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Dump bed question

Discussion in 'Other Earthmoving Equipment' started by Rickyb1968, Jan 14, 2022.

  1. Rickyb1968

    Rickyb1968 Active Member

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    I have a hydraulic dump bed on a Mack 613. I have noticed as the day goes on my dump bed falls really fast the first foot coming down and then slowly retracts the rest of the way down. I think I have three cylinders. Any idea what is causing this? It doesn’t do this on first dump but after that it does it the rest of the day.
     
  2. 1693TA

    1693TA Senior Member

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    Sounds like air trapped at the top of the lift cylinder(s). Has the filter or strainer been changed or looked at lately? Most cylinders will have a bleeder port at the top of them to purge air when installed. You leave this plug loose an cycle the dump body until hydraulic fluid starts escaping. There is no need to raise the bed. Tighten the plug, raise the bed full stroke, let it back down, then bleed again repeating the scenario.

    If your suction strainer is restricted the pump can cavitate inducing air into the system and in a dump truck environment these need looked at fairly often. Cavitation shortens hydraulic pump life significantly if allowed to persist.
     
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  3. Rickyb1968

    Rickyb1968 Active Member

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    1693ta thanks for responding. If the valve is at top of cylinder how do you get to it with bed down? I’m guessing thru dog box?
    Where would the strainer be on the pump body? Big line come from tank to pump and smaller line going to cylinder.
    Thanks Ricky
     
  4. 1693TA

    1693TA Senior Member

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    I'm thinking you have a three stage cylinder and not three individual cylinders. I've not seen a three cylinder setup except on something like Bristol-Donald bodies that were common in the 50's and 60's before rock tubs became popular.

    Yes, with a three stage cylinder the bleeder port will be in the "dog box" under the removable panel. The strainer will usually be inside the hydraulic reservoir. The suction line leading to the hydraulic pump attaches here. The hydraulic filter is usually on the return line prior to the reservoir. I've seen external strainer installations also, but what I've mentioned is far more common.

    Post up photos if you need additional suggestions. Just going by generics with knowing what is being worked with.
     
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  5. Rickyb1968

    Rickyb1968 Active Member

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    Here is my pump below truck. No very good service so I have to do this in stages. Your right it’s a 3 stage cylinder my ignorance.
     

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  6. Rickyb1968

    Rickyb1968 Active Member

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    There is my set up. Don’t see any filter and no idea where the screen would be?

    talking about bleeding the cylinder. Open dog box and loosen bleed valve. Do I raise the bed a foot. Do I need to be in the bed and have some raise it until I see hydraulic oil come out then close it off. Not understanding the steps your talking about to purge it. Sorry for asking detailed question but first dump I have ever owned.
     

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  7. 1693TA

    1693TA Senior Member

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    Looks like a Commercial Shearing or Intertek dump pump with an air shift PTO, and controls. In your photos I don't see a strainer and you may need to remove the top cover the reservoir for access. Let me see the top of the reservoir. If there is a plate large enough to remove and reach to the bottom, there should be a strainer. If just a fill cap, then there would be no strainer included.

    You do not need to raise the bed to bleed the cylinder. It would be easier and less messy to have someone watch the open bleeder port as you commanded the bed to raise, but you can do this from the truck also as when it starts losing oil from this port and dripping down the cylinder, you'll know you've purged the air. Tighten the port, raise the bed full stroke three or four times, break the bleeder port loose again, and see if there is any air. If not, button it up. Air will always rise to the top of the cylinder and a leaking gland seal is always a culprit. There is a vacuum created as the cylinder retracts and older seals do tend to pull a bit of air into them with age. They don't always leak fluid on the extension cycle either. Your cylinder looks as if it's leaking a bit from the photos so could be time for a repacking. I don't see a filter assembly in your setup as it would need another line only used on the return side. You only have a single line system meaning a suction and return line from the reservoir, and a single high pressure line to/from the cylinder. Both lines are used in both capacity's. Most systems incorporating a filter use a separate line for return to tank and this is where a filter would be incorporated. Doesn't appear you have one.
     
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  8. Rickyb1968

    Rickyb1968 Active Member

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    Thank you for your time and response to my question. I don’t know much but I’m willing to learn. Pretty sure purging air out of the cylinder I can do. I don’t know if I would even have a clue on repacking the cylinder. It still has the tag with serial number so that will help. I also only have a vent cap on top of reservoir

    one more question. How much hydraulic fluid should that reservoir hold. Do I check it with dump all the way up, halfway up. And when I check it how much fluid should be in the tank.
     
  9. 1693TA

    1693TA Senior Member

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    Level needs to be checked when the bed is down. I've never seen a vent in the top of a hydraulic cylinder. Only a cap or plug and this is where you loosen the fitting to bleed the system.

    Repacking a cylinder is best left to a hydraulic shop to ensure all parts are good/serviceable. These things are damned heavy and a forklift is a must to handle them. Most dump bodies require cribbing the body up securely to remove the cylinder also.

    A job I strongly suggest you have performed.
     
  10. gwhammy

    gwhammy Senior Member

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    The top doesn't have a lot of oil volume. Mine comes down fairly quickly on the top stage also then slows when it hits the second and first stage. I would think if there's much air it would give a spongy raise on the last stage.
     
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  11. Rickyb1968

    Rickyb1968 Active Member

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    1693TA I was describing the reservoir tank has a vented cap. I haven’t taken the dog house top off. It’s a Parker cylinder and still has the manufacturer tag on it.
    Gwhammy mine does the same thing. I think it’s a good idea to bleed the air off so maybe it will help.
     
  12. 1693TA

    1693TA Senior Member

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    I read right over the part about the vented cap on the reservoir; sorry. Yes, the reservoir has to be vented to allow the free flow of air pressure and vacuum as the oil is displaced.

    Most of those cylinders will accumulate a bit of air over their usage lifetimes citing the phenomena I stated earlier of the vacuum created as the cylinder retracts. The third, or last stage of the cylinder on extension, or retraction displaces the least amount of fluid and therefore will fall faster than the next two stages which are progressively slower due to moving more oil through a fixed orifice. You don't really notice that much because the bed as it declines in angle increases the weight impressed upon the hydraulic cylinder forcing the fluid from it faster than when the bed was more vertical.

    Never hurts to bleed, or burp the cylinder of entrapped air when it's compressed.
     
  13. DMiller

    DMiller Senior Member

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    Other issue is linkage/cable wear/movement where may not be reaching full stroke on Down request to the Spool.