Hydraulic Motors

[oldpilot]

Hello; I need to rotate two tables with hydraulic motors 6rpm each. Motor volume 12in3/rev each. Pressure
at least 1200 psi. Tables are under water at 180 deg f
and are loaded with 1200 pounds above the water level.

Could I use one three way motor spooled valve and
hook up both motors in series? or parallel? or use a
separate valve for each motor from the pump??
This is my first post. Thanks for a response.

Mike

 

[hydromech]

Mike...

First thing is the speed is very low. I guess you will be using reduction gearbox because 6 RPM is below the minimum operating speed of nearly all motors. You would find they keep stopping and starting.

The minimum speed is usually around 50 RPM.

The most cost efficient way to synchronise multiple motors is to use a rotary flow divider and run the motors in parallel. A good flow divider will give 95% accuracy.

You cannot run them in series, it won't work.

One pump, one control valve, one flow divider and two motors.

If you need independent control of each table you will need some sort of auxiliary control system...

Can you let us know how you intend to control the tables?

Regards

Adrian

 

[oldpilot]

Thanks for the reply. I have decided to buy larger
motors at 24 ci/rev each and live with about 30 Rpm.
These tables are 36 inches in diameter and the parts
are evenly distributed for spray washing at 2000 psi
130 degree water. The most economic way to run might
be two separate motor spool valves with flow controls
on each for speed adjustment. Pump is set at 2.5 GPM
and max pressure is 1500psi. Both tables don't have to
be the same speed.
What do you think of this system now??

Thanks;
Mike

 

[hydromech]

Mike...

They may not need to run at the same speed, but if they need to run at the same time you will still need a flow divider otherwise the table with the least resistance will rotate and the heavier table will not move.

Even if they are evenly and equally loaded, mechanical stiffness of the assembly, efficiency of the motors, temperature, oil viscosity...they can all make the system unreliable.

There are things that can be done to artificially increase the working pressure of the motor to make them run at the same time, but that depends on what type of motors you use. Internal/external casing drain.

If you need to run the tables completely independent of each other, then it is a very simple system. You just need two directional control valves and some flow controls.

Most motors will work upto 176 deg F.

The published minimum speed of most motors is about 50 RPM. Satisfactory performance at lower speeds cannot be guaranteed. They might work at 30 RPM they might not, you would have to ask the motor manufacturer.

Regards

Adrian

 

[budt]

Another option is to use a Double Pump and feed the motors from their own pump. Same results as a Flow Divider but less plumbing.

Add a 4-Way, 3 Positionn Open Center Directional Control and have the option of running One at a time, Both together and in same or opposite rotation.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[oldpilot]

Hello;
Thanks for the reply. I called the motor manufacturer this morning and he guaranteed that the
motors could be hooked up in series and operate off
one motorspool control valve with flow controls on
ports a and b. He said that that's the way car wash
brush systems are set up--interesting!
I guess I will just give this a try in the next few
days and will post results...
Thanks;
Mike

 

[hydromech]

If your man at the motor manufacturer is correct, and the pressures you stated in the earlier posts are correct, the first motor will take 1200 PSI and use that energy to turn the table.

The pump only gives 1500 PSI, where will the energy to run the second motor come from?

There is a big difference between turning a brush to clean a car and turning 1200 pounds.

I can't see it working, but I'm interested to see what other people have to say.

Adrian

 

[hydtools]

Running motors in series will work, IF:
The pump and driver will deliver 3000 psi to meet the motor demands for pressure. Motor pressure demands are additive for motors in series.
The first pump in series has a case drain or a shaft seal capable of full pressure so it will tolerate the pressure demand of the second motor. And the first motor shaft seal will experience full pressure if the second motor in series stalls.
Both motors are rated for 3000 psi, full system pressure.

Ted

 

[hydromech]

Everything after IF: does not apply here...

He only has 1500 PSI from his pump...1200 is used on the first motor. The 300 PSI left over won't do much.

It won't work!

'least not with a 1500 PSI pump.

Adrian

 

[PNachtwey]

Why won't it work? If the flow goes through two motors in series then how can one motor move without the other motor moving since the same oil is flowing through both? It should NOT take 1200 psi just to turn the motor. The pressure from the pump will be divided evenly across the two motors UNLESS there is a difference in the motor loads. In this case the pressure drop across the motor will be proportional equal to whatever torque is require do to keep it moving. Since these motors are just spinning brushes in a car wash there should be much load unless a brush spindle hits a car and stalls. In the normal state the two motor should have a pressure drop of 1/2 the pump pressure. It shouldn't take 750 psi to turn the brushes either.

