Basic Hydraulic question

[Tim M]

I am considering the purchase of a 25-35 HP tractor for mainly landscaping a slightly sloped 3.3 acres. I have a few rookie questions on hydraulics.

Most tractors and skid steers spec hydraulic pump flow rate but not psi. It strikes me that the psi that a hydraulic pump can generate is also very important. Yet I cannot seem to find any specs on psi.

It is my understanding that flow rate determines the speed of an implement (FEL for example), but wouldn't psi along with cylinder diameter determine "IF" a given load can be lifted at all ?

The bigger the cylinder the slower it moves (assuming constant flow rate) but the less psi required to lift a given load? yes?

Is there a standard psi (perhaps 2500 psi) that pumps generate and what seperates small from big is the flow rate? Which is then matched to cylinder size to give a reasonable lift rate and of course the cylinder diameter and/or relief valve would be sized so as not to cause any of the steel involved (bucket, lift arms etc...) to break?

Do most implements (FEL for example) have built in releif valves? Preventing the pressure from getting too high?

Thanks for listening,
Tim

 

[Wayne County Hose]

[

QUOTE=Tim M]I am considering the purchase of a 25-35 HP tractor for mainly landscaping a slightly sloped 3.3 acres. I have a few rookie questions on hydraulics.

Most tractors and skid steers spec hydraulic pump flow rate but not psi. It strikes me that the psi that a hydraulic pump can generate is also very important. Yet I cannot seem to find any specs on psi.

I would think they have to be there somewhere. Contact the dealer/manufacturer. Flow rates are spec'd because many hydraulic implements have a minimum flow rate for acceptable performance, psi is usually a given. If I am not mistaken, most compact tractors operate at close to the same psi. This is from the hoses and hydraulic parts I have seen in my shop, I may be wrong.

It is my understanding that flow rate determines the speed of an implement (FEL for example), but wouldn't psi along with cylinder diameter determine "IF" a given load can be lifted at all ?

Yes, you are correct. Flow rate determines speed, psi determines power.

The bigger the cylinder the slower it moves (assuming constant flow rate) but the less psi required to lift a given load? yes?

Correct. There are other minor technical variables in there, but for the sake of simplicity, let's just say yes.

Is there a standard psi (perhaps 2500 psi) that pumps generate and what seperates small from big is the flow rate? Which is then matched to cylinder size to give a reasonable lift rate and of course the cylinder diameter and/or relief valve would be sized so as not to cause any of the steel involved (bucket, lift arms etc...) to break?

Depending on the style of pump, there are maximum PSI rates that can be obtained. You really don't need to be concerned with that on a tractor. As far as flow rates, I'll let the tractor guru's here that know all these spec's handle that one. Almost any hydraulic system has a relief valve to prevent breakage. Any tractor you would be purchasing has relief valves installed in all the correct places.

Do most implements (FEL for example) have built in releif valves? Preventing the pressure from getting too high?

Most implements have relief valves built into the valve bodies. Nothing to worry about, they are very dependable.

Thanks for listening,
Tim

Anytime, Andy.

I see this is your first post, welcome to TBN. Just to let you know, I openly admit that my advice is worth as much as you paid for it. Not that I have ever charged for advice, but if I did, I would frequently have to give refunds.

 

[RonMar]

The PSI should be listed in the specs somewhere, it may be worded something like "system operating pressure". If it is a foreign tractor, it may also be listed as MPA(MegaPascAl) instead of PSI. 2320PSI = 16MPA so you might not immediatly recognize it as a high pressure specification.

The system operating pressure is usually the pressure that the safeties are set at. The pump itself may be capable of greater pressure, but the system pressure is that which the maximum structural loads(with a safety margin) are designed around. By setting the system pressure at less than what the pump is capable of, you relieve stress on, and in theory extend the usefull working life of the pump.

I would expect a typical modern tractor to operate around 2000 PSI, but I am sure there are exceptions to this.

