Pascagoula Run

The slow one might have air in its trim system.
Happened to me, after I bled the air out of my slow one, the speeds were close to the same.

Audiofn

The problem with that is I often run my tab and drives at the same time......

propwash

Another way to control cylinder speed is with a in-line hyd. flow control to slow down the faster drive to match the slower one. It's done all the time on hyd. systems. Just make sure it's installed correctly to work properly for you. Propwash.

ljsmith1

With the right volume/pressure pump, and the proper solenoid control valve system, it would be possible to run tabs and trim at the same time. A flow control/check valve system would dump excess pressure/volume if only tabs or one drive is used.

Yep...the more I think about it, a single pump would work very well.

-Larry

Falcon

It would work but you would probably want a pressure accumulator, seperate up and down electric valves for each item, and of course a larger reservoir. Probably more expensive than the current stuff. But I think it would be pretty nice. Any slight differences in restriction and load could still cause differences in speed of trim. An automatic system, combination supply pump or not, would be a big improvement.

ljsmith1

All that would be needed is a bank of 4 double-acting solenoid control valves, a pump and motor combination, a flow control regulator, a few check valves, and a reservior. Not too expensive or complicated. The pump sizing is key as well as the system pressure control. The flow would be limited by piping size and system pressure. If you had a pump running at say, at a 300psi constant, and your drives and tabs only took 200psi to work smoothly and uniformly, the flow control regulator would dump the excess flow (100psi) back into the reservoir. If the loads (flow requirements) change, this valve would automatically dump more or less to keep the system pressure constant - no matter how many solenoids were calling. The pump would have more than enough capacity to handle all loads at once (drives + tabs).

I have seen injection molding machines with multi-actuating cylinders driven off of one HPU (hydraulic power unit). The cylinder stroke speed does not change if one or more cylinders are being actuated at any given time.

Ain't hydraulics and pneumatics FUN!?

-Larry

Falcon

Part of the problem is the pressure needed to trim out against the thrust of the prop. That's why the drive pump trim out pressures are set higher than 2000 psi. That's another reason why there can be different trim rates, the pumps are working at near max pressure. I'm not sure if the tabs would usually need high trim down pressure. With a common pump/supply you would need enough pressure for the highest need.

CObarry

ljsmith's idea is good, but I would add a second pump to generate more pressure under high demand and also to build in some fault tolerance. If one pump fails, everything still works....

Pascagoula Run

With a common pump, the drive that moves the easiest will get the most fluid, thus move the most.
Two independant pumps would be cheaper than the flow control devices needed to utilize a single pump. Not to mention the constant adjustment of the flow controls that would be needed to keep the movement speeds the same with a single pump.
My Formula uses four pumps, one each for each drive and K-plane. I would hate to try running at speed in rough water while constantly trimming drives and tabs with a smaller number of pumps.

ljsmith1

The pressures generated by a single pump are not the issue, nor are the costs for standard hydraulic control valves. If the setup could be packaged properly (i.e. USCG approved, marinized, etc.). I would bet it would cost less than 2 drive pumps and a trim tab pump setup. Actuator friction will always be an issue no matter if you have one or two pumps. As long as the friction is normal (i.e. no binding or corrosion issues with movement), there will be no appreciable difference in speed. The cylinder speed is controlled by pressure and volume. If the pressure is kept constant via the automatic pressure compensating valve, the volume will be a constant because of the finite pump output and the size of the hydraulic lines going to the cylinders. If two drives are actuated at the same time, less excess pressure is dumped to the reservior and directed to the drive movement. The drive should move at the same speed as if only one is being actuated at a time.

We are not talking about large volumes of fluid here, or extremely high system pressures....

-Larry