VMC retrofit Z axis assist?

[wyobmf]

Hello,
I am retrofitting a Hitachi Seiki VK-55 II. It is a large VMC weighs 24K lbs.
The original z axis had a double acting hydralic assist cylinder on the Z axis.
I am not sure how the logic worked on it the lines are disconected and the control is dead so I can not see the logic.

What type of control should I use to help assist the z axis servo?

Thanks for any sugestions.

[DareBee]

Were the hydraulics used instead of a counter weight or in addition to?
If it was used "in addition to" I would think a larger servo or a slightly less steep starting ramp would suffice.

[wyobmf]

There is no counter weight, Just a hydraulic cylinder. There is alot of casting on the Z axis it is very heavy.
Thanks

[Geof]

It is possible to rig a hydraulic cylinder with a hydraulic accumulator so that it carries the weight of the head through the full length of the travel. As the hydraulic fluid is displaced into the accummulator the nitrogen pressure increases and this changes the force lifting the head. The amount of pressure change can be minimized by using a large accumulator compared to the cylinder size or by using an extra tank as a ballast tank.

[wyobmf]

Geof,
Let me know if I am on the right track, I am not familiar with hydraulic systems.

You tube the hydralic cylinder (high pressure side) to an compressed gas accumulator. The pressure of the accumulator is set with a gas regulator on a N2 bottle or air supply. As the Z servo motor lifts the Z axis the pressure drops and the regulator adds volume to keep its pressure SP this assits the servo motor.
As the Z axis drops the pressure increaces and the regulator would have to vent.

Is this correct? What do I do with the other side of the double acting cylinder?
Thanks

[Geof]

Originally Posted by wyobmf ... As the Z servo motor lifts the Z axis the pressure drops and the regulator adds volume to keep its pressure SP this assits the servo motor. As the Z axis drops the pressure increaces and the regulator would have to vent. Is this correct? What do I do with the other side of the double acting cylinder? Thanks

The way you describe it is correct except using a regulator to keep the pressure constant means you gradually empty your high pressure nitrogen cylinder that is supplying the gas; every time the Z axis drops you lose a little bit of gas.

You can avoid losing gas if you simply use the nitrogen tank as a reservoir; if the combined volume of nitrogen in the accummulator and reservoir tank is many times the volume of hydraulic fluid displaced from the cylinder into the accummulator the pressure change is very small when the head moves. In this case the tank is connected directly to the accummulator without any regulator and you start with an empty tank of course and charge the whole system once it is installed.

I cannot answer your question about using a double acting cylinder; I am not familiar with Hitachis machines. I cannot think how a head needs any assistance going down, gravity takes care of that problem. Does the machine have a hydraulic pump? Perhaps it was a complete live hydraulic system with a pump providing the pressure to lift the head in which case a double acting cylinder simply reduces the size of the hydraulic tank. The 'retract' side of the piston is never pressurized but fluid flows in and out as the piston moves.

[samco]

Our big mill uses cylinders to counter balance the y axis. IIRC - it uses a constant pressure pump - (or maybe a regulator) that keeps a constant pressure on the cylinders. It actually preloads the head up. Then when the head moves down - there is an over-pressure valve that bleeds it out. No real control at all except for maybe 1 solenoid for enabling it.

[wyobmf]

Yes, the machine has a hydraulic system, mostly for tool change functions.
There was alot of parts robbed off the hydraulic system and I cant tell what went where. I am trying to figure out how to replumb everything.

So could I just install a hydralic regulator and set the pressure just enough to take most the weight off the Z axis? I guess the pressure down port could just be returned to the tank in case of a seal leak.
Thanks for all you help