OT: Would like advice on sizing hydraulic fittings to prevent cavitation of control valve on backhoe

[Randalthor]

Background: I recently bought a Bradco 509B attachable backhoe. It was made in 2019, but has never been used. The company has discontinued manufacture of attachable backhoes and customer support has been disappointing. I did not purchase it from a dealer, so there is no dealer support either.

Since the unit has never been setup to use, I am in the process of getting ready to have the primary input/output hydraulic hoses made. Normally, this would be very straightforward for me. However the operator manual has a fairly strong warning which states, "The most common cause of premature wear and malfunctioning of hydraulic system components is the ingress of contaminants and incorrect high pressure inlet and low pressure return connections (cavitation)." (emphasis added). The manual offers no advice for the correct inlet and return connections.

The inlet and outlet connection points came from the factory with flare adapters on them, and caps, in order to keep dirt out of the lines until the hoses are made and installed by the customer. The flare adapters reduce the I.D. by about an 1/8".

The primary input and outlet have multiple fittings. The first fitting is a 1/2" NPT female. Then screwed into that is a 1/2" NPT male X 1/2" JIC male. The JIC male end has a female JIC cap on the end to keep dirt out.

As I mentioned, the JIC fitting restricts the I.D. orifice size by about 1/8". An 1/8" may not seem like much, but on 1/2" fittings the flare fitting reduces the cross sectional internal area by more than 50%. The high pressure inlet has an inline filter built in, which can be seen in the first picture. Here are a couple pictures of the factory inlet/outlet fittings (outlet fittings are identical-except the outlet has no inline filter of course). The first picture is of the inlet, with the protective cap off, the second picture has the inlet and outlet pictured with the inlet cap on.





I'd like to take the flare fitting adapters off, and just have the hydraulic hoses made with 1/2" NPT male ends to allow for less restrictive fittings, but I don't know enough about cavitation of control valves to have confidence to remove the flare fittings the manufacturer installed. I've probably read just enough about cavitation on control valves to be dangerous. What I've read more or less indicates cavitation occurs where there is a substantial pressure drop in the system. Is that perhaps why the manufacturer put the more restrictive flare fittings on before the inline filter on the inlet? Perhaps in order to reduce the pressure before the hydraulic oil enters the system?

Perhaps they have the more restrictive flare fittings on the low pressure outlet to prevent the pressure from dropping below the "critical" level on the low pressure side?

The manufacturer obviously went to extra expense to put these fittings on. Perhaps it was just to make it more convenient for the customer to install the inlet and outlet hoses? If so, it doesn't add much convenience for me, as the hydraulic hoses I intend to have made, aren't very long, so it wouldn't be difficult to screw them into a NPT fitting. The other ends of the hoses will have quick disconnect (i.e. breakaway) fittings on them which plug into auxiliary hydraulic ports on a tractor.

Lastly, the specs for the backhoe state that the backhoe requires 12 gpm oil flow at a pressure of 2500 psi. The tractor puts out 14 gpm at 3000 psi.

My gut tells me it's OK to remove the more restrictive fittings, but the explicit warning in the owner's manual about cavitation gives me pause. In my experience, some of these things are counter intuitive, so I don't necessarily trust my gut in this case. Any help on this matter would be appreciated.

 

[Gordon Heaton]

Never heard of cavitation being a problem in a control valve. To my knowledge its a pump problem and as long as the pump inlet/supply is adequately served there will be no cavitation.

 

[gbent]

Pipe threads in hydraulic service are a tool of the devil to make you break your religion. Add in Chinese threads that won't even seal in water service and you have a perpetual leak. Keep the flairs.

 

[dkmc]

If your tractor doesn't have an adjustable flow control on the aux hyd. line, you might be wanting to install one. Those backhoe attachments
can be a jerky hard to control mess with full flow running to them at any engine speed other than idle. I'd guess cavitation will be the least of your worries at the lower flow rates that will make that attachment usable instead of a PITA.

 

[MwTech Inc]

I see your problem is a bit too much flow and pressure.
Factory book says consult them if over 12gpm.
per bradco book on a 365 hoe

 

[Deerehauler]

I think the cavitation they backhoe maker might have been concerned with would be cavitation of the main hydraulic pump by dumping the return line from the hoe into the sump. The return circuit from a tractor function control valve does not just get dumped into the sump, but rather gets redirected into the suction line of the tractor main hydraulic pump. This helps the tractor pump because it does not have to suck 12 GPM out of the sump, but rather just enough to satisfy other hydraulic needs.

What kind of tractor are you connecting to this backhoe?

 

[Randalthor]

Thanks for the replies. I really appreciate the comments.

