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Discussion Starter #1
I maintain the dock levelers at work but I don't understand how they work. Could some one help a confused one out ??? There is two cylinders. One lifts the plate and one lifts the lip... I drew out exactly the way the hose's run. Both cylinders move out when you push the up button... How can this work with the drawing below? The second cylinder looks like it should pull in not out. And the first cylinder looks like a dead short or a brick wall... The Big number 1 cylinder moves out first and then #2 moves out...

 

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Hmmmm, I'm not sure either.

I'm assuming the green line is pressure and the blue...eventually is the retun line?

You said when you push the 'UP' button that lifts the deck up. What does the other functions of that switch do? If there's a down button, I would think that's activating a double action cylinder...or relaxes the pressure and the opposed weight collapses both cylinders?

Yeah, I know...I'm not much help.

Mark
 

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Cylinder #1, despite having the same line connected to both ports, pushes out because the pressure on the back side is greater, due to the piston rod on the front side diminishes the piston area where the fluid presses against. Therefore, the piston extends.

Cylinder #2, pushes out for the same reason. Both the blue and the green lines must be pressure lines, until the pump reverses or stops, then they both become leak-back return lines.

Otherwise, I don't see how it would work either.
 

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Pounds per square inch. With equal pressure in both lines the larger cylinder will move first because it has a larger piston surface and it will move out because the rod takes up some of the piston surface on the rod side, ie more square inches on the base end of the piston.
 

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Looks like the first one pushes a scissor type mechanism and the second operates a cantilever arm , which just might pull as you said. What type mechanical mechanism does it operate to lift the dock l plate ? To hook them together like that is an equalizer for operation to be unison. not all the info is here to give a good conclusion. so thats my 2 cents.
 

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i use them too ours has 2 buttons one rasises it the other pushes the lip out after u rase it not sure how they work only security cams inside so no pix they also have automatic dock locks to wayne
 

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Collector of many tractors
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Discussion Starter #10
Got A question... What exactly is the sequence of events?

I've used dock levelers where, when you push the button, the leveler rises, then when you release it, as it begins to fall, the lip extends.

Does the lip extend while the button is being held? or after it's released?

Dave
The lip extends while the button held and after the dock plate raises to the top... After you let go of the button the dock plate drops...


 

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Neat little system. Cylinder sizing is critical to the correct operation. I think that there is an open centre four way solenoid valve missing from the circuit diagram between the pump and the first cylinder and a pressure relieved check valve between the first cylinder and the solenoid valve that allows free flow from the pump to the first cylinder but restricts flow on the return. Or it could be electrically operated as well. Have to think on it, I've been away from this stuff too long. The relief valve is also missing from the diagram, but it is system protection, not operation, so it can be discounted for this discussion. The blue line is the pressure line going up, the green line is pressure going down. They are switched by the solenoid valve.
Hope I didn't add to any confusion, and I hope this twigs someone to come up with a simple and complete answer. I think Stash has the best "how" so far and Mark the explanation of "why". Anytime you get into a hydraulic sequencing circuit, it goes from simple to complicated on the hardware side at warp speed.

Bob

p.s. Sure wish I knew how to make and post diagrams like that!
 

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Discussion Starter #12
Nothing missing... We have 44 units I need to maintain and I drew it out exactly like it is... thats why it's got me puzzled ...
 

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Hi Kevin can you post some pictures ??? I'm NOT a hydraulic expert by any means ,but isn't the lip cylinder a "slave cylinder " ??? My 2 cents,Jim

BTW,I also maintain 6 Dock levelers that use no hydraulics and no electricity ...they are all mechanical and work on springs ..."Six BIG Springs" and when one spring goes the dock leveler doesn't....I then get a phone call then....
 

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Your diagram makes the assumption that both cylinders have internal pistons typical of dual acting cylinders. The smaller cylinder could contain a deliberately "leaky" piston. Pressurizing the system via the blue outlet on the pump (with the green inlet preventing oil return) would then cause both cylinders to extend. Similarly, to drop the leveler, shutting off the blue outlet and opening the green inlet would cause the cylinders to retract.

A check valve on the bottom inlet of each of the cylinders would then prevent catastrophic failure if any of the hoses were to develop a leak.
 

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Stash has the best answer. The pump is not a closed loop as in a traditional system. It is providing pressure on both lines. The cylinder is not equal because the ram displaces oil and reduces sq in of piston area. Essentially, the oil pressure is pushing against the atmosphere and the weight of the dock.
 

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Had some time this morrning to play around with some air rams I have and they have quick disconnects. The way you have it drawen , the blue line makes the small ram retract. this happens when the large ram is at mid point. If the small ram is turned around the the blue and green lines are the same the small ram extends as the large ram is charged. as far as the 2 rams retracting the green line is entergized and pushes the rams to the retracted state.
 

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That is a good one!

Would the cylinders have check balls in between the cylinder body and the Tee's?

If so, it looks to me like the two cylinders hooked together has been done to get the two pistons the pistons to move together and a calculated speed proportional to each other when extending. Then the motor changes direction to reverse flow on the pump for a powered return instead of using gravity. Doing so assures that the two cylinders retract fully.

I could be completely off base here, just thinking about it and wondering too.
 
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