27 Jun 12 19:23
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?