To answer some questions.

I completely agree pipe threads are the devil in sealing higher pressure oil lines. Especially Chinese threads. They are horrible. To my embarrassment, I've bought Chinese threaded fittings/pipe before. I've thrown some of them away. I can't understand why they can't thread something even as good as a manual threader, like my old Toledo manual threader. I've threaded tons of pipe with a manual threader with a good fit. I wonder what is so hard for Chinese manufacturers to make fittings which have a decent fit?

However, the problem is that the backhoe machine already has pipe threads as the initial connection coming out of the input/output connections, so I can't get away from NPT connections, no matter what I do.

The tractor has auxiliary flow, which can be turned off or on at a flip of a lever. So I can lower the flow. And of course I can idle down the tractor to reduce flow. I'm not too concerned with too much flow from the tractor. I've owned an attachable backhoe before, but it was a Case D100, which was a much smaller backhoe, and much less powerful.

Really my main concern was if I removed the more restrictive flare fittings would I be risking a cavitation issue in the control valve that the operator's manual warns about.

The backhoe unit doesn't have a PTO pump, so my only concern is cavitation of the spool valves on the backhoe unit. I suppose it's possible the hydraulic pump on the tractor could cavitate, as Deerehauler points out, but I don't get that the warning from the backhoe manual. The Bradco 365 is a much lighter unit than the 509B I purchased so I'm not too worried about to much flow to the 509B. These 500 and 600 series units are marketed to construction folks with heavy skid steers producing heavier hydraulic flows and pressures. Nevertheless, I have the shop manual for the tractor I plan to examine for possible cavitation of the auxiliary hydraulic pump on the tractor, so thank you for that idea. It has the hydraulic circuit mapped out, so I plan to check that.

 

[Deerehauler]

Since you have the tractor manual, you will be able to tell whether the hydraulic system is closed center or open center. This is important for a couple reasons.

An open center system has a gear pump that has constant flow at all times. When a hydraulic function is used, all the oil from the pump gets diverted into the circuit demanding fluid. If the circuit does not require all the flow, the relief bypasses the remainder of the fluid to the sump. Most smaller tractors have this system. When there is no demand on the hydraulic system oil, some of it goes through a filter and cooler, with the rest going back to the sump under very little pressure.

A closed center system has a pump capable of variable flow. Older tractors (read John Deere farm tractors) had a valve in the main hydraulic pump that would limit the pressure to 2200 psi. A charge pump in the sump supplies 200 psi oil from the sump to the main hydraulic pump. The MHP has either a swash plate or pressurizes the case, varying the displacement of the pump to meet the pressure requirement demanded. When a function is activated, full system pressure is available at the function's valve. 2200 psi oil will go into a load (cylinder) and the return oil gets put back into the system after passing through a filter. This oil gets put back into circulation AFTER the charge pump, so the charge pump only has to pump oil for the demand required from the load. Under no hydraulic load, the MHP is flowing a very minimal amount of oil (cooling the pump) and the oil from the charge pump gets routed though the oil cooler and filter, and back to the sump.

I suspect the valve on the backhoe is open center. An open center valve will work poorly on a closed center system, but a closed center valve WILL NOT work on an open center system. In selecting fittings for hooking up the hoe to the tractor, the pressure side needs to be strong enough to withstand full system pressure. The return side needs to withstand the return system pressure of the tractor. I am not familiar with a valve cavitating. If the return pressure was near zero (dumping directly in the sump) I could see the valve functioning harshly. There would be essentially no return 'back pressure' allowing the oil to flow very freely. It might be harder to modulate a gentle swing or other bucket action with such a pressure difference. I suppose under certain circumstances air could be drawn into a hydraulic circuit through an open return, which would also cause issues.

 

[john.k]

Why a flare fitting is any different ?......the flare doesnt fit in isolation.....in fact the only hydraulic fitting that does fit direct to a casting is the Caterpillar type with a 4 bolt split clamp........anyhoo,I dont see where the valve bank is supplied with this ...........backhoe valves are quite varied according to function.......for instance ,the slew function must have a check system to prevent the bucket runaway sideways downhill......other valves need an anti cavitation/anti dieselling function.

 

[technocrat]

12gpm through a -8 hose puts you at about 20 ft/sec which is the maximum recommended hydraulic velocity for hoses. Too fast and you will get turbulent flow which drops pressure and creates excess heat. The big danger of incorrect sizing is on the suction side of the pump, too small and you create cavitation which will trash the pump in no time.

