Hydraulic drive for small tractor 3

[fluidlearning]

Hi. I need to transfer 12 Hp from a gas engine to a differential transmission, it is too difficult to do it by mechanical means, and so I chose hydraulic drive. I have a gear pump that will couple to the engine, and would like to use a gear pump as a motor to drive the transaxle input. It looks like the transaxle input will be around 1500 RPM. Can I use a gear pump as a motor? I got the hydraulic parts off Ebay and don’t have the specifications, so I will have to wing it. Any suggestions would be appreciated.

 

[Icanfix]

I would start with trying to get the make and model for the hydraulic parts, and do an internet search to find out what you have. Some of the manufacturers are pretty good about giving information for designing with their products.

You can use a gear pump for a motor. If it is in good condition it should be fairly efficient. The output speed of the motor will be a function of the displacement of the pump vs. the displacement of the motor.

The project can be as simple or complicated as you want to make it. You can just plug the parts together and see what they do, or you can design everything on the board before you even touch the parts.

You are probably dealing with keyed shafts on the motor and the transmission so you will need couplings. You will also need something to take the place of the clutch that the belt also acted as. You will need an open center valve for this. Northerntool.com has some formulaes and explainations in their hydraulic section. Enough to get you pointed in the right direction.

 

[fluidlearning]

Thanks, The info off the pump (it's a dual and I was just going to use the larger bottom section) is Bosch D86 1518 222 000 I have not had any luck searching the web or Bosch/Rexroth for that number, it must be an older numbering system. The other pump that I plan to use as a motor has RK 040 656876 665004 stamped on it, no manufacture.
The engine is chain driven to the pump with a 2:1 ratio. I plan to use a spool valve O/C for the clutch, if not I will make an idler clutch between the motor and transaxle, which will be belt driven. If I can get the return tank set up, I hope to pump some oil this weekend and see what happens.
I certainly appreciate you help, and will send you a picture of it when completed.

 

[kcj]

with fixed pump, you will have the equivalent of a gear drive, not a variable speed. still need a clutch to start and stop.

with variable, reversible, piston pump you can do hydrostatic drive, but thats beyond the parts you have
kcj

 

[fluidlearning]

That will be great, because of the difficulty in getting the mechanical linkage from the motor in front to the transmission in the rear, fluid power will be the way to go. I want to make sure that I can deliver the horsepower to the transmission
As far as variable speed, I can vary the engine RPM right? I have the transmission gearbox to select the speed range. I will try a spool valve (open center) for a clutch, if not, I will fall back on an idler clutch at the transmission.
Thanks, Bob

 

[fluidlearning]

I have decided to use an idler clutch at the transmission, which means the pump connected directly to the motor.
But I was trying to figure out the horsepower and something was not coming out right, The formula for HP is flow X pressure over 1714, right?
I ran a rough estimate on the pump / motor I will be using and I figure I will be supplying less than 5 HP to the transmission from my 12-horse engine. Can this be right?
Thanks

 

[kcj]

Could be quite correct.
psi x gpm / 1714 is correct, then correct for efficiencies, maybe 90-92% in each of pump and motor.

the 12 hp engine is factory rating at 3600 rpm most likely. You will need to ratio it for rpm (assuming fairly constant torque), and subtract some for engine wear or factory 'optimism'.

Also, the engine is maximum hp, but the load is what determines the actual hp in the drive. If you have a small pump and motor, coupled to a 500 hp cummins diesel, the pump and motor determine the pressure and hp transmitted.
Your pump and motor could indeed be transmitting 5 hp.

k

 

[fluidlearning]

kcj,
I don't understand how RPM figures into the equation (per the spec sheets). I know if RPM goes up, then GPM goes up. Let's take a typical gear pump, the spec sheet says displacement .144 - RPM 1800 and at 1500 PSI yields .71 GPM.
What is the formula for figuring GPM from these three variables?

