl setup between his toplink and drawbar.. when he raised his 3pt lift.. it compressed the cyl, and forced the trapped oil thru the hose to another hyd cyl on his front snow blade, raising it. Using 1 hyd line.. 2 SA cyls, and some oil, he had made a closed loop hydro system. compressing 1 cyl extende dthe other.. made his 8n look pretty spiffy, using the 3pt to lift his snow blade.. )</font>

    Soundguy,
    We learn something new everyday. That's a cool use for an SA cylinder. I'll bet bleeding it was a pain though! [img]/forums/images/graemlins/blush.gif[/img]

  7. #7
    Epic Contributor Soundguy's Avatar
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    Default Re: Single vs Double acting hydraulic cylinder.

    I'll bet it deffinately was... must have had a bucket of oil att he end and ran it disconnected till it cycled full a few times.. then must have compressed the front cyl, and then partially filled it with oil, hooked up the lines loose, let the bubble come to the top, and then bleed excess oil out that fitting... bet it made a mess.. but looked spiffy in the picture.

    I've seen variations on that as well. One fellow used a rather large cyl on back to move two smaller cyls on front.. saw similar using 2 or 3 cyls on back, just for more oil volume to seperate cyls/circuits.. etc.

    Soundguy

  8. #8
    Veteran Member Steve_Miller's Avatar
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    Default Re: Single vs Double acting hydraulic cylinder.

    Not to get too technical but if you use a SA cylinder you need a bigger reserve of fluid (i know it is sort of irrelevant here) unless you are using two SA's back to back. Two examples: On power steering (on my Kioti) the two cylinders are back to back so when one is retracting the other is extending, this way the identical amount of fluid that is pushed into one cylinder is pushed out of the other back to the tank to keep the level constant. This, I think is the way most truck plows work with the exception of the 'V' plows which need DA's.

    Steve

  9. #9
    Bronze Member KenVT's Avatar
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    Default Re: Single vs Double acting hydraulic cylinder.

    I think you are asking why most plow set ups have 2 single asting cylinder, why not just 1 double acting on 1 side....

    I also have a cheapy snow plow, it's a snow bear, and I figured with 1 dual acting cylinder and a QA plate welded to it, I could use my remotes to power angle it, instead of getting off the tractor to manually position it...

    If I ever get to it, I will post pics


    Ken




  10. #10
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    Default Re: Single vs Double acting hydraulic cylinder.

    OK, I'll take a shot at explaining why snow plows use two SA cylinders as opposed to one DA.

    If you look at the geometry of snow plow, the cylinders have to angle the blade from a somewhat disadvantaged position and there is no gravity to help it move. It has a limited hydraulic supply as well.

    The two SA cylinders provide the most force (equal) in both directions - no rod taking up part of the cylinder volume in one direction of movement like with a DA cylinder. So the SA cylinders can be specifically designed for their purpose - meaning they are less expensive because they are each much smaller than a DA would have to be to give adequate force in both directions.
    If you notice the cylinders for snow plows have rather large shaft diameters compared to the cylinder body, this is by design as well; gaining the most piston force in the smallest, cheapest package. The way they are mounted on a plow, one cylinder extending causes the other to retract - preparing it for its next extension.

    I may be off base on my explanation of this issue, just my observations.

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