Different Pressure Demand on Different Pumps

[ZacMech]

Hi, hopefully someone can clarify something for me. The application is the use of a cement mixer as a vessel to mix compost (on my father's farm). The drum of the mixer must rotate at a slow speed (~3rpm) and is powered by a hydraulic motor. Three different pumps have been used so far on the system.

1) Stock cement mixer hydraulic pump driven by electric motor - this was swapped because it was very large and akward (centrifual pump).
2) Allis chalmers tractor (auxillary function). This was used for testing (not sure of the pump type).
3) Muncie pump driven by electric motor (gear pump).

With all three pumps, the speed was adjusted to obtain the same drum speed of 3 rpm (using pulley's where an electric motor is involved and the throttle on the tractor). The material in the drum was not changed between the different tests. Also, there is a pressure gauge mounted directly before the motor inlet. The pressure reading for all three pumps was quite different.

1) ~1000psig
2) ~1700psig
3) ~1450psig

I was under the impression that pressure will build in the system until it meets the load requirement. Since the load didn't change between tests, why is there such a variation in pressure? Is the shape of the pump curves enough to explain this? Thanks for any clarification as I am really curious about this.

 

[budt]

What was the pressure gauge location in the circuit?

If it was at the hydraulic motor inlet and the outlet was free flow to tank and the load was the same it should have read the same for all the pumps.

If it was at the pump outlet and there was any change of the flow resistance, such as a Flow Control to adjust the speed, then you were reading the Flow Controls resistance and sending excess flow over the relief valve at its set pressure.

A ciruit schematic would be a big help in diagnosing your situation.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[hydromech]

As Bud says, a schematic diagram would help.

At this stage is seems that the pump are supplying far more oil than the motor needs to make 3 RPM.

The pressures you are seeing at the motor are the load + the back pressure caused by trying to push too much oil through the motor. Pump number 2 looks to have the most output.

Can you slow the pump at all?

Adrian

 

[IceStationZebra]

You didn't measure the pressure drop across the motor, which should have been the same for all 3. The tractor most likely had the most back pressure due to the ISO couplers and probably more adapters/hose.

ISZ

 

[ZacMech]

Ok, I checked into a few things. First off, the schematic for the present setup - with the Muncie pump - is attached below (apologies if there are any small inaccuracies, but hopefully the idea is clear).
budt - for pumps 1) and 3) the gauge was connected directly before the motor. For the tractor it was connected at the outlet of the tractor, then about 6 ft of hose ran to the motor.
Adrian - the pumps were slowed down (using pulleys when coupled to electric motor and throttle when running from tractor) in order to obtain the required flow through the motor to give 3rpm for the drum.
ISZ - that is true, only the inlet pressures were measured in all cases. Also worth note, in the concrete pump circuit there were no valves (apparently the pump had a built in flow control) so pump outlet in that case ran straight to the motor and then to tank.
Could the explanation be changes to the system after the motor? Would the differences in the return line from the motor account for the fairly large changes in pressure required?

 

[hydtools]

The plumbing on the return side can make a big difference in your single point pressure measurement at the motor inlet. The backpressure from the motor outlet to the tank adds to the pressure reading at the motor inlet. It also determines the pressure on the motor shaft seal.

Ted

 

[budt]

Do you have a Model Number and manufacturer for what appears to be a Priority Flow Divider that the pump outlet goes diretly into? The one that has a Tank connection and a flow line to Directional Control Vlve.

Also, that valve has Port Markings such as "P" "IN" "T" "OUT" etc. Show the markings on your schematic to see if the valve is plumbed correctly.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[ZacMech]

The flow divider is an FCR-51-1 from Brand Hydraulics (it is the 0-50gpm model) which is grossly oversized for this application, but it happened to be readily available. A properly sized one is on order. Also, a detent valve is going to be used instead of the present spring return. It is easy enough to hook a pressure gauge to the outlet of the motor, I will do this ASAP and check the differential pressure across the motor.

The mention of this valve leads me to another question. It is not really there to regulate flow (i.e. to control the speed of the motor). Presently, when starting the system (full load in the drum) the flow is all diverted to the EF port, then the CF port is slowly opened until all flow goes through the CF port. If the system is started with the CF port already fully opened, the electric motor draws too much current and will blow the breaker. Everything runs fine as long as the flow is "eased" on with the FCR-51.

