what to do with a free scrap dumpster find ?

I was able to put the head back on the machine to see how things move on these plain split bearings and and things move well but the weight back drives the axis on the ball screw pretty easy, so i need something to solve this? i don't have a brake on the servo so some kind of counter balance? i don't think the servo's will have trouble holding things when powered but once the power is off the head will drift downwards until it hits something which is not good

a counter weight would solve this but is going to be bulky and take up to much space. gas struts don't cycle fast enough and the life would not last very long. i seen some VMC's use a gas over oil counter balance which i started to look into and some of the big routers use air cylinders to balance a heavy axis.

the air over oil method seems like i nice way to keep the system sealed and it would be a stand alone system not requiring to much up keep once it's all sealed. they do loose pressure over time but it's usually slow. the air cylinder counter balance would require constant air supply. air leaks faster than oil so this is a down side.

a down and dirty cheap solution would be some kind of head lock to set when the power is off, it only take a slight bit of pressure on the ball screw to hold things in place.

long story short i'm open to ideas if anyone has any input

Counterweights double the inertia. Not ideal.

If I ever had this issue to address I would use pneumatic cylinders with relieving regulators. (the "relieving" part is important, they will let gas out as the cylinder compresses, to give constant force)

i was thinking of a pneumatic cylinder honestly. setting it up like a air over oil system in a closed loop. i have an old air compressor tank around 15 gallons or so that i could use for a reservoirs tank. add a Schrader valve and pressure gauge and some check valves to control flow direction and you would have an adjustable system. i don't expect it would hold air forever but if checked once in a while to maintain the pressure it should work the same.

i seen some sheet metal shears that work like this.

since i had the head all together i can see what kind of real travel i ended up with and see what i needed for an air cylinder. i was able to find one with a 14  inch stroke length with a 2.5 inch bore size for a little over $40. so at that price point i got one to play around with to see what i can come up with. the air cylinder is plenty big enough to only need one of them. i think it will be pretty easy  to do a central mount off the back of the machine. a couple of simple mounting brackets should be simple enough to make.

as for travel things worked out ok. i lost some travel since the table got lifted with all the linear rails but it's rigid and it's working out how i wanted even if the travel is not as much as i was hoping for. i ended up with 12" to the center line of the spindle from the table top and around 2" below the table top to the center line of the spindle. so 14" total travel on the head. i knew the work space was not going to be huge but was after something rigid and able to cut steel well. i think i should end up with that.

7.5 months later and still have a ton of work to do on this project. but here is my starting point for the air cylinder counter balance.

  

The attached component may help. It designed to trigger an air ram when the Z axis is moving up.

sudo halcompile --install liftaxis.comp

You will need to have linuxcnc-uspace-dev installed to get halcompile

i don't think this will need anything that complex but it's still good information to know for other applications.

basically all the ram does is even the forces out between lifting and lowering for what i had in mind. this gets rid of the back driving by finding a neutral state, you can add or relieve pressure from the air tank until you get the weight evened out on the axis. the ball screw still does the lifting and lowering with this kind of set up. it's a closed loop low pressure system. all you are doing is moving air between one side of the ram to the other.
i need to get the air cylinder in hand so i can see first hand what is going on and how things are put together to see if i can recycle the exhaust on it or need to modify it to do what i want? this is not something i just cooked up on my own Haas Automation has done this with an air over oil type design that works like this but their system is costly and designed for heads a lot bigger than what i am dealing with. i think they might even use nitrogen over compressed air?

like Andy said the relieving regulator is important if the system has a constant air supply feeding it but with a tank to recycle air and check valves to control the flow back to the tank you create the constant force in both directions without the need to regulate the system. the regulation is done with your tank pressure based of the weight of your head and the diameter of the air cylinder bore. the air cylinder i got can lift over 600lbs but that's only running an max pressure. if you just ant to lift 200lbs you can adjust by running at a much lower pressure. you can find info on the calculations on-line pretty easy for the pressure and force generated by air cylinders.

people add gas struts and counter weights to help with the lifting because the axis motor is under sized and can't lift the axis on it's own, that's not the case in my situation i just want to avoid back drive if power is lost to avoid a crash. the added bonus is a counter balance takes some stress off the lead screw too.

if it works well i will post pictures and get into more details if anyone wants to do something like this?