Introductory Hydraulics Questions - The Forestry Forum

Hello all,

I've been browsing through the forum for a little while, but I figured I would join.  I'm building my own bandsaw mill, and I'm now considering the hydraulics I would like to eventually have.  Initially, I would like to power: head travel, head height adjustment, and clamping dogs; and eventually a chain turner, backstops, and toe-boards.  I've done a decent amount of research on hydraulics, but I still have many questions, so I was hoping folks out there "in the know" might be able to lend me a hand?

I would like to build a portable hydraulic powerpack, that I will connect to the mill with quick-connects to the valves (permanently mounted to the mill).  I have an industrial quality TEFC 2hp 220v electric motor that I would prefer to use as the hydraulic power source for two reasons: (1) I already have it, and (2) the amp draw is low enough that I can run it on a decent length of extension cord from a 15amp 220v outlet.  So that being said, now I'm trying to size a pump for this motor.  My mill is just a hobby mill, so hyd speed is not of utmost concern, but I am shooting for realistic cyclic speeds.  I was reading threads and it seems between 2 and 3 GPM at psi is a good place to be for the mill's functions.  This is where I begin to get lost though...  I figure that psi would be around the pressure I need, so I use the following formula to deduce HP required: GPM x PSI / = (gas) HP.  I divide that again by 2 to get the electric-equivalent HP (because I was told on average 1hp electric = 2-2.5 hp gas).  So to me, driving 3gpm at psi is within the realm of possibility of a 2hp electric motor, correct or no?  Does this all sound right so far?  Does 2-3 GPM sound like an alright flow-rate for a bandsaw mill?

So when I begin to search out hyd pumps, there are a myriad of them.  I've found many that are around my desired flow-rate but much higher psi, or vice versa.  So is it possible to take a hyd pump and reduce the output pressure of the pump to reduce the power needed to drive the pump?  Or does the pressure stay constant and I have to reduce the RPM of the pump to reduce flow (so if I turn a pump at a slower speed: is the pressure or GPM reduced)?  I'm assuming there is a formula out there something like: "GPM = PSI x RPM" but I have yet to find it...

I would be most appreciative of anyone's help!  Thanks! 
There's a few confusion factors, 3 gpm gear pump, the simplest and cheapest type will serve your purpose nicely. For all practical purposes, you do not need to be concerned about pressure ratings. That spec is a MAXIMUM rating, not necessarily working pressure. RPM is relevant. A 3gpm pump turning rpm will only deliver 1 1/2 gpm at . Using an external electric motor is the best way to go unless you need to operate the mill as a portable. If your motor is rpm, couple that to a 3gpm gear type pump, should be just about optimum. Gear pumps are rotation sensitive, needs to match your motor. A control valve bank will have a pressure relief valve, there is none in the pump. Be sure to specify open center control valves. I have added hydraulics to 3 different manual mills, be glad to elp any way I can. You might update the info in your profile so we have some idea where you are located and what facilities you have available. Thanks for the replies all, and thanks for the warm welcome.  I've been out of town for a few days, so my apologies for not replying sooner (I wanted to get all my ducks in a row before coming back with more questions).

I am building a heavily-modified Linn-style mill up here in Canada; I will post some photos soon as I can.  As for the hydraulic system, I would prefer to stay away from a 2-stage pump because I'll be running motors and I don't want the speed randomly changing while I'm operating those motors.  So I'm limited to a single stage pump (also easier on the math end of things).  The powerpack will be stationary (re: not mounted to the mill head), so I will run hoses to the head for the up/down motor... everything else will be attached to the log deck I figure. 

So the general consensus here is that ~3 GPM at-or-below psi will be sufficient for my needs yes? 

Pineywoods: thanks for your offer to help, so I will pick your brains a little further if I can.  Why should I not be concerned with pressure in the system... is it because I will rarely ever reach the max operating pressure using typical functions?  If I do find I reach that limit, then I could just use a larger bore cylinder (which would reduce pressure needed for the same task, but also reduce cycle time) yes?  I have attached a drawing of my proposed setup that I was hoping you could review.  I want to make a powerpack with two quick disconnects which could be hooked to various valve banks on different hydraulic applications (re: not the mill).  I've included a relief valve in the system to dump the pressure when the power pack is not connected to any valves... is this the correct way to do it?  I have shown my two pump choices: which do you think would best utilize the power of the 2hp electric motor without exceeding its maximum amp draw (I'm thinking the 2.73 GPM pump might be best)?  Can you offer any other suggestions for the design of this hydraulic circuit?

Rougespear, I said you needn't be concerned about pressure, based on my experience building 3 of the pineywoods hydraulic turner/clamp systems. That 3 gpm gear pump will stall the 2 hp drive motor long before you bust anything.
It appears to me you are assuming a fully pressurized system, where with the pump running and no control valves actuated, your pressure gauge would show psi. For us doityourself types  there's a better way. Use open center control valves and you will see 25-30 psi under those conditions. Open center valves have an open passage to allow oil to flow freely through the valve bank, through the filter, and back to the tank with very little pressure involved. When the valve is actuated, this passage is blocked and the passage leading to the hydraulic cylinder is opened. Closed center valves have that passage permanently blocked. Pressure will then be directly proportional to the load on the cylinder. Much less stress and wear on everything. Go to the sawmill and milling forum and search "manual to hydraulic". Lots of pics and autocad drawings. If you can't find it, holler and I'll post a link.
Chain log turners are not my favorite, they are expensive, work well, but no good for anything else. I opted for a claw type turner, with a little tweaking, it makes a combination turner and a very nice 2 plane clamp.
External power pack is a good idea if you are fixed location. I seriously considered using a log splitter for a power pack. One enterprising sawyer in Oklahoma built a homemade system and used a hydraulic outlet on his farm tractor as a power source. Said it worked well, but climbing up on the tractor to turn a log got to be a pain..

