Is the 3pt hitch holding capacity greater than it's lifting capacity?

[rfc143]

Kubota 5240; read the manual on the 3pt hitch weight lifting limitations. But if you place a load on the hitch after it's raised, can it hold more than it can lift? Is there a ball check or other device in the hydraulic system that would allow it to hold the load?

 

[mikester]

Rated capacity is rated capacity. Sure you can overload it beyond it's rated capacity limits. Why would you want to? Eventually you risk stripping the 3PH lift splines and mechanical failure. Search 3PH backhoe.

 

[Gale Hawkins]

When I have done that in the past the failure was typically at the hitch itself. I had a Zetor that when you pulled up the lift the arms were coming up or the front end was coming up or the 3PH was collapsing but they use 90 weight oil not the 10-20 weight UTF.

Most stuff will just go into bypass mode and some too quickly to my liking.

 

[oosik]

Mikester said it - the rated capacity is what it can lift and hold. Damage from overloading the 3-point system can end up being expensive.

Point being - if you need to lift more that your 3-point is rated for - you need a bigger tractor or some other system for lifting.

 

[Dave M7040]

Everyone is forgetting to mention the safety valve which many makes, including Kubota have in the hydraulic circuit for the 3 pt hitch.

The safety valve will not allow you to put too much weight on a 3 pt hitch which is already at the relief valve pressure. It will open and the hitch will go down.

The relief valve protects the pump. The safety valve, which is rated at considerably higher pressure, is to protect the 3 pt assembly under conditions such as a heavy implement bouncing up and down as you travel.

The safety valve has to be set up when removed from the tractor. You use a pump made to test fuel injectors so you can reach it higher pressures.

See this typical 3 pt hitch hydraulic circuit.

Dave M7040

 

[rfc143]

Everyone is forgetting to mention the safety valve which many makes, including Kubota have in the hydraulic circuit for the 3 pt hitch.

The safety valve will not allow you to put too much weight on a 3 pt hitch which is already at the relief valve pressure. It will open and the hitch will go down.

Dave M7040

Thank you Sir. This is excellent information. I'm building a crane attachment, have done the math on capacity at the load distance from the hitch; have no intention of "pushing the envelope", but wanted to know what would happen if I exceeded the capacity. This information tells me everything I need to know.

 

[Dave M7040]

Thank you Sir. This is excellent information. I'm building a crane attachment, have done the math on capacity at the load distance from the hitch; have no intention of "pushing the envelope", but wanted to know what would happen if I exceeded the capacity. This information tells me everything I need to know.

I am very pleased that I was able to help and further that you are taking the time to design your crane to be safe.

Although the 3 pt hitch hydraulic system has a safety valve + a relief valve, a FEL does not.

When building hydraulic circuits, you always need to be aware of a phenomenon called hydraulic amplification.
This occurs when the pressure in 1/2 of a double acting cylinder can rise far above the relief valve pressure.

This schematic was made for me by 100 td, a frequent poster. He used a hydraulic modelling program to double check my manual calculations.

What is shown is a situation which has happened to a number of owners. The retract line quick coupler on a double acting cylinder is not fully seated. The fluid in the retract side is trapped and the push side is at a max pressure of 900 psi. ( just an artificial number) the pressure in the retract side of the cylinder jumps by 50% to 1,400 psi.

I offer the above to illustrate how matters can be more complicated than many of us realize and we all need to have large factors of safety in whatever we build.

Dave M7040