Knuckleboom Crane: Additional Hydraulic system thoughts

Back in China, and now I’ve got more time to research to determine the root cause.  One thought I had was that the pump on the truck is a much lower capacity than recommended.  Williams Machine and Tool lists several pumps, and they all look identical, but have various flow rates.  I needed the model number!  Fortunately, my mother was up to the task of sliding under the truck, and getting the model number off of the pump and emailing me the information.  Once I had that, I made short work of looking up the specs on William’s website.  The pump I have is a Model 30, which is now replaced by the Model F98 pump.  That has a rating of 10 GPM, which exceeds the specs that IMT recommends of 9 GPM.  However, the pump is pushing fluid through the valve body, which is a Model 4 control valve, and that has a flow rating of 6 GPM.  So that is definitely choking the flow some but it still seems like it should be faster than what it is.  IMT lists the main boom as lifting in 24 secs, this would increase with a smaller flow rate to ~36 secs, which would be acceptable for me, but the reality is that it was much longer than that, more like 5 mins!!!!

After doing a bunch of internet research, and some calculations on pressure drop, the ¾” hose draining the oil from the secondary tank back to the primary tank is causing the issue.  There’s just no way that it can flow much oil when there’s no pressure pushing it back.  This causes the secondary tank to fill up with oil, and starve the primary tank of oil, which then the pump is sucking air, which further causes problems.

tank comparison 1

So what I need to do is increase the size of the drain-back passage from the secondary tank to the primary tank.  I’m thinking 1-1/2” should be about right! That should solve my issues.

tank comparison 2

All I can do right now is get the parts on order, and then wait until I’m back at the shop to give it a try.  Here’s my detailed plan:

upgrade to hose-fittings

I’ve already gotten the third tank (extra oil capacity), and some of the black iron fittings.  I’ll order the 1-1/2” hose and barb fittings from discount hydraulic hose online.

Once I have a chance to put it all together and try it, I’ll post updates on the results.