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Slide 13 isn't quite right. Hint the problem is with the left cylinder.

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Originally posted by Bud T:

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Slide 13 isn't quite right. Hint the problem is with the left cylinder.

I give up Peter, What is wrong with the slide????

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The gage reads 0 psi. What about the load induced pressure?

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'What ever' is not an engineering term.

I knew that, But the untrained seem to undestand the term.

Since PG1 is at 0 PSI there must be a lot of pressure dropped across the counter balance valve.

You can read about Counter Balance Valves and their function in Chapter 14 Pages 3-6. Essentially they are a Relief Valve used in a diferent circuit location to control flow in relation to pressure. They are set 100-150 PSI higher than the Load Induced Pressue and must be opened by pump flow to the retract side of the cylinders. This keeps the cylinders from moving until the circuit wants them to regardless of the Load force. Been around for years and can be Internally Piloted as shown, Externally Piloted and Internally/Externally Piloted to achieve different end results. That means the Runnibg Away Load will not move any faster than the pump flow is forcing it to. Stop pump flow and the cylinders stop.

If you were right about the pressure being at 0 then the left side of the platen

Pressure is not at ZERO on the left side of the Platen it's at ZERO at PG1 on the Right Side (Outlet) of the Counter Balance Valve.

Pressure on the Left Side of the Coubter Balance Valve is Whatever Load Induced Pressure is while the platen is stopped and Whatever the Counter Balance Valve Pressure Setting is while the Cylinder is being pushed down as fast as the pump supplies oil to its given area on the retract side of the piston. That means the Load will not fall butonly move as fast as the pump pushes it. The Counter Balance Valve causes an Overrunning Load to act like a resistive load and the resistance can be set as desired.

would try to fall with an acceleration rate of 12400/8000 or 1.55g which is faster than what the load on top the platen would fall. That would be interesting to see. I am sure the 'guided' platen would bind first and the resistance of twisted metal would slow down the left actuator. BTW, 1 lb force ( lbf ) applied to 1 lb mass ( lbm ) will accelerate at 1g. ( It would be nice to use metric units. )

There is no indication what the pressure really is on the top right cylinder but lets just do some math.
The force pushing down is

Pb*10+4500-100

where Pb is the b port pressure to the top of the right cylinder
and the force pushing up from the bottom in your diagram is still 4500 lb. If you do the math we have

Pb*10+4500-100=4500 so Pb=10 psi. That means there must be a lot of pressure drop across that land of your Bang-Bang valve. If there is not pressure drop across the valve then the next force would be

2000*10+4500-100=24490 lbf

If I divide that my the load on the right side I get an acceleration of 24490/4500=5.4422g which is MUCH faster than the load on top of the platen will fall. You can see that the right side will fall much faster than the left side but again I am sure the resistance to motion due to twisting metal will slow things down.

Actually, the accelerations will be worse than I predict because after the system starts accelerating down the load on top of the platen will not be in contact with the platen so the hydraulic force will be applied to the platen only.

Acceleration will only be as fast as the pump moves the cylinder since the Counter Balance Valve must be forced open to allow flow after its pressure seting is reached. Anytime Pressure at the Counter Balance Inlet drops below its setting it will start closing and throttle flow at its setpressure.

The point is that both of these cylinders must go up and down at the same speed and will accelerate at the same rate ( ignoring oil compression ). This means the force relative to the masses must be the same while accelerating and the the net forces for both actuators must be 0 at a constant velocity. I suspect you are counting on a lot of structural stiffness in the guided platen to 'balance' the forces.

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Bud: From what I understand, this model was used as an educational purpose and although the numbers may not be exactly on, it displays the required task of conveying the type of system logic required to preform series cylinder operation.