 

[Soundguy]

Yep.. many agg applications are 2500psi and under. My NH 7610s runs 2500 psi.. etc.

soundguy

 

[flusher]

I am considering the purchase of a 25-35 HP tractor for mainly landscaping a slightly sloped 3.3 acres. I have a few rookie questions on hydraulics.

Most tractors and skid steers spec hydraulic pump flow rate but not psi. It strikes me that the psi that a hydraulic pump can generate is also very important. Yet I cannot seem to find any specs on psi.

It is my understanding that flow rate determines the speed of an implement (FEL for example), but wouldn't psi along with cylinder diameter determine "IF" a given load can be lifted at all ?

The bigger the cylinder the slower it moves (assuming constant flow rate) but the less psi required to lift a given load? yes?

Is there a standard psi (perhaps 2500 psi) that pumps generate and what seperates small from big is the flow rate? Which is then matched to cylinder size to give a reasonable lift rate and of course the cylinder diameter and/or relief valve would be sized so as not to cause any of the steel involved (bucket, lift arms etc...) to break?

Do most implements (FEL for example) have built in releif valves? Preventing the pressure from getting too high?

Thanks for listening,
Tim

You're right about the hydraulic pressure spec being somewhat hard to dig up at times.

Neither the operator's manual or the service manual for my 2005 Kubota B7510HST (21 hp engine) has this spec. But I know it's less than 2200 psi because that's the max pressure for the LA302 FEL manual. That's the FEL on my 7510.

The operators manual for my 1964 MF-135 diesel (45 hp engine) is more informative. It says that the hydraulic pump is a constant running, scotch yoke, piston type pump, max pressure 2800 psi, 3.8 gpm @ 1700 engine rpm and 1500 psi, 4.5 gpm @ 2000 engine rpm and 1500 psi, oscillating control valve on inlet side of the pump.

A while ago I was interested in the JD 3020 diesel (70 hp engine). The operator manual says the max hydraulic system pressure is 2250 psi.

 

[Soundguy]

Kubota is real weaird about putting usefull info and specs out for some reason. My old ford, and NH manuals show hyd press / gpm specs.. etc.

Soundguy

 

[Yankee Clipper]

Pumps do not create pressure - they displace fluid. The moving fluid pushing against a load is what creates pressure in a hydraulic system.

Pump flow rates on positive dipslacement pumps - usually gear type on compacts - are given at rated engine speed as that is where the engine HP rating is developed. You throttle up to get more hydraulic system speed - meaning more flow.

Most tractors use a system relief valve that is higher in pressure than the circuit (loader / backhoe) relief. There is a spread betweenn the two settings to avoid RV "cross talk" - where they both are attempting to relieve the pressure.

The system relief is to protect the pump and components from being overpressurized. The implement may or may not have a circuit or system relief valve as well, to protect it from structural damage or over pressurization.

Probably clear as mud but I have worked with it for years and if I can understand it, it has to be straigtforward!

 

[bigbull338]

the reason they list the gpms on hydro pumps is because certain equip requires a certain flow rate to operate.you can only operate equipment as fast as your hydro pump compacity.

 

[Soundguy]

Pumps do not create pressure - they displace fluid. The moving fluid pushing against a load is what creates pressure in a hydraulic system.

Keep in mind that the 'load' can also include restrictive orifices, and lines, and PD across connectors, besides the actual intended laod.. like a cyl or motor.. etc..

Soundguy

 

[Tim M]

Yes, yes hoses and fittings can drop pressure just like a resistor drops voltage. Hoses and fittings need to be sized to match flow rate.

So it appears here that the pertinent spec is the tractor's system relief valve pressure. If it is higher than the implement's relief pressure then the implement can perform to it's published specs (lift capacity).

Let's look at an example. Kubota B7800. It's 402 FEL lifts 1060 lbs. at the bucket pin. This FEL could be repalced with a woods LC102 which lifts 1650 at the bucket pin at a pressure of 2500 psi.

That's a huge jump in capacity but I have no way of knowing if the B7800 can actually provide this pressure.

And if it could then the next question is will it damage the B7800 frame.

Thanks for all the input. Great forum.
Tim