Creating a Bi-Directional Hydraulic Movement for a Personal Project

[mmmcheechy]

I'm a machinist and i want to make a hydraulic circuit for a personal project. I'm sure the type of movement i want to make already exists i just don't know what it would be called. I want to make a bi directional movement that from a center postion encounters a constant resistance as it moves away from its resting position (like a dampener) but then moves freely back to its rest position and if continued to move from the rest position (in the same direction as the free movement and opposite of the first movement) would once again encounter the same type of constant resistance.
the pic is a diagram of what i would like to achieve.
in the left illustration the black bar represents the piston (or whatever will move) the small arrows indicate that the piston will encounter resistance it it is pushed in either direction. the right illustration shows that the piston having moved down would continue to feel resistance if it kept moving down but would move freely back to the center position (indicated by the large arrow) then would once again encounter resistance in either direction from there.

also i don't mind if it requires more then one piston and i would like to avoid having a reservoir

 

[DaveC426913]

This sounds kind of like a standard shock absorber in a car. But you say you don;t want a reservoir.

What's wrong with springs?

 

[mmmcheechy]

what i am trying to make is similar to a shock absorber except that most shocks don't have any free movement, there probably are types of shocks that will dampen in on direction and move freely in the other but that not what I am going for. and springs won't work either as i want a constant resistance to movement and springs exert greater resistance the farther you move them from their rest position. (you calculate the increase in resistance with increase in deflection by a value known as the spring constant) I have drawn a circuit that uses porting to achieve what i am trying to but i haven't built or tested it yet. I'm just wondering if know if this type of device already exists and what its called

 

[tvavanasd]

mmmcheechy,
I would suggest either a double rod cylinder or a rodless cylinder (magnetically coupled maybe). Either option eliminates the need for a reservoir since the total volume inside the cylinder is basically independent of stroke. A double rod takes up more room, but will probably be cheaper and have lower seal friction (not sure if it's an issue in your application).
Note that any way that you try to solve the problem, there will be a short zone at centre with either low or high resistance depending on design (maybe small, but I don't think that it can be eliminated). There won't be an exact centrepoint, it will be a centre zone.
Assuming that you want your design to be completely mechanical (no sensors or solenoids), some sort of moving mechanical flag will be required in order to open or close at least 2 separate flow circuits, based on which side of centre you are on. Each of the flow circuits will require a variable flow control with check valve.
Perhaps I've said enough to get you moving, if not, let me know and I'll see if I can elaborate or sketch something up (it's only in my head so far).