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Thread: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

  1. #41
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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.



    This was the video I was referring to. At the 3:38 mark they show you the Z counterbalance cylinder. It does now look like there is a port on the bottom as well. At least there is an air line disappearing around the back in that general location. So its definitely a double acting cylinder. If its a double acting then it probably uses a special regulator.

    There is a Thompson Pneumatic center here in town. I'll check with them tomorrow and see if they know what type of regulator would work for an application like this.

    Did anyone else notice the Z axis drive motor? It looks like a Hybrid stepper motor. They made a point of using mitsubishi servo's on the other 2 axes. This is probably the reason why they need a balancing cylinder.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Doing a little research and it looks like the key piece is an Air operated precisionregulator.

    https://www.smcworld.com/upfiles/pgp...54X-008E-1.pdf

    See the Single workpiece and balance selection: 2 pressures selecting circuit in the linked pdf.

    I think that method would require a constant source of pressurized air. It might be able to be done without a constant source of air with a pilot operated check valve.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Quote Originally Posted by 1Jumper10 View Post
    Doing a little research and it looks like the key piece is an Air operated precisionregulator.

    https://www.smcworld.com/upfiles/pgp...54X-008E-1.pdf

    See the Single workpiece and balance selection: 2 pressures selecting circuit in the linked pdf.

    I think that method would require a constant source of pressurized air. It might be able to be done without a constant source of air with a pilot operated check valve.
    Hey Chris,

    My goal was a sealed system, but with the experience I had with the SMC precision regulators I tested with they would let out small burst of air to maintain the psi setting with pneumatic cylinders movement. With the compressor that was inputting, I had that regulator set to 10 psi but it still would go on with a lot of Z movement.

    What I seem to not understand and hoping someone will explain is why would the air feed be on top? With the cylinders I had that would be pushing down and add more pressure on the spindle to raise.


    Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.-preregulators-jpg



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Hi Dan.
    The air port flows both ways. The top port needs to let air in as the cylinder is pulled down. In this application it won't have any pressure on it.

    Some cylinders only have one port; single acting spring return is how they're called. Pressure goes in the port and when it is great enough to overcome the spring force, the cylinder begins to extend. Remove the pressure and exhaust the port and the spring retracts the cylinder.

    It sounds like you had the regulator. If you have the air source it's probably the best solution. But a pressurized air reservoir with a flow controller might work to. Got a request in to tech support to see what they say.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Quote Originally Posted by 1Jumper10 View Post
    Hi Dan.
    The air port flows both ways. The top port needs to let air in as the cylinder is pulled down. In this application it won't have any pressure on it.
    Ohhhh... just clicked, like holding top up straw when filled. Duh

    Thanks



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Quote Originally Posted by Dan911 View Post
    Ohhhh... just clicked, like holding top up straw when filled. Duh

    Thanks

    I will never speak of it again. Just as long as you never bring up button numbers and Macro's.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Quote Originally Posted by 1Jumper10 View Post
    I will never speak of it again. Just as long as you never bring up button numbers and Macro's.
    Never!!! we all been there and did that.

    I feel silly because all along wanted a system with out a compressor and purchased many regulators and now realize all I needed was a vacuum regulator.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    I talked to an engineer today. He called me based on a request for tech support I submitted on their website. It took a good deal of explaining before he understood what I wanted to do. For method 1, just using an air reservoir, it's in effect just an air spring. Without a way to add energy to the system it can never be anything that actively controls. But I still think if you size your system right you would get satisfactory results. And if you used a flow controller or two you could add some dampening and fine tune the response.

    For method 2, using a source of compressed air, all you need is a standard precision regulator and an adjustable check valve. Nothing special. You would set your pressure based on how much load you want to balance and the regulator will maintain it. The adjustable check would be set to relieve at greater pressure than the regulator is set to supply. Both would be connected to the same port on the cylinder. The other port would be left open to the atmosphere. As the z axis moves up and the cylinder volume increases (assuming the cylinder is mounted in compression not tension), the regulator would flow and maintain a steady pressure and therefore constant force on the cylinder assisting the upward travel. As the z axis descends and cylinder volume decreases and pressure goes up, the check valve would vent and maintain a steady pressure. So then the effect would be constant pressure on one side of the cylinder even as the volume changed. Counterbalance.
    Shop Sabre looks like they are probably using a balancing solenoid and it's mounted close to the cylinder to minimize the bounce effect. Also their cylinder bore looks to be about 2". So they are probably working with ~ 50 - 100 psi? I'm sure they put some thought into sizing the components so they work optimally. Lower pressure would probably need larger cylinder volume to work effectively.

