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.

 

[NormL]

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.

That 3.4 GPM may well be all I can handle while I learn the operation of a backhoe. Once you start practicing on a new-to-you machine, reality gets real close and sometimes real ugly if you're not careful. So I hope to go slow but strong as I gain proficiency.

 

[NormL]

I picked up a new pump yesterday and took a look at the challenges I face installing it where I had the original pump. That is going to take some planning!! I'll have to cut away the mounting brackets and weld others into place because this pump, although smaller displacement, is physically larger in all dimensions other than body length. Also, the rotation is going to have to be reversed. Fortunately I was able to look up the instructions to do that and they're not too complicated. Yes, the "fun" never stops!

 

[NormL]

I picked up a new pump yesterday and took a look at the challenges I face installing it where I had the original pump. That is going to take some planning!! I'll have to cut away the mounting brackets and weld others into place because this pump, although smaller displacement, is physically larger in all dimensions other than body length. Also, the rotation is going to have to be reversed. Fortunately I was able to look up the instructions to do that and they're not too complicated. Yes, the "fun" never stops!

The new pump is installed after reversing the rotation and making room for the larger dimensions. Not as challenging as I expected once I got into it. I did use the restrictors I made up and it all seems to work the way I had hoped. I have yet to try digging with it but that will have to wait till tomorrow. I spent the day correcting a couple of control valves by switching the away and back hookups. There were also some seepage issues that needed correction. Careful use of Teflon Tape should take care of them. I say "careful use" because any tape that wraps beyond the threads winds up clogging valves downstream. Taking apart scroll valves is not my idea of a fun or relaxing afternoon.

 

[NormL]

The new pump is installed after reversing the rotation and making room for the larger dimensions. Not as challenging as I expected once I got into it. I did use the restrictors I made up and it all seems to work the way I had hoped. I have yet to try digging with it but that will have to wait till tomorrow. I spent the day correcting a couple of control valves by switching the away and back hookups. There were also some seepage issues that needed correction. Careful use of Teflon Tape should take care of them. I say "careful use" because any tape that wraps beyond the threads winds up clogging valves downstream. Taking apart scroll valves is not my idea of a fun or relaxing afternoon.

I'm pulling this thread back up to get some feedback and ideas. First the positives: after changing out the tired and inconvenient vertical shaft engine the Sears/Craftsman GT18 that the backhoe is built on with a horizontal shaft 13hp as described in the thread titled "From vertical to horizontal - proposed engine swap?!" in the Build it Yourself forum, I have finally been able to do some digging with it. The ability to mount the hydraulic pump directly has vastly improved digging ability. The belt drive just kept on slipping no matter what I tried. The tough conditions of our soil following our summer of drought conditions has caused numerous weaknesses in my design and workmanship to show up as breakdowns one after another. No real biggie though because now the hoe is better and stronger for it!! The negative(s): the hydraulic cylinders all do not hold position. Some are older used salvaged from other implements but the new ones are also leaking down. The former I realize I will need to overhaul and I can do it over winter. The new ones I suspect something else is the cause - I allude to it in the sentences in bold in the above quote. Careful as I tried to be when I used the Teflon tape to stop seepage I may have messed up on one or a few! Can anyone advise me what I can expect and need to be wary of when I disassemble scroll valves? The scroll valves were purchased new but sat unused for almost ten years while I waited for funds to become available to finish the build. I don't think that should be a factor but I mention it in case it is. None of the hoses and connections are leaking so the leak-down is not from that. Anyone have experience/advice to share? Also other possibilities that would be responsible for this?

 

[oldnslo]

All Spool valves leak due to machining tolerances and are rarely damaged by Teflon tape. Which way are the cylinder’s drifting extending or retracting? If extending possible the piston seals in cylinder are leaking. If retracting then leaking across spool.

 

[NormL]

All Spool valves leak due to machining tolerances and are rarely damaged by Teflon tape. Which way are the cylinder’s drifting extending or retracting? If extending possible the piston seals in cylinder are leaking. If retracting then leaking across spool.

The drifting is occurring in the direction the resistance is pushing. For example: the stablizers keep dropping the wheels back on the ground. The boom cylinder drops the dipper and bucket till they drag along while I'm driving between the shed and dig site. those are the most annoying examples. The stablizers are the ones I will overhaul. The boom cylinder is new.

 

[oldnslo]

I suspect stabilizer should have pilot operated checks to insure they hold position.

this does sound like directional spool leakage. Some valves leak more than others.

 

[NormL]

I suspect stabilizer should have pilot operated checks to insure they hold position.

this does sound like directional spool leakage. Some valves leak more than others.

The pilot operated checks are something I have not heard of till now. Are they a solution for the boom cylinder as well and where can I find them?

 

[4570Man]

I suspect stabilizer should have pilot operated checks to insure they hold position.

this does sound like directional spool leakage. Some valves leak more than others.

Backhoes don’t have those.

 

[oldnslo]

Norm
No to P.O. check on boom cylinders