I'm curious as to why this is from a fluids perspective. The flow divider is acting like a clutch, but I'm not really sure how it can be explained from a pressure/flow perspective. Thanks, this is all very educational!

 

[budt]

I have used this Hose Break Valve for Mobile Equipment to eliminate Starter Motor failure due to the diret driven pump not letting the engine start running at a high enough speed before the pump sees pressure.

http://www.hydraulicspneumatics.com/200/eBooks/Article/True/67859/

It is in a line teed into the pump outlet and is plumbed back to tank. You buy it set for approximately 75% of your pump flow. This valve alllows the pumps electric motor to be almost full speed before it sees pressure and starts pulling high amps.

The other option is a Manual Ball Valve rated at your pressure in place of the Hose Break Valve and open it when starting the pump and closing it after you are up to speed.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[hydtools]

Bypassing all the flow with the flow control allows the electric motor to come to speed with low pressure demand, low horsepower demand. Full flow, but low pressure. The same could be done with a three-port diverter valve ahead of the flow control and leave the flow control set. Or a two-port valve to bypass flow when it is open. Use it instead of the closed-center four-port, three-position motor valve you show in your schematic. Do you need reverse flow?

Your schematic shows the motor connected to the P port of the flow control. The CF port should go to the motor. Maybe that is a schematic error.



Ted

 

[budt]

I just noticed my link to te Hydac Hose Break valve was not right. Here is the right one:

http://www.hydacusa.com/valves/hose_break.htm

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[ZacMech]

Thanks for the responses, that makes perfect sense, always nice when that happens! A question though, since we do need reverse flow, could a tandem center directional control valve be used on its own (with no flow control)?

That way, when the pump is started the flow goes through the directional control valve and empties to tank (which lets the motor/pump come up to operating speed). Then use the directional valve to turn the drum in whichever direction is required. This seems like it would work, and reduces the number of components/fittings/possible leakage points etc. but am I missing something with this?

Also, I was thinking about a rotary mounted handle for the directional control with a metered spool so the drum speed could be controlled somewhat if ever necessary (I know Brand makes these as well, as do many other manufacturers I'm sure).

Ted - The pump is meant to be on the top right of the schematic and is mounted to an electric motor. Have I used the incorrect symbol for this arrangement? The motor is meant to be on the bottom right.

Bud - I took a look at the hose break valve you spec'd. The idea is that it closes when the flow approaches operating flow, thus diverting the full flow of the pump on to the hydraulic motor, have I got that right? If so, then they seem perfect for this application. Have you found these to be reliable for this use in your experience?
Also, I've been doing some extensive reading of your articles, thanks very much for such an amazing resource for us newbies.

 

[budt]

" took a look at the hose break valve you spec'd. The idea is that it closes when the flow approaches operating flow, thus diverting the full flow of the pump on to the hydraulic motor, have I got that right? If so, then they seem perfect for this application. Have you found these to be reliable for this use in your experience"

I have used them on seveal circuits and never had any negative feedback. One was similar to yours on a piece of Mobile equipment that destroyed startes because the pump would have enough resistance, even in an Open Center valve that the engine could'nt get up to speed before it died. The Hose Break valve is actually a reverse Check Valve in its physical makeup and is sized to a flow you specify that is 10-20% less than Max Flow of the Pump. Once it closes it stays closed under very low pressure.

The only thing that might mess them up is contaminated fluid.

"The idea is that it closes when the flow approaches operating flow, thus diverting the full flow of the pump on to the hydraulic motor, have I got that right?"

It dumps approximately 80-90% of full pump flow before closing. Each one is orificed to mathch something less than maximum flow expected if there is a line open after it.

I suggested to the truck shop that had the problem with destroying starter motors since they originally tried the Shut-Off valves but had no control over the operator using them.

Another fix for your application would be a Normally Open Solenoid valve that would dump pump flow to tank and then close after a timer. Or probably better a Normally Closed valve that was powered open from the Motor Start circuit and de-energized after a short delay to allow the motor to get upto speed.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[hydtools]

Sorry, I misread your schematic. Your flow control connections are correct.

I would change the valve spool to a full open center position spool so that the motor ports are not suddenly blocked when you turn the valve off.

Ted

 

[ZacMech]

Thanks all for the excellent advice, I will try to update with a photo when the project is completed. Again, very much appreciated.