hydraulic design review

[bdw9862]

Good afternoon: I am building a towable backhoe (I will post some pics) and am getting to the point I need to begin the attachment of hydraulic cylinders as well as motor, tank etc.... Most of the fab and welding (other than cylinder mounts, valves mounts, etc) including frame, boom, pivot, outriggers, wheels and tires is done.

I have attached the hydraulic design for your review and thoughts. I have already ordered and received the 4 outrigger cylinders as I want to weld the rear outriggers prior to the installation on the frame (they run straight down). The front outriggers are already on the frame.

One thing that has me puzzled is the source and return line sizes for the tank. I am looking at a 15 gallon tank online and the openings are 2 inch 1 1/2 inch and 3/4 inch NPT. I am not sure what size line to use to run to the gear pump and also the return lines. I would like to use 2 wire JIC hose. I am also not sure where to get fittings to move from these size NPT to smaller JIC and SAE.


Any constructive help would be appreciated.

 

[Dave M7040]

bdw....

Your project aroused my curiosity.

The two commercially available towable backhoe products I found all had hydraulic flows around 2.7-3 usgpm so why is yours @10 usgpm so much higher?

Having high flow rates may appear to be a good thing but at the same time smooth gentle control may be very difficult.

If you plan on using much larger cylinders than the commercially made products, remember, you are limited in your digging force by the weight of your machine.

As far as fittings to go from NPT to a hydraulic fitting, any hydraulic shop can supply you with both custom length hoses and fitting.

This link is to a nomograph to select hose sizes can be your guide.

https://products.kuriyama.com/Asset/Kuriyama-Hydrualics-Cat----Hose-Sizing-Nomogram-9-27-12GK.pdf

I have added yellow and blue lines to the nomograph to guide you.

The most important design parameter is the speed the hydraulic fluid will be traveling in the hose. For the pressure hose (yellow on the nomograph) a good number to use is 25 feet per second. A higher speed will do two things, first erode the inside of the hose and second start causing pressure losses at high flows

For the suction line (blue on the nomograph) the velocity should be 4 feet per second to avoid cavitation.

Dave M7040

 

[LD1]

I agree, 10 gpm for the 2" or 2-1/2" cylinders found on a small unit seems pretty fast.

 

[bdw9862]

I agree, 10 gpm for the 2" or 2-1/2" cylinders found on a small unit seems pretty fast.

Thanks for the input, when I do speed calculations on the cylinders utilizing the tools on surplus hydraulics 3 to 4 gpm pumps appearr to crawl. In addition I would like to run two cylinders at once with joysticks. The gear pump will run from 7 to 12 gpm depending on rpm.

 

[bdw9862]

The boom will be 3 to 3 1/2 cylinders

 

[HarryVanderpool]

What do you have in mind for a steering cylinder?

 

[bdw9862]

Thank you Dave that is very helpful

 

[bdw9862]

I am going to use one 3 inch double acting cylinder, I will have about 180 degrees movement.

 

[HarryVanderpool]

I recommend that you use a double ended cylinder, even if one end is a dummy end.
That will eliminate rod differential.
Then place a normally open 12 volt valve conected to both sides.
When the towable is running the valve is actuated and closed.
When the towable is not running, the valve is open allowing the steering wheels to trail you as you tow.
This will not work however with a single ended cylinder.

 

[CincyFlyer]

Thanks for the input, when I do speed calculations on the cylinders utilizing the tools on surplus hydraulics 3 to 4 gpm pumps appearr to crawl. In addition I would like to run two cylinders at once with joysticks. The gear pump will run from 7 to 12 gpm depending on rpm.

Unless you are running a closed-center hydraulic system, you will need special valves to allow more than the one that is furthest upstream to get any of the flow.