Think i would blow it up? - Stump Grinder

[woodlandfarms]

There is a used Bobcat Stump Grinder for sale in my hood up north. From what I can tell it is rated for a smaller bobcat, but I don't know much about them so...

Stump Grinder Attachment - Bobcat Company

It looks like the machines these hook up to are rated around 12 to 15GPM. I am at 18GPM but we both share the same PSI (within a couple hundred MB).

Just don't know enough about Hydraulics to ask...

 

[Gravy]

You could use a flow divider or some such, or you could just be careful to keep your engine below full throttle. Probably wouldn't blow up anyway...

 

[J_J]

There is a used Bobcat Stump Grinder for sale in my hood up north. From what I can tell it is rated for a smaller bobcat, but I don't know much about them so...

Stump Grinder Attachment - Bobcat Company

It looks like the machines these hook up to are rated around 12 to 15GPM. I am at 18GPM but we both share the same PSI (within a couple hundred MB).

Just don't know enough about Hydraulics to ask...

What Gravy said plus. At whatever rpm your engine is running, it is putting out pressure and GPM. The hydraulics on the attachment is designed for a certain rpm/psi, and will have a continuous rpm settings, and an intermittent rpm limit. It will also have continuous and intermittent pressure limits. If you drive the motor faster than design limits, then more heat will develop, and damage the motor. You have to look at what they use for rpm and pressure, and try and match it. You can divert some of the fluid, or reduce engine rpm. If the attachments uses a lower pressure, than your pump, you have to reduce the pressure. Relieving it generates a lot of heat. A flow control valve might be your best selection to control GPM's, and that valve has a relief. My PTO is flow controlled, by one of these valves. Newer version of the PT use a solenoid valve, and when activated, it is full on. The hydraulic motor of the attachments should be close to the pressure of the driven pump.

 

[woodlandfarms]

What Gravy said plus. At whatever rpm your engine is running, it is putting out pressure and GPM. The hydraulics on the attachment is designed for a certain rpm/psi, and will have a continuous rpm settings, and an intermittent rpm limit. It will also have continuous and intermittent pressure limits. If you drive the motor faster than design limits, then more heat will develop, and damage the motor. You have to look at what they use for rpm and pressure, and try and match it. You can divert some of the fluid, or reduce engine rpm. If the attachments uses a lower pressure, than your pump, you have to reduce the pressure. Relieving it generates a lot of heat. A flow control valve might be your best selection to control GPM's, and that valve has a relief. My PTO is flow controlled, by one of these valves. Newer version of the PT use a solenoid valve, and when activated, it is full on. The hydraulic motor of the attachments should be close to the pressure of the driven pump.

Solenoid operated.

But here is a question JJ... I assume both pressure and GPM are interrelated to RPM of the engine, That if the engine runs slower the GPM and the pressure will be lower. Is this a fair assumption?

 

[RickB]

Solenoid operated.

I assume both pressure and GPM are interrelated to RPM of the engine, That if the engine runs slower the GPM and the pressure will be lower. Is this a fair assumption?

Systems with fixed displacement pumps (as most SSL's do) have flow characteristics directly related to pump (and engine) RPM. System operating pressure is dictated by the work being performed, and ulitmately the system relief valve setting. If the OP operates the grinder at no more than 83% of his machines rated ERPM, he will safely maximize the performace of the grinder. It is quite likely the grinder has a relief built into the motor assembly.

 

[J_J]

A relief valve is for pressure. GPM is dictated by speed of the pump. If you were to run that pump and suddenly close off the output line, then in an instant, the pump will blow out a fitting, blow out a hose, or burst the pump open.

Yes, you are right, that if no work is being preformed, then there is very little pressure.

You have to regulate pressure to the motor, in order to keep the pressure from blowing out the seals. The motors are rated for continuous pressure, and intermittent pressure.

Either way, if you run the hydraulic motor faster than it is designed for, it will break. If you over pressurize the motor, then it will break.

You always try to keep the hyd motor, within the hyd pump rating.

 

[J_J]

Carl,

The pressure is dictated by a force interrupting the free flow of the hydraulic fluid. That force could be a hydraulic motor, a cylinder, a relief valve, a needle valve. anything that will block the fluid.

 

[Gravy]

Carl,

RickB & JJ have covered much of this, but here's my take:

Slowing down the RPM's limits the GPM output in a fairly linear fashion. The pump moves a certain amount of fluid with each revolution. Turn it slower and it pumps less fluid.

PSI is different. The pump pushes fluid into a motor or cylinder. The fluid pressure rises until the motor or piston moves. If the motor or piston moves easily, the pressure will be low. If it doesn't move, the pressure will rise until something gives - a relief valve will open, the pump will stall, or maybe something will break.

That kind of pressure spike can be pretty dramatic. As long as it's spinning, the pressure in the stump grinder circuit may only be a few hundred pounds because the fluid is moving through the motor and back to the tank. If the motor stops spinning when a grinder tooth hits a particularly tough spot, the pump will suddenly be trying to compress a pretty incompressible fluid. Most likely, a relief valve will open and you'll hear an unpleasant whine. OTOH, if you are running it wide open, the pressure may rise faster than the relief valve can open. In that case, you might blow out a hose or seal.

From a practical standpoint, the differences in PSI & GPM don't seem really severe. If you don't run wide open, the reliefs will most likely soak up the spikes.

Disclaimer: I'm not any kind of engineer. I've never played one on TV, and I've NEVER stayed in a Holiday Inn Express...