    Sent from my Pixel XL using Tapatalk



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Please correct me if I'm wrong, but this appears to solve the problem...

    Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.-eqck-pop1-jpg


    Equa-Check Valves - Aladco



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    Default

    Quote Originally Posted by bobmagnuson View Post
    Please correct me if I'm wrong, but this appears to solve the problem...

    Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.-eqck-pop1-jpg


    Equa-Check Valves - Aladco
    I don't think that will work. It might, it's been awhile since I've designed air circuits and they weren't for balancing, but I think that equa check valve is more for a work station where pneumatics help a worker lift a heavy object. They want $110 for the cheapest model. All you need is a decent regulator and an adjustable check valve. At least, I can't see why that wouldn't work but I've never tried it.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    From looking at the videos, the shop sabre pneumatic cylinder looks like one of the two I have and that is 32mm bore, but it does look like a larger bore, I'm guessing 40mm as a standard size unless it uses Imperial units. You can see from this video when they hook up the pneumatics. I am curious about the air system, reservoir size, regulator, flow rate requirement for the compressor that's always dumping air....etc. Router Bob, come to the forum and answer some questions about Shopsabre? LOL, who knows, he may be a member here. Also curious why they have hoses hooked up to the top and bottom of the cylinder.



    Lower pressure needs a larger piston area to provide the same force, the volume of the piston effects the stroke, and could effect the needed reservoir size. When using cylinder with the rod in tension, the area is the bore area minus the area of the rod, in compression, you just use the bore area for the calculations.

    Smaller bore cylinders need higher pressures to get the required force, but can also use smaller reservoirs, and that is essentially what a traditional gas strut is. Higher pressure, small bore, small reservoir that is small enough that it is built into the spring. Dampening that is built in, and non-adjustability, and higher force differences at top and bottom of stroke, are the problems there, although some can let air out to make it adjustable in one direction. I would have considered gas struts if the dampening wasn't there, you can buy some without dampening but I think that is lots of $$$. Also I am thinking if the spindle I have doesn't work out for me, I may use a lighter one, and this way I can adjust the force for the new weight and fine tune the pressure for the best balance.

    Traditionally, these pneumatic systems are used to do other things. The example Chris posted, I believe this is used to be able to help lift up different objects of different weights, so there is a level of complexity there that doesn't necessarily need to exist in counterbalancing something of fixed weight, although it was interesting to look at, and I appreciated the post!

    Chris, you were talking about adding dampening on purpose? To my knowledge, dampening is bad for a CNC application, but you know what, a little bit of it might not be? The easiest way I can think of to be able to have adjustable dampening is simply to add a simple screw based flow control valve(s) to the pneumatic cylinder(s). Making the exit hole for the unpressurized side of the cylinder restricted with a much smaller opening will dampen the system. Although doing this will make it fight against your ballscrew on fast moves. There may be some dynamic responses that I am not considering which is why I'm saying I don't know if a little bit of dampening is better.

    Bob, the sample you posted was an example of a rod in compression, what I'm doing, but I'm already thinking it's not the best, that system will be continually dumping air with each stroke and will need a compressor that is constantly running to make it work, which may be fine, if that's what you want.

    My thoughts are that you should design your system based on the force you need to counterbalance and the reservoir size that gives you an acceptable force difference at top and bottom of stroke. 10-20 lbs difference, is it important, I don't think so....you can always add a regulator and turned on compressor into the system later if it leaks or doesn't work to your expectations? It's not like sizing a gantry tube where you are stuck with it unless you want to make another. The regulator is small and could be mounted in a number of places....so why not try a closed system first?

    If you could tune the compressor only to come on below your optimum mid stroke pressure value and the regulator to only dump air at a value higher than the calculated extremity, then you might have a nice "closed - open" system that only takes care of leaks and has a safety release, so that the compressor isn't constantly running and air isn't being constantly dumped.

    A few questions I still don't know the answers to that could change everything.....are these cylinders prone to leaking...does using a lower pressure make them less prone to leaking....?

    So how much weight do you want to counterbalance?

    Let's do an example.......

    Let's say you want to counterbalance 120lbs with a stroke of 300mm and a single gas spring (rod in tension), and you want to use a 2 gallon air reservoir.

