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Old 01-21-2020, 02:41 PM   #1
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Compact Piston Hydraulic Pump Options

Im in the process of refreshing my bow thruster system and am running into an issue with the hydraulic pump.
I had wanted a piston pump which would eliminate the current electric clutch system I have that I see as a pretty big failure point. The problem is that the hydraulic shop is telling me that they don't have hydraulic piston pumps that will fit as the engine fits pretty close to the forward fuel tank. I don't fully understand since removing the clutch should give another few inches of room but I may not understand how much bigger a piston pump is.
I need to produce about 20hp (18gpm at 2500rpm) to run the new thruster, has anyone else looked at changing to a piston pump or are most people using clutch systems since gear pumps don't have compensators?

Thanks
Arthur
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Old 01-21-2020, 03:10 PM   #2
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If you're using a Pitts electric clutch there is not much to worry about. I had one hanging off the front of an 1800 RPM gen set driving a 25 HP hydraulic pump. We worked that setup hard running a crane and winches. I estimate 10 - 20 engage / disengage cycles per 12 hrs. It had well over 20,000 hrs on it when I left the boat with just one failure. For a bow thruster hydraulic pump on your main you could throttle down to idle for the engagement and probably never wear out a Pitts clutch.
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Old 01-21-2020, 05:57 PM   #3
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So you are talking about a variable displacement pump, vs fixed displacement?

Just keep in mind that without a clutch any leak will dump all your fluid and shortly after trash your pump unless you get the engine shut down fast. Then to restart you need to physically disconnect the pump which can be easier said than done.

It’s all doable, but I generally see people wishing they had a clutch rather than wishing they didn’t.

If the goal is to unload the pump except when needed, a solenoid controlled dump valve might be an easier way to go. When you don’t need the hydraulics, the valve is open and the pump just pumps back to the tank without developing any pressure, which means minimal heat and power consumption. Then when you need the hydraulics you close the valve, the system pressurizes, and you
can put it to work. The clutch can then be more of an emergency disconnect.
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Old 01-21-2020, 06:05 PM   #4
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I was told by my Cat Cummins rebuild shop that continuous operation of hydraulic gear pumps was fine in the Bypass zero pressure mode. If I had chosen to switch to hydraulics during my refit, the final choice was direct driving the pump off the ZF transmission. It's a straight bolt up with their adapter assembly.

Ted
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Old 01-21-2020, 06:22 PM   #5
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Quote:
Originally Posted by O C Diver View Post
I was told by my Cat Cummins rebuild shop that continuous operation of hydraulic gear pumps was fine in the Bypass zero pressure mode. If I had chosen to switch to hydraulics during my refit, the final choice was direct driving the pump off the ZF transmission. It's a straight bolt up with their adapter assembly.

Ted


And that’s exactly why the dump valve would accomplish...
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Old 01-21-2020, 08:26 PM   #6
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I have a Cessna variable-volume pump and it is always turning when the engine is running. You need a reservoir but not much fluid is circulated when the pump is idling but the case drain continuously returns a small amount of fluid and some is circulated for cooling. Actually, the amount of case drain fluid increases as the pump ages and reflects the condition of the pump. Anyway, my pump is 26 years old and works fine. You turn on a switch in the wheelhouse to open a solenoid to start to circulate the fluid through the systems and then you just control them at their location.

The bronze plate on the left is my fire pump.
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Old 01-24-2020, 11:31 AM   #7
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So I think I have been convinced to keep the electric clutch and just upgrade to a larger gear pump. Now I need to see if there is an automated way to ensure it doesn't run at too high an RPM...
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