Running two motors with one pump keeping speed constant.

[MEM1]

Hello All.

I am working on a soil sampling unit that will be run by two motors and one pump. I want the motors to run at constant speeds regardless of the load. I know nothing about hydraulic circuits and I'm looking for advice on the proper components to make the system work. The two motors will run at different speeds and loads will vary based on soil type. The unit will be mostly automatic using limit switches and electric valves. I don't want either motor slowing down because the load changes. The two motors are identical. One will run at 300 RPM and the other around 600 RPM. I was researching pressure compensated flow control valves. I understand how they work with one motor but with two.

Any help is greatly appreciated.

Thanks
Scott

 

[PNachtwey]

How accurate must this be?
The phrase "regardless of load" is very vague. It is possible to stall hydraulic motors. The motors cannot do more work than the energy supplied by the pump.

You may need a hydraulic servo controller. They can control two motors at any speed combination you want that is possible within the limits of the HPU. Controlling one motor to run at 300 and the at 600 RPM is no problem

Limit switches are not good enough. You need to have an encoder and servo quality valve for each motor.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[hydtools]

Instead of one pump use two tandem pumps.

Ted

 

[MEM1]

Thanks for the insight. The load won't change a lot. I currently use a unit (geoprobe 5400) that runs a hydraulic cylinder and an auger motor. The auger is only 1" in diameter and I am only going 24" deep. The problem I have now is if I am in a clay soil that has a little moisture the auger motor bogs down so I am continually feathering the valve for the hydraulic cylinder to keep the auger spinning at maximum rpm. This is what I am trying to avoid. The current setup only rotates at about 60 rpm. The pump on the geoprobe is a 10 gpm pump and the pressure relief is set at 2150 psi. I have a 20 gpm pump at 2000 rpm rated to 2500 psi. The motors will go 890 rpm at 14 gpm. I am building the power unit from a 22 hp Honda engine. I don't want the engine to run full speed all day long. It should provide the required flow at idle or just above idle. The two pumps would work if I have the room. I will have to check to see if I do. If I can go that way, the 20 gpm pump is a lot bigger than what I would need. Ideally this will be run with a plc, encoders and servo valves at some point (when funds allow). I'm wanting to go with limit switches right now to work all the bugs out of the hydraulics and sample collection sides before I start adding complexity with the electronics.

Again thanks for the insight. It is greatly appreciated.

Scott

 

[hydtools]

The engine needs to run at an rpm above its peak torque rpm or the hydraulic system will bog down.

Ted

 

[EngineerTex]

It should provide the required flow at idle or just above idle.

This setup is not going to do what you want it to do. @hydtools is correct.

Look at the torque vs. speed (rpm) chart for the motor. The 22hp output is going to be at a speed close to the upper end of the chart. Start there and work your way back. How much power does it put out when at or just above idle?

Also, why do you not want the engine to run at full speed all day long? Under no or low load, the engine is using very little fuel.

As @hydtools says, you want the system to run such that when the system hits increased resistance, the engine slows a bit to a point of higher torque. This way, it will go a little slower, but be able to put more oompf into the dirt.

You may also consider a two-stage pump (log-splitter pump) that automatically cuts out part of the flow when the load exceeds a certain level. Yes - this will cause the motor to go slower when meeting increased resistance, but would be more suited for mating to an engine running at low speed.

Engineering is not the science behind building. It is the science behind not building.

 

[MEM1]

Peak engine torque is at 2500 rpm. I can run slightly above that. I just don't want to run it at 3600 rpm if I don't have to. It isn't about fuel consumption. I figure the easier I can run things, the longer it will last. I'm liking the idea of two hydraulic pumps. I think I can make two fit. I'm going to try with the one bigger pump to see how it operates. If I don't like it, I will add a second pump. Thank you all for your help. I am not an engineer and know nothing about designing hydraulic circuits. I spoke with the local hydraulic supplier. They said they could design the system and the plc/controls, but it would cost me between $15,000-$20,000 to get it done. I have a very small environmental consulting firm that can't afford those prices. How many times can I screw it up before I spend more than that? I'm guessing not many.

Again thanks for the help.

 

[hydtools]

These engines are meant to run at high speeds. Bearing oil film is better, air cooling is better, they run smoother. Lugging, demanding excess power at low speed is harder on the engine and you will be disappointed in the overall system performance.

Ted

 

[EngineerTex]

Just a thought or two... check out

http://www.surpluscenter.com

for a lot of hydraulic items for one-off and limited-quantity hydraulic parts if you're not building a production run of hundreds of them. They have pump stacks of multiple pumps there as well. The two-stage pumps (log-splitter pumps) have all of the circuit selection logic built into them and they're pretty cheap and reliable for something like this.

Engineering is not the science behind building. It is the science behind not building.

 

[TimSchrader2]

Hello

Once you get the pump and eng worked out what flow controls you use depends on how accurate the speeds need to be. Two press compensated flow controls could be used to control GPM and speed to each motor to a certain accuracy. If more accuracy is needed you could use a gear type flow divider sized to each flow (probally plus or minus 2% depending on press drops of each motor). Or for the most accuracy use the PLC's and encorders as already mentioned above.

Or to adjust during operation you could use a press compensated adjustable flow control (with elec paddle controls) to each motor and adjust speed on the fly as you operate. If it the motors get jumpy you may need a additional two needle valves (not PCV) on the downstream or outlets of the two motors. To provide back pressure and stability

 

[MEM1]

Accuracy isn't of major importance. I just don't want a system that starves one motor over the other. Eventually encoders and servo valves will be installed after I get the kinks worked out. My issue is the dividing the flow. I don't know what components are available. Are there dividers available that will allow for say 60% of the flow to go one way and the other 40% the other and stay that way regardless of pressure?

 

[hydtools]

An old fluid power expert, Yeaple, advised divide the pump, not the flow.
But, yes gear type flow dividers can be had in other than 50-50 ratios.
Ted

 

[MEM1]

The system is being designed to accommodate two pumps. I believe this will be the easiest and cheapest way to go. Know anyone that wants to buy a 20 gpm clutch pump? It is most likely a lot more pump than I need if I go with two pumps.