How large a reservoir do you really need?

[PutnamVictor]

As I understand the reasons for a hydraulic reservoir are, heat displacement through additional volume,
storing additional oil need for the difference of the pull and push stroke.

I have a small FEL and am building backhoe for it. The FEL has 2 2"-16" stroke, and 2 2-14" stroke cylinders. The backhoe will gave 6 2"-10" stroke cylinders.

I have a one gallon tank right now. Will adding the 6 new cylinder require more?

 

[Rocwin]

The generic rule of thumb for a reservoir is 3 times the output of your pump.

From what you are saying it looks like that tank is way too small for the cylinders you are using. The 6 cylinders would take about .8 gallons (actually less but I don't know the rod size)

 

[J_J]

The reservoir used to be 3 to 5 times the pumps capacity in GPM. However mobile applications have reduced that down to a reservoir to equal the pump GPM. The purpose of the reservoir is not only to store fluid, but to let particles settle out, and any air that gets in to come out of suspension, and to give off heat to keep the temp below about 200 degrees. When you are using the PTO to turn the hyd pump, the GPM are still flowing through the open center valve, and with heavy usage, the fluid will get hot, and performance will degrade.

 

[PutnamVictor]

First, full disclosure, I have no experience, this is my first hydraulic project.

The cylinders are double acting so it's only the rod size that would differ from full out to full in. Am I wrong?

The pump is 4-GPM. Even 4 Gal sounds like a lot, I don't have that in my car.

 

[J_J]

You probably should use a 5 gal hyd tank.

 

[20 20]

I agree with J J you will need atleast a 5 gal tank.

 

[BruceWard]

A Kubota B21 has over 8 GPM flow and a 7.5 gallon reservoir.

 

[J_J]

How did you know he had a B21?

 

[BruceWard]

How did you know he had a B21?

I dont, just thought it might be a good example that its reservoir is about the same size as the GPM flow.

 

[Ford tractor]

The reservoir used to be 3 to 5 times the pumps capacity in GPM.

???

Is that military spec or heavy industrial application?

I've never heard a 3-5x the pump capacity being used for reservoir volume. There is no way I would want a tractor with total 20 gpm pump that needs 100 gallons of fluid. A GOOD mobile application that is worked at 100% power all the time will have volume = capacity. A decent one will be 1/2 and many are 1/3 or less the capacity.

A typical small tractor like a JD 4300 has a 12.9 gpm pump capacity for implements and PS. Trans case / diff case volume is about 6.9 gal. That's about 1/2 of capacity. Don't forget that there is also a hydrostatic transmission pump using the same fluid and that is the same or higher volume capacity than the implement/PS pump! That means the reservoir is about 1/4 of total capacity. Few will say that JD makes an unreliable tractor. Yeah, more would be better, but there is a practical limit.

I have a one gallon tank right now. Will adding the 6 new cylinder require more?

Calculate the difference in volume between full retract and full extend for all cylinders. You will need enough fluid in the tank so that when you go from full retract to full extend on all you still have ~1-2 gal in the tank. (enough so that you never suck air, even at any expected machine operating angles.) The tank also needs to have air space so that when the fluid heats up to operating temps of about 150F to 170F, it does not expand and puke. (but not so much air space that it brings in moist air.) I would guess a 5 gal tank with 4 gal high fill (all cyl retracted) level, would be a good starting point. If operating temps get over 170F, add a fluid cooler on the return line.

 

[ovrszd]

I have a 42 Ton Splitter with a 28gpm pump operating a 6"x30" splitter ram and two 2"x12" rams for log lift/wedge lift with 20 gallon of oil in the reservoir.

This fall I split for 90 minutes with two people so the splitter was moving a cylinder almost constantly in 80 degree heat. The oil temp when I stopped was 170 degrees in the tank.

To stay under JJs recommended 200 degrees I was pushing the limit. I'm sure the oil temp within the pump and cylinders was greater than the tank temp of 170.

My tank capacity is 28 gallon. I could probably add another 5 gallon of oil and drop that operating temperature a little.

 

[J_J]

If you wanted to cool the fluid more, then a radiator and fan assembly could be used, as long as the engine could produce fan voltage. 170 degrees temp is fine. A temp sensor on the radiator hose could activate the fan.

 

[IceStationZebra]

I agree with FordTractor. I have worked for 4 different off-road OEM as an engineer on new products and none of them had a reservoir 3-5x the system flow. Only a few were over 75%! Now I will say getting down to 25% is starting to cut it close depending on the reservoir shape and the angles of operation.

I can also say from personal experience that the normal rectangular reservoir does a poor job of letting particles and air settle out. We were having problems with a prototype machine, so we cut a window into the side of the reservoir. We started the machine and watched all the air bubbles come in from the return line. Several minutes of running at idle didn't help. We shut off the engine and watched the bubbles SLOWLY rise to the surface - it took ~15 minutes to get rid of ~90% of the bubbles. System design is the best way to beat aeration. And I like to put magnets in my reservoirs for cheap insurance.

As for cooling, the reservoir makes a very poor heat exchanger. You have limited surface area vs. volume. Adding more volume only adds a little more mass to heat up, which will only slightly delays your time to temp. And adding surface area to the reservoir is only slightly better, especially when you consider that most reservoirs are covered on several sides. If you need cooling adding a small cooler is the way to go - or try to increase your system efficiencies.

JJ's recommendation of 200ｰF max temp is about right (for equipment you care about ). With regular hydraulic oil every 10ｰF over ~160-170ｰF (depending on the base stock and additives) you double the oxidation rate which halves the life till change out. Regular NBR seals and low end hydraulic hoses are good to ~215ｰF. The problem with hydraulic systems is that over-temp just decreases the life expectancy, so it ends up being a ticking bomb.

ISZ

 

[Rocwin]

We design and repair mobile hydraulic work equipment, it is rare to see a properly sized hydraulic system but they are around. Generally you see 1-2x the flow rate, the equipment that uses tanks less than 1x flow have very serious problems in the long term. It seems those systems have have major issues with pumps and drives, along with cylinders needing overhaul serious containmination.

 

[AKKAMAAN]

We also have to remember that constant flow displacement systems (fixed pump) turn around fluid constantly, while in a constant pressure system, the variable pump only deliver flow on demand to actuator.....so a 10gpm fixed pump will definately need larger tank than a 10gpm variable pump.....
Most mid size and full size mobile equipment has variable pumps today....