Hydraulic Question on Reducing PTO Pressure and Flow (2024)

Hi JJ,

I apologize in advance for the length of this post.

I thought I would get your input on some general hydraulic flow questions and am posting it in the forum so others can learn from your input if needed. If you do not know some of the answers, I am not asking you to spend time researching them. Others, feel free to chime in.

Many implements require pressures and flows lower than what my PT produces. One option is to use the auxiliary PTO circuit but I am using that for so many controls that I would likely have to use the main PTO circuit.

1.) The PTO pump is always running and dumping to the tank (correct me if I am wrong on that). Does running the flow through a pressure relief valve on an implement generate more heat than when the PTO is just dumping to the tank?

2.) If an inline flow restrictor is used, does that generate more load on a pump than a pressure relief? Basically, the question comes down to, should a pressure relief be put into the circuit before the flow restrictor?

I was going to buy some things to play around with and install with quick disconnects so that I could use them on various pieces of equipment. I am not going to worry about reverse capability at this point.

3.) Would the system run cooler or the same using a priority flow divider with the auxillary flow going to the tank versus a flow restrictor? This would give the advantage of being able to run an additional circuit if necessary (unlikely) and if it was cooler, that would also be a plus. An example is Surplus Center - 3/4" NPT 30 GPM ADJ PRIORITY DIVIDER VALVE RD575
I find it strange that the 3/8, 1/2, and 3/4 ported models are rated for the same flows - 30 gpm through a 3/8" port?

4.) Is there a big cooling, noise , etc advantage using a ball and spring pressure relief valve versus a poppet style? If there was, i might give up adjustability of the poppet style like this one Surplus Center - 1/2" NPT 20 GPM 1500-3000 PSI RELIEF VALVE RV-H4 versus the fixed Surplus Center - WJL-50-2000 HYD RELIEF VALVE

I am sure there are other questions I have not thought of.

Thanks for any and all input.

Ken

Yes, using the relief will generate more heat, as the orifice is probably much smaller that a port.

A flow restrictor is designed to reduce the volume of fluid, thereby reducing speed of operation. A pressure relief is designed to protect something, a cyl, pump, or whatever. A pressure relief works best if located close by the cyl or motor.

As you know, QD will reduce the flow, less than the source, and that is why they recommend to use one size larger QD's.

Ball and spring relief are used for fast acting and occasional relief, whereas, the poppet relief is used where you need a continuous acting relief. They are quiet and smooth acting.

Thanks, JJ. So it sounds like the best thing would still be an adjustable flow valve without a built in pressure relief with the excess flow plumbed to the return line to the tank. Follow that with a 3 port poppet pressure relief valve also connected to the return line and then a Tee for a gauge for setting purposes, maybe even on a long hose back to the cab if i want to monitor while using things and not have to worry about branches hitting it.

For a typical implement that wants to see only 2000 psi, it will be interesting to see how often the relief opens.

To confuse things for me more, some implements get plumbed one way if in an open center system and another way if it is a closed center system. The PTO is an open center system if i am understanding things correctly - flow goes to the tank when the solenoid is not activated.

Ken

SpringHollow; said:

Thanks, JJ. So it sounds like the best thing would still be an adjustable flow valve without a built in pressure relief with the excess flow plumbed to the return line to the tank. Follow that with a 3 port poppet pressure relief valve also connected to the return line and then a Tee for a gauge for setting purposes, maybe even on a long hose back to the cab if i want to monitor while using things and not have to worry about branches hitting it.

For a typical implement that wants to see only 2000 psi, it will be interesting to see how often the relief opens.

To confuse things for me more, some implements get plumbed one way if in an open center system and another way if it is a closed center system. The PTO is an open center system if i am understanding things correctly - flow goes to the tank when the solenoid is not activated.

Ken

Click to expand...

You need at least one relief, in the hyd path.

As far as the relief pressure, the item that is using the fluid will develop the pressure by the resistance to the flow of fluid. A lot of times you never get close to the relief pressure. You get the most power when operating at or near the relief pressure.

If the item you are using operates at a lower pressure, you need a relief for that item and anything downstream.

My PTO operates using a flow control valve, with relief, and with variable speed. The fluid not used is returned to tank. If your PTO is solenoid operated, the fluid flows to tank with no voltage, and you have full flow to the PTO when activated.

Attached is a simple block diagram of the hydraulic PTO circuit and what I would add. At "W", it is still a "closed" circuit and any implements added at this point should be plumbed for a closed circuit?

I realize the pressure reliefs are protecting the pump from being dead ended.

Ken

The relief must go before the flow control. Or else purchase an adjustable priority flow control with the relief valve built in. Relief down stream of the flow control only protects that down stream circuit. It does NOT protect the pump if the flow control malfunctions.

Another way to reduce flow is run the engine slower so the PTO runs slower. Flow controls work Via pressure drop, pressure creates heat which wastes energy.

Roy

I did not draw it but I thought the PTO had a pressure relief valve in its circuit but I do not see it any where in the parts list so I think you may be right. Thanks for catching that! I also see I have to correct the pressure and flow numbers.

Ken

OK, here's the corrected one.

I do realize you can lower rpm's but since this is driven across the ground by hydraulic wheel motors, there is a limit to how low the rpms can go.

Ken

I will have to try and remember to ask Terry if there is one already or not.

Carl or any of you other guys know?

Ken