Backhoe Does RPM effect PSI to backhoe or just speed?

[shepherd02769]

I am new to this forum so forgive me if I am somehow not posting correctly.

I just purchased a 2008 B2320 with a Woods BH70X backhoe - love it . The 540 PTO is 2800 RPM but when I set it there while working with the backhoe, movements can get a little jerky. I'd like to slow things down a little but without the loss of power. I thought I had read somewhere that the PSI to the backhoe is constant regardless of RPM, but the flow will change correspondingly, thus speeding/slowing operation. So I was thinking I could just lower RPM to around 1800-2000 while keeping maximum power for digging.

Are my assumptions correct? Also, is there a minimum PSI where power really does begin to drop (assuming my above assumptions are correct, of course)?

Thank you in advance for any insight!

 

[radioman]

I am not sure about your pto driven pump - but on mine when rpms is slowed, the speed and power (psi) lowers proportionally. I assume it exactly the same with rear pto pumps. You won't be able to keep max power up though - but it should be enough for you. As time goes on- you will be more experienced with the controls you can run it at full pto rpms.

 

[Dave M7040]

A couple of basic hydraulic facts. Until a cylinder encounters resistance, hydraulic pressure will be low. Once the resistance is encountered the hydraulic pressure will build up until one of two things happen. The first is that the cylinder will again move and the pressure will remain constant at that level as long as the cylinder continues to move or two, the pressure will increase until the relief valve setting is reached and even at that pressure the load on the cylinder may be too great for it to move. The rpm only affects the quantity of oil pumped. Assuming your system has minimal internal leakage, you will reach maximum relief valve pressure at idle if the cylinder has a load it cannot handle.

You are correct with your concept of slowing the RPM to 1800 to 2000 but you may want to more thoroughly understand what you have read. Flow dividers are used in hydraulic systems to share the volume of fluid the pump can deliver while allowing each circuit beyond the flow divider to have access to the maximum pressure the relief valve will manage.

Hope this helps a little.

Dave M7040

 

[murphy1244]

:welcome: to TBN

 

[sqidget]

You are correct. PSI is regulated by the relief valve, regardless of how fast the pump spins. Flow as in gallons per minute drops with pump speed as will the time it takes to reach full pressure. I figure the lowest rpm will be found when it no longer moves fast enough to work with.

 

[art]

Most things like that run off the PTO or even for tillage type work I tell people the most economical operating range is about 2/3rds top 3/4 throttle. The jerkyness is normal with the woods equipment at full throttle.

 

[shepherd02769]

The gentleman that I purchased this machine off of stated that this was the forum to be on - he was so right!

Thank you for the kind reception and the exceptionally useful, quick responses - much appreciated.

 

[rbargeron]

Many pto and engine-mounted hydraulic pumps are gear type. The output flow is directly tied to rpm. If the pump is in good condition, lowering the rpm should reduce the flow, but not the pressure.

The gears in the pump fit extremely well. As they turn, the meshing gear teeth cause oil to be drawn in one one side and forced out on the other. When the pump is new there is very little internal leakage and pressure is maintained, even at low rpm.

As the pump wears the internal clearances become greater, allowing more internal leakage. This shows up as reduced pressure at lower rpm. Raising pump speed can compensate for this somewhat - but for a pump in good condition higher rpm is not intended to produce higher pressure. Hope this is useful. Dick B

 

[J_J]

You are correct. PSI is regulated by the relief valve, regardless of how fast the pump spins. Flow as in gallons per minute drops with pump speed as will the time it takes to reach full pressure. I figure the lowest rpm will be found when it no longer moves fast enough to work with.

The relief valve has no function until the set relief pressure is reached.

It is essentially a safety valve for the components in the hyd system.

Load or resistance to the flow is what develops pressure.

Pressure is variable throughout the range of the load, as the load varies, so will the pressure.

GPM/volume dictates speed of operation.

 

[shepherd02769]

I understand. My machine likely has a single relief valve for the entire hydraulic system that keeps it from damaging seals, hoses, couplings, etc. (I wonder if that's adjustable or factory preset only?). So I suppose if someone wanted to alter their true operating pressure, they'd adjust (shim, as I understand it) the set relief pressure, which I'm assuming is a separate adjustment either on the pump itself or somewhere in-line and common to all sub-systems.

Thanks again for all of your detailed answers - I haven't worked on hydraulic systems since my time on C-135's in the USAF back in the early 90's. This has been a tremendous help and will certainly make me a better backhoe operator. I'm not a professional operator so speed isn't my biggest concern, just the power issue.