 

[kcj]

I am a bit confused by where you mean by rpm, but:

1. the engine rpm figures into the hp. Say 12 hp at 3600 rpm, this is 210 in-lbs of torque, assuming the engine puts out full 12 hp, not marketing hype.

If this engine is run at 2400 rpm, assuming same 210 in-lbs of torque, the hp is only 2400/3600 x 12 or 2/3 x 12 or 8 hp.

2. .144 in3/rev is pretty small for a gear pump, but using those numbers.
Output is .144 in3/rev x 1800 rpm = 259 in3 of fluid out every minute. (in3 = abbreviation for cubic inches)

Since 231 in3 = 1 gallon, 259/231 = 1.12 gallons per minute is pump output at 100% volumetric efficiency.

1.12 gpm x 1500 psi/1714 conversion of units = 1 hp, so this pump only takes 1 hp to turn it at 1800 rpm against a 1500 psi load. If the engine is capable of more, it doesn't supply it, just thrttles back to supply 1 hp to the shaft.

If the chart indicates actual outut is .71 gpm at 1500 psi, then volumetric efficiency is .71/1.12 = .63 or 63%. Very small pumps have low efficiency, as the perimeter and side leakges around the gears is high compared to the output, but even then 63% is very low.

3. If you are good at Excell I have a hydr calcs spreadsheet with lots of common formulas I could send you.

kcj

 

[fluidlearning]

Yes, the formulas would be helpful.

Thanks for your insightful information; it will be a great help in understanding what’s happing.

ps: I used those numbers from a mfg. spec sheet, I have not been able to find any information on the pump I have (although it is bigger than the one I used for the example).

I am going to have to hook it up and see how it works.

 

[fluidlearning]

OK, now that I have gotten further along on this project, thanks to your help. I now need to use the hydraulic power to do other work.
The pump is directly (fluidly) connected to the motor, and I need to add a valve to power a cylinder.
My question is, can I "tee" off the output of the pump, or should I put the spool valve for the cylinder in series with the motor (they both won't be operating at the same time).
Let me put it another way, without using a divider or other device, should I run the flow is series or parallel with the driven units?

 

[kcj]

Think this through from basic principles: If you tee into the pump line, where does the flow go? Into the leg of the tee with equal pressure each way. Basically, whichever path has lowest resistance. Through the open center spool valve at low resistance, hence no motor pressure or torque when you try to move the tractor.

Depending on the pump and motor, you could put a selector valve in the pump line to send it to either motor or cylinders. You will need a separate cylinder spool valve then. Alterneate: a power beyond center spool valve for the pump outlet line. This was described in a previous posting topic. Flow not used by the cylinder circuit continues on to the motor. The motor would turn whenever engine is running unless there is already a motor on/off or F/R valve of some sort.

Caution: you need a relief valve at the pump either way. The selector goes through blocked center condition when shifting, and the power beyond spool needs a RV for cylinder protection. usually that is built into the valve inlet section.

kcj

 

[fluidlearning]

Thanks, I have a video (mpeg) clip of a backhoe I built, I would like to send you, or include in post if that is possible. Any idea's how?

 

[kcj]

I have no idea, I am not very computer savvy. On another board (motorcycles) I frequent, they post a link, clok on link and it goes to the video.
kcj

 

[fluidlearning]

kcj,
To see the video go to

http://www.qsl.net//wd5bjw

and click on backhoe, that should download it. It was built with a positive displacement pump (old pressure washer), using glycol as the fluid. It was driven off the lawn mower engine, which was also coupled to the transmission. The problem with the unit was, even with my weight, it was not heavy enough to drive the bucket deep into the ground. I still have the arm and plan to incorporate it into the tractor when I finish it.

