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Floydster

620 loader problems

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Floydster
The 700 Ark loader on my 620 Allis has gradually come to the point where it will not lift any load. I think it my be the pump, I pulled the pump off and it is a Cessna H 24203 RAG 6NC-41-HS. The loader operates at 1250 P.S.I. I took the pump to a hydraulic shop and they told me $275.00 for a new pump. Fleet Farm has pumps about the same size but higher P.S.I. ratings for$90.00. Could I use a pump like this? I know enough about hydraulics to be dangerous. Any help sure would be appreciated Floydster

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Guest
I know NOTHING about an AC 620, so consider that when reading the following... Did the hydraulic shop confirm the pump was the cause the problem? Is there a pressure relief in the system that may have gone out of adjustment? Is there a filter in the system that may be clogged? Does the Fleet Farm pump rotate correctly, or is it bi-rotational? Does the Fleet Farm pump have a compatible flow (GPM)?

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Floydster
Dutch, thanks for the reply. The shop did not confirm my pump was bad, they said I should put the pump back on and check the cylinders for internal leaking, which I am in the process of doing. I don't think there is a pressure relief valve in the system,if there is I don't know where to look. Also there is no filter system. The Fleet Farm pump is bi-rotational,and the G.P.M. is from 4.5 to 7.8 depending on the R.P.M. I don't know the G.P.M. on the old Cessna pump, I can't find any info as these pumps are no longer made from what I know. Thanks for any help, Floydster

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Bumble_Bee
Hydraulic cylinders can develop internal leaks, but I've never seen two go at the same time. If one cylinder is leaking it won't apply as much force as the other and the loader arms will twist and rack since one cylinder is lifting more. Relief valves are often incorporated in the control valve. Check your control valve and see if there's a thing that looks like an "Acorn" nut. If there is, that is probably the cover for the relief valve adjusting screw. Although it is not an accurate method of diagnosing, you can get a general idea of pump condition by “TEE”ing in a gauge on the pressure side of the system (before any relief valve) and SLOWLY restrict the pump flow with a shut off valve at WOT. You can check flow by placing a system hose in a 5 gallon bucket, check the time required to pump 2 gallons at WOT, and do some math. Wear old clothes and have rags handy, it can (and probably will) be a mess. Just remember that pressures lower than 1500 psi can inject fluid right through your skin. Use extreme CAUTION. Also try to find out whether the AC 620 uses a “closed” or “open” system.

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Bumble_Bee
Floyd I doubt you have a pump problem. All hydraulic systems have a relief valve. The pump produces much more pressure than your system operating pressure. The relief valve will be found on the valve body. To check for proper operating pressure attach a gauge rated above system pressure (I use a 5000 psi gauge) to the line going to the lift cylendar and pull the valve body lever as to raise the loader. If you read your proper operating pressure then the problem is not the pump or the relief valve. If the pressure is low my best guess would be trash, rust etc under the valve seat. t can be taken apart and cleaned.Diesel fuel works well. drain and flush the tank and iinstall a filter on the return line to. You should also check the suction line for obstructions or even a pinched line. markpogue@hotmail.com

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powermax_paul
I agree with Mark that the pump is probably not the problem and the relief valve probably is. As I understand it, your loader runs at 1250psi. But that is only when you have enough load in your bucket to require 1250 psi. First you must understand that liquids such as hydraulic oil cannot be compressed like air and gasses can. With open center control valves like the Cessna, the pump feeds oil to the cylinders to move them, kinda like a motor/screw feeds threaded stock. The pump is a positive displacement pump that pumps a certain amount of oil per revolution REGARDLESS OF THE PRESSURE. This pump would probably have to have extreme wear of its gear teeth for it to leak enough internally to not be able to lift the loader. There's really very little pressure until you put load on the system. When the valve is centered, the oil runs straight thru the valve and back to the reservior. Operating the valve redirects the oil flow from the pump to one side of the cylinder and directs oil from the other side of the cylinder to the reservoir. Normally, the relief valve stays closed unless you try to lift something that's too heavy for the loader. That amount of weight creates a pressure on the cylinder pistons that equals or exceeds the relief valve setting. When you reach the end of travel of the cylinders (load or no load), the relief valve will open because there's no other place for the oil from the pump to go. Therefore, the relief valve is in the system to prevent the pump from simply stalling the engine and lines from bursting when loads are too high. 1.If you have a broken relief valve spring, the poppet in the valve would operate prematurely which would make the relief pressure lower or zero. 2.If the adjustment inside has backed out, the pressure would also be lower. 3.The relief valve poppet could be worn to the point that it is not seating completely and some of the oil is going to the reservoir during normal operation. (I think this is the most likely) 4.The spool valve itself could also be worn to the point that it is leaking back to the reservoir, but I doubt it. If in the end it is the pump, you will have to be carefull about pump & pulley sizing, because it, along with engine speed, determines the speed of the hydraulic cylinders. When I got my Danco loader it didn't have a pump. I had to make a pump assembly and purchase the right size pump and pulley. You need to calculate the volume of the cylinders and how fast you want them to move in cu.in./sec and convert that to GPM. Then get the specs on the pump ie. GPM at RPM and figure out what RPM it needs to run. Finally, size your pulley to get that RPM on the pump. Hope that helps, Paul

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dmcluckie
Men, this is a GREAT thread on hydraulics!!!! Thank you thank you thank you. One of these days I have some minor hydraulic system troubleshooting to do on our Ford 3400 utility tractor with loader. This info makes a lot of things clear. I don't know if it's closed center or open center. Can you explain that for me? Thanks again, -Don

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Floydster
I want to thank everyone for the wealth of information on this thread, you guys are great! I basically know the operation of this hydraulic system because of all your input, this is a super club. The first thing I am going to check is the relief valve for any problems, then I will go from there and let you know what I find out. I was always in the dark on hydraulics but I am learning, I can't thank you enough for all your help. Floydster

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