Engine speed and 3PH lifting capacity...

[fredhargis]

I haven't asked a really stupid question for a while, so though I'd bring up this one: is the lifting capacity of the 3 PH related to the speed the engine is running? I ask because I have one attachment that is one the edge of my tractor not being able to lift it (Kubota B2920). This thing (an aerator) with blocks weighs about 850#, and is within 24" of the ends of the arms. Adding one block (33#) makes it too heavy to pick up, at least at about 1600 rpm. So if I ran it at 2200 RPM, and opened the 3 PH speed valve will it then lift the heavier weight? I'd try this out, but it's a lot easier to ask here and make a fool out of myself. BTW, I'm not interested in adding more weight to the aerator...more of a curiosity question.

 

[suprnova]

According to the new (not sure what year yours is) B2920 spec sheet, the pump capacity is 8.3GPM, providing a lift of 1356lb @ lift point/1058lb at 24" from lift point. If you're certain your aerator only weighs 850, it may be safe to assume that 1600rpm doesn't provide the maximum pump capacity. I don't know the information for that particular pump though to figure out where maximum capacity is achieved.

 

[corey9212]

I haven't asked a really stupid question for a while, so though I'd bring up this one: is the lifting capacity of the 3 PH related to the speed the engine is running? I ask because I have one attachment that is one the edge of my tractor not being able to lift it (Kubota B2920). This thing (an aerator) with blocks weighs about 850#, and is within 24" of the ends of the arms. Adding one block (33#) makes it too heavy to pick up, at least at about 1600 rpm. So if I ran it at 2200 RPM, and opened the 3 PH speed valve will it then lift the heavier weight? I'd try this out, but it's a lot easier to ask here and make a fool out of myself. BTW, I'm not interested in adding more weight to the aerator...more of a curiosity question.

It should be able to lift that with no problems if the weight is only 850# when I had my BX2660 it lifted 1200# square bales with a 50# bale spear and moved them off of the 3 point hitch easily.

I do know the rpm's increase speed and response so I would imagine it will increase lift capacity as well.

 

[k0ua]

In my "opinion" it should not matter if you are at idle or if you are at full throttle, the pressure should be able to be built to maximum pressure up to the relief valve pressure. Hydraulic pumps don't make pressure anyway they make fluid flow. the pressure is a function of restriction. In other words if you put a pressure gauge on the output of the hydraulic pump you will see very little pressure, just the small restriction of the hoses to the flow of the fluid. now when you move a control valve to push the flowing stream of fluid to look into a cylinder like the one in your 3pt. you will now build up a head of pressure and the cylinder piston will start to move and lift the load. The load will lift slower at a lower RPM because the flow rate of the pump will likely but much less at a lower RPM than a higher RPM. but the pressure that can be achieved will likely be the same. Now if the pump is worn, and has internal leakage, I "suppose" it may not be able to deliver maximum pressure as set by the relief valve until the RPM is increased enough to increase the flow rate to overcome any leakage. Keep in mind my knowledge is theoretical, not practical in this field, and I hold no qualifications in this field whatsoever. But now you have heard my "opinion"..

James K0UA

 

[Don87]

I didn't see anything stupid about the question. But I would like to know the answer.

Try it and let us know.

 

[SPYDERLK]

In my "opinion" it should not matter if you are at idle or if you are at full throttle, the pressure should be able to be built to maximum pressure up to the relief valve pressure. Hydraulic pumps don't make pressure anyway they make fluid flow. the pressure is a function of restriction. In other words if you put a pressure gauge on the output of the hydraulic pump you will see very little pressure, just the small restriction of the hoses to the flow of the fluid. now when you move a control valve to push the flowing stream of fluid to look into a cylinder like the one in your 3pt. you will now build up a head of pressure and the cylinder piston will start to move and lift the load. The load will lift slower at a lower RPM because the flow rate of the pump will likely but much less at a lower RPM than a higher RPM. but the pressure that can be achieved will likely be the same. Now if the pump is worn, and has internal leakage, I "suppose" it may not be able to deliver maximum pressure as set by the relief valve until the RPM is increased enough to increase the flow rate to overcome any leakage. Keep in mind my knowledge is theoretical, not practical in this field, and I hold no qualifications in this field whatsoever. But now you have heard my "opinion"..

James K0UA

Nothing wrong with theory.
All pumps will have internal leakage. Worn will be worse. Hot, thin hyd oil will leak faster too. ... When cool the system will probably develop full relief set pressure at lo idle. When hot the increased internal leakage will likely prevent reaching full pressure at idle even on a new system. Now at 1500 rpm you should be able to get full pressure even hot. Things might be very slow because the hi press leakage flow will use a lot of the pumps potential and there wont be a lot of net/output flow. More engine speed will get you the hyd speed back. I would only start getting concerned if I couldnt get full relief pressure until at least 1500RPM. Even at that there should still be a lot of life left in the pump.
larry

 

[k0ua]

All pumps will have internal leakage. Worn will be worse. Hot, thin hyd oil will leak faster too. ... When cool the system will probably develop full relief set pressure at lo idle. When hot the increased internal leakage will likely prevent reaching full pressure at idle even on a new system. Now at 1500 rpm you should be able to get full pressure even hot. Things might be very slow because the hi press leakage flow will use a lot of the pumps potential and there wont be a lot of net/output flow. More engine speed will get you the hyd speed back. I would only start getting concerned if I couldnt get full relief pressure until at least 1500RPM. Even at that there should still be a lot of life left in the pump.
larry

Thanks for providing some more dimension to the explanation of pump leakage, and considering the temperature of the fluid. Something I didn't even think about. I know much more about electronics than hydraulics, but thanks to TBN, I have learned a little!