 

[kcj]

I cn only see a very small clip, but looks nice. Looks like the trailer style one that is sold by Northern and other outlets.
'Glycol' meaning water glycol hydraulic fluid? we use the Citgo water glycol fluid up to 5000 psi in heavy duty systems. The older style WG fluids are only 1000-2000 psi rated, but for any one faced with fire issues or environmental reasons, the premium new WG fluids are good stuff.

Spent a lot of time in Jefforson parrish LA in my past.
tks for info and good luck on the tractor prject.
kcj

 

[fluidlearning]

A quick question. I have a dual pump (cannot find specs on it) but of course one has a larger displacement than the other. Since I am using one pump to drive another pump as a motor, should I use the first stage of the dual pump as a charge pump and feed the 2nd stage of the pump, getting higher pressure and lower volume or should I just go straight from the larger Primary pump to drive the motor?
Since I don't have specs, here are my obseverations of the pump and motor. The primary pump has a larger displacement than the pump I am going to use as a motor, The secondary pump (on top of primary pump) has less displacement than the motor. The pump number is Bosch D86 1518 222 000, but I have not been able to find anything about that number.
Thanks, Bob

 

[kcj]

Charge pump is not done for higher pressure. Max pressure is limited by the mechanical strnegh of gears, bearings, etc. of the larger pump and you can't exceed without problems. Charge is done to get the fluid into the inlet without cavitation. Or for holddown reasons of a hydrostatic piston pump. Charge requires an external relief valve set depending on the circuit, to 15 psi up to 400 psi.

You can't charge a larger pump with a smaller pump, in fact it would be worse than atmospheric inlet as the larger pump cannot pull enough fluid through the smaller one.

Some issues with using a pump as motor: The shaft seals are typically connected to inlet port on a pump, expecting no line pressure at all. On a motor, either a separate case drain is required, or a high ressure shaft seal with a shuttle valve to drain to the lower of two ports. If you are reversing this direction, or if you have two motors in series, or having any meter out load control, the downstream port may be toohigh for the shaft seals.

You didn not say if this dual pump is a two stage log splitter pump, or simply two independant gear sections in one housing. Dual pump sections can be used as you wish for size and pressure, but the two stage (kickdown) pump will give you fits. the large dispalcement section will shift in & out as pressure changes. Tow stage pump must alwasy push against a postive resistive load. Any motor or overrunning load will cavitate badly when the pressure rises enough to unload the large section. Motor is running at rate of say 20 gpm, when suddenly pump shifts down to 5 gpm. Motor cavitates and coasts, pressure drops, pump comes back onto 20 gpm, etc. Chatter, cavitation, line shock, etc can happen.

There is a limit to what we can help in patched together stuff without vendor data.
kcj

 

[fluidlearning]

Well, I got it all hooked up, basically like this Tank-pump-(guage)motor-return to tank. I found that the motor would not always start on it's own, after a push it took off running. With the gas engine at idle and the motor stalled, the pressure was about 300 PSI, I could feel the outlet hoses tense up, I give the motor a push and it took off spinning and the pressure drops (lower that the guage could read)this is what I expected. The problem is the motor is not self starting and will stop under the slighest load. Note, I am using a gear pump in reverse- I am putting pressure into the small port (normally out) and the large port returns to the tank. I tested the motor in this way with compressed air and it turned as needed. This is a Parker pump, and on their website I found instructons on reversing the direction when using it as a pump, but it seems these modifications are not necessary as I am using it with reverse flow, am I missing something?

 

[kcj]

300 psi to start a mtoor is high but not out of the question. I would expect 50 or 100 psi.

More signficantly: why is the pressure only 300 psi? if the motor is stalled and fixed pump is running, flow is produced, and pressure is a result of flow across a load. Since it is not turning a motor, it has to be leaking big time somewhere. Either pump is bad, motor is bad, relief valve is too low (you did not mention there was RV in circuit) or a big leak somewhere.

Assuming a gear pump, if the bearing drain cavity is seeing pressure side connection, the pressure can apply to the side plates and bind up the gar set.

kcj