Estimating Hydraulic Push Needs

[Bargeguy]

Hello - On a barge, we are rigging a formerly electric operating A-Frame hoist with two hydraulic cylinders for smoother operation. It must lift from about a 30 degree position (up from horizontal) to slightly past total vertical. The idea is to lift a load through the A-frame, just past vertical for safety and to relieve the cylinders...then the electric hoist will lower the load to the deck. Each 4" x 3/8" thickness square tube A-Frame arm is 14'-6" long and weighs about 500lbs. We would like to lift up to a 5 ton load with the A-frame, so each cylinder should be capable to lift 5,000 lbs each, in theory. However, because the cylinders connecting points at the A-frame arms are 1/3 up from the a-frame deck hinge axel (5 feet exactly), there is extra leverage needed to accomplish the lift. At the lowest postion, the cylinder will be at 90 degrees in relation to the arm and hinge anchored at the bottom clevis. The cylinders would then be at a 45 degree angle in relation to the horizontal barge deck. The ram extension would be 36" to accomplish the total lift / swing. Speed is not critical, but should be at least the 36" in about a minute or less. How much force does each cylinder need to be capable of? Any takers for this question?

 

[insult2injury]

F=ma

Get out the dynamics text book.

I2I

 

[kcj]

Not a dynamics f=ma problem, but a static = sum of forces and moments problem.

Draw out a scale dwg, apply your geometry for the effective distances the loads and cylinders act on (moments about the pivot point on deck) and you will get the maximum cyl force at some pont through the arc.

It's a fairly simple problem, not trying to avoid answering, but if it's beyond your background or training it would be best to have someone locally review it in person. There may be other issues overlooked here that aren't evident on a BB. One thing that comes to mind is counterbalance or overcenter valves on the cylinders to prevent loads running away when it goes past center, or for safety in case of a hose break or accidental valve shifting.

kj

 

[insult2injury]

It's definitely a dynamic problem. The A-Frame needs be accelerated first. If only statics are performed, only the force required to hold it still in a single position is determined. Movement implies dynamics.

I2I

 

[FOETS]

Force will not be the main determination of the speed as it is a function of flow.

This may be looked at as a torque calculation as you are rotating about the pivot applying forces at several different radii. As above some basic geometry needs applying.

You would also need to know what Factor of safety to apply to this particular device. Being a heavy industrial application be over generous in safety factor for use and abuse. This will also help overcome the acceleration requirement

Simplest is what rating was the electric hoist and did it do the job?

 

[Cyclefrog]

Over-size the cylinders and use hydraulics in both directions. Check valves and flow controls work wonders. Consider using a gear-type flow divider to keep the cylinders in sync.

 

[Bargeguy]

Thank you for your ideas and input folks! The electric hoist was a Ramsey 8,000 lb hoist (12VDC). It pulled the load up to the stop on the A-Frame, then continued to pull the loaded frame to 2 degrees past vertical and stopped. Then we could lower the load safely down to the barge deck for securing.

 

[HydroMET]

All the above and more.

supply a DWG of the geometry and you would get some real answers.

I recommend looking at counterbalance valves mounted directly to cyl ports, (code for lifting around people)

I agree do it hyd both directions

hydroMET

 

[PNachtwey]

Since the motion is very slow and therefore the acceleration rates are low, this is more of a statics problem. The force needed to hold position at each point along the arc of motion. Likewise the ratio of the force applied on the load to the force exerted by actuators is required. It isn't clear where the load is attached but if it is at the top of the A frame then the actuators will be at about a 3 to 1 mechanical disadvantage. This means that the actuators would have to apply about 15 tons of force to hold the same angle. Remember that the sum of torques around the clevis must be 0 to hold position.

Check valves and flow controls work wonders. Consider using a gear-type flow divider to keep the cylinders in sync.

Can a simple flow control make sure the difference between the applied and load torque will provide the desired angular acceleration? This is not a simple problem.

There has been no mention of a servo controller. I don't see how you can control this with out because of the changes in the required forces/torque due to the changing geometry. A servo controller can also synchronize the two actuators.

The hydraulic system gain is proportional to:

Kvpl*sqrt(Ps*Ape-Fl) A portion of the VCCM formula.

The Fl or load force that the actuators see changes due to the changing geometry so the controller gain must also change as the function of geometry. The controller gain changes inversely as the hydraulic gain changes.

Don't ever use counter balance valves with servo valves and controllers. This is a very common mistake. Use blocking valves that are energized open.

 

[ScottyUK]

Unless you have a fantastically accurate servo system the two servos will always be slightly out of synch and will produce an undesirable torsion stress across the A-frame. Whether it actually matters probably comes down to how good the sevo controller is and how stiff the A-frame is.



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Sometimes I only open my mouth to swap feet...

 

[kcj]

If I understand the frame correctly, using feedback and servo, or flow dividers, seems vastly overkill. The mechanical structure is the equalizer, if the frame is stiff enough to handle one complete cylinder force without damage. One cylinder on each side, lines teed together, like a farm tractor or wheel loader (except with counterbalance added on each cylinder) is where I would go.

kcj

 

[PNachtwey]

"Unless you have a fantastically accurate servo system the two servos will always be slightly out of synch and will produce an undesirable torsion stress across the A-frame"

Not a problem. Fantastically accurate servos have been around for years.

I don't think the servo controller is overkill considering the geometry changes. Smooth motion counts for a lot.

 

[hydromech]

I desinged hydraulic systems for launching boats from the side of US Navy LSD's and USCG Medium Endurance Cutters. Although they were not 'A' frames, some did have two arms connected by a 60ft boom and the arms always had to be synchronised.

Some had flow dividers, some had proportional controls and some just had directional control valves.

I found that the only way to get truly reliable performance is to use a good flow divider.

1 pump, 1 valve, 1 flow divider and 2 cylinders. That's all it took to keep these things working in all weathers.

Using servo valves is the "sexy" way to do it, a rotary flow divider is the best way!

Hydromech