GPM Question

[NWOutlawMX]

I've been looking at excavators to buy and have rented a couple to test. The Bobcat 337 was about as small as I'd want to go. It is 48hp with a GPM of 47. Found a Komatsu 75uu2 which is what I am looking for (16-22000 lbs with steel tracks) and it is 68 hp but only has 44 GPM.

My question is would the Komatsu have stronger hydraulics with the higher hp even though the GPM is lower, the Bobcat struggled picking up some of the rocks I need to move with a 47GPM. I would think that the higher GPM the better hydraulics regardless of HP.

I've heard that Bobcat has some of the best lifting capabilities of mini's, even though I'm looking for a 20K lb machine

 

[J_J]

Power is a function of pressure. If both pumps produce the same pressure and use the same cyl, the power to the hyd will be equal. Now having said that, the mechanical setup may give one unit an advantage over the other.

GPM dictates the speed of operation.

A 3 in cyl at 3000 psi will produce about 21,206 lbs.

A 4 in cyl at 3000 psi will produce about 37,699 lbs.

 

[xtn]

GPM does not effect hydraulic power. PSI does. The magic formula for how much a cylinder can push/pull depends only on pressure and cylinder diameter.

GPM only effects the speed that the cylinders will move.

xtn

 

[J_J]

47 GPM at 3000 will require about 97 HP.

If the figures you quoted are correct, the the pressure has to be less. About 1500 psi. So your overall lifting capacity will be less

Still, comparing 1500 psi to another machine with 1500 psi, using same cyl, power would be the same except for the mechanical setup.

 

[SPYDERLK]

GPM does not effect hydraulic power. PSI does. The magic formula for how much a cylinder can push/pull depends only on pressure and cylinder diameter.

GPM only effects the speed that the cylinders will move.

xtn

I guess you mean force. Otherwise it would be just as true to say the reverse.
larry

 

[NWOutlawMX]

Interesting, So it would be reasonable to believe that the Komatsu would be stronger as it is a higher class machine and more HP so it should have a pump that produces more pressure. It would be the same swing speed, arm and bucket speed etc.

Thanks for the quick reply, for the condition and price, it's not going to be around for long.


Bobcat - 48 hp @ 2500rpm 47 GPM

Komatsu - 68 hp @ 2300rpm 44 GPM

 

[J_J]

It is not pump size that dictates pressure, but the material and design of the pump.

This is a 3000 psi pump, .194 cu in displacement. 0.194 cu in BARNES 1003044 HYD PUMP

This larger pump , 3.17 cu in displacement, at 2500 psi. http://www.surpluscenter.com/Hydrau...cu-in-PRINCE-HYD-PUMP-SP25A52D9H2L-9-8031.axd

2500 psi, 4 in cyl, = 31,416 lbs

3000 psi, 4 in cyl, = 37,699 lbs

So smaller pump in size and displacement, with the same cyl will develop more power.

Essentially, you have to pick up the same load, same distance.

Again, GPM affects speed of operation.

 

[MHarryE]

Are you sure your hydraulic pump flow is all from one pump? Haven't looked at your excavator's specs but even my little Cat 2 ton excavator has a double section pump. Like JJ pointed out if the excavator at which you are looking had a single gear pump that put out 47 gpm at 3,000 psi it would kill the engine. If, however, you have variable displacement piston pump(s) you could get the higher pressure but at a lower flow rate. Out of curiosity I looked at the Caterpillar 320D specs and they give cylinder diameters and pressure limits of each system. It uses 2 piston pumps and at maximum flow and pressure of both systems the engine would need to be more than twice as powerful as the one installed. From my experience the only way to know for certain how the machine will work for you is to try it and compare. Excavators are too dependent on lever arms of the various systems (bucket, stick, boom) to even get an accurate comparison knowing cylinder diameter and max pressure.

 

[glastron23]

Hi guys the Bobcat is 49HP@20GPM@3750PSI with a pressure compensated/load sense/torque limited piston pump ,they also have a 20GPM@3000PSI auxiliary system off the main pump. 4 inch boom cylinder/3.5 inch arm cylinder
the Komatsu is running 68HP one pump 44GPM @3840PSI is a variable piston for the boom/arm/bucket and the second is a fixed 15 GPM @3000psi displacement pump for the swing and blade

 

[glastron23]

here is a nice video on the a10 pumps, if you ever wonder just how they are built PC75UU 2 A10V17 - YouTube