Hello All,
I am designing numerous vertical linear actuating systems designed to lift 500 lb loads. The system will utilize a 60" stroke 2" bore hydraulic cylinder to do the lifting. My question is in regards to creating my hydraulic system. What kind of system should I use, and what are the benefits and disadvantages of my different options? (By kind of system I am referring to things like open vs closed center and other option like flow dividers and accumulators.) Here are some more details:
# of cylinders in system: 4-8 (to be determined)
positions: 3 (completely extended, 3/4 extended, and completely retracted)
# of repositionings in a given work day: 2-5 per actuator
Speed of positioning isn't a factor as long as it is relatively quick.
Calibrated positioning is of interest (for the 3/4 extended positon) and ideas on how to best achieve this is of interest!
thanks for helping,
I'll keep an eye on this thread in case more details are needed!
The number of cylinders will be dictated by the layout of the moving frame the load is located upon. If it is long and thin then you may need ten cylinders. If it is 12" diameter a single cylinder might suffice.
A trip-valve at the 3/4-extended position would suffice, but with the sketchy information you have supplied any one of probably twenty different methods may be better or worse.
Slotted holes will help setup of 3/4-extended (I assume that's what you meant instead of "calibrated')
as far as the static load it is already determined that 1 cylinder per lift will suffice, the 4-8 cylinders referred to whether or not we decide to upgrade the 4 smaller lifts from 18" stroke hand pump powered to hydraulic (since we are putting in the hydraulics anyways).
Theoretically, my plan was to have a setup as follows: reservoir -> relief valve -> check valve -> accumulator -> 4-8 in parallel cylinders operated by control valves...
In more detail, I want the system to position the load at 3 different locations:
1. fully extended stroke --> this is for when the load is offline, and not being used in production (0, 7, or 14 hours per day)
2. 3/4 stroke --> this is for when the load is online (0, 7, or 14 hours per day), I used the word calibrate because I was thinking there is a way to calibrate my control valve? However I like the idea of a trip valve.
3. fully retracted--> this is for when the load is offline, and is being cleaned (once per day)
I have never used hydraulics before and do not know all the options that are out there, thanks for your help.
If you have never used hydraulics before you are jumping into a pretty deep pool for a novice. I would STRONGLY suggest that you partner up with a trusted local hydraulics distributor for design advice. There are so many design options it will make your head spin. All of the answers to your questions will be found in the details of your performance spec, probably in details you haven't even dreamed of yet. Do these actuators operate indepently of each other or are they tied to a common frame? What would happen if they all didn't operate at exactly the same speed every time? Would that break something, or stress it? Will they see any side forces? Will they all operate on the same pressure? Are you going to use a pressure compensating pump? (Do you know the features of a pressure compensating pump?) How accurate and repeatable does that 3/4 stop position have to be? Does it have to be held hydraulically, or could it be a mechanical stop? How will it be adjusted/fine tuned? There will be some internal leakage in the system. How do you account for it and/or control it? The questions about "open vs closed center and other option like flow dividers and accumulators" all depend on how you want the system to perform. Will each cylinder be manually controlled or will a PLC be involved? Are you going to use sub-base mounted or manifold mounted valves? Are you familiar with hydraulic schematic conventions? Who will build and install the system? . . .
I could go on and on. I'm not trying to impress you. I'm just trying to make the point that you will need the assistance of an experienced hand to help you avoid the land mines and anticipate problems/questions. You will also need that person AFTER the system is installed. Your management and operators will see that it does exactly what they told you they wanted it to do, and they'll say, "That's not what I wanted! It needs to do X." I guarantee that will happen.
You can get some help here and I'm glad you asked, but in my opinion the amount of help you will need goes far beyond the capabilities of an online forum.
Just my opinion.
"You will also need that person AFTER the system is installed. Your management and operators will see that it does exactly what they told you they wanted it to do, and they'll say, "That's not what I wanted! It needs to do X." I guarantee that will happen."
^ haha aint that the truth... Thats why you always have to not only meet their expectations, but exceed them!
As far as me being over my head, sure I am, I am a young engineer and need to start somewhere so that someday I can be the one giving advice. I am only asking for design ideas and brainstorming if you will. Worry not I will be presenting my finished design concept, BOM, ect. to our senior engineer and then after that we will get the opinion of our vendor I am sure.
As far as performance spec questions you raised:
- all actuators operate independently, and it can be assumed that at MAX two cylinders could be operating at once (independent of each other of course)
- They are not tied to a common frame
- Same speed does not matter
- the cylinder will only experience axial loading
- all cylinders to be used have the same operating range of 3000psi, so I would assume operating at the same pressure is the way to go.
-I am not sure if a pressure compensation pump is necessary. I understand it to be useful in situations where the system is under multiple loads, however, I would say that 95% of the time only one cylinder will be moving at once, do I need it for that 5% of the time where two could be operating at once? I do think that a variable displacement pump should be used to increase efficiency?
-As far as how accurate does the 3/4 position need to be. Ideally I would like it to be accurate within +-3 inches, the load is a feeder for a plastic extruder and I need the feeder to be close to the extruder obliviously.
