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Discussion in 'Heavy Equipment & Pavement' started by AWJ Services, Oct 13, 2013.
In theory I am rich from doing this work. Reality is I am not!
One of Factory tech guys gave the number of 15gpm @ 3000 psi for the TB260. He said that is from a flow meter. If it will do that then it will be well above the pack for a 6 ton machine.
That machine is out of spec on the high side.. Not sure what to make of that. That's would line up with the average of 2500 I was trying to remember for what my machine was putting out. That's a huge jump from 382 to 1000 and only 2 gpm difference.
I don't see how 8.6 hp grinds up 12" logs though. :}
Here is the plot for the data provided for the T770. Notice the linear drop in pressure from 0 - 31.2 gpm? The pressure decrease over this range is only 800 psi. Clearly this is not exponential as you declared earlier. Would you like me to plot this on a log graph?
I don't have the schematic for the T770 but apparently from the brochure it runs gear pumps. A gear pump is a fixed displacement pump. The aux flow apparently has two configurations: variable flow and max flow. Which mode was the aux circuit running in? What was the rpms that the flow was taken at?
Get me the hydraulic schematics and I'll tell you what is going on.
from 36.5 to 40.4 is 1700 psi drop. I call that big. maybe it does mean anything but it's a bigger drop than the linear drop from 0 to around 32.
The machine is out of factory spec by +/- 4 gpm. Don't rpm's affect gear pump performance?
Bobcat says their hydraulic hp on a high flow machine is about 75-76 hp.
Cat used to state 78 hp. I felt like my Bobcat had my hydraulic hp than my CAT but that is subjective and it took awhile to come to that conclusion.
The operator has a lot to do with the auxiliary performance. In a mini, keeping other tasks to a minimum to keep flow going to the aux circuit is a very important. In a skid steer, which has more power to support the aux circuit, it is still important to limit other functions. I am very cognizant of what I'm doing when I'm running the mulcher, trencher, stump grinder, or other even low flow attachments. There is a finite amount of horsepower available and it needs to be put to good use. People who watch me mulch probably think I'm 100 years old because I'm crawling and have deliberate smooth actions but in reality, I'm trying to keep as much flow to the cutting head as I can so that I can cut faster and reduce stalling. Took a long time to get good at that and that's why I had a hard time believing the drops in psi and gpm when my mulching performance is telling me otherwise.
I spoke to the svc manager. The machine is in spec- all reliefs are correct, the oil was at about 160 degrees and theachine was at Max engine rpm when he started turning the valve to increase resistance through the flow meter. The reason that there is a "linear drop" is because he stopped at every 100psi to log the flow.
He has reinforced what I was saying in that none of the high flow machines he has ever put a flow meter to will put out the high flow gpm shown on the brochure at te max pressure listed on that same brochure. I've known this guy since I was selling Bobcat machines in my past life and he doesn't have a dog in this hunt so the numbers are what they are.
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I know this was AWJ's thread and it got off topic but I appreciate all the extra information. I had a talk with a Bobcat factory rep awhile back. One of things we discussed were pressure and flow and he told me that some changes were coming to Bobcat. I wasn't supposed to say anything about it at the time but since then it's been rumored elsewhere. I know the manufacturers monitor these threads and to be honest, the chart that was posted (both) don't match up with manufacturer claims. Performance-wise, I would say the charts don't match performance in some applications that I'm in. Maybe a flow meter isn't the best tool for measuring performance since there are SO many types of pumps and configurations on many types of attachments in excavators, skids, and other loaders.
For example, my mulcher is pretty powerful with it's 2 speed 110cc motor but the trencher, which is similarly spec'd, has a lot more low end torque...so much so that it pulls my 9k lb machine around. My stump grinder, which is rated for the 37 gpm claimed by bobcat, doesn't have hardly any torque and it's motor is puny compared to the trencher or mulcher yet it seems to have a very high rpm and just enough torque to do it's job but it will stall easily if you push it.
What rpm does the head turn unloaded?
It has nothing to do with stopping to take a reading. Explain to me why it would? I've said it in a previous post and I'll say it again, it's a system curve and it entirely depends on how the engineers have designed with the circuit. The maximum power available to drive anything on a machine <= bhp of the motor. How the engineers configure the system to capitalize (or not) this limitation is up to them.
YellowDog, the relationship between torque and angular velocity is the same as psi and gpm, power is the product of the two. So, for a fixed power you can increase torque but then you have to proportionately decrease rpm or vice versa. This is why the stump grinder spins fast but has no torque and why the mulcher does.
thx for the clarification with the graph mxrider
The graph clearly shows that the maximum advertised gpm (40.4) and the max advertised psi (3500) are not on the same vertical line
Therefore max advertised gpm (40.4) and max advertised psi(3500) cannot be seen at the same time
neither could they be observed as per flow meter readings