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Author Topic: Air Cylinders VS Gas Springs  (Read 325 times)
Bob La Londe
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« on: June 19, 2018, 19:52:25 pm »

I have three small bed mills that use gas springs to support the head.  One works great.  I had to add lead bars to the other two in order to get moderate performance like I wanted.  Basically to counter the gas spring. 


We have talked here in the past about using an air cylinder with a regulator to perform similarly.  I think it might be better than weighting down the gas spring or even using pullies and counter weights simply because there is less mass to move. 


If anybody here has done something like this in the past how well and how quickly did the regulator equalize pressure when compressing the air cylinder?  Its not going to work very well if the head is constantly changing from positive load to negative load on the lead screw.  I must remind you I do a lot of high cycle rate 3D milling so something that might not be an issue for some people might be a major issue for me.  I was thinking if I did it I'd crank up the pressure slowly until the head had negative pressure on the screw (was lifting) and then drop the pressure until the math says I have about 40 lbs downward force on the screw.  (Drop the PSI based on area of piston surface (not) straight PSI.) 
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lloydsp
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« Reply #1 on: June 19, 2018, 20:15:16 pm »

Bob,
My opinion is that 'response time' isn't an issue.  That head cannot move as fast as a decent regulator can equalize the pressure, more-especially if it's put very close to the cylinder, and both have ports of sufficient diameter to move the necessary air.

I'm always in favor of a 'balancing cylinder', vs a gas-spring.

LLoyd
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Bob La Londe
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« Reply #2 on: June 19, 2018, 21:15:18 pm »

Bob,
My opinion is that 'response time' isn't an issue.  That head cannot move as fast as a decent regulator can equalize the pressure, more-especially if it's put very close to the cylinder, and both have ports of sufficient diameter to move the necessary air.

I'm always in favor of a 'balancing cylinder', vs a gas-spring.

LLoyd

Well I am concerned mostly because I run very short moves in Z at a high cyclic rate.  In order to reduce job time I often set the clearance plane at .05 inches and sometimes on repeat jobs as little as .01 inches.  You believe the regulator can respond quicker than that?  Or perhaps if the setup is right that the increase and decrease in pressure from such short moves will have negligible affect? Of course I suppose that depends on the volume of the cylinder at the operating height.  The higher the volume the less affect. 
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lloydsp
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« Reply #3 on: June 19, 2018, 21:26:18 pm »

Bob,
If there's enough 'head space' in the cylinder (so there's already a sizable column of compressed air in there), the regulator's response time isn't terribly critical.  The 'cushion' pressure provided by that column will reduce or increase only a tiny bit with a few tenths of an inch of of relief or compression, and will still supply substantially the same force.

Lloyd
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Garyhlucas
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« Reply #4 on: June 19, 2018, 21:59:06 pm »

Bob,
I have a lot of experience with air regulators from building pneumatic packaging machines. You need to choose a relieving type regulator, not all of them are. Even if you get that type this isn’t a good way to do it. What you really want is a tank between the regulator and the air cylinder and use a large tube to connect it all. If the tank volume is ten times the cylinder volume the pressure change over the whole stroke will be only ten percent of the pressure needed to balance the head. The bigger the tank the better this works. The regulator then simply keeps the system pressure in the right range. No air is used unless there is leakage.
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