-I need it to be repeatable, that is the only reason for me to go to the trouble of creating a positioning function, otherwise i would just give the operator a joystick and pray that they don't smash the feeder down onto the extruder...
-It could be held at 3/4 hydraulically or mechanically or both... safety is a MUST! I have had trouble brainstorming a good mechanical stop. The load is mounted on 4 plain bearing slides that are mounted to 2 parallel precision shafts.
-leakage does concern me. It makes a mechanical stop seem vital.
-I think that manual control vs. PLC will depend on how the 3/4 position problem is solved... perhaps plc is the solution?
-I do not know where I will mount the valves... manifold is I guess what I was thinking but I am interested to hear what you think
-as far as schematic conventions yes I am, however I do not know benefits of one system over another is some instances, so suppose I am not too familiar.
all I'm asking for is simple concept ideas, in the end I will have our vendor and my supervisor help me with the meat and potatoes of assembling the system, however, I refuse to think that the system I am trying to set up is complexed, I think it is a very simple straight forward system.
Could you have two cylinders in-line that operate as a two-stage movement? One cylinder fully extended holds the "work" position with the second retracted. Both cylinders extended is the "get out of the way" mode and both retracted for "clean."
How repeatable accurate does that "work" position have to be? Relying on trip or stop valves is not a good idea if you are looking for 1/8" accuracy. Also holding force will be questionable too. This does not sound like a project well suited to hydraulics unless you really have to use them for some reason. I would be opting for an electro/mechanical solution for position stuff.
you maybe onto something with the two cylinder idea. I ruled out pneumatics early on based on the realization that I want 3 different positions, however, perhaps I should revisit the idea with a two cylinder approach. I have not looked into electro/mechanical actuators at all, do you think I could even find a 60in stroke length in electromechanicals?
btw, I'm only looking for accuracy within a few inches (plus or minus 3 inches is what I said earlier)
The two cylinder approach will get you positive and accurate stops and save a bunch of money if you use pneumatics over hydraulics.
Electro-mechanical I was thinking of a worm-gear driven lead screw using quadrature rotary encoder.
OK, I think I'm getting a picture now. Each cylinder will be actuating its own mechanism independently of the other cylinders, on an as-needed basis, right? And each will basically be doing the same job? So, you could design one system and copy it 4 to 8 times? All systems would share a common power source, whether the plant air system or a single hydraulic pump. So, ideally each cylinder would be operated by a single manually operated 3-position valve to achieve full-down / mid-stroke / full-up positioning.
You mentioned a 500 lb load per cylinder, 2" bore, and 3000 psi. That information doesn't jibe. You'll want to re-examine that.
I personally think you have a pneumatic system here. And I like Dave's suggestion of back-to-back cylinders. Many cylinder manufacturers offer that construction as a standard configuration. Look around, you'll find it.
A couple design tips:
1> One valve cannot adequately control the positions of two cylinders, but ... one 3-position manual primary valve can control two air-pilot secondary valves, each of which in turn control one cylinder section.
2> You are dealing with some relatively long strokes. When long stroke cylinders are fully extended they can be prone to buckling failure. Internal stop tubes can control that and I would strongly recommend that you consider using them. Any good pneumatic distributor can put you in touch with an application engineer at a cylinder manufacturer who can help you figure that out.
Thanks for your help, I am going to propose a pneumatics as the actuator in my two cylinder lift design. (although I will have to be careful because it was my senior engineer that proposed I use hydraulics in the first place!)
Also thanks for the design tips, do I need to worry about buckling considering that I will have the load actuating on two parallel 3/4" dia precision shafts, and the load exerted on the cylinders will experience virtually no torque, only axial loading? I assume the answer is yes, that i should still look into internal stop tubes. I just wanted to clear that up.
In case you are interested, my other design is to lift the load using an acme screw mounted at the top of my frame, I wouldn't be surprised if that is what we end up going with anyways!
If the load is totally supported by the two rails and the cylinder has no loads other than push-pull then standard no-tube approach should be fine.
Not knowing the exact details of the entire thing, it must be up to you to make that final decision though. You know exactly what you are trying to do, we on the other hand are hoping we see the same mental picture as you.
When you are examining the side load, don't forget the load imposed by the weight of the cylinder itself. If it isn't perfectly vertical then it will see some side load. When it is fully extended a very small side load can damage the rod gland, seals, and score the inner cylinder barrel.
I see your point. With a large stroke even the smallest deviation from a perfectly vertical lift will result in a significant side load because the stroke is so long.
JB is trying to point out that it's not so much the actual side-load causing a problem, it is keeping the piston straight in the cylinder as it nears the full extended position that is also important. That's the idea of using stop-tubes to have the piston travel, mechanically stopped at say, six inches from the end of the actual cylinder length. For a 60" stroke, the cylinder would actually be 66" long but the piston will stop at the 6" stop tube leaving it with plenty of rod to keep it square within the cylinder.
That means the piston and rod have a fair chance keeping the piston perfectly perpendicular to the cylinder walls for a good seal.
That would depend to a large degree on how well your support rods carry the load and how much, or how little the cylinder-rod has to support itself. Again, I repeat, "it must be up to you to make that final decision though."
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