Backhoe Building a thumb for a model 48 Backhoe

[downsizingnow48]

I use Discount Hydraulic Hose never any issues.

For hose routing consider using a tube return on the thumb cyl base end fitting. To keep that hose out of the way.

Steel wire hose guards prevent damaging the hose.

I used a work port relief set at 1200psi. The incremental cost to the valve was about $60 several years ago. I thought it was cheap insurance.

 

[chevy791979]

IMO. Every thumb I've set up for customers I ALWAYS set the retract RV at whatever pressure it takes for the bucket to curl into the thumb and the thumb retracts with mild force. Aka the thumb retracts circuit is weaker the the bucket. The relief for the head end (extend) should not be able to move the bucket. Just my $.02

 

[DanMc77]

Nice to see that Discount Hydraulic Hose is good. I like their prices and the website looks like its easy to specify what is needed. I'll have to look around for those wire hose guards. The hydraulic tiller for my JD318 has those on the lines -- they just drag in the dirt and those springy covers do a great job of keeping them from getting torn up.

The relief valve would need to be on the extend line, not on the retract, right? When the extend line goes into bypass, then the retract side will pull a vacuum?

I don't have any pressure gauge for setting the relief valve. Maybe I'll need to install one? That might actually be handy to know if I'm getting the full strength out of my machine.

 

[chevy791979]

Nice to see that Discount Hydraulic Hose is good. I like their prices and the website looks like its easy to specify what is needed. I'll have to look around for those wire hose guards. The hydraulic tiller for my JD318 has those on the lines -- they just drag in the dirt and those springy covers do a great job of keeping them from getting torn up.

The relief valve would need to be on the extend line, not on the retract, right? When the extend line goes into bypass, then the retract side will pull a vacuum?

I don't have any pressure gauge for setting the relief valve. Maybe I'll need to install one? That might actually be handy to know if I'm getting the full strength out of my machine.

Really need it on both (circuit relief) think about an outside force i.e. your bucket hitting the thumb. Then think about the direction the piston will be force in from that force. If you are curling your buck into the thumb you will be forcing oil backwards on the extend circuit.

You can do it with out gauges. Curl the bucket into the thumb and back the relief off slowly until the thumb let's the bucket over power it. There's a fine line here but it's done easy with eyes and feel. Don't make it so weak it won't hold objects but weak enough you won't hurt the frame work or pins

 

[chevy791979]

I k ow there is a valve block for both circuits to pass through with RV's in them so you won't need to run another line to tank. It releases oil on the given return flow side when needed

 

[chevy791979]

As for vacuum yes but most RV's are a form of anticavitation valve that eliminates the issue

 

[texasranger556]

A cushion valve

Prince Double Relief Cushion Valves: Prince No. DRV-4HH, 15-3 PSI Adj. Range, 1/2’’ NPT Port Size, 2 PSI Relief, 22284

 

[DanMc77]

A cushion valve

Prince Double Relief Cushion Valves: Prince No. DRV-4HH, 15-3 PSI Adj. Range, 1/2’’ NPT Port Size, 2 PSI Relief, 22284

This is the one I bought. That was before you guys helped me to figure out that the cushion valve is for a double-acting cylinder.
Prince RV Series Differential Poppet Line Relief Valve: Prince No. RV-2H, 15-3 PSI Adj. Range, 3/4’’ NPT Port Size, 2 PSI Relief, 222786

I could either return the single RV and get the double, or live with the single. If it's pulling a vacuum on the retract side when the bucket pushes the extend side of the thumb into bypass, that's only 1 atmosphere (15 PSI) of negative pressure. That's like nothing compared to 2500 PSI in the other direction. Unless the cylinder is sitting in water and there was a risk of sucking it in, I don't see why that would be an issue. Help me on that one.

 

[texasranger556]

I'm thinking physical space will be more the issue. Have you decided where to mount the control valve?

 

[DanMc77]

I'm thinking physical space will be more the issue. Have you decided where to mount the control valve?

No, I don't have things worked out yet to that level, but it would seem best to have it down below close to the existing valve, with a foot pedal and linkage to run the thumb. I'll be working out the layout of the hydraulics the next time we are up in NH where the tractor is, and I'll get the whole week of July 4 to do that. Sadly, we spend most of our time in MA where I look forward to escaping away to our off-grid 28 acre sanctuary in the wooded hillside. When we are in NH I tell people that we are refugees from the madness of MA.