Help on simple circuit design 3

[pmf]

Go easy on me guys, I'm a novice. I'm building an attachment for a Bobcat skid loader. The Bobcat's PTO will deliver 18 gpm at 2800 psi. My motor has a capacity of 15 gpm continuous, 20 gpm intermittent, at 1300 psi max continuous, 1500 psi max intermittent. Obviously, I need to control the pressure and flow to the motor. I'd also like to vary the speed of the motor. What do I need for a safe circuit?

 

[kcj]

Work backwards from the load: what are you trying to accomplish?

Torque required determines displacement and pressure of the motor.
Speed required deteremines flow to the circuit.
The load thus deteremines what the Bobcat circuit must supply.

Flow comes from the pump, but pressure is determined by the load themotor pushes against.

Any further info on the bobcat work ports? are they constant flow (at a given engine speed)of 18 gpm? that would be a gear or other fixed pump. Pressure would vary with load to a maximum of 2800.

If it is variable flow, 0 up to 18 gpm, there is either variable pump, or some sort of valving that controls flow and dumps the rest to heat in the tank line.

I have some spreadsheets you are welcome to, but first please define the application more, and your background.

Spreadsheets are not a substitute for understanding, they only automate the tedium. The real task of engineering is to understand what is to be accomplished, the options/advantages/disadvantages of how to do it, and the details in carrying it out.

glad to help, just need to know more info.

kcj

 

[pmf]

Thanks for the reply kcj. As for my background, I'm a non-degreed designer with over thirty years of experience dreaming up things mechanical. Just haven't dealt much with hydraulics.
The application is straightening bent rebar; mild steel ranging from 1/2" to 3/4" dia. I plan to mount a drive wheel on the motor with an adjacent idler wheel to grip the rod, and draw it through a die. So there should be very little load until a kink hits the die, when the forces (and pressure) will spike. I just don't have a very good feel for how high that spike will be. I'll want to feed the rod in very slowly, then stand back and crank up the speed.
This project is for a friend, so I haven't actually inspected the Bobcat yet. I'll check that out tomorrow. But he assures me that the PTO has nothing more than an on/off valve. I find that very hard to believe, since Bobcats come in a wide array of sizes and capacities, with an amazing assortment of available attachments. If the machine itself doesn't have any controls, each attachment would have to come with it's own. Come to think of it, maybe that's not so hard to believe. It insures more expensive attachments. Anyway, I'll find out tomorrow. In the meantime, I'm having so much fun with this, that I'm forging ahead with a worst case scenario. Any adivice will be highly appreciated.

 

[kcj]

good info.
The forces to draw through a die might be very high, given the rusty and embossed surface of the rebar. If you can get some idea, (I personally don't have a clue) what those forces would be, that determines the pinch roller/gripper forces required to grab it.

The continuous roller idea would be very fast and efficient. I suspect won't be enough pinch to puol it through a smooth fixed die.

some other ideas:
-a series of rollers, above and below, and another set 90 degrees from that, like used on wire straighteners- Those are commonly used for straightening rod and wire off rolls and making it into straight shafts. The pinch rollers pull the bar through these rollers instead of a fixed die.

-a cylinder pushing at the point of kink, with support at two places, one on either side of the kink. press brake or push pipe bender in reverse. That would be slow production, but would handle some pretty thick bar, and also straighten it in place int he forms if necessary. Your contractor problably already has this technology though, for making the bends in the first place, so wouldn't be asking for help if that tool worked. I am not very familiar with concrete world.

-a cylinder with jaws that pulls, release, retract, grip and pull again. This would be slow, unless it was a 'hand over hand' like used on some oil winch applications. Those have two cylinders, each gripping and releasing in phase. Gets complicated in controls.

If you can get the mechanics figured out, AND a value for force or torque, the rest is easy. Lots of help on this board, as a diversion from the daily jobs.
k

 

[pmf]

Believe me, kcj, I've considered all the ideas you have proposed. They're good ideas, but either too slow or too complicated (expensive) to be cost effective.... except for the rollers (wire straightener). I plan to add those into the next iteration if/when necessary. I've managed to convince myself that the components I have will deliver enough force to do the job. And I already have the components, with very little invested. It's a pretty robust little motor. It'll deliver 4000 lb-in. So why not just try it out and see if it works? I just don't want to blow the motor. With the pressure and flow under control, the worst I'll do is stall the motor and bruise my ego.

 

[pmf]

All right. I've got a little more info. I spoke to the service tech at the Bobcat dealer, and he told me that the output of the auxiliary hydraulics is fixed. So I've got to reduce the flow from 18gpm to 15gpm max, and limit the pressure from 2800psi to 1500 psi max. He also mentioned that I'd probably better dump the excess back to the tank via a third line, to avoid back pressure in the return line. Can somebody help me out with this? A flow control and pressure relief certainly can't be to complicated for all you fluid power wizards.

 

[maytag]

pmf,
You should be able to get the parameters for the RPM for your hydraulic motor and tach the shaft and come up with a designated engine RPM that would put out the max continuous motor RPM. Because of the inefficiencies of both the hydraulic pump and motor 18 gpm may not be too far off from the max continious rpm of your motor. A pressure reducing valve in one of your motor lines would solve your pressure problem providing that the motor will only run in one direction. Maytag BTW-the RPM of the motor will decrease as the load increases and more wear in your pump and motor-with no feedback from the shaft to verify the actual RPM this would only be an approximation

 

[95yukon]

pmf,
The efficiencies of the motor and pump should not be hard to come up with, Bobcat probably uses Sauer-Danfoss, Parker, or Eaton products. I grew up near the Bobcat plant and a guy I graduated with is a mechanical engineer there. It's hard to keep up with the suppliers now because many smaller companies have been bought by those listed. I work with those products on a regular basis.