Two motors in series makes sense to me. Simple plumbing and only one flow to control. I like Bud's idea of using one electrical motor to drive two pumps. The pumps can be put in parallel to achieve high speed or just one can be used for low speed.

 

[hydromech]

Equating pump output to power...

Most of that power is used up by the first motor in turning the load. The oil exiting from the first motor will be low pressure until it reaches resistance in the second motor. At that point the pressure becomes cumulative.

There is not enough power(pressure) from the pump to turn both motors in series. They will rotate freely when unloaded, but there will not be enough power from the pump to turn two loads together.

 

[hydtools]

The 36" tables are under water. That is a lot of drag even at low speed. If the motors demand only 500 psi each, then the system will have to deliver >1000 psi. The first motor seal still has to deal with the 500 psi demand of the second motor plus the pressure to return the fluid back to the tank. If the back pressure is 200 psi, then the demand on the pump is 1200 psi. It could work. But the motor loads are still an unknown.

The IF was to lead to those conditions to be considered when running motors in series. The actual numbers depend on the motor demand and system design.

Ted

 

[kcj]

Sounds like the real IF is what are the actual loads?

if the 1200 psi was from actual test, with the motor size listed, then you know what size motor and pump and electric motor to pick. Getting the brushes up to rpm could be signficant inertia, meaning high RV setting. Spinning might be low load. Stalling is what load? So I think the key is to take a simple psi reading, and get the motor size and reduction ratio/drive, then run the numbers.

and at risk of opening the can of worms again, there is no such thing as a 1500 psi gear pump! There are pumps that are rated to maximum pressures of 1500 psi, or 3000 or whatever but the pressure the actual circuit sees is a result of the load, not the pump nametag.

Personally, I'd vote for buds idea: single electric motor with two section gear pump, two individual solenoid valves. Independant control, can reverse is desired, less case drain potiential issues, and maybe cheapest? Cost of the divider is similar to cost of the pump as they are about the same hardware, so no need to have pump + divider.
I would not use a spool divider at all unless the motors would never run separately. Too much heat if only one is runing. But two section pump is efficient even with one or two motors on.

If you do flow control, I would use either a bleedoff FC or priority divider, but putting FC on the A & B ports after the control valve means the pump would be operating at full RV setting all the time, and venting the excess flow at full pressure. i.e. lots of heat and wasted engergy and cost.

kcj

 

[hydromech]

I think it's all academic now anyway...

Oldpilot seems to have given up and gone.

As always there are probably several ways to get it done, but with sketchy details everyone ends up arguing about things that probably does even exist.

Regards

Adrian

 

[oldpilot]

Hello All! Havn't given up just didn't know that there would be such a great response for this problem.

The pump is rated at 3000 Psi at 2.5 gpm. I will have
to use the flow controls to slow the tables down to around
25 RPM. Both pumps are under water on a framework with
sealed roller bearings supporting the weight. Both table
tops are above the water line so drag is minimal.
I will try using a sol valve for each motor and see if
it is successfull. Motors are rated at 2000 psi each.
Appreciate all the advice on this problem and will post what solution is found mid next week when it is all
put together.
Thanks;
Mike

 

[Newmanite]

Many mobile hydraulic blasthole rigs use in line system(in series)motors for their rotation of the drilling equipment, and to confuse the issue many also go the parallel route.A set of fixed displacement motors and a variable control valve and a single or dual pump set up works great I have worked on both types for years.You just have to decide which is more practical for your application

 

[oldpilot]

Great NEWS;
I was able to install the hydraulic unit with
both pumps under their own three way sol valve. At a
flow rate of 2.5 GPM the flow controls were almost
all the way closed but RPM was dropped to 18 for each
motor. Both are run at the same time during the five
minute cleaning cycle. The pressure was high almost
2500 psi to do this.

Once again, thanks for all your responces to this
problem.

Note: I first hooked up the two motors in series and
could only get them to operate at high rpm and
high pressure. Under load the second motor
would stop and start also.

Thanks again;
Mike

 

[PNachtwey]

Note: I first hooked up the two motors in series and
could only get them to operate at high rpm and
high pressure. Under load the second motor
would stop and start also.

This doesn't make sense. How can one motor stop if the other motor is going? Where to the oil go that passes through the first motor go if it didn't go throught the second motor?

 

[kcj]

Is this a gear pump system?

Using a two ported, inline FC on a gear pump is why it will run at full relief valve psi-the extra oil has to go somewhere else, and only alternate is across relief.

Or, are you using a three ported priority flow divider? Then it should only throttle the extra oil enough to run at load pressure.

If you only have a two ported flow control, a bleed off arrangement, where it takes oil around the motor to tank, will keep the pressure down. have to see the schematic to know how to do two motors/valves if running off one pump.

kcj