Hydraulics 101 for my Mini Backhoe Build

[ptsg]

If it ends up being close to a new pump in price, just go for the new pump. It makes for simpler setup, less hoses, less fittings, less complication.

On my homemade backhoe, I run a 17 cc (1.037 ci) hydraulic pump powered by a 3 cylinder Kubota diesel 16hp engine. It's way too big for this setup but pretty like you, it what I had at the time. Run it 1:1 ratio, would make the backhoe crazy fast, resulting in jerky movements, plus the engine had not enough power to run the system at some useable pressures.

I ended up slowing the pump using pulleys with a ratio of 1:1.33 if I recall correctly and I set the system pressure to 150 bar or 2175 PSI. This gives me a nice and comfortable speed and enough force to do some serious work. I still have some RPM left on the engine in case I'm in an hurry and need some more working speed.

Considering what I learned with this backhoe, if I were to do everything all over again, the ideal pump setup for my backhoe with the current engine would be: three 5.5 cc hydraulic pumps, one pump would feed a 2 spool valve for two functions of the backhoe, the second pump would feed another 2 spool valve for the remaining backhoe functions and lastly, the third pump would feed a single spool valve for the auxiliary circuit for the thumb. Then for the drive, I would probably add another valve and figure out a way to combine flow of two pumps to give me two speeds on the hydraulic motor.

Here is a rock I pulled a couple weeks ago:

 

[NormL]

If it ends up being close to a new pump in price, just go for the new pump. It makes for simpler setup, less hoses, less fittings, less complication.

On my homemade backhoe, I run a 17 cc (1.037 ci) hydraulic pump powered by a 3 cylinder Kubota diesel 16hp engine. It's way too big for this setup but pretty like you, it what I had at the time. Run it 1:1 ratio, would make the backhoe crazy fast, resulting in jerky movements, plus the engine had not enough power to run the system at some useable pressures.

I ended up slowing the pump using pulleys with a ratio of 1:1.33 if I recall correctly and I set the system pressure to 150 bar or 2175 PSI. This gives me a nice and comfortable speed and enough force to do some serious work. I still have some RPM left on the engine in case I'm in an hurry and need some more working speed.

Considering what I learned with this backhoe, if I were to do everything all over again, the ideal pump setup for my backhoe with the current engine would be: three 5.5 cc hydraulic pumps, one pump would feed a 2 spool valve for two functions of the backhoe, the second pump would feed another 2 spool valve for the remaining backhoe functions and lastly, the third pump would feed a single spool valve for the auxiliary circuit for the thumb. Then for the drive, I would probably add another valve and figure out a way to combine flow of two pumps to give me two speeds on the hydraulic motor.

Here is a rock I pulled a couple weeks ago:

Wow!! That rock looks like it could weigh as much or more than your hoe! Around here folks use them for lawn ornaments, the rock, I mean!

The pump I have in mind is .244 cubic inch (3.99cc) and can be run at 600 to 4000 rpm. At high engine speed I get about 1700 rpm to the pump which is lots for that pump. The GPM is around 4.2 which I think should be enough. What you described as your ideal pump setup sounds a lot like a pump I have from a Toro lawn mower that had all its functions hydraulically driven. It has three separate outputs for the various cylinders and drive motors. I'm not about to try it on my backhoe. Too many outputs I can't use.

 

[ptsg]

Your 18 HP engine should be able to handle that pump easily with it directly mounted on the crankshaft (no pulleys).

I would suggest that you did some calculations so you can figure exactly what you had versus what you're going to run. I've been on the trial and error with my backhoe too and it gets expensive really fast, so it's definitely better to run some numbers and figure exactly what you need.

SurplusCenter has a very good calculator: Surplus Center

Do you know the displacement of your current pump? And do you have a pressure gauge to see at what pressure you're working with? This would help with the calculations.

 

[NormL]

Your 18 HP engine should be able to handle that pump easily with it directly mounted on the crankshaft (no pulleys).

I would suggest that you did some calculations so you can figure exactly what you had versus what you're going to run. I've been on the trial and error with my backhoe too and it gets expensive really fast, so it's definitely better to run some numbers and figure exactly what you need.

SurplusCenter has a very good calculator: Surplus Center

Do you know the displacement of your current pump? And do you have a pressure gauge to see at what pressure you're working with? This would help with the calculations.

Thanks ptsg, that calculator helps a great deal. If I installed the pump I have in mind without changing anything else, such as pulley sizes and engine speed, I could expect 1.8 GPM of flow with around 2500+ psi. As for direct mount of the pump, that would be a "project" all its own!! The engine is a vertical shaft and the pulley is on the underside of the tractor where it used to have a mower deck mounted. The pulley to the tractor tranny and the one to the mower deck which is now my drive pulley to the pump are some 4-5 inches apart welded to a tube which after forty years is solidly fused to the output shaft of the engine. So the 5" pulley is what I have to work with and therein lies the challenge.