James K0UA

 

[fredhargis]

Thanks guys. I may try it out just to see ...the tractor is 2010, and now has 140 hours. It's hard to imagine any internal problems with the pump; but I guess stranger things have happened.

 

[SPYDERLK]

It could be the relief valve is set low.
larry

 

[daugen]

I'm not sure there is a direct relationship here but since both the FEL and three point hitch run by hydraulic
power, then I would expect some commonality of response. I've never had a problem picking up my new very heavy LP
84 inch mower with my Kubota at fairly low rpm. Personally my experience with the FEL grapple picking up heavy trees was that
I needed between 1800-2000 rpm to pick up the heaviest logs. Maybe it would have picked them up but just more slowly at lower rpm, but several times I went nowhere at a lower rpm and when I goosed it up to 2000 rpm, I clearly picked up more capacity.
But maybe my tractor's sweet spot is 1800 rpm, I don't know.

Add the hydraulic demands of an HST trans to the mix while using the FEL or 3PH and it's interesting to see how my tractor reacts, slows down usually, if I increase the demand on the system. Might make an interesting "science project" to test it, both with light and full capacity loads.
There are a lot of electro/hydraulic nannies operating on the Kubota, though I have managed to stall it several times. Always when the rpm was fairly low. And yes I thought I had stall guard on. I'm sure op error, new tractor, still learning its personality.

 

[MHarryE]

The relief valve on your system is a direct acting relief. A spring presses on a poppet and when the pressure is high e ought, it lifts the poppet allowing oil to flow back to tank. If you increase the flow rate it needs to lift the poppet higher and therefore the pressure increases. If you look at hydraulic supplier catalogs they often have a flow vs. pressure chart. So you will be able to lift more at high idle, but will it be enough?

 

[GManBart]

The load will lift slower at a lower RPM because the flow rate of the pump will likely but much less at a lower RPM than a higher RPM. but the pressure that can be achieved will likely be the same.

In a closed system like this (fairly simple setup), pressure is directly related to flow rate. Higher flow rates equal higher pressures, because the amount of resistance/restriction, (or potential resistance) remains constant. In other words, if you raise the 3pt lever, the valves are at the same setting regardless of flow rate. If you increase the flow rate, the pressure will increase, up to the relief setting. In a complex system it would be possible to vary valve setting/controls based upon flow rate, and have multiple pressure relief settings, etc, but that's likely far beyond anything seen in normal tractors. If the pump can produce max flow at idle, then the system would almost always been running with the relief open somewhat. It would be interesting to see what RPM produces max flow rate, but I doubt we'll ever be able to figure that out. I'd guess something like 50-60% of rated speed for max low rate.

Anybody know if the Nebraska tests covered that?

 

[OldMcDonald]

I am a McDonald not a McAnic, so I learn something new almost every time I lurk around TBN, but special thanks to you all for today's lessons.

 

[adirondackmtnman]

I am a McDonald not a McAnic

HA! That's clever!!!

Sent from my LGL35G using TractorByNet

 

[Stonehaller]

The short answer is running the engine at rated or max engine RPM will give you slightly more lifting capacity.

Your pump is positive displacement so each revolution of the pump generates the same volume. The faster the pump turns, the more flow it generates. The more flow, the faster a cylinder will move. Pressure equates to how much force it takes to move a cylinder. The more load on the cylinder the higher the pressure. The relief valve limits the maximum pressure and therefore the maximum load the cylinder can move. The relief pressure is mostly independent of the flow. Two good points raised here by others (SPYDERLK & MHarryE), internal leakage takes some of the pumps output and the poppet relief valves have some non-linearity with flow and oil temperature. High flow and cooler oil will give slightly higher relief pressures. To get the maximum lift capacity, run the engine at rate speed and do the heavy lifting before the oil gets hot.

If suprnova is correct that your lifting capacity is 1058 lbs at 24 inches sand you cannot lift 883 lbs, then you may have other issues. But first, is the center of the 883 lbs within 24 inches? (Pictures are nice) If the center of mass of your attachment is beyond the 24 inches, then you may be exceeding the lift capacity.

Are there multiple holes for the lift rods to connect to the hitch arms? If there are, you may need to move them further from the tractor. That will reduce lift height range, but increase lift capacity.

Your signature says you have a backhoe. Do you switch a valve or connect a hose loop when the back hoe is off? Double check that you have that correct and quick disconnects are fully seated.

Your relief pressure could be low. You would need a gage and a fitting to connect the gage to your backhoe supply quick disconnect or one of the FEL quick disconnects. If it is low, you will need to get the relief valve adjusted.

You could have an internal leak in your 3 point cylinder or valve. Will it hold up a load with the tractor off? Note that the 3 point speed control only affects lowering speed and has nothing to do with raising or lift capacity.