hydraulic prerotator

[Doug Riley]

I thought I heard Ernie say at B-Days that you can get more performance out of his hydraulic prerotator by using a non-vented filler plug on the reservoir. I bought my Dominator used and, of course, there's a vent hole drilled through the plug. It actually works quite well as is (250 RPM on 28-foot blades without pushing it very hard) -- but most of us are RPM-greedy.

Has anyone compared the two versions? Mike Boyette, do I have this story straight?

[Dean_Dolph]

Interesting! Do you have any idea why?

The only possibility that I can see is that temperature rise and subsequent expansion of the hydraulic fluid in an unvented reservoir would cause back pressure against the hydraulic drive motor and thus the hydraulic pump discharge. But, by the same token the pump now has a higher suction pressure so it would seem there wouldn't be any net gain/loss. The circulation rate which determines rotor rpm shouldn't change.

The pump pressure will go up as the load on the motor increases from rotor drag but the beauty of hydraulics is that the circulation rate isn't affected until you run out of driving power and/or reach the pressure limits of the system. In this case belt slippage on the pre-rotator drive pump is the limiting factor.

The commercial systems I'm familar with use unvented reservoirs but as far as I know that is just because a pressure system isn't required to make them work and they are easier to fill as needed. These are two pump systems that use a low pressure pump, with an open reservoir, that feeds a high pressure pump with relief devices to prevent exceeding pressure limitations. The flow circulates from the low pressue pump to the high pressure pump to the driven hydraulic motor and back to the reservoir. The hydraulic prerotator uses the same circulation but without the extra pump.

I just don't see how a vented reservoir would have any effect on the systems operation. Then again I don't completely understand hydraulics other than the circulation rate is determined from the drive pumps displacement and rpm. And the driven motors rpm is determined by its displacement and the circulation rate.

[Chuck Irby]

Doug, How many r's is your engine turning as you pre-rotate to 250?

My cap is not vented. I have never tried to see how many r's I could pre-rotate to. I normally stop at 160 to 200, as I have plenty of runway.

This is an interesting topic.

[Mike Hook]

There are two types of systems, Open systems that are vented to the atmosphere or closed systems that are pressurized reservoir. To tell any difference in the two you need to substract the difference between the pump pressure and the reservoir pressure. To calculate the pressures and torque and HP use the below formulas.

Horsepower = Pressure ( psi ) x GPM / 1714

Fluid motor torque ( iin lbs ) = Pressure ( psi ) x displacement ( cu in ) / 62822

Fluid motor speed ( rpm ) = 231 x GPM / displacement ( cu in )

Mike

[Friendly]

How about a picture of the hydro prerotor setup??

[MikeBoyette]

Doug,
I just spoke to Dad abd he said that is correct. By having the system closed it keeps pressure on the return tank. Which gives him better performance. Many people have told him over the years that the way his prerotator is set up doesn't make any sense. He isn't a Hydraulic engineer, this is just what he has found works best. There were many testing sessions done on different confirqurations, and this worked best for the particular application. Hope this helps.

[Doug Riley]

Thanks, Mike. I'm off to the plumbing store for a new plug.

Chuck Irby, I don't know offhand what the engine RPM is. I don't watch the tachs much on takeoff, relying more on visuals and feel instead. At 250 rotor RPM with the current vented setup, I'm near (but not quite at) the limit of braking capacity with the Azusa cable-actuated band brakes.

Ernie tickled my "RPM greed" by talking about 300 RPM prespins with his unvented setup.

[Dean_Dolph]

There are two types of systems, Open systems that are vented to the atmosphere or closed systems that are pressurized reservoir. To tell any difference in the two you need to substract the difference between the pump pressure and the reservoir pressure.
Mike

Mike, I'm aware of the closed systems, which work the same as open systems. But I don't see how a system that has an unvented, or pressurized reservoir, is going to have a different supply pressure or circulation rate than one that is vented.

The hydraulic supply pump used on pre-rotators is a relatively simple positive displacement type and as long as the suction requirements are met it will provide a flow rate that is linear with the rpm. The discharge pressure only changes if the load is increased. Care to explain how subtracting the reservoir pressure would affect the flow and/or pressure?

[Dean_Dolph]

Your right, Mike, I also say it doesn't make any sense! I'm not a hydraulic engineer either but if it works it works. But if it works better with pressure then there is something going on that isn't immediately obvious. Maybe Mr. Hook will give us an explanation.

[Mike Hook]

Dean

Mr Hook died back in 1972, so it.s just Mike.

John Deere is a good example of a closed system. They use it on all their hydralic systems. One thing to look at is that they run at a much higher pressure somewere around 4,000 plus lbs. One advantage I see and this is just my opionion is that with a closed system and the higher pressure you dont have a big difference in the pressures inside of the system so the seals only sees what ever the difference is. If you had a open system and ran 4,000 lb pressure you would probably blow the seals inside of your motor or doubleacting cylinder. A open system runs pressure anywere from 1,2,00 to 2,500 psi.

No matter you still need hoses and pump and cylinder seals rated for the higher pressure. Open systems are just cheaper to operate and build and so is replacement parts. Then again you can always just screwwwwwwwwww in that main hydralic releif valve.
Hmmmm guess that is why I am hunting for a swing motor for my track hoe now someone already did that and now I split my motor down the side.
Some arab over seas has the last one in the world according to Case equipment and now wants $15,000 for the motor. Grrrrrrr

I am not a engineer either but have had years of working on hydralics in the mines and on the farm. I did get some hydralics classes in the early 70's

[Aussie_Paul]

One problem that the hydraulics seem to have every now and then, is the drive belt. Not perfect yet sort of ok.

Have the belt continuously driving a clutch similar to an air con unit. No slippage straight engaged. Then have a valve to supply the oil(power) to the rotor gradually. Then, as you roll, release the air con clutch so the pump is not pumping.

Just a thought to avoid slipping clutches and belts.

Aussie Paul.