hydraulic actuators-mechanical synchronization

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Thread: hydraulic actuators-mechanical synchronization

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    Question hydraulic actuators-mechanical synchronization

    I am currently doing one project on hydraulic actuator synchronization using mechanical coupling( rigid coupling) as part of my internship. it is cost effective, don't have any complicated controls. just pump the fluid. since it is rigid and the mass is centralized the volume and pressure should be equal to both actuators.
    This is the basic structure -->
    when I was doing my research, I came across so many models which using other techniques like servo valves, but I hardly saw anything about the mechanical coupling., What are the disadvantages of this system?
    PS- the connection is rigid, the weight is centralized, might have slight leakage (assuming, since it is for practical use).

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    Community Moderator Al_The_Man's Avatar
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    Default Re: hydraulic actuators-mechanical synchronization

    I guess the disadvantage is you rely on the hydraulic pressure to be perfectly equal in each cyl, or that the weight will be the same on each cyl.
    Although if coupled, the pressure should equalize?
    I was once involved in one system that had four cylinders that were used for raising a R.R. box cars, one cylinder in each corner and four valves, so a encoder on each ram was used to monitor each lift rate and modify flow to keep each equal, but there was quite a high degree of sophistication for this method.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Default Re: hydraulic actuators-mechanical synchronization

    Cylinder with least resistance is going to move first or faster. If load is not perfectly equal on each cylinder, or internal resistance of cylinders and mechanical components are not exact it will not work. Been tried many times on large and small scale, in a perfect world it it very difficult to achieve. Been involved in many molding projects with 4 cylinders that would not move all corners at the same exact time. lots of "engineers" that argued to just make all hose connections, lengths equal and all will be fine. After countless hours and money, not so. Al has a good solution, but not easy to apply.



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    Default Re: hydraulic actuators-mechanical synchronization

    The easiest solution which is often used on industrial machines such as press brakes that have to cope with uneven loading is to use a transfer line principal. This implies pressurizing one cylinder (primary) as normal and then feeding the hydraulic oil that is displaced from it and using that to drive the other cylinder (secondary) and connecting the secondary outlet back to the valve. If the cylinders have piston rods on both ends the displacement will be the same, otherwise the cross sectional area of the secondary will have to be proportionally smaller ie the cross sectional area of the primary piston need to be the same as the cross sectional area of the piston rod end of the secondary cylinder. Just an idea. Dave



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    Default Re: hydraulic actuators-mechanical synchronization

    actually, the mass is always centered through another mechanical set-up.
    and about the series connection even though both the actuators in sync, but the pressure difference and the force generated will be different right? because of the diameter difference....!!
    I know if I'm using a flow divider or a servo valve it will be in sync. but I'm doing a research( i.e. I'm not trying to find new means to synchronize them, I just wanna know why this rigid coupling can't do it) in theory, this system is capable of synchronizing itself without any external control. but because of some reason, this is not using in heavy load lifting. instead, the industry using more complicated and expensive servo valve systems........
    I want to know the reason, that's all
    thanks for your comment....!!!!!!!



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    Gold Member Jim Dawson's Avatar
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    Default Re: hydraulic actuators-mechanical synchronization

    You can use a rack & pinion system. A rack & pinion on either side with the pinions connected to a common shaft. It's not as accurate as using encoders and servo valves, but is pretty good and inexpensive. This is a common way of synchronizing the ends of a long moving fence in the wood products industry.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: hydraulic actuators-mechanical synchronization

    I am not sure what you are really trying to achieve, industrial solutions to this situation are many and and varied. With a few exception hydraulic cylinders and the piston rod ends there will always be some form of flexible joint to
    allow for some degree of movement. Even the most rigid of structure will flex when under load and ideally this potential misalignment should not be passed on to the piston rod. There will always be a reason why servo, balance valves or
    some other kind of flow control is used. If the project you mention uses a platform that is mechanically kept perpendicular to the stroke then maybe you do not need additional control but it would always be best to have some degree
    of flexibility in the joint connection so as to avoid unnecessary load on the cylinders. If there is sufficient mechanical control then why not use a central single cylinder instead ?.



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    Default Re: hydraulic actuators-mechanical synchronization

    I wonder why industry does not use multiple cylinders. Could it be that 100+ years of practical experience has found it does not work?

    The cylinder movements never track properly. The structure twists and jams - and then distorts.

    Cheers
    Roger



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    Default Re: hydraulic actuators-mechanical synchronization

    Even the large 1000 ton or more power presses that are used in car body plants for example only use one cylinder, the bolsters that the tools are
    mounted on run on cylindrical rods that keep the tooling parallel. Balance valves work to a point but still need a degree of angular misalignment
    for them to operate which may or may not be acceptable, the only other way is to use linear encoders driving servo valves, expensive and often
    need a simplistic motion controller for them to work well. The only commonly used application I encounter that use two cylinders is on guillotines
    which are used with some form of mechanical linkage and on press brakes which use either transfer lines (uncommon) or double pumps each
    driving a cylinder and or a balance valve connected to a pulley system which pulls on the valve control arm depending on misalignment. Nearly
    all CNC machines use servo control. As Roger just pointed out a century of experience albeit often empirical is usually a good guideline.
    Dave



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    Default Re: hydraulic actuators-mechanical synchronization

    Sure, it's an old thread, but I think it reasonable to weigh in on this. This can be accomplished with a stiff system under certain circumstances. Use a stiff frame and have a very stiff connection between the rod ends and the moving load. This is used with zero synchronization widely in the snubbing industry. The following video shows a system that has four cylinders run without synchronization, but the same setup is used on smaller units with only two cylinders. None of these systems use any type of synchronization besides sheer stiffness.



    Now about those certain circumstances... In the video, you can see what these circumstances are. The load itself is a pipe string that does not tend to contribute to system instability when slightly off-center.

    Note: You should have thick wear rings in the cylinder gland and on the piston.



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    Default Re: hydraulic actuators-mechanical synchronization

    Hello,
    I worked many years on this kind snubbing unit to run and pull completion assy, etc. I just realized that the four cylinders run without synchronization, I thought that the hyd pump will pump hyd fluid equally so that synchronize fluid flow to 4 direction.



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