self contained, passive top link

[ford860]

I'm fabricating a rear end loader for a 40 HP tractor. I want to be able to lift and dump the bucket using a self-contained hydraulic top link that I have (dual action; 2" bore; 15" stroke; 21" retracted length). The top link will hold the bucket position when the valve is closed and used to dump the bucket when opened. Resetting the bucket and top-link will be done by lowering the bucket and rolling the tractor forward. The valve will then be closed to hold the bucket position. An example of what I have in mind can be seen about 2/3 of the way through this video:

http://www.jiffyhitchsystems.com/jh_bb.wmv

Essentially, the hydraulic cylinder will function passively (not with active hydraulics) and the pressure, even with the loaded bucket will not be that high as the tractor lift arms will more directly bear the weight. Nonetheless, the cylinder, hoses, valve, etc. will be rated for 2500 psi.

I already have the cylinder (described above), but would appreciate recommendations on the best way to fabricate the other components. I was thinking the the following beginning with the rod end port -> hose -> 2 way ball valve -> hose -> reservoir, with a second hose linking the reservoir to the 2nd port? A pressure relief valve could be in-line between the hose and the 2 way ball valve if needed; but when not in use I'd leave the valve open.

Any thoughts, recommendations, concerns, etc. would be appreciated. If a reservoir is needed, suggestions about what I might use would be most helpful - it won't need to be large.

 

[Brad_Blazer]

Pretty interesting. I think you need a reservoir and a 3-way valve that connects or isolates all three connections. I can't seem to find such a valve online. A T-ported ball valve connects but doesn't seem to isolate. Alternatively 2 normal valves operated in unison with the first one connecting the 2 ends of the cylinder and the second one leadiing from a tee on the base end to the reservoir.

If you connect the cylinder ends and just valve the reservoir it should hold in compression with pressure proportional to the rod area but will cavitate at a tensile load of about 15psi x rod area.

Hope that makes sense.
B

 

[jinman]

If you don't need the full range of the cylinder, then just plumbing the ends together and placing a valve inline will work. If you fill the cylinder when it is extended fully, you will not be able to retract the cylinder completely because the retract side has less volume.

While this will work with a cylinder, I'm curious why you don't just build a toplink with a trip function that lets it extend/dump? If you don't need to lock the toplink in various positions or have a controlled extension speed, I think a tripping toplink would work. Of course, cylinders are pretty cheap and you might be able to use a cylinder with less hassle than designing and building a trip mechanism.

 

[ford860]

Thanks Jinman,

That's the kind of information I'm looking for. To optimize the range, should I fill the rod side of the cylinder when it's fully retracted?

The reason why I'd like to fabricate it this way is to have some control of the speed of the dump, especially if I want to spread the contents of the bucket - not to mention that I already have a cylinder with a 15 in stroke.

 

[Brad_Blazer]

If you don't need the full range of the cylinder, then just plumbing the ends together and placing a valve inline will work. If you fill the cylinder when it is extended fully, you will not be able to retract the cylinder completely because the retract side has less volume.

Have you tried that? I don't see how it would behave well. If you fill it solid fully retracted and then seal the system with the valve open, you will have to pull hard enough to form a void to extend it to connect the top link. Where the void forms is indeterminate but some oil must flow into the base end. When the valve is shut and outside force is released any void in the base end will tend to collapse, causing some retraction. The remaining void space in the rod end will allow slop in tension when force is applied.

Again hope that makes sense. If you've done it and I'm full of it let me know.

 

[mjncad]

The Jiffyhitch systems is similar in concept to the DeltaHook products.

Both products are interesting.

 

[jinman]

Have you tried that? I don't see how it would behave well. If you fill it solid fully retracted and then seal the system with the valve open, you will have to pull hard enough to form a void to extend it to connect the top link. Where the void forms is indeterminate but some oil must flow into the base end. When the valve is shut and outside force is released any void in the base end will tend to collapse, causing some retraction. The remaining void space in the rod end will allow slop in tension when force is applied.

Again hope that makes sense. If you've done it and I'm full of it let me know.

Brad, I think you may be missing that the retract and extend sides will have a hose connected so the fluid can freely flow between them. By adding a valve in the line, Ford860 can control the point where the cylinder locks.

Ford860, I would not just fill the retract side because you would then have extra air in the system. You must fill the extend side and try to ensure there is nothing but fluid in the lines through the valve. Any air will cause the cylinder to be "spongy." More air will make it worse. I doubt you will see that much loss in retraction by filling the extend side and lines; maybe an inch, but no more.

 

[Brad_Blazer]

I understand that the 2 ends are connected but the total contained working volume of the system will change by the volume of the rod as it enters and leaves the cylinder. Fully extended the working volume equals (cyl area x stroke). Fully retracted the working volume equals ((cyl area x stroke) - (rod area x stroke))

(I say "working volume" to neglect the volume of plumbing and clearances.)

 

[ford860]

Brad's point makes sense. As the rod moves in an out of the cylinder the total volume within the system may change; moreover, temperature changes will cause expansion and contraction of the two fluids (air & oil) in the cylinder. This is why I asked if it is necessary to have a reservoir; if so, any design suggestions?

 

[mwb]

Maybe you could use an old fire extinguisher as an accumulator. Put a schrader fitting in the top so you can preload it with low pressure air (10 psi maybe?). This is the same principle as a house water tank (except they have a bladder). Have a flex hose from the rod end of the cylinder, through your valve to the extinguisher reservoir; vent the other end of the cylinder.
With the air charge the cylinder could have enough force to return the bucket to the upright position by itself.

Extinguishers are cheap, lots of volumes available and it can be mounted anywhere. You will need to consider the volume of oil that has to be stored vs the volume of the reservoir. You may not need any (or very small) head pressure; when the cylinder is extended the accumulator pressure will increase...

I would also install a safety valve to prevent the extinguisher from being over pressurized (say if you filled it to the top with oil). There are isolating valves (shuts off flow) or vent valves to do this and it could be installed anywhere on the low pressure side of your valve. Install a pressure gauge so you can monitor it also (extinguishers come with a gauge that may be good enough).

I have a system like this for oil spraying equipment (undercoating cars).

Regards,
Mike