Slow Dump on LA351 Loader

[hayden]

Sorry I missed this thread last summer when it was posted.

My LA351 did exactly this. I went to the dealer about it and they said it was normal. My LA402 does exactly the same thing, so that plus the experience relayed here makes me believe that it truely is normal.

Although I don't totally understand the cause, it's clearly related to the regenerative cycle that makes up the fast dump. A regenerative cycle is when you connect both the ports on a cylinder together and apply hydraulic pressure to both at the same time. On first glance you would think the workd would come to a screeching halt if you tried that, but here's what really happens:

- Equal oil pressure is applied to the two ends of the cylinder.

- The end with the rod has a smaller surface area to apply oil pressure against when compared to the non-rod end because of the area lost to the rod itself. This means the net mechanical force (pressure x area) on the non-rod end will be greater and push the rod outwards (dumping the bucket).

- Because the two ports are plumbed together, the oil expelled from the rod end flows to the non-rod end increasing the oil flow rate into the non-rod end.

- The increased oil flow (combination of pump flow plus expulsion flow) makes the rod extend faster than it would under "normal" conditions.

- Net result is faster dump because the rod is moving out faster, but dump force is less since it's the difference between the force on the two ends of the cylinder.

Now this may be interesting cocktail conversation, but it still doesn't explain the pregnant pause before the dump starts happening. The only thing I can figure is that oil has to get moved around inside the control differently to do the regenerative dump and that takes time, but who knows. I don't think it's air since there are no other indications of air in the system, an dif there were I woudl expect it to bleed out with repeated cycles.

Peter

 

[Anonymous Poster]

MY LA 401 DOES THE SAME THING. IT DID NOT HAPPEN WHEN THE TRACTOR (2710) WAS NEW. IT ALSO DOES NOT DO IT ALL THE TIME. IF I MOVE THE STICK ALL THE WAY OVER TO THE RIGHT (FAST DUMP )TO REMOVE STICKING MATERIAL, IT WILL SOMETIMES DUMP SLOW. I WILL THEN MOVE THE STICK LFT - RT (ROLL BACK - FAST DUMP ). SOMETIMES THIS WORKS, SOMETIMES NOT. WHEN I DO THIS THE LOADER IS NOT GOING UP OR DOWN. THE LOADER VALVE IS ALSO A 4 POSITION AND NOT THE STANDARD 3 POSITION. THE FLUID AND FILTER WAS CHANGED, SCREENS CLEANED @ 50 HRS. (NOW HAS 90 HRS.) I WOULD GUESS THAT SOMETHING IS STICKING IN THE VALVE.
FRED

 

[CraigM]

Interesting puzzle. I'd like to offer a possible explanation. Let me start by saying I'm no expert in hydraulics, so this may turn out to be way off base.

Pks says that the problem is worst when changing from slow to fast dump with the lift arms in motion. I don't know what the lift motion has to do with it, but after reading hayden's explanation of regenerative cycles, I have a thought on why the hesitation is worst when shifting from normal dump to fast dump.

With the bucket in motion on a normal dump, fluid flows from the pump to the non-rod side of the bucket cylinder. The fluid on the rod side of the cylinder is exhausted through the valve to the tank. Pressure in the exhaust side of the circuit is esentially zero. Now engage the fast dump. Wham! The moving fluid in the exhaust circuit is hit by full pump pressure in the opposite direction. The rod side of the piston now sees a force that wants to curl the bucket. This force is smaller than the dump force on the other side of the piston, so the bucket will still dump. Before engaging fast dump, there was motion happening at a steady rate to dump the bucket. Hitting the exhaust side of the cylinder with pump pressure has to interrupt this motion until the higher force available from the non-rod side of the piston prevails and gets things moving again. Think of it this way. A 300 pound football player is barreling along at full speed and gets hit head on by a 150 pounder. The big guy's forward progress is momentarily slowed until his greater strength and momentum can overcome the little guy. If fast dump is engaged from the start, then both sides of the piston see the same pressure at the same time and the dump motion starts immediately in response to the imbalanace in force between the rod and non-rod sides of the piston. Sound reasonable?

Craig
Hershey, PA

 

[Pks]

Craig,

I think you and Peter Hayden have identified the reason for the pausing.

This means that I should decide whether I want a fast dump or a slow dump BEFORE I begin the dump cycle. I think this is pointing out my wishy-washy gean.