Hydraulics 101 for my Mini Backhoe Build

ptsg · Mar 23, 2021

I agree with Rich.

NormL has this thing pretty well figure out.

NormL · Mar 23, 2021

I must apologize for not providing more information. This backhoe build is documented on the Build it yourself forum here: It's time; either get started or.....

But to answer the question, the backhoe is built on the back of a Craftsman/Sears GT18 garden tractor. so it is self propelled. The dozer blade is adapted from an attachment from another similar garden tractor and serves where a FEL would otherwise be needed. The BM30 bank of valves is only in operation during set up and set down. In addition, the dozer will be useful for refill after digging is done. Thanks Rich and ptsg for your helpful comments. I appreciate and give serious consideration to all the advice I have received on this thread as well as on the above mentioned thread. It has been immensely useful, let me assure you. Cheers!

NormL · Apr 28, 2021

Since I last posted to this thread, I have installed and tested the hoses to all the cylinders. There were a few leaks which I have taken care of. One of the potential problems I was concerned about was whether the hydraulic pump would still be functional as it was a salvage item from an old bale loader. As it turns out that pump is more than I need for this machine. It delivers a volume in excess of the amount required and as a result I'm not able to easily control the movements of any of the cylinders. I'm running the engine as slowly as I can without stalling it when I move a valve. I already slowed the pump as much as I can without adding another set of pulleys and a belt. I would have to do all of that underneath the tractor - not a pleasant prospect! I have looked for restrictor fittings but have come up blank. There must be a source for them because I have seen some. I'm open to ideas here....

lennyzx11 · Apr 29, 2021

NormL said:
Since I last posted to this thread, I have installed and tested the hoses to all the cylinders. There were a few leaks which I have taken care of. One of the potential problems I was concerned about was whether the hydraulic pump would still be functional as it was a salvage item from an old bale loader. As it turns out that pump is more than I need for this machine. It delivers a volume in excess of the amount required and as a result I'm not able to easily control the movements of any of the cylinders. I'm running the engine as slowly as I can without stalling it when I move a valve. I already slowed the pump as much as I can without adding another set of pulleys and a belt. I would have to do all of that underneath the tractor - not a pleasant prospect! I have looked for restrictor fittings but have come up blank. There must be a source for them because I have seen some. I'm open to ideas here....

You can make restrictor fittings by welding/brazing them shut then drilling a smaller hole in center. You only need one in each circuit to slow the fluid.

I would only do this on the individual circuit lines though. If on the main supply or return lines then the constant flow may cause fluid to heat.

Your valves/controls really should match your pump in flow per minute also.
Example
A 10 gpm(gallon per minute) pump .
With a valve that flows 10 gpm the halfway open flows 5gpm in theory.
But a valve rated at 20 gpm halfway open will flow 10gpm .harder to make delicate adjustments as each bit of handle movement makes a larger adjustment in the flow volume.
And a valve rated at 5 gpm (too small) though slower at halfway (2.5 gpm) is restricting the constant flow causing heat from friction AND the pump to be working harder constantly just
to move the fluid in the system.

Slowing the pump though slowing the flow also lowers the pressure produced.

ptsg · Apr 29, 2021

You can also use a Pressure Compensated Flow Control valve and adjust the flow to your needs and dump the excess to tank. Or just get a smaller pump now that you have an idea how the system works.

Link to an example of a flow control valve: https://www.princessauto.com/en/pre...w-controls/product/PA1000000773?skuId=8252785

NormL · Apr 29, 2021

Thank you to lennyzx11 and ptsg for your helpful advice!! I was considering the flow control valve concept and I also realized it would never do to restrict the flow of the pump without a release provision as lennzx11 cautioned me. As for mating the flow rates and valve capacities, I'll have to work with what I have as best I can for now. The idea of fashioning restrictors is the solution I'll try first if my search for suppliers continues to hit the wall. I have some galvanized street elbows to practice on as I'm taking seriously the warning to not use those for hydraulics. If I succeed in modifying them, I'll go on and make up a batch of restrictors for the seven cylinders that need slowing down. The ones I practice on can be drilled back out and used as normally.

PS: Yes, I also realize how careful I need to be inhaling fumes while welding galvanized material. I have read horror stories about that!!

NormL · Apr 30, 2021

I've revisited the possibility of slowing the pump some more and I think it can be done. I've posed the same issues on my build site (see post#12) as I have done here and it was suggested I use a similar setup to the one I slowed down one of my drillpresses. I'll give that a try as well as the restrictors. I've done a couple to see how brazing works and I think it will work. I'm starting with 1/16" openings as that is what I see on some fittings I have but are the wrong size. I can drill them bigger if need be.

Xfaxman · May 1, 2021

lennyzx11 said:
--------------------------
Slowing the pump though slowing the flow also lowers the pressure produced.

Can you explain how?

My machines will generate maximum pressure at idle and same pressure at max RPM.

NormL · May 19, 2021

Rich B1 said:
I would simply use a T at your 3 spool valve, with sufficient size hose to reduce back pressure. If I am reading your schematic correctly, you are using the valve as a T. IMO, that will create some back pressure on your primary valve (4 spool).

I am having a problem that I originally suspected to be a worn out pump scavenged from an old bale loader/stacker. Thinking it over and remembering this comment about possible backpressure and the issue I am having with the movement of the boom as well as the dipper and bucket to a lesser degree, I will investigate this possibility further before changing out the pump. Thanks in advance, especially if this proves to be what is happening here

. Cheers, NormL

NormL · May 28, 2021

ptsg said:
You can also use a Pressure Compensated Flow Control valve and adjust the flow to your needs and dump the excess to tank. Or just get a smaller pump now that you have an idea how the system works.

Link to an example of a flow control valve: https://www.princessauto.com/en/pre...w-controls/product/PA1000000773?skuId=8252785

I have isolated the difficulties I'm having with the hydraulics and it is that the pump is delivering a much higher GPM than I need and when I try to dig, the engine stalls. It's an 18 HP Briggs and Stratton and it should be more than enough engine for the task. I picked up a flow control valve much like the one you suggested but by the time I add in the hose and fittings I'll need for it, I can almost buy a new pump that matches the job. Does "pressure compensated" mean that the pump will continue to deliver the same pressure to the cylinders while dumping excess volume of fluid back to the tank?