Piston pumps usually have higher volumetric efficiencies (~95%) than gear pumps (~90%) or so. Keep in mind - not all gear pumps are the same. Early gear pumps were of the fixed clearance type, where there was a fixed clearance between the gear tips and the body of the pump. They were not efficient as the newer pumps. Many higher performance gear pumps use pressure plates on the side of the gears to limit leakage and the gears themselves are used to make the final cut in the body when they are manufactured. I've seen gear pumps on test stands with 97% volumetric efficiency.

If you live in a part of the country where log splitters are sold, go have a look at the valves they use. It sounds like you are looking for something similar to what they come with. A simple flow control valve with an integrated relief if available should fit the bill.

Again, the suppliers above should have the product you need. I used to work for Sauer and created some of their gear product catalogs in the 90's (but worked alongside the piston product desktop publisher). They publish efficiency data and formulas in their catalogs that you can use to determine what you need - both for motors and pumps.

Here are some links you might find helpful:

http://hydraulics.eaton.com/products/pdfs/303c.pdf

http://hydraulics.eaton.com/products/valves_mobile_flow_controls.htm

http://www.sauer-danfoss.com/Literatur/index.html

This option from Parker may help:

http://www.parker.com/frameless/def...DivID=74&GID=0&Type=2&ID=74&TechID=&PSID=6941

or this page of valves:

http://www.parker.com/frameless/Default.asp?substDivId=&FamID=1080&DivID=74&GID=0&type=2&img=1&id=74

 

[danhelgerson]

Pmf contacted me outside the forum to get some information specific to his needs. We had some dialog and I am placing some of the information here in the forum for all to see. I hope you find it useful.

Hi Dan,
Can you please give me some help on this simple question? I've been
fishing for a week on Fluid Power Professionals Forum, and mostly all
I've gotten is a critique of my mechanical design. I'm not worried
about that. I just need to nail down the hydraulic part. I've pasted
the entire thread below.
Any input would be greatly appreciated.

Thanks,
Paul

Paul,

There were some good points in the thread but it is sometimes hard to sort out the wheat from the chaff. Let me see if I can help.

In your application, it appears you have a fixed displacement pump and you are limited to the 18 gpm. The pressure in your new system will be determined by the resistance to flow as the hydraulic motor sees the load. As long as the pressure setting on the Bobcat is above that required by your system, you will be OK.

When you have a fixed displacement pump and you want to vary the flow to your motor, the best way to do it is with a valve that uses a bleed off circuit. Let me explain. Your pump is going to produce 18 gpm no matter what you actually need to use. All unused flow will be directed back to the reservoir. There are basically three ways in which to vary the flow; you can use a simple flow control; you can use a by-pass priority flow control; you can use a bleed-off flow control. A simple flow control, whether it is pressure compensated or not, will cause all unused flow to pass across the upstream pressure control valve. A priority flow control will direct an adjustable flow to your motor and will direct the excess flow to the reservoir. However, the pressure upstream from the flow control will be the load generated pressure plus a control pressure of about 80 psi. With a bleed-off circuit, two paths are open for the fluid. One path is directed toward the motor, the other is directed to the reservoir. By simultaneously squeezing off the tank flow and opening the motor flow, pressure builds to the point where it can move the load. Upstream pressure is governed only by the load. There is some wasted energy as the unused flow is directed to the reservoir but it is the smallest energy loss of the three options.

There is a directional control valve that will do all these things. It is now made by Parker but was originally developed by Gresen and is used in the fishing industry. It contains an integral relief valve and a rotary spool system that allows you to vary the flow to the motor in either direction. It is called an SP-W4.

Some things to consider: This valve is open through its center cavity when in the neutral position. That means that the fluid will pass right through to the reservoir unrestricted. When you hook it up you will probably use some quick connect fittings to disconnect from the existing system and connect to this valve. You will not be able to operate any other functions when this valve is connected because all the flow will be going through the valve to the reservoir. If you need to operate some of the other functions of the Bobcat while the valve is attached, you will need a slightly different configuration called power beyond. This option has an additional port that allows the fluid to pass through the valve and on to another system.

Also, there was a suggestion in the forum that you add a pressure-reducing valve in the circuit. This can be very dangerous. A pressure reducing valve is designed to protect an actuator (your motor) from being pushed too hard. It does not control system pressure. If there is no relief upstream from the pressure-reducing valve, there can be a catastrophic failure of the pump.

I hope this is helpful. Let me know.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[maytag]

There was no intent to give any dangerous advice. The Bobcats I am familar with have a dedicated aux. pump complete with relief valve with which to set system pressure for that circuit and with a set of QD. If the rebar attachment was the only aux. equip. used then it would be simple enough to tweak pressure down to an acceptable level. For multiply use the easiest fix would be a reducing valve incorporated into the motor circuit for the rebar straightner. Maytag

 

[danhelgerson]

Maytag,

I am not suggesting that there was any intentional dangerous advice. I am just giving a general warning about the use of pressure-reducing valves.

As you know, a pressure-reducing valve is really a pressure sensitive variable orifice. It is a normally open valve that begins to close as pressure builds to its pressure setting. This causes a pressure drop across the valve that maintains the set pressure down stream. This valve is capable of closing off altogether if conditions are right. If this occurs and there is no upstream relief valve to protect the system, the pressure will build upstream until the weakest component fails. This could be a pump that splits open, a hose that bursts, a fitting that flies apart, or a coupling that shatters.

It is important to note that a pressure-reducing valve does not provide any protection for the components upstream.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com