So, with those numbers on the table, I realize that the cylinders could be very slow indeed. Too slow? The logical option to speed things up would be run the engine faster and/or change out the 10" pulley on the pump for a smaller size. Thoughts?
PS: I have the pump on the bench right now, so I'll see if I can get it to meter out an idea of the ci/cc it puts out. I have an idea it could be quite a bit. When I had it open, the cavity was about an inch deep, give or take, and the width may have been double that. So I'll try to measure out a rotation worth of oil to get a clue.

 

[ptsg]

It's very hard to figure exactly the working speed without having used the machine for a while and get a feeling for everything.

So I would say, make so what you think it's the optimal speed is around 75%. This gives up 25% more in case you find it too slow or need to work faster.

I didn't realize you had a vertical shaft engine, so yes, direct mount is way too complicated. Go with the pulleys and then if you don't like how it performs and have extra pulleys around, you can always play with those.

The Surpluscenter calculator also has a tool to figure out the displacement of the pumps, although it still needs the pump taken apart to measure the gears.

 

[dfkrug]

When I had it open, the cavity was about an inch deep, give or take, and the width may have been double that. So I'll try to measure out a rotation worth of oil to get a clue.

There are online calculators for estimating pump or motor volume, based on dimensions. Just measure the dimensions, rather than using fluid and a pipette.

As for your vertical-shaft engine, you can still direct-couple the PTO to a pump, using a Lovejoy-style coupler. Properly sizing the pump to your flow needs eliminates the need for pulleys/belts/chains/sprockets. Riding mower engines often have 2 pulleys on the PTO, but they are removable.

 

[NormL]

It's very hard to figure exactly the working speed without having used the machine for a while and get a feeling for everything.

So I would say, make so what you think it's the optimal speed is around 75%. This gives up 25% more in case you find it too slow or need to work faster.

I didn't realize you had a vertical shaft engine, so yes, direct mount is way too complicated. Go with the pulleys and then if you don't like how it performs and have extra pulleys around, you can always play with those.

The Surpluscenter calculator also has a tool to figure out the displacement of the pumps, although it still needs the pump taken apart to measure the gears.

I have done an approximate measure of the displacement of the pump I have. In ml it averages 23 over several tries. That converts to 1.4 cubic inch according to the conversion scale on Google (where else!!??). That appears to be 25 - 28% more than the one you are running. No wonder it was stalling under load. What do you think of the capabilities of the .244 ci pump working cylinders that are 2" diameter? That pump is capable of putting out 4.2 GPM but That is at 4000 RPM. I could reasonably expect almost 2GPM at the RPM's I plan to run. Would that be too slow?

 

[ptsg]

That pump is huge!!! That will put out 10.5 GPM at 1700 RPMs and most likely requires 25 to 30HP to work well at 2500 PSI. However the small pump will also be too small.

My boom and bucket cylinders have a 50 mm bore or 2" basically. The swing cylinder has a 60 mm bore or 2 3/8" and the dipper is 70 mm bore or 2 3/4". More or less similar with your backhoe, although the pivot points and leverage will have take a big part on speed. I'm getting about 8 GPM with the pump spinning at about 1800 RPMs.

Now, you can get a bigger pump and run it slower, or buy a smaller pump and run it 1:1 with the engine. Either way, I would be looking somewhere around 5 to 8 GPM at 2500 PSI.

 

[NormL]

That pump is huge!!! That will put out 10.5 GPM at 1700 RPMs and most likely requires 25 to 30HP to work well at 2500 PSI. However the small pump will also be too small.

My boom and bucket cylinders have a 50 mm bore or 2" basically. The swing cylinder has a 60 mm bore or 2 3/8" and the dipper is 70 mm bore or 2 3/4". More or less similar with your backhoe, although the pivot points and leverage will have take a big part on speed. I'm getting about 8 GPM with the pump spinning at about 1800 RPMs.

Now, you can get a bigger pump and run it slower, or buy a smaller pump and run it 1:1 with the engine. Either way, I would be looking somewhere around 5 to 8 GPM at 2500 PSI.

From what you are suggesting my best move will be two sizes up at .488 ci and 8 GPM @ 4000 RPM. I'll see how that performs at the present speed setup of approximately 1700 pump RPM. Thanks for this .

 

[ptsg]

No problem! I'm just trying to help and hopefully prevent you from spending huge amounts of money on trial and error. I've been there and it's not a good thing and can really bring the morale down on a project like this.

That would put around 3.4 GPM at 1700 RPM and you still have the option to run it at higher RPM if that turns out to be slow. Sounds like a plan to me.