Re: Z axis drive

--- In CAD_CAM_EDM_DRO@yahoogroups.com, gary <garyclick1@c...> wrote:

>
> Many machines even with mechanical counterweights use an electric

brake

> on the Ball Screw that is tied to the Servo Drive "Drive OK"

output.

> This brake is engaged in its default state to prevent the weight of

the

> head from back driving the ball screw and free falling and requires
> power to be applied to disengage. This power is controlled by the

Servo

> Drive "Drive OK" and sometimes by the CNC "Drive Enable" outputs in
> series. If either output goes low the brake voltage is dropped and

the

> brake mechanically engages preventing free fall. Usually this will

occur

> so quickly that the brake is set before the drive output has fully
> decayed and the head will nt drop at all. Similarly when the

machine is

> restarted the drive is fully energized and in control of the axis

before

> the brake is released.
>
> On large machines that I have retro-fitted where the head could be

over

> 7,500#, we generally use a hydraulic counterbalance. One example of

this

> was a 24 Foot Farrel Vertical Boring Mill. The z-axis ram was 20"
> square, have a 50 HP Spindle inside of it and would extend out 9

feet,

> it weighted 16,000#. This used a simple ram circuit with the

cylinder

> mounted parallel to the ram and connected to the end pushing up on

the

> ram. We apply a variable hydraulic pressure to the hydraulic

cylinder

> ram to take a major part of the head weight and allow the cylinder

to be

> back filled by the pump as the head rises maintaining constant

load.

> When the head stops the cylinder ram maintains a constant load on

the

> head. When the head is driven downward a check valve closes the

supply

> line from the pump cylinder and the ball screw drives against the
> cylinder increasing the hydraulic pressure. A separate pressure

relief

> valve in he circuit then opens to vent the cylinder back to the

tank.

> Again there is usually a normally closed solenoid valve in this

circuit

> behind the relief valve that is held open by the Drive OK/Drive

Enable

> Signal which closes if the signal goes low maintaining

counterbalance in

> the event of a power failure or machine health problem.
>
> The biggest reason that we use a counterbalance however is to

balance

> the dynamic reactions of the machine in the up and down direction.

On a

> heavy head without a counter balance it is not unusual for the

servo

> drive to be firing upward all the time even when the machine head

is

> going down or at rest. It is easy to understand that when the head

is

> being raised that the drive has to fire upward, what is not so

obvious

> is that as the head feeds downward the ball screw essentially

offers no

> resistance to back driving and the head tends to free fall. The

servo

> has to act like an engine brake on a truck to maintain control. You

can

> see this if you monitor the current up and down. The counterbalance
> allows us to balance the drive current and performance

independently for

> both up and down travel. The pump pressure is varied to assume a

large

> portion of the head weight to keep the up currents and

accelerations

> reasonable. The pressure relief valve in the return circuit is then
> adjusted separately to prevent head free fall and offer enough
> resistance to match the down current and accelerations to the up

travel.

> This allows us to run a much tighter error limits and higher acc

and dec

> ramps with faulting the machine with a following error.
>
> For lighter weight heads we have also used a hydraulic ram connect
> directly to a nitrogen accumulator where the nitrogen pre-charge
> maintains the load.
>
> gary
>

That's pretty much what I've seen, just not quite that big. I've seen
the smaller Kitamura and large AWEA machines.

Pete C.