790 Rear Hydraulics/Adding Hyraulic Top Link

[N9PDL]

I'm wanting to add a hydraulic top link to my 790. Can I do this with the "power beyond" kit or what else do I need? I do not want to have to control it with the SCV because I leave the loader on most of the time. Can anyone give any advice based on their own setups? Any help I could get as far as what exactly I will need, I would appreciate.

Thanks.

 

[creeper]

I can't give you specifics,but it is possible.Look for info on990 also, there have been some recent post on tnt for other tractors.My dealer said you were better off to just buy hyd lines and fittings for the power beyond, I think the kit is about $200 and is a continous loop, so I believe you would need a valve that would let fluid return constant. Others have also posted about electric diverters to work accessories. Sorry not more specific,but it doesnt look like you are getting much help.

 

[RobS]

I've never seen remotes on a 790 but Deere does/did show it as an option. I have the power beyond but as creeper mentioned it's just hydraulic flow to the rear with no control. You could use it for the source and add your own valves. Check with tomrscott as he is in the process of adding rear remotes to his 790 /forums/images/graemlins/smile.gif

 

[N9PDL]

Thanks, Rob. I sent him a private message asking for info. If that is, in fact, a continuous supply then I would like to add a two spool valve with two ports so that I could use a hydraulic top link and have another port available for miscellaneous (grader blade tilt, etc.). I just keep thinking I've heard someone say that it can't be set up like this, though....I'm thinking someone said the rear hydraulics are dependent on the SCV.....maybe I'm wrong about this, though.

 

[BGL990]

I just installed the power beyond kit on my 990 and I'm going to add my own valve bank. I'm not sure how close the 790 and 990 are as far as the existing loader valve hookup and power beyond kits are, but here's how it works on my 990:

The loader SCV valve block is mounted on the side of the transmission housing with a short pressure supply line coming from the transmission housing to the valve block. The return line comes out of the valve block and goes back into the transmission housing somewhere.

The power beyond kit basically just stretches out the pressure line to the loader SCV and brings it out back with a quick connect and hose so you can add your own valve into the loop. Remove the existing steel supply line to the loader SCV, run two other steel lines to the back of the tractor (one from the pressure port on the transmission housing and one back to the SCV). Join the two steel lines together with a short length of flexible hose with a quick connect on one end so you can split it and put your own stuff in. Pretty simple.

I'm not sure where the 3pt fits in - it is either the first or last thing in the loop but regardless, all the valves are basically in series. Pump -> power beyond -> loader SCV -> 3 point -> back to tank. (or the 3 point is between pump and power beyond)

The external valve you add must have a power beyond capable return port because (at least in my configuration) the fluid coming out the return port of the PB valve is the supply to the loader SCV. When the loader is used, whatever is plugged into the power beyond ports will feel the full hydraulic pressure the tractor is capable of producing. If the normal return-to-tank port from the external valve block is used and it sees that much pressure, you might blow out seals or damage the valve.

The multi-spool valve blocks I can get locally at Princess Auto can be converted to power beyond operation so their return can safely be routed to the input side of the existing loader SCV on my tractor. I plan to get a 4 or 5 valve block and mount it over the right fender with a set of quick connects at the rear of the tractor for each valve. Should be a nice mess of hoses ...

 

[N9PDL]

So would buying a "Power Beyond" kit and adding an open center spool valve to it work? I'm kind of ignorant on hydraulics, so bear with me.

Also, are you saying that the power beyond port constantly has pressure applied to it?

 

[BGL990]

Yup, the power beyond kit plus your own external valve should do the trick (that's what I'm doing so I hope so ... /forums/images/graemlins/tongue.gif)

As far as the PB port constantly having pressure - sort of. Your hydraulic pump is always pumping fluid whether or not you are actually using a valve for anything. All this fluid goes through the center of all the valve blocks in the loop and back to the storage tank (your transmission housing) - a continuous flow whenever your tractor is running.

When you pull on a valve lever, some or all of this fluid is diverted into a hydraulic cylinder causing the rod to move. The rod movement pushes fluid out the other side of the cylinder back into the valve body where it is directed out the return port of the valve body to re-join the regular fluid flow on it's way back to the tank. If you extend a cylinder all the way or otherwise block it's movement AND you have the controlling valve spool fully shifted, the fluid will be unable to flow and the pressure relief valve will come into play.

The pump is unable to stop pumping and if the fluid can't follow it's normal path, the pressure will rise until the relief valve opens and provides a path directly back to the tank, bypassing the normal hydraulic loop. While this is happening, however, the hydraulic loop up to the point where the valve spool is trying to direct fluid into the stalled cylinder will be under maximum pressure.

So, if you have no valves activated, the PB port will have lots of flow through it but very little pressure. If you shift a valve downstream of the PB port and stall a cylinder, the PB port will have no flow through it but will be pressured up to the relief valve setting of your tractor (probably 2000 - 2500 psi). If you shift a valve but it's cylinder isn't stalled (ie. raising the loader), you will have the same flow as a no-valves-shifted situation but the pressure will be somewhere between zero and max. (exactly enough to move the cylinder against it's applied load).

Having valves in series like in this application means that the upstream valves can effectively have priority over and can disable downstream ones. In my case, after I get my external valves hooked up, if I stall a cylinder connected to one of them, my loader and possibly 3pt hitch won't function while I'm holding the remote valve fully shifted. In fact this is already the case with the loader and 3pt. Which ever one is first in the loop will cause the other to lose power if it gets stalled and you try to use them both at the same time. In practice this will never really be a problem - just let go of the valve handle and everything is good to go again.

Hopefully this all made sense. If not, I'll try again ....
/forums/images/graemlins/laugh.gif

 

[N9PDL]

It actually makes a lot of sense, thanks!! Here is what I want to do: I want to get a power beyond kit and a two spool valve. I found one on Northerntool.com that claims to have "power beyond" capability (part #2033). The first spool will be to run a hydraulic top link. This will be a two-way cylinder, so I will run two hoses, one for each direction of the cylinder. My next question is this: For my second spool, I want to run a single-acting cylinder on an old pull-behind bushhog. The weight of the bushog will push the cylinder back down, but I will use the pressure to raise it. How exactly will I plumb this? What it seems to me is that when I push the valve in the opposite direction to "lower" the bushhog, since I have nothing connected to the second port of that spool, it will send the system into relief. Does that make sense? I'm very new to hydraulics.....Thanks in advance, you are being very helpful!!

 

[BGL990]

Hmmm... I'm not actually sure if you can use a single acting cylinder with a double acting control valve. I think you are right that you'd have to plug one of the work ports on the valve and then when you shift in the direction to lower the bushhog you'd send the system into relief.

I know the valve blocks I'm looking at have replaceable spools for different applications. Perhaps you can get a single acting spool for your valve block.

Maybe an even better solution is simply to use a double acting cylinder on the bushhog. You can probably find a surplus one for cheap, or even new they aren't terribly expensive for the size I would expect in that application (I'm guessing 2" or so diameter with maybe an 8" stroke??)

<thinking ....>

I just had an idea. What if you plumbed the second work port directly to the return line (Tee it in). When you shift the valve in the direction required to lower the single acting cylinder, "pressurized" fluid will be flowing out the second work port, but will just go straight into the return flow under no pressure. Might be worth a try.