 

[john.k]

If an apparent 1/2 NPT connection is loose in the thread ,it may be 7/8x14 tpi ,sealing with an O ring ...........this is a common mistake often made by professionals (well ....hose doctors)

 

[Randalthor]

Again some good comments, and food for thought. Thank you.

The tractor has an open center system. The backhoe is also set up for open center, but can be switched to closed center with a little bit of minor re-plumbing.

The tractor is capable of 14.5 gal/min, but that is at max PTO speed and with combined flow. It has a lever to combine the flow of the auxiliary hydraulic pump and the lift pump to get a total output of 14.5 gal/min at max PTO speed. If the lever is placed in the non-combined position, the auxiliary pump only puts out 10 gal/min at max speed.

The back hoe may be a little jerky, so I plan to experiment with the hydraulic flow selection to get the smoothest operation. The backhoe manufacturer claims the machine has cushion valves on all the main cylinders for smoother operation and less jerkiness.

It sounds to me, from the comments, that more issues result from too much oil flow restriction, vs. the other way around. So I think I'll go ahead and remove the more restrictive flare fittings and have the hoses made to screw directly into the NPT fittings on the backhoe. I removed one of the flare adapters to make sure it's not a boss O-ring fitting. It's definitely NPT on one end and flare on the other. I once bought a piece of equipment which someone had connected the wrong fitting to a boss fitting and it came apart under pressure, so I know that can happen.

I'm still puzzled why the manufacturer warned about cavitation, as one of the main causes of failure to the spool valve assembly, resulting from incorrect hydraulic connections. I wonder what type of connections would cause cavitation in the spool valves? If I could answer that question I might have more confidence in the proper selection of hydraulic hose ends.

The manufacturer warning was in the paragraph describing the control valve, so I assumed they were referring to cavitation of the control/spool valves, but perhaps they just stuck it in there as a general warning for the hydraulic pump on the tractor/skid steer?

Here is the complete paragraph from the manual:

"CONTROL VALVE
The hydraulic control valve maintenance in normally limited to
replacement of O-ring seals, cleaning and the replacement of relief valve
cartridges.
To convert your monoblock valve from open center to either
closed center or power beyond, the optional plug #45938 must be
purchased and installed in your valve.
Remove plug #45921 from the end of your valve and install plug
#45938 inside port (approximately 1”) followed by the fittings and hose
from the existing return port. Install plug #45921 into the existing return
port for closed center or use this port as your power beyond.
The hydraulic control monoblock valve maintenance is normally
limited to replacement of O-ring seals, cleaning and the replacement of
relief valve cartridges.
The most common cause of premature wear and malfunction-
ing of hydraulic system components is the ingress on contaminants and
incorrect high pressure inlet and low pressure return connections
(cavitation).
Observe a high standard of cleanliness when doing valve
maintenance."

The duplication of the sentence about monoblock valve maintenance is in the original text.

 

[Hobby Shop]

Is the BH hooked up with standard quick couplers? If so, they’ll restrict more fluid than JIC fittings and need to be oversized. Is the diverter down stream from the loader valve to the rear remotes? If so, it’s at least 3rd in line for fluid. My first BH attachment was a Woods. Nice little BH for what it was and 10x better than dying on the end of a shovel. A BH likes to be first in line for fluid from the pump. I replumbed the Ford 1715 tractor at the diverter when I got the attachment. BH first and power beyond to the loader, remotes, 3pt hitch, etc.

I have a few backhoes now and each one is a little different. Its smooth as silk all day for me, then watch a buddy or brother attempt to run the machine and it bucks like a raging bull. I cringe but they gotta learn.

Maybe you already know but if you’re not familiar with the numbering system in hydraulic fittings they’re X/16 then reduce the fraction.

(#8) 8/16=1/2”
(#12) 12/16=3/4”
(#14) 14/16=7/8”

 

[Randalthor]

Is the BH hooked up with standard quick couplers? If so, they’ll restrict more fluid than JIC fittings and need to be oversized. Is the diverter down stream from the loader valve to the rear remotes? If so, it’s at least 3rd in line for fluid. My first BH attachment was a Woods. Nice little BH for what it was and 10x better than dying on the end of a shovel. A BH likes to be first in line for fluid from the pump. I replumbed the Ford 1715 tractor at the diverter when I got the attachment. BH first and power beyond to the loader, remotes, 3pt hitch, etc.

I have a few backhoes now and each one is a little different. Its smooth as silk all day for me, then watch a buddy or brother attempt to run the machine and it bucks like a raging bull. I cringe but they gotta learn.

Maybe you already know but if you’re not familiar with the numbering system in hydraulic fittings they’re X/16 then reduce the fraction.

