Hydraulic Pressure

[Joee]

Hi,

I have a 20 gpm Prince relief valve, between a 21 gpm pto pump
and motor controller, there is also a 3000 psi gauge after relief
valve.

My question is I'm not sure about how to adjust relief valve. With
pto engaged and turning about 540 rpm, all the pressure I can get
at the gauge is between 500 & 800 psi. This is no load pressure
and I am turning adjuster on relief valve.

The Prince relief valve has an adjustable range of 1000-2500 psi.

Advise needed, Joe

 

[kennyd]

That may be OK for the "no load pressure". You need to operate the valve and deadhead the pump to set the pressure relief valve.

 

[Joee]

Hi Kenny,

What do you mean by deadhead?

Joe

 

[JerryG]

Hi Kenny,

What do you mean by deadhead?

Joe

If your system has a cylinder, hold the control valve either direction under pressure. When you do that you are dead heading and your gauge will show the relief setting pressure.

 

[RonMar]

In an open center system, fluid ALWAYS flows. if it dosn't, the pump usually breaks or a hose blows to allow the fluid to flow or relieve the pressure. when you move a valve, that flow thru the valve is diverted to a cylinder. When a cylinder strokes, fluid flows into one end and pushes the piston and rod. the fluid on the other side of the piston is forced out the other end and back to the system so there is still flow. When the cylinder reaches the end of the stroke, if you are still holding the valve, the flow stops when the cylinder does and the pressure builds quickly(nearly instantly) This is when the reliefe valve must open to maintain flow and no more than max system pressure to preserve the system components and structure. When you are using the cylinder for work such as to split a log, the safety valve limits the maximum system pressure applied to the cylinder under load and therfore the maximum ammount of force that can be applied(pressure X piston surface area). There have been people seriously injured and equipment destroyed by misinformed operators who adjust up pressure limits to get more work out of a machine. Start with the reliefe set low and work the pressure setting up to the rated pressure. Be carefull and wear eye protection. Hydraulic fluid injections are no fun.

 

[JerryG]

There have been people seriously injured and equipment destroyed by misinformed operators who adjust up pressure limits to get more work out of a machine. Start with the reliefe set low and work the pressure setting up to the rated pressure. Be carefull and wear eye protection. Hydraulic fluid injections are no fun.

That can't be stressed enough.

 

[Farmerford]

Joee:

I built a vaguely similar machine a couple of years ago, though it is not as complex or finished nearly as well as yours.

I also used a Prince 5.7 cu in/rev PTO pump and adjustable relief valve. My motor is a Char-Lynn 4.9 cu in/rev. At idle speed with no load the pressure is about 500psi cold and 350-400psi hot. The motor drives a 48" Bush Hog Squealer, and it is not hard to stall the motor with a heavy cut. When the motor stalls, the pressure builds to the 2000psi preset limit on the Prince relief valve. The pump is rated for 2250psi and I intended to raise the relief pressure to that limit. But after using it a while I decided that the power I got at 2000psi (14 hp at the motor shaft) was plenty, so I left it alone.

If your relief valve is preset like mine at 2000psi you might just wait until you are using the mower. Watch the pressure guage and if you don't reach the 2000psi relief setting in normal use, you could leave it set at 2000psi to limit the load on the motor, driveline, blade, etc.

I am most interested in the motor and blade. What kind of motor are you using? Is the blade spindle a manufactured setup with shaft, bearings, and housing, or did you make it? How is the blade attached to the shaft: ie, is the shaft splined or keyed; is there a separate hub for the blade to bolt to?

That is quite a project. I am looking forward to seeing pictures of it in action.

Chuck Ford

 

[Joee]

Hi, thanks for all the help and now I know what deadheading is.
What if I lock down the blade would that work for getting a
relief pressure reading?

The blade is just a 33" Snapper stock part and the spindle is
off an old heavy BushHog finishing mower.

I want to start out with a little lower pressure to see if there
is a weakness before I go full tilt. Hopefully I can head off any
trouble before I break something expensive.

Thanks again for the great info,

Joe

 

[Farmerford]

Joee:

Locking the blade should work, and should not harm the motor, pump, or other hydraulic components if each is rated for the maximum pressure you apply and if the motor case is drained properly. But if the weakest link turns out to be the LoveJoy coupling or the blade attachment the maximum pressure might damage them.

You can calculate the motor torque for each pressure and the maximum torque for the Lovejoy coupling should be published, but the capacity of the spindle/blade attachment is probably unknown.

Your relief valve is so easy to adjust, can you just turn it down (to low pressure) most of the way and try the mower? You should be able to stall it in heavy grass/weeds and when it does stall turn the relief valve up a bit and try again. That incremental process might let you identify the weakest part of the drive line before you turn the pressure up enough to break it.

That's going to be a first class machine!

 

[Joee]

Thanks Chuck I didn't think about Lovejoy could be weak link.
I think your suggestion about backing the relief off and slowly
tighten till I get what I need.

One more thing about relief valve, loose means low pressure and
tight means more pressure. Am I close?

Thanks again, Joe