Did anybody understand the hydraulic article? I thought it was OK but I don't like the word sometimes because it doesn't specify when that sometimes occurs. Does sometimes mean some designs or under some conditions? I prefer speak in terms of forces since it is really force that moves objects. An object will accelerate or decelerate as long as there is a net force acting on it. The net force must take into account the load and its orientation. Changes in the load and orientation can affect the speeds at with the sum of forces become 0. So can friction. The article is trying to say that normally the actuator will extend because the net force of 0 is reached at a higher speed when extending than when retracting BUT a load opposing the extending motion can slow the the maximum extend speed down to where the system extends slower than it retracts. If the pump is flow limited it may not be able to maintain a constant pressure. If the supply pressure is less extending than when it is retracting then actuator may also retract faster. The VCCM equation will predict the maximum velocity as a function of supply pressure and load.
It is force that causes things to move. Newton's first law applies here. See the VCCM equation. The VCCM equation calculated the maximum velocity of an actuator. By find the velocity were the sum of forces acting on the actuator are 0 and therefore aren't accelerating.
For steady state conditions, bottom line boils down to: is the system pressure/force limited or volume limited?
Pressure and force limited is the VCCM equations and orifices in the valves. The sum of forces on the piston = loads and accelration, basic Summation of F = MA
Flow limited is simple open loop gear pump and cylinder or motor. Big valve, smaller supply. Speed determined by pump and pressure drops are heat but not limiting the final velocity or forces.
Jacks curves show the parabola from Stall force at no speed, to slewing speed at no force. Just depends on if the pump/supply limits the circuit before it gets to pressure limited, or the pressure/valve/cylinder limit the circuit by pressure drops.
Accel and dynamics are Peter's world....
Posts: 151 | Location: Minneapolis MN USA | Registered: 02 November 2005
Not just my world. Anybody that want to compete and win will be asked to move faster, smoother and more accurately.
BTW, the George Keller book, Hydraulic Systems Analysis, in the H&P book store covers a form of the VCCM equation. It is a bargain at $35 but the math is probably more than what most people can understand. The first edition was written way back in 1964 for Boeing engineers. This is long before Jack Johnson dubbed the term VCCM. I wonder who really worked out the VCCM equations ( force balance ) first. I just know Jack Johnson wasn't the first but he did a lot to make it known.
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