GPM Question

NWOutlawMX · Dec 6, 2013

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 · Dec 6, 2013

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 · Dec 6, 2013

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 · Dec 6, 2013

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 · Dec 6, 2013

xtn said:
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 · Dec 6, 2013

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 · Dec 6, 2013

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 · Dec 6, 2013

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 · Dec 7, 2013

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 · Dec 7, 2013

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

J_J · Dec 7, 2013

Even with those figures, there doesn't seem to be enough Hp to run everything.

However if he only runs the 20 GPM pump, it would take about 51 HP.

If he only runs the the other 20 GPM @ 3000 psi, that would take about 41 HP.

Total is about 92 HP if both pumps are running at max.

If the machine is only using low GPM's from each pump, then the engine could run without stalling.

The Komatsu pump running at max and at max pressure would require about 116 HP.

You think they are fudging on the ratings?

glastron23 · Dec 7, 2013

JJ, your thinking in fixed displacement pump requirements. With the a1 pumps, pressure/flow compensated/torque compensated pumps, you can have high flow/low pressure, as the stain on the hydraulics increases the pump will start to shift the swash plate from high flow/low psi to a lower flow/higher psi. this requires less horse power to run. as far as having the extra pump for the blade/swing its highly unlikely to run both at the boom and blade at the same time. check this from Rexroth The compact variable pump A1VO - YouTube

J_J · Dec 7, 2013

I understand that, but at some time, the load will call for max GPM and pressure.

I have a variable speed pump on my Power-Trac, and when I jump on it, it will load up the engine, and if I am doing anything with one of the other pumps such as the gear type PTO pump, I have to back off the pedal to recover.

mjcountrybumpkin · Dec 7, 2013

The 75 will bury the bobcat that simple. Flow , hp , pressure. They do not matter comparing these 2 machines. No matter home much lift capacity you have if you don't have enough counter weight your going no place quick. If you need or want the bigger machine then that's the way to go.
Happy digging

npalen · Dec 7, 2013

Probably should distinguish between the terms force and power.

glastron23 · Dec 7, 2013

npalen said:
Probably should distinguish between the terms force and power.

YUP ..
JJ with a compensated pump you will not get full GPM and MAX psi at the same time, as soon as the swash plate piston starts to see high PSI it has all ready starts to destroke .. it has to

Arc weld · Dec 7, 2013

Can almost guarantee the Komatsu is a much more refined and sophisticated machine than the Bobcat. Komatsu makes some of the most advanced excavators in the world...Bobcat, not so much. There are variable displacement pumps and then there's load sensing, pressure compensating and a host of other advances in hydraulics.

NWOutlawMX · Dec 9, 2013

Great info, more in depth research leads me to the Komatsu being a grey market machine - nothing to do with hydraulics - but in the final decision. Here's a picture of the rock I tried to pick up and the stump I need to tackle. I like the mobility in tight quarters with the mini's but on the other side (as you can see) I need something in the 20-30K lb range. There's a lot more of these stumps and rocks. There was another rock slightly smaller that I moved (slowly) but don't know if i could pick it up and put in a truck.