Hydraulic drive for small tractor
Hydraulic drive for small tractor
(OP)
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.





RE: Hydraulic drive for small tractor
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.
RE: Hydraulic drive for small tractor
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.
RE: Hydraulic drive for small tractor
with variable, reversible, piston pump you can do hydrostatic drive, but thats beyond the parts you have
kcj
RE: Hydraulic drive for small tractor
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
RE: Hydraulic drive for small tractor
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
RE: Hydraulic drive for small tractor
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
RE: Hydraulic drive for small tractor
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?
RE: Hydraulic drive for small tractor
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
RE: Hydraulic drive for small tractor
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.
RE: Hydraulic drive for small tractor
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?
RE: Hydraulic drive for small 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
RE: Hydraulic drive for small tractor
RE: Hydraulic drive for small tractor
kcj
RE: Hydraulic drive for small tractor
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.
RE: Hydraulic drive for small tractor
'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
RE: Hydraulic drive for small tractor
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
RE: Hydraulic drive for small tractor
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
RE: Hydraulic drive for small tractor
RE: Hydraulic drive for small tractor
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
RE: Hydraulic drive for small tractor
I think you are correct, by the way the motor was acting the pressure must have been equal on both sides of the gear, so that it would not start, and when I gave it a push the flow started but when it became loaded, the pressure equalizes around the gears and it stalled.
I found instructions on the web to reverse the motor (when it was a pump) maybe this will direct the flow without causing the problem of binding up the gear set.
I am hunting for a 'real' motor to complete this project. Thanks
RE: Hydraulic drive for small tractor
Also, same comments. If no RV, then there is a reason the pressure only builds to 300 psi when pump is turning but motor is not. Something is seriously worn.
RE: Hydraulic drive for small tractor
I ran a test and with the motor stalled, quite a bit of fluid did flow through it. I cannot see how it can get that much flow past the gears and wearplates, I dissambled it and do not see anyway it can pass that much flow.
So, I'll keep looking for a motor.
Meanwhile can I test the system by putting a loop on the spool valve cylinder ports (it does have an RV), and take the output (power beyound) back to the tank, bypassing the motor? Then can I operate the valve which will go through a blocked state and expect to see higher pressures, until the RV operates? This will test the pump and also check what the RV is set at.
Thanks for your patience.
RE: Hydraulic drive for small tractor
I don't follow your second valve description. To check RV settings, just shift the spool against the plugged port and force all pump flow across the RV.
However, the RV should be first in line next to the pump, not after the motor. Caution: full pressure applied to both ports of themotor may or may not daucse damage, depending on motor ratings. It puts twice the thrust loads on bearings etc.
The RV in the spool valve should be set lower than manin RV, if you with the cylinder circuit limited at a lower pressure
RE: Hydraulic drive for small tractor
Let me see if in underatand the function of the RV (standalone) I used one on the first backhoe I built. It basically was a spring valve that would open when it reached the pressure setting and bybass the output of the pump back to the inlet. It was inserted into a tee in the pump outlet that fed the cylinder valves.
Aside from the motor functions.
The spool valve I am using has an inlet and outlet port and two smaller ports for the cylinder that I am going to control, it has an adjustment on the side that I assume is an RV and will bypass any excess pressure through the valve and out the outlet port. (Maybe I should put an RV at the pump outlet).
The motor should be in about two weeks. I will post pictures of the tractor and hydraulic systems to my web page later this week so you can see what I am talking about. Don't laugh, I have to make do with what I have, this is a hobby for me
RE: Hydraulic drive for small tractor
Need an RV at the pump, or inlet to motor valve, or anywhere ahead of the first load. If the motor stalls, the oil never reaches the RV in the second valve for the cylinders.
kcj