Farmerford
Platinum Member
- Joined
- Dec 9, 2006
- Messages
- 733
- Location
- Columbus, Georgia
- Tractor
- Kuborta B2400, L2900, L4330; Caterpillar D3B, John Deere 455D
Bob:
The combined advice of the posters covers just about all the considerations except perhaps one: heating of the hydraulic fluid. As Duff said a pressure relief valve causes the fluid to heat up when the valve opens and bypasses excess fluid back to the tank. The pressure reducing valve he shows in the "down" circuit between the directional control valve out port and the motor inlet does not bypass, so it causes little heat.
But I suspect that you intend to use a detented directional control valve so that you can leave the valve full open and control motor pressure with the pressure reducing valve rather than by feathering the directional valve (since that will require constant attention). Isn't that going to cause the system pressure relief valve that Duff shows between the pump and the directional control valve inlet to open? I assume the hydraulic motor will turn very slowly, if at all, when the machine is drilling; therefore, very little, if any, hydraulic fluid will flow through the pressure reducing valve. But the pump is fixed displacement, so the 8gpm it produces must flow somewhere. I believe the pressure between the pump and the pressure reducing valve will build to 1500 psi (approx) and then the system pressure relief valve will open and let most of (all) the 8gpm flow to the tank.
The concern is that the fluid will heat up as it passes through the system pressure relief valve. The rule of thumb is 1 degree Fahrenheit temperature increase in the typical mineral based hydraulic fluid for each 140psi of pressure drop across the orifice (pressure relief valve).
Assume your hydraulic reservoir, lines, etc. hold 16 gallons of hydraulic fluid. The 8gpm pump will circulate all the fluid through the system pressure relief valve one time every two minutes. And the pressure drop across the system pressure relief valve is 1500psi. 1500/140 is about 10, so the fluid temperature will rise 10 deg F every two minutes. After 20 minutes the fluid will be 100 deg F hotter than when you started. So, on an 80deg F day, the fluid will reach 180deg F in 20 minutes, which is considered by many a maximum acceptable long term temperature.
I realize that the machine probably has some sort of fluid cooler, but I doubt that it is sized to remove the heat generated by effectively transforming the entire pump output into heat. And of course heat will be dissipated through the lines, tank walls, etc. But, there are likely to be other sources of heat to the fluid: the pump itself is probably bolted to the engine block where it collects heat, the flow of the fluid through the pump, valves, pipes, and hoses creates heat, and if the hydraulic sump is also the transmission sump significant additional heat will come from the transmission.
One solution may be to insert a second pressure relief valve set at about 500psi in the circuit between the directional control valve out port and the pressure reducing valve. This pressure relief valve, that is only in the motor down circuit rather than in the entire system, is called a circuit pressure relief valve. That way, when (and only when) the directional control valve is shifted to send the 8 gpm to the motor to put down pressure on the drill, even if the pressure reducing valve blocks flow to the motor completely after the motor pressure reaches 400 psi, the circuit pressure back to the pump will not exceed 500psi because the additional circuit pressure relief valve opens at that pressure. The pressure drop across the circuit pressure relief valve of 500psi will generate one-third of the heat generated by a 1500 psi drop across the system pressure relief valve.
If the additional pressure relief valve is not sufficient to keep the fluid temperature below 180deg F, consider accepting the inconvenience of throttling the diesel engine back to slow idle while drilling in order to reduce pump volume, and then increasing engine speed when raising the drill. If the pump is producing only 2gpm, the it takes 8 minutes for the fluid to circulate once and thereby it receives one-fourth the heat for the same period of time. This gives the system hydraulic fluid cooler and other escape routes for the heat more time to deal with the heat.
Additional relief (pardon the pun) may be had with the more expensive synthetic hydrauliic fluids that will tolerate higher temperatures than the typical mineral oil based fluids.
Of course, it is possible that the excavator hydraulic system can handle the heat that will be generated in the fluid. If it does not have a hydraulic fluid temperature gauge you should install one or use a hand held infrared thermometer. But I believe it quite likely that you will have a heat problem without the additional circuit pressure relief valve, and you may well have one with it, only smaller.
