Tom Jenkins
Lifetime Supporting Member
Well pardner, I can't speak for Alaric, but I is often wrong. Not this time, though. (Actually, I was kinda feelin' ornery and did my post partly jist to gitTom and Alaric are both wrong.
yer goat, Peter.)
Tom and Alaric are both wrong.
An actuator will accelerate until the sum of forces acting on it are 0. At that point there will be no more acceleration and maximum velocity is achieved.
Well, yes 'n no. Ya gots to remember, Peter, thet not all hydraulic systems are highly engineered precision motion control packages like the ones you deal
with. (And I might add you is undoubtedly a world class expert in thet field.)
Some sysetms is jist brute force producers. Dynamic considerations is usually negligible. A good example is the old manure bucket I used as a kid back on th'
farm. It was an open loop system, with a single acting lift cylinder and a gear pump direct driven off the engine and a relie valve that dumped plain ole 10W
engine oil direct to the hydraulic reservoir.
Now, when I shifted the spool in the valve they was obviously accelleration, but the time frame was imperceptible to the human eye. The cylinder hit max
velocity durn near instantly, seein' as how max velocity was very slow. If I wanted more flow I'd open the throttle on the engine and the bucket would go
from very slow to jist slow. More FLOW meant more GO, on accont of cuz the pressure capability of the pump was way more than the load induced by the bucket
of @*$&^! (Let's jist call it a fragrant dairy by product).
So, You is right, Peter, that the load would accellerate when flow was given a path to the cylinder. The force and rate of acceleration was determined by the
pressure setting of the relief valve and the area of the cylinder, up to the point that the terminal velocity of the cylinder was reached. This hyar teminmal
velocity was determined by the flow rate of the pump. When terminal velocity was reached, then the pressure x area inside the cylinder equalled the load of
dairy by product so accelleration stopped. Flow made the cylinder go, and more flow made the cylinder go more faster!
Now if I added load to the bucket while lifting (say, by driving the tractor into the pile of dairy by product while raising the bucket) my cylinder wouldn't
slow at all. Thet's cuz the load still weren't enough to exceed the pressure x area of the cylinder. The gear pump kept pumpin' at a constant rate and the
cylinder kept movin' at the same speed.
On the other hand, if'n I hooked the bucket under the corner of the barn, the load exceeded the force capability of the cylinder at the max relief pressure
setting. I had force, of course, but not enough to cause accelleration or movement. The net force wern't zero, but there weren't no accelleration cause the
net force was in the wrong direction for cylinder movement.
A net force causes acceleration
Yep - no doubt about it. And when the net force is zero - like my bucket at steady state lift velocity - acceleration is zero too, but velocity ain't.
Acceleration is integrated to get velocity.
Well, thet is jist a typo on your part, Peter. a = dV/dt ....which ain't integration. No change in velcotiy means no accelleration. Wen my bucket was
climbing towards the sky with full pump flow to the cylinder velcity was constant so accelleration were zero.
Flow is caused by a difference in pressure.
Well, actsually more often the change in pressure is caused by flow. The pressure drop and movement of oil through them hoses and tubes caused a frictional
pressure drop between the pump and the cylinder. If I hooked a hose on a tree branch and tore it loose at the pump, I had all kinds of flow but the pressure
drop was negligible. Thets cause the pump kept pumping, but hte system had no restriction so the discharge pressure was zero. Thet is also why the spool
vents direct back to the reservoir in center position - full flow, but small restriction so small pressure drop and less wasted power.
It works the other way, too, of course. With thet ole single acting cylinder the wieght of the bucket and product dropped the bucket when I moved the spool
to zero. The pump flow was diverted direct to reservoir. The weight in the bucket created pressure in the cylinder, which created a pressure difference,
which created flow. I could add more restriction to flow by feathering the valve. This would decrease the downward velocity, since the bucket was a constant
pressure source. Forces would balance because the pressure drop through the valve had to equal the pressure in the cylinder. Pressure drop through the valve
would vary as the square of the flow. So pressure difference was creating flow, at the same time flow was creating pressure differnce. Cute!