General question about tractor hydraulics

[jmc]

I was looking at iron workers (flat metal punch, shear, bend, etc) which are basically hydraulic cylinders attached to various mechanisms. The salesman pointed out that the hydraulic pump is designed to pump at a high rate until the cylinder meets resistance, then it pumps at a lower rate, higher pressure. He didn't say if it was just a 2 speed pump or continuously variable. It's a great way to cut the cycle time on a long cylinder stroke if the load is light. Seems like it would be good for tractors too, considering how often loaders are raised and lowered with very little load. Anyone know why tractors don't have a variable displacement pump?

John

 

[slowrev]

what you saw was probably a 2 stage pump, common on log splitters.
I am not sure if the 2 stage pump approach would be good on a tractor or not, I sort of doubt it.
Variable displacement pumps are used on closed hydraulic systems , mostly on larger tractors and equipment. Most CUT sized tractors use fixed displacement open center systems.

Ben

 

[SkyPup]

One reason is that variable displacement pumps would not provide the same speed of operation, the hydraulic cylinders would all slow down considerably when under any load and speed up without any load. That could become a serious safety problem with a loaded FEL...

 

[jmc]

Slowrev, Skypup,

Well, if they use it on large tractors, the hydraulics must be manageable. Seems that excessive drop speeds could be avoided with flow restriction and the operator/joystick. I was thinking of a pump with a normal flow at high pressure and maybe 30% more at low pressure. Nothing too radical. Saving 30% in lift time would seem pretty quick. It probably comes down to cost.

John

 

[MadReferee]

</font><font color="blue" class="small">( It probably comes down to cost.)</font>

Exactly. Having a fixed displacement pump that works directly of the engine is much cheaper and easier to implement than having to get a variable displacement pump to work correctly with everything that can be connected to it.

For CUTs in the <$30,000 range cost is a significant factor in purchase choice whereas in the >$50,000 tractors it is not that big of a deal. From what I understand the larger (>100 hp) JD's use variable displacement pumps.