Quote from: Rougespear on April 05, , 09:01:29 PM
Woodenhead: any word on whether the HP formula you posted is applicable to gas or electric hp?  I've read that it is for electric hp, but when I apply the formula to an off-the-shelf gas powerpack it works out right for them too.  I always thought electric hp was about twice that of gas...

The formula is for an electric driven pump.  My gas powered unit has a 13 HP motor on it.  The formula indicates that I would need about a 6.5 HP electric motor to drive it.  So the 2 to 1 rule seems to prove correct.

My unit is much more than I need, but a local surplus store had a sale - $450 for gas engine, pump, 5 gal reservoir and pressure relief valve.  The intent was to use it in case I needed to go mobile.  Right now I'm using it for my new stationary setup.  I also have a 3HP electric unit with a 3.5GPM pump.  I have the pressure relief set to PSI for that setup.  When I get the opportunity, I will move it over to my new mill location for stationary operation.

Hydraulic Valves, which are used to control hydraulic power by regulating the flow of fluid, are as complex as they are vital to a hydraulic system.

For more information, please visit Huade Hydraulic.

As control managers of the system’s flow, valves are capable of redirecting pressurized fluid, controlling flow to specific areas, or completely closing a line.

Valves have the ability to manipulate flow, the direction of flow, and perhaps most importantly, the amount of pressure coursing through a hydraulic system.

Hydraulic pressure relief valves not only limit the maximum pressure within a hydraulic circuit; they also create a new path for flow when that pressure exceeds a limit.

Let’s examine further how pressure relief valves operate to dispense built-up pressure, what can happen when they fail, and how to problem-solve any common issues.

What Do Pressure Relief Valves Do?

Pressure Relief Valves limit maximum pressure in a hydraulic circuit by opening an alternate path that relieves pressure that exceeds a preset level.

All fixed-volume circuits require a pressure relief valve to avoid excess pressure that can cause system failure. They can be direct-acting or pilot-operated to perform this function.

Pressure relief valves are the most widely used type of pressure control valve in nearly every hydraulic system.
How they work is pretty simple:

1. The relief valve opens when fluid pressure exceeds the set limit
2. Excess pressure is relieved
3. When the pressure falls, the valve closes

For more understanding, let’s examine how pressure relief valves appear on a schematic and in a simple circuit.

credit: learnmech.com/pressure-relief-valve-diagram-working/

In this example, a poppet is held inside the valve by a heavy spring. The poppet is forced off its seat when the pressure exceeds a set value, and flow is released through an outlet into a tank.

Are you interested in learning more about Superimposed Relief Valve ZDB10​​? Contact us today to secure an expert consultation!

Why Are Hydraulic Relief Valves Important?

A pressure relief valve has the critical task of protecting a system from any overloads of actuators that can compromise the system. 

When a pump unloads or actuators are in motion, fluid movement is not a problem. A relief valve is essential when the actuators stall or to balance hydraulic force with spring force, opening in response to pressure and releasing heat. 

Without a pressure relief valve, when pump flow is stalled or stopped, the system would sustain damage, power wastage, overheating, or even failure due to excess pressure. 

Sticking, Leaking, and Common Pressure Issues

Relief valves can last up to 30 years when maintained properly. However, problems can arise that may need some troubleshooting. 

In the meantime, look for signs of common pressure issues, such as sticking or leakage, that can lead to pressure relief valve failure.

If the system is not reaching the designed pressure or is exceeding the maximum pressure, it can be a sign of valve issues. Checking the relief valve for common problems can determine the problem.

Here are some reasons these common issues may occur:

• Wrong calibration – the relief valve is calibrated to the wrong pressure
• Not closing fully – the valve can be worn or damaged after years of service, or debris may be preventing closure
• Sticking – Contaminants like dirt, dust, and corrosion can cause the valve to remain stuck in the closed position
• Leakage – If the valve is not the proper size, is not closing, or is damaged from debris or extreme temperatures, leakage can occur

Wear and corrosion, excess debris, and leakage may lead to pressure relief valve replacement. 

However, to avoid these issues, calibrate and install pressure relief valves properly and plan on performing routine checks and adjustments.

Here are 6 steps to use while setting, checking, and adjusting a hydraulic relief valve:

1. Consult schematics and locate the relief valve, taking note of which circuit may need adjusting 
2. Locate and remove the hydraulic hose on the system side of the relief valve
3. Connect a pressure gauge between the valve and pump (use an adapter if there is not a port installed)
4. Loosen the pressure relief valve as far as it will go until the pressure reading is as close to zero as it can get
5.  Adjust the relief valve
   • Turn the adjuster clockwise until the gauge pressure matches the schematic to “crack” pressure (the pressure at which the relief valve opens)
   • Tighten the adjuster lock nut securely
6. Shut down machinery and remove JIC plugs and caps, reconnecting any hoses removed in step 2, then start machinery and test the relief valve by starting the circuit and checking that pressure matches the set pressure from step 5

Knowing More About Pressure Relief Valves

With their ability to relieve the pressure coursing through the system, hydraulic pressure relief valves are vital preventative instruments that function to make sure damage is not done if pumps and actuators are halted.

Without a pressure relief valve present, the system can sustain damage, power wastage, overheating, or even failure due to excess pressure.