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Discussion Starter #1 (Edited)
I am in process of repairing my peculiar small tractor from 80's, which has hydrostatic all-wheel-drive with hub motors. In normal use all four motors are working parallel in a closed loop circuit. For slippery conditions there is "diff lock" kind of function, which is controlled by a hand valve. The the "diff lock" is made in a 4WD control block, where all the hub motors are connected. There is no documentation or schematics available for the block anymore, and I am not willing to disassemble the plugs and cartridge valves on the block. Therefore I'm trying to make reverse engineering by an educated guess.

The 4WD control block looks this:


In the upper part of the block there are two through bores closed with the big plugs visible in the picture. I assume there are flow divider/combiner valves between those plugs. The pressure lines for the hubs are connected right beside on both sides of the block. Additionally there is one through bore perpendicular in the middle of the block. For a perfect solution there should be a third flow divider/combiner to make all four wheels spinning at same speed. However I understand that there has to be the pilot operated spool valve somewhere for connecting the flow to the flow divider/combiner valves. Therefore my assumption is that the third pair of plugs hides the spool valve and there are only two flow divider/combiner valves.

Based on what I explained and what you see in the picture, would this schematic diagram make any sense? The port codes on the drawing are same as stamped physically on the block. I omitted the hub motor leakage lines from the picture.

Or can there be such flow divider valve cartridge, that have an integrated pilot operated control to enable/disable the function? Then there could be proper three divider setup.
 

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Logic would suggest that there should be 5 lines connected to that tandem pressure compensated flow divider. The pic appears to show more than the necessary 5 lines leading to the possibility that there is another single flow divider located elsewhere.

There is also the possibility that the porting inside that divider is such that there is enough restriction to each spool to ensure that the flows from a single source are balanced.

A search for flow divider images will generate pics of the various body styles as well as pics of the spools and some schematics.

For the uninitiated, a video of how a pressure compensated flow divider works.

Pressure Compensated Flow Divider - YouTube
 

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Discussion Starter #3
Thank you Tudor, you are the hydraulis specialist I actually was looking forward to reply my post.

The flow divider/combiner is integrated inside the block as cartridge type valves. Therefore the lines to the double flow divider/combiner are channels inside the block, not external connections.

There are total 15 lines connected to the block: two pressure lines for each four motors, two lines from the pump, pilot line controlling the "diff lock", tank line and three other leakage lines, which go to the tank line via the block. The tractor was completely disassembled few months ago. Every hose and pipe was out, so I can assure there are no hidden components between the block and hub motors.

Do you think my drawing of the assumed diagram looks viable and plausible?
 

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Your diagram is very much simplified, but it does show enough to get the basic concept across. If I saw the real schematic of that valve block, it would probably take me a week to get my eyes uncrossed. I'm a retired industrial mechanic, not a hydraulics engineer, so I have a preference for simplified schematics unless I absolutely require the details and have the time available to puzzle them out.
 

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Discussion Starter #5 (Edited)
The diagram is simplified mostly because internals of the block are unknown and based on my assumption only. But also as you said a detailed schematics is seldom used or needed in complete vehicle hydraulic diagrams. And that's my aim: to make a complete hydraulic diagram covering both original drive hydraulics and the improved accessory hydraulics. Luckily there are no black boxes in accessory hydraulics.

Maybe it is also so that manufacturers do not tell every detail inside the blocks they are selling. Below is an example how a new Bucher differential lock valve block diagram is presented in manufacturer's spec sheet.

2492360
 

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Your diagram will provide 'diff-lock' from front to back only. The lines connecting A1/A2 and B1/B2 are redundant since the flow dividers are connected at their inlet.

Unless there are orifices.

So, the tractor has four hubs? There is no mechanical differential? Is it front wheel steer or 4-way steer?

Edit; I see you have a lengthy discussion on this. I will read it through and will probably have the answers to my questions.
 

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Discussion Starter #7
More about the tractor here. It has four hubs with a hub motor on each, there is no mechanical drive train and it is front wheel steered.

Yes, the diagram provides "diff lock" from front to back, and as I mentioned in the first post that is my assumption based on the plugs seen on the block. I wish I had the diagram from manufacturer.

When I have been using the tractor I haven't been in situation where I could notice "diff lock" between left and right. I certainly need to test it better when the tractor is back in use.

Aren't the lines connecting A1/A2 and B1/B2 needed when the "diff lock" function is not used i.e. motors are running parallel without flow divider?
 
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