Good luck and keep us posted, with pictures.
The combined advice of the posters covers just about all the considerations except perhaps one: heating of the hydraulic fluid. As Duff said a pressure relief valve causes the fluid to heat up when the valve opens and bypasses excess fluid back to the tank. The pressure reducing valve he shows in the "down" circuit between the directional control valve out port and the motor inlet does not bypass, so it causes little heat.
But I suspect that you intend to use a detented directional control valve so that you can leave the valve full open and control motor pressure with the pressure reducing valve rather than by feathering the directional valve (since that will require constant attention). Isn't that going to cause the system pressure relief valve that Duff shows between the pump and the directional control valve inlet to open? I assume the hydraulic motor will turn very slowly, if at all, when the machine is drilling; therefore, very little, if any, hydraulic fluid will flow through the pressure reducing valve. But the pump is fixed displacement, so the 8gpm it produces must flow somewhere. I believe the pressure between the pump and the pressure reducing valve will build to 1500 psi (approx) and then the system pressure relief valve will open and let most of (all) the 8gpm flow to the tank.
The concern is that the fluid will heat up as it passes through the system pressure relief valve. The rule of thumb is 1 degree Fahrenheit temperature increase in the typical mineral based hydraulic fluid for each 140psi of pressure drop across the orifice (pressure relief valve).
Assume your hydraulic reservoir, lines, etc. hold 16 gallons of hydraulic fluid. The 8gpm pump will circulate all the fluid through the system pressure relief valve one time every two minutes. And the pressure drop across the system pressure relief valve is 1500psi. 1500/140 is about 10, so the fluid temperature will rise 10 deg F every two minutes. After 20 minutes the fluid will be 100 deg F hotter than when you started. So, on an 80deg F day, the fluid will reach 180deg F in 20 minutes, which is considered by many a maximum acceptable long term temperature.
I realize that the machine probably has some sort of fluid cooler, but I doubt that it is sized to remove the heat generated by effectively transforming the entire pump output into heat. And of course heat will be dissipated through the lines, tank walls, etc. But, there are likely to be other sources of heat to the fluid: the pump itself is probably bolted to the engine block where it collects heat, the flow of the fluid through the pump, valves, pipes, and hoses creates heat, and if the hydraulic sump is also the transmission sump significant additional heat will come from the transmission.
One solution may be to insert a second pressure relief valve set at about 500psi in the circuit between the directional control valve out port and the pressure reducing valve. This pressure relief valve, that is only in the motor down circuit rather than in the entire system, is called a circuit pressure relief valve. That way, when (and only when) the directional control valve is shifted to send the 8 gpm to the motor to put down pressure on the drill, even if the pressure reducing valve blocks flow to the motor completely after the motor pressure reaches 400 psi, the circuit pressure back to the pump will not exceed 500psi because the additional circuit pressure relief valve opens at that pressure. The pressure drop across the circuit pressure relief valve of 500psi will generate one-third of the heat generated by a 1500 psi drop across the system pressure relief valve.
If the additional pressure relief valve is not sufficient to keep the fluid temperature below 180deg F, consider accepting the inconvenience of throttling the diesel engine back to slow idle while drilling in order to reduce pump volume, and then increasing engine speed when raising the drill. If the pump is producing only 2gpm, the it takes 8 minutes for the fluid to circulate once and thereby it receives one-fourth the heat for the same period of time. This gives the system hydraulic fluid cooler and other escape routes for the heat more time to deal with the heat.
Additional relief (pardon the pun) may be had with the more expensive synthetic hydrauliic fluids that will tolerate higher temperatures than the typical mineral oil based fluids.
Of course, it is possible that the excavator hydraulic system can handle the heat that will be generated in the fluid. If it does not have a hydraulic fluid temperature gauge you should install one or use a hand held infrared thermometer. But I believe it quite likely that you will have a heat problem without the additional circuit pressure relief valve, and you may well have one with it, only smaller.
Good luck and keep us posted, with pictures.
