Towable backhoe

[Farmerford]

The rule of thumb is two gallons of tank capacity for every gallon of flow per minute. You said your pump is 11gpm, which would call for a 22 gallon capacity tank; leaving space for air, etc., you would need a 25 gallon physical capacity tank.

But that is big and expensive, and one rarely sees the two to one rule observed in consumer products or light industrial equipment. For instance, I use a 15 gallon capacity tank on an 11gpm pto pump driving a rotary mower; fluid temperatures don't exceed ambient plus 50 F (which is still 150F in mid summer here in southwest Georgia), and I don't have a noticeable aeration problem.

You can affect the temperature with your manner of operating. If you spend a lot of time with the cylinders against the stops and the fluid fllowing through the relief valves you are really heating up the fluid because all the energy produced by the engine is turned into heat. For 1,000 psi drop in pressure you heat the fluid up 7 deg F. So if you have a 2,000psi pump pushing fluid against the relief valve, the fluid temperature rises 14deg F each time it passes through the relief. Your 5 gallon system full of fluid will pass through the relief twice each minute, so the fluid temperarute will go up 28deg F every minute.

I realize that most of us don't just sit there and hold the lever back while the dipperstick stalls against a root for a full minute; but if you become sensitive to just how often in steady, heavy digging you do trigger the relief valve it will surprise you. At least it surprised me.

The design of the tank also matters. A baffle reaches from an inch or so above the bottom to an inch or two from the top does several things. It reduces sloshing of the fluid, the pump intake side is somewhat isolated from the return flow entering on the other side of the baffle, and the fluid from the return flow side gets to the pump side by flowing under the baffle, which means the fluid with fewer air bubbles, which is heavier, gets to the pump first.

So, my guess is that a 10 gallon tank with a baffle and an intake strainer with a 3psi relief valve will probably be okay. A similar 15 gallon tank will almost certainly be okay.

I notice the loop in your suction line in one of the pictures. Straighten out and shorten that line if you can; it won't make a lot of difference, but it will reduce the vacuum at the pump a bit, which never hurts.

Tell us how it works out.

 

[Yooper75]

That hydraulic fluid looks like it may have water in it if you look at the bottom of the container it's almost clear compared to the rest of the fluid.

 

[David Devuono]

To me the geometry looks ok for the boom cylinder connection. Be careful as you try different things as the tubing is not as strong for lifting laying on the flat and its good that you are lifting the hoe body as a warning, instead of pretzeling the boom. Remember that even when not working the backhoe, if you are running at 3600 RPM you are dumping 11GPM through the system so you will increase the air entry in the oil with that volume of flow. I can't imagine how fast those booms would be able to swing with no load and 11GPM available. You need to reduce the flow by getting rid of the 2 stage pump and going for something like a 3GPM pump. Then you can play with engine RPM to get a good boom operating speed. Personally without knowing more about the specs on the boom tubing, I would stay with pressures no more than 1500 PSI. Towable backhoes are great at digging, but not too good at lifting. For digging you are always pulling material towards the tractor and pivoting the bucket when its full, not pushing into the ground and ripping chunks of dirt out. That is why the optimum design for a towable has stabilizers pointing backwards at a 45 degree angle with large spiked pads. My tractor is about 4,000 lbs and my homemade backhoe will pull it backwards and anyone who has seen my videos would see that my 40 year old tractor hydraulic system is very weak.

 

[3v0]

I found it surprising how little engine power it takes to run my portable backhoe. It has a Subaru 7 hp and works best between 1/2 and 3/4 throttle. More engine speed may make it move faster but does not seem to build any more pressure. It has a single stage 2,788 GPM pump. Claims to dig to 78 inches.

On a new machine I would change the hydraulic oil filter after maybe 20 hours.

 

[bcp]

7 hp, pumping 2,788 GPM

Those Subaru horses must be really strong!

Bruce

 

[12barber12]

When it comes to Hydraulics I am lost everybody keeps talking about a relief valve where would that be on the control valve

 

[PILOON]

12abarber12

In your pix, the rt body has a screw and nut device to the left of the left hose. That should be the relief and screwing in will increase pressure and out lessen it.

 

[sparc]

I have spent many hours looking at towable backhoes and 2 years ago I bought a used 6 ' 6 " arps / JD and mounted it under a 26 HP Kubota. Most towable hoes have arms that extend well backward of the boom swing point. This is necessary because they do not have enough down weight ie BIG Chassic. Look on the net and you will find many makers of your style and copy what you think will work the best. The arms will get slightly in the way of your excavation but they will move your center of gravity well to the back.

Craig Clayton

This is how my Dig-it is setup. I was about to make the same comment you did then saw your post.
There is little counterweight on these type machines. I have been thinking about adding a rack that I can hang some tractor weights on, or maybe add9ng some weight in the frame tube steel.

 

[PILOON]

While I don't recall pump size, I owned a ToeHoe 'Cricket' that only used a 5 hp Briggs that was awesome.
All the cylinders were only 2" with 3/4 shafts.
I could dig trenches 5 ft deep and 40-50 ft long in about 1/2 hr, haul out stones bugger than the 18" bucket.
There was enough power that once on a large rock it pulled the factory welding apart at the frame pivot point
And it was NOT slow, in fact swing was so fast that one had to feather the valve or overshoot.
Only real down side was that it used 4 valves (would have loved to convert to 2 'joy sticks' and they had installed internal limiter in the elevation cylinder,( probably for stability) so that I could not load a trailer or truck.

I post simply that you know that design is more important than HP and large cylinders.
While not an expert, I have operated many compact excavators and assure you that other than mobility issues that small 5 HP ToeHoe compared favorably in most aspects.

All

 

[3v0]

When I ordered the dirt master I purchased a 16" inch bucket thinking it would only be good for moving loose dirt or material handling. I just finished digging a 4 foot deep pond with no problems using that bucket.

As mentioned earlier the lack of weight is a downside but you get creative and learn what works and the job gets done.

I was bummed about its lack of power up till I noticed I had caved in the oil filter. Much better now

After purchase I made one upgrade. An electronic hour meter. It is too easy to kid oneself about how long it was since you last greased it.