Counterbalance Valves pressure drop?

[kgwhipp]

We're troubleshooting a Scissor Lift that has four cylinders each with a counterbalance valve to prevent uncontrolled drops in the case of a line failure. They're also important to hold the load solid when the lift is unpowered but raised.

The lift will move itself up and down without issue. When you put a load at the bottom of the lift when it is lowered (aka, the point where we're expecting the most force needed from the cylinders) the lift doesn't move. If you reduce that load then it will carry it.

We're using CB valves with a 1:3 ratio, have an HPU with output of 1250psi, and have calculated maximum load at about 712psi at the cylinders (~20,000lb with 6" bore)

Here's the kicker, we have CB valves that are technically "wrong" for our application, they are set at something between the operating range of 1015psi and 3480psi. THe manufacturer recommends a 1.3x setting at max load. This would be 925psi, JUST outside the range of the CB valve.

Despite this, the lift had functioned before in our shop. After being setup on site, however it doesn't seem to be able to fully raise a load. (it can lift it just fine once the lift has risen a bit) The only thing the technician says has changed has been the counterbalance valve, and as much as I wish it were the problem, I'm pretty doubtful our problem resides here. What would a "too high" CB valve do in our situation?

IN SUMMARY: Is it possible that an incorrect setting on the valve will RESTRICT psi on the extension stroke, thereby weakening the force the cylinder is able to exert?

Thanks!



-Kevin

 

[Oldhydroman]

Hi Kevin

What will happen if your CB valves are set too high? Not much - you will need a slightly higher pilot pressure to get them to open when lowering the load so your system efficiency will be slightly worse. Think about it this way ... you set the CB valve to 1.3x the worst case pressure (highest load) which means that when you're NOT at the worst case pressure your valves are [theoretically] set too high - and yet the system still works.

When you are raising the load then the flow will be directed into the cylinder via the check valve part of the CB valve so the actual setting of the CB valve has no impact or influence on the pressure required to lift (assuming you only have single CBs - just on one side of the cylinder).

I would look carefully at the maximum pressure you can get from your pump - has the relief valve been inadvertently set lower than previously? Or, has one CB valve been inadvertenly installed the wrong way round (so each of the remaining 'active' cylinders is trying to lift a third of the load when they should only lift a quarter)?

If this isn't the answer then post a circuit diagram along with the model numbers (and make) of the valves in the pertinent parts of your circuit. There's no need to divulge confidential information.

DOL

 

[pob786]

Hi Kevin,
Is it possible that when the scissor lift is in the "low" position, and the additional weight when "loaded" has put the pivot points of the cylinders/linkage on the same axis, so that you have lost any mechanical advantage?. This could be tested by introducing a "Packer" for the scissor lift to land upon (not go as low) and then see if it will lift the load from the revised position.
John

 

[Cbrunner]

Do you have CB valves on only one side of the cylinder, or on both ends (blind and rod)? What are the differences between the CB valves originally installed and field installed?

As the others have already said, a properly installed valve will produce no restriction in one direction.

The value of a counterbalance valve is usually set/calculated to increase stability. It is possible to use a counterbalance valve with a setting much greater than necessary, losing only the energy required to overcome the valve. On the other hand, using a CB valve with too low a setting will allow the valve to open long before any load can be supported.

I would first check on the valve installed, and the pressure developed by the HPU. Check to make sure the machine is setup in a manner for which it was designed (not on a slope or with unequal loading). Given that the device worked in the shop you need more data about the field parameters - it could be a case as simple as the fluid being warm enough your pump slip has increased to the point you can't develop max pressure.

Chris Brunner
Brunner Equipment LLC

 

[hsbcn]

Another note on this issue is regarding the valve installation; ensure it's mounted in the correct flow-configuration, and levelled out. That way you'll avoid restriction, or at least you won't tilt your restriction readings in one direction or the other.

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