Z Axis Counter Balance???

[bio_eng]

What are your thoughts on counter-balancing the Z axis weight?

This would be a system in which the moving z-axis weight is counter-balanced by an equal weight placed on the opposite side of the gantry connected such that when the router moves up, the counter-weight moves down and vice-versa.

This can be accomplished via hydraulics, pulley system, or a constant force spring (http://www.springcompany.com/jes2/cfframe/cf.html ).

I read that the HUGE boring mills...i.e. the 40,000 pound ones use a hydraulic counter-balance system to offset the wieght of their 100+ pound milling head.

What are the advantages and disadvantages to such a system?

This is what I can think of...(I of course could be wrong)

Advantages:
1. less wear on Z axis ball screws.
2. equal power required to move the z axis up vs down. Therefore, equal speed and acceleration potential for up vs down motion.
3. MORE???

Disadvantages:
1. Complexity of system...although a simple "constant-force spring" system can be devised.
2. Cost
3. MORE???

Thanks!

[buscht]

I think that a gas spring would be a perfect solution, cheap and easy to add to any slide.
Here's a link
http://www.reidtool.com/PDF/170.pdf
Check out the adjustable models. You can lower the pressure to match you needs.

They would solve all of your disadvantages but gain all the advantages

[HomeCNC]

Some more advantages:

Smaller motor needed to lift the Z axis head.
Less likely that the head will fall when the motor power is turned off.

[bio_eng]

I think when reidtool refers to "fixed force" they mean that it is not one of thier adjustable models...as opposed to meaning that their gas springs are "constant force".

Maybe?

Any other advantages or disadvantages?

Originally posted by buscht I think that a gas spring would be a perfect solution, cheap and easy to add to any slide. Here's a link http://www.reidtool.com/PDF/170.pdf Check out the adjustable models. You can lower the pressure to match you needs. They would solve all of your disadvantages but gain all the advantages

[buscht]

I'm pretty sure that gas springs are constant force. That's the whole point of them. Fixed force means that you cannot adjust the pressure. You buy a 30lb spring and that is what you get. The adjustable allows you to (one time) remove pressure down till you get it right. If you go too far, you are out of luck.

For a Z axis counterbalance, you don't need a perfect floating counterbalance, maybe 90%. The stroke would be the main thing, make sure you specify a spring with adequate stroke.

[bio_eng]

"Constant Force" means that force does not change with stroke as opposed to a normal gas spring that has more and more force as it is compressed.

That is why reidtool specifies the force of their "fixed force" springs at their extended length (theie P1* note)...because the force increases as it is compressed.

As we know of gas: PV=NRT...so as the volume decreases, the pressure increases->greater force

Originally posted by buscht I'm pretty sure that gas springs are constant force. That's the whole point of them. Fixed force means that you cannot adjust the pressure. You buy a 30lb spring and that is what you get. The adjustable allows you to (one time) remove pressure down till you get it right. If you go too far, you are out of luck.

[buscht]

You are right about the gas springs not being constant force.

I found off the shelf springs that gain about 20% in force over the entire stroke of the spring. You could get a longer spring than needed. Say 6" if you only need 3" of stroke. Size the spring 10% under your load. Then the spring would balance perfectly at the bottom of stroke and lift 90% at the top.

I noticed on this website http://www.indgassprings.com
that you can get special springs that only gain 4% force over the stroke length.

I use these springs as counterbalances on doors, lifting tables, and they are very easy to install and work well.