Atos QVZ Speed valves.

The QVZ is a proportional throttle valve with a pressure relief valve built in.

The main cartridge consists of a throttle assembly.

The spool has a restricted hole in the bottom that lets oil up to the top area.

On top there's a cap, that sits on top of a spring, that holds the spool in it's closed position.

There's a control port and a tank port around the middle of the cartidge.

The pressure in the control port pushes the spool open against the pressure of the spring.

More control pressure, the further the spool moves, and the wider the opening that controls the flow.

To control the valve the solenoid produces a control pressure using the control spool.

This control pressure.inside the control spool balances the force produced by the solenoid current.

For the whole thing to work you need a pressure source.

That is provided by a pressure compenstion cartridge.

This is pushed into base of the top section of the valve block. (Can be pulled out using an M3 bolt down the middle.)

It works by allowing system pressure up to the control section of the valve.

Should the main pressure suddenly drop, then the spool in the compensating valve can shut off to keep the control section still active.

The as the system pressure increases the compensating spool moves back and the system pressure can recharges the control section.


In the bottom block there's a pressure relief cartridge.

This is held closed by a spring and pressure to it is supplied via a restrictor in the top section of the valve.

If the restictor gets blocked the pressure spool will lift, dumping all pressure.

There are a couple of restictors in the top block, shown in purple.

To get to these you need to remove the blanking plugs in the block.

One of the plugs is located under the name plate.

Most often the plugs dont come out as they're made of brass. It's often easier to just drill them out using a 3.3mm or 4.2mm drill. then run a tap down to clean out the thread.

Once the restictors are clear then plug the ports using a grub screw with hydraulic thread seal.

As the valve works with pressure balances throughout, it's important that all the resrictors are clear, and that all the seals are good.

Any leakage past a cartridge outer will cause a drop of pressure and the valve wont work.

Also the valve must be bled of air. There is a bleed port in the middle of the solenoid, and another on top of the throttle cartridge.