    Option 1 is a 32mm bore with a 10mm rod

    Option 2 is a 40mm bore with a 10mm rod

    Option 3 is a 63mm bore with a 20mm rod

    ^^^^^^^^^^^^^^^^^^
    Area1 is (3.14 x 0.63^2) - (3.14 x 0.197^2) = 1.12 in^2

    Pressure 1 is 120 lbs / 1.12 = 107 PSI

    Cylinder volume 1 is 11.81 x 1.12 = 13.2 in^3

    2 gallons is 462 inches cubed

    P1V1 = P2V2, so assumed balanced at bottom stroke (in reality you might want to do mid stroke). The 2 here refers to the top of the stroke, not example 2.

    107 x 462 = P2 x (462 + 13.2), P2 = 104 PSI

    104 x 1.12 = 116.5 lbs

    So there is a top to bottom force difference of 120 - 116.5 = 3.5 lbs

    ^^^^^^^^^^^^^^^^^^^

    Area 2 is 1.83 in^2

    Pressure 2 is 65.5 PSI

    cylinder volume 2 is 11.81 x 1.83 = 21.61

    65.5 x 462 = P2 x (462 + 21.61), P2 = 62.6 PSI

    62.6 x 1.83 = 114.5 lbs

    So the force difference between top and bottom stroke is 120 - 114.5 = 5.5 lbs

    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    Area 3 is 4.34 in^2

    Pressure 3 is 27.6 PSI

    Cylinder volume 2 is 11.81 x 4.34 = 51.25

    27.6 x 462 = (462 + 51.25) x P2, P2 = 24.84 PSI

    So the force difference is 12.2 lbs.

    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    So really you see that as you lower the pressure and increase the bore, the force difference between top and bottom stroke becomes greater, however, with a 2 gallon reservoir, it still doesn't matter that much. Smaller reservoirs magnify this effect.

    On beer 4 now guys, may have made a math error, check it out for yourself, may have another look later and subsequent edit.....

    My thoughts for you Bob, size the thing and build it, add a regulator and compressor system later on if you need to.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Hi Nic -

    It sounds like you've got it a handle on it like always. Only thing I can offer is decent quality cylinders like ShopSabre used (Festo, SMC, Parker and others) leak very very little. Just use teflon tape and pipe dope on your fittings and any leaking will be inconsequential. You'll probably see a greater pressure fluctuation due to temp changes more than anything else. And the closer you can mount your reservoir to the cylinder the less bounce it will have.



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Quote Originally Posted by 1Jumper10 View Post
    Hi Nic -
    And the closer you can mount your reservoir to the cylinder the less bounce it will have.


    What is this bounce you speak of? I haven't accounted for it. Current plan is to join the two cylinder lines with a T fitting at the Z and then run a single line through the entire cable chain to a reservoir located outside the frame.

    It makes sense that there will be some flexibility in the line that could cause some resonance or oscillatory effects.

    No idea if it is an issue.

    But now your post is making me think that a much smaller reservoir directly mounted to the back of the Z, my original idea, is better? I'm so confused. LOL. I'm not an expert on this. I'll call myself one when my project is finished and I've done it successfully, until then, I'm just a noob.

    The shopsabre has it's pneumatic lines routed through the entire cable chain??



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    Default Re: Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

    Quote Originally Posted by NIC 77 View Post


    What is this bounce you speak of? I haven't accounted for it. Current plan is to join the two cylinder lines with a T fitting at the Z and then run a single line through the entire cable chain to a reservoir located outside the frame.

    It makes sense that there will be some flexibility in the line that could cause some resonance or oscillatory effects.

    No idea if it is an issue.

    But now your post is making me think that a much smaller reservoir directly mounted to the back of the Z, my original idea, is better? I'm so confused. LOL. I'm not an expert on this. I'll call myself one when my project is finished and I've done it successfully, until then, I'm just a noob.

    The shopsabre has it's pneumatic lines routed through the entire cable chain??
    Yes. You're essentially just making an air spring. Air is compressible, and there is always some cylinder friction to overcome so there is likely to be some bounce and that will worsen with the length of your air lines. But your it sounds like your reservoir is right sized and I think you'll be fine. Your servo motor should be able to handle it.

    And, I run my air lines through my cable chain. It's not uncommon.



    Sent from my Pixel XL using Tapatalk



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Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.

Z axis counterbalance, pneumatic cylinders, air reservor, fittings, etc.