(#8) 8/16=1/2”
(#12) 12/16=3/4”
(#14) 14/16=7/8”

Thank you. I hadn't actually checked the flow rates of the couplers, until just now. It would make sense to me the quick connect coupler Pioneer type (iso 5675) would have more restriction than the JIC fittings. The tractor and my other equipment all have 1/2" Pioneer quick connects, which are rated for 12gpm, which if I ran the tractor wide open, would exceed the capacity of the fitting and the hose. That's a good to know and thanks for pointing that out.

Your compact tractor's hydraulics must work differently than larger tractors. The tractor I plan to use doesn't work the way you describe yours. It has an auxiliary pump which is really two pumps inside of one casing. They call it a dual element pump. One of the pumps runs the steering, pto clutch, four wheel drive solenoid, and differential lock solenoid. On that pump, steering is primary. The other auxiliary pump runs only the remotes, and trailer brakes, if a trailer is hooked up. According to the shop manual, this auxiliary pump puts 10gpm flow at the remotes.

A third pump is the lift pump for the three point. It has a selector valve which will divert it's flow to the remotes (rendering the three point in a locked position, until the selector valve is switched back to the three point). This is supposed to give an additional 4.5 gpm directly to the remotes. At one point I split the tractor and rebuilt the lift pump with new seals, pistons and cam blocks, so I have visual confirmation the pump works the way the shop manual describes. (The pump really didn't need rebuilt, but typically whenever I tear into something, I replace anything with the slightest wear because parts are less valuable than my labor of tearing into the same thing again because I didn't replace something the last time I tore into it.)

I may not use the combined flow, if the auxiliary pump flow alone is fast enough to run the backhoe attachment.

I am familiar with the A/N system, but thanks for posting that. Someone in the future may read this thread who doesn't understand A/N is a base 16 numerical system. I used to know why it was base 16 because I read about it on this forum, but I've forgotten.

I agree everyone is rough on equipment at first. I was a young man when a kind farmer, who was also friend of mine, put me on his tractor attachable backhoe to dig burial holes for his dead sows. I did it for free to learn how to use one. I remember how jerky I was. He was wise enough to show me the controls, give me the safety warnings, and realize as a neophyte I would learn faster if he walked away and did not watch me. Now, having owned and/or operated heavy equipment I do the same thing.

The last piece of heavy equipment I operated was a new Cat 120 AWD blade, which I rented from a local Cat dealer. I trained a good employee to run it. He had never run anything. I had him ride in the cab with me for a while till he knew all the controls. Warned him repeatedly not to cut the tires with the moldboard. I road with him in the driver seat for just a little bit, then got out of the cab to let him learn without someone watching over him. He couldn't move dirt or grade super fast, but he was a help and let me have a break from the machine.

 

[DDoug]

I relate "Cavitation" to the suction side of a pump.

 

[Doozer]

This seems like a lot of chatter for worrying about
a cavitation problem that has not happened yet.
Seriously.

-D

 

[technocrat]

This seems like a lot of chatter for worrying about
a cavitation problem that has not happened yet.
Seriously.

-D

yep, and thousands of backhoes use this pump, hose and fitting combination

 

[WizardOfBoz]

Never heard of cavitation being a problem in a control valve. To my knowledge its a pump problem and as long as the pump inlet/supply is adequately served there will be no cavitation.

Naw, its a big deal in chemical/petroleum control valves. Cavitation turns the valve seating surfaces to something that looks like a meteor surface. It sounds like gravel is running through the valve. For cavitation to develop in a hydraulic valve one would have to have a very low discharge pressure. Near its vapor pressure, which is 2 or 3 psi absolute. Not sure I buy it happening in a hydraulic system. Maybe others, with hydraulic experience, can correct my view. But you're not going to be anywhere near that vapor pressure (so cavities in the fluid won't form).

 

[garychipmaker]

The only thing I can think about is when you drop the stick into a deep hole does this create cavitation because the cylinder is being extended by the weight of the hoe faster than the oil can be pumped into the cylinder by the pump.

 

[idacal]

i have one of those little backhoes it has 1/2 hoses i stepped up the quick connects to 3/4 just ease flow restriction and to match everything else that runs on the skidsteer. just plumb it so it works and run it most likely their was a lawsuit and its just easier to put the warnings in. The only time i have had cavitation on equipment is when a pump is starting to go or a valve is leaking And sucking air in.
Im not sure how a pump creates cavitation bubbles without air but somehow it does must heat the entrained air and they explode out of the oil ?
1/2 lines with whatever fittings are fine for 14 gallons a minute on any flow chart i have looked at. Plus you will most likely be idling.