Tractor Sizing How Much Can A Sub Compact Tractor lift?

[dragoneggs]

I have a Kubota loader rated at around 2500 lbs at the pins. (not a scut, sorry, but the principle is the same). I have lifted with it a calculate-able load at around 3000 lbs centered about 2' forward of the pins. Kubota is conservative. It is about geometry, cylnder diameter, rear weight and available hydraulic pressure. The remainder is smoke screen snake oild marketing.

I will agree with that. Unlike some sales specs (in general) Kubota's numbers seem easily defendable.

 

[Egon]

Are not hydraulic pumps of a positive displacement type which means the pressure will keep rising when the pump turns unless some other device will divert flow?

 

[check]

I will agree with that. Unlike some sales specs (in general) Kubota's numbers seem easily defendable.

That doesn't seem to be true with the BX 25. The rated maximum FEL lift is 745 pounds, but the real lift max in practice seems to be closer to 500 pounds.

 

[gladehound]

Are not hydraulic pumps of a positive displacement type which means the pressure will keep rising when the pump turns unless some other device will divert flow?

Yes. When you are not operating anything hydraulic on your tractor the system is open and fluid constantly returns to the reservoir. Very little pressure builds because the pump can freely move the fluid.

Once you start lifting or curling or both, the fluid is diverted to the cylinders and the cylinders move at a rate that is determined by the pump flow and the size of the cylinders.

Resistance to moving the cylinders creates pressure on the hydraulic system. As resistance increases, pressure in the system increases. Resistance increases as a result of changing loader geometry as the loader height increases with a given load, or will be increased with heavier and heavier loads at any given loader position.

If the load at a given position causes enough resistance to cylinder movement to generate a pressure exceeding the system relief setting, fluid once again returns to the reservoir so that the positive displacement pump does not over pressurize the system and blow something in the system.

At the point that the relief valve opens, you have reached your max lift at a given loader position. This max can be increased by increasing the relief valve setting or increasing the size of the cylinders.

 

[LD1]

:mur::mur::mur::mur::mur::mur:

If someone has a gauge on their lines as you do and:

1. Pumps produce the pressure your gauge shows but

2. Your pressure goes higher at height

3. Then either
a. Something else is creating that pressure (static support from fel arms in more vertical position ???) or
b. Something allows the pumps to create more pressure because of weight relief on the pumps as the load goes to more vertical fel arm support.

What else can explain greater pressure being created. Gravity at 6 foot is the same as gravity at 3 foot . . but static support of gravity is much improved at 6 foot compared to 3 foot because of fel arm positions . . Isn't that true?

Either you or I can hold more weight up 30 inches over our head than we can at a 3 or 4 foot height 30 inches away from our body. Its the same weight but much differently supported. Right?

Pressure goes higher as height goes higher BUT you still cannot exceed what the relief is set for.

IE: my relief is 2600psi.

raise the loader 1' off the ground with a load that requires 2400psi it lift. Darn near my limit.

As I continue to raise, at 2' height my gauge might be reading 2500psi. Then at 4' i might quit lifting all thogether because it NEEDS more than 2600psi to lift any higher.

THUS, you can either say the higher you lift, the more pressure is needed. OR the higher you lift, the less weight you can lift. Same thing.

ITs not rocket science, its accepted by every MFG, its a fact. loaders have more power at lower heights than at max height. Hundreds of stories on here of only being able to lift something a few feet when working near their max, many people with gauges like mine that show the same thing.

To me this is as silly as debating 2+2=4. So I am done. I have explained it many times and my words just get twisted to somehow magically pumps making more pressure than relief

 

[jenkinsph]

Yes. When you are not operating anything hydraulic on your tractor the system is open and fluid constantly returns to the reservoir. Very little pressure builds because the pump can freely move the fluid.

Once you start lifting or curling or both, the fluid is diverted to the cylinders and the cylinders move at a rate that is determined by the pump flow and the size of the cylinders.

Resistance to moving the cylinders creates pressure on the hydraulic system. As resistance increases, pressure in the system increases. Resistance increases as a result of changing loader geometry as the loader height increases with a given load, or will be increased with heavier and heavier loads at any given loader position.

If the load at a given position causes enough resistance to cylinder movement to generate a pressure exceeding the system relief setting, fluid once again returns to the reservoir so that the positive displacement pump does not over pressurize the system and blow something in the system.

At the point that the relief valve opens, you have reached your max lift at a given loader position. This max can be increased by increasing the relief valve setting or increasing the size of the cylinders.

Good description of the more common open center hydraulic system. I agree with this post.

 

[k0ua]

Guys lets try one more time.. Pumps don't make pressure, they make flow. Or to say it another way, the fluid that comes out of the outlet port of a pump is not pressurized. Only when it meets an obstruction does the fluid pressure rise. Until you get this concept firmly planted in your head and accepted, only then will you start to understand hydraulics.

 

[k0ua]

Yes. When you are not operating anything hydraulic on your tractor the system is open and fluid constantly returns to the reservoir. Very little pressure builds because the pump can freely move the fluid.

Once you start lifting or curling or both, the fluid is diverted to the cylinders and the cylinders move at a rate that is determined by the pump flow and the size of the cylinders.

Resistance to moving the cylinders creates pressure on the hydraulic system. As resistance increases, pressure in the system increases. Resistance increases as a result of changing loader geometry as the loader height increases with a given load, or will be increased with heavier and heavier loads at any given loader position.

If the load at a given position causes enough resistance to cylinder movement to generate a pressure exceeding the system relief setting, fluid once again returns to the reservoir so that the positive displacement pump does not over pressurize the system and blow something in the system.

At the point that the relief valve opens, you have reached your max lift at a given loader position. This max can be increased by increasing the relief valve setting or increasing the size of the cylinders.

Excellent description.

 

[Egon]

Thankyou Gladehound.

The pressure going up as the load is lifted just may be due the increase in effective length the load lift arm goes thru as it is raised.

 

[AxleHub]

Yes. When you are not operating anything hydraulic on your tractor the system is open and fluid constantly returns to the reservoir. Very little pressure builds because the pump can freely move the fluid.

Once you start lifting or curling or both, the fluid is diverted to the cylinders and the cylinders move at a rate that is determined by the pump flow and the size of the cylinders.

Resistance to moving the cylinders creates pressure on the hydraulic system. As resistance increases, pressure in the system increases. Resistance increases as a result of changing loader geometry as the loader height increases with a given load, or will be increased with heavier and heavier loads at any given loader position.

If the load at a given position causes enough resistance to cylinder movement to generate a pressure exceeding the system relief setting, fluid once again returns to the reservoir so that the positive displacement pump does not over pressurize the system and blow something in the system.

At the point that the relief valve opens, you have reached your max lift at a given loader position. This max can be increased by increasing the relief valve setting or increasing the size of the cylinders.

Glade, now that is a nice description.

Now if you could explain why so many care about maximum height weight. If I'm interested in picking up a heavy load and transporting it to another location . . Im going to transport at 1 foot or 2 foot or 3 feet above ground and then setting it in a truck or in a building or wherever. Nobody transfers a heavy load at max height . . They might pivot a few feet to dump at max height but thats it.

Yet evey brochure and most posters are always concerned about weight handling at a height they use the least.