Hydraulic circuit changes for load independent controlled speed lowering

[DarrenTBros]

Hi Fluid Power Forum,

I work more in the electrical control system domain with only a rudimentary understanding of hydraulics.

We are having a reliability problem with the speed regulation of a simple hydraulic system.

Please find attached a hydraulic schematic of a weight lowering station based on our examination of all the components and interconnections.

The aim of the station is to lower a stack of weights via a cradle attached to a cylinder. At the start of lowering the cylinder travels at normal speed and then transitions to slow speed on final approach. The weights are conveyed away, the cradle raises and collects a new set of weights.

During the slow speed phase to full down the weights are sequentially unloaded from the cradle i.e. the cradle travels at slow speed carrying two weights (150 Kg + cradle), then only one weight (100 Kg + cradle) and then no weight (cradle only) until it reaches full extension (cradle fully down).

The problem we are having is that in hot weather the cradle is not always travelling to the full down position as the weights progressively unload.

We have not ruled out electrical problems, however, by inspection, the lower motion is achieved entirely by gravity. The initial lowering speed is determined by One Way Flow Restrictor Z and then once the cradle reaches the Slow Approach To Full Down limit switch, Flow Restrictor X is placed in the return line by energising 3/2 Valve X, further reducing the flow.

We believe the problem may be that the lowering circuit has to deal with varying load conditions i.e. at the start of lowering (normal speed) the load = Weight A + Weight B + Cradle, then it transitions to (slow speed) where the load is still Weight A + Weight B + Cradle, then Weight A disconnects and the load reduces to Weight B + Cradle and then Weight B disconnects and the load reduces to Cradle only to full down. When this is factored in with it being gravity driven, changes in temperature and being one subcircuit to a larger hydraulic system all sharing the one pump and tank, it may be asking a bit too much.

Our questions follow :

. Is this an example of 'not great' hydraulic circuit design for this application ?
. Would it be better to achieve the lowering and raising motions via positive fluid control ? If so, could we rearrange the circuit with the same number of elements only replacing Flow Restrictor X with a pressure compensated type - may need to be one way type (for Slow Approach To Full Down) but maintain use of One Way Flow Restrictors Y and Z (but moved accordingly) ?
. Is there some other elegant solution which might minimise the amount of physical circuitry and hardware to be rearranged ?


We look forward to your response.

 

[HPost]

2 assumptions

1) You don't want to spend a fortune

2) The system worked OK at some point

On the basis that the above are true, then the suggestion is to check the mechanics. You have mentioned that the problem occurs in hotter weather. If the hydraulic system was creating the problem, it would be easier in the hotter weather as the oil viscosity would be lower, hence the flow resistance would be lower and therefore less force required to reach the full lower position.

Question - Will the cradle lower from the top position with just 1 weight on it? If so, then this would suggest that there is an issue with the mechanical assembly at the bottom, possible run out.

There is nothing inherently wrong with the hydraulic system. Yes, it could likely be made better if you have the time and money to spare, but that won't translate to better running, especially if the fault is mechanical.

I'd suggest that you look for a mechanical issue, to see if there is some side loading of the cylinder.

Only 2 things will stop the assembly reaching the full lower position, flow resistance and / or mechanical resistance - friction. On the basis that oil gets thinner with heat and metal expands...I'd be inclined to check the mechanical assembly.

 

[Compositepro]

One possibility is that the weight is not generating enough pressure to overcome the check-valve cracking pressure and the cylinder's internal seal friction.
The simple solution is to add some dead weight to your cradle.

 

[HPost]

That's not a simple answer at all. Adding additional mass will require validation of the new safe working load. It will also require more power to lift the load. Bolting on more mass to overcome any friction is just pushing the problem somewhere else. Not an elegant solution at all.

Anyone can just bolt mass on, it would be nice to engineer a solution, surely?

 

[Compositepro]

That is just plain silly. He is using gravity to lower the load, so dead weight is critical for the system to work at all.

 

[HPost]

Not silly at all. It worked at some point in time, now it is not. Find out why, then fix it.

Is that not the art of engineering? It's what separates engineers from the the masses.

There is nothing for free, more mass to make it go down, means more mass to lift.

 

[hydtools]

What is the hydraulic fluid you are using?

Ted

 

[3DDave]

As HPost mentioned, if it isn't traveling to full down either:

A) there is something preventing fluid from leaving
B) there is a load on the cylinder that is supporting the weight.


The cylinder may be tilted slightly relative to the load path and too much friction is available. Checking to see if the cylinder is plumb will answer part of it, but one must also check that the CG of all the weights is close to the axis of the cylinder as well so it isn't binding. Since the cylinder stops when it is more extended, that would provide a greater moment arm to increase binding.

If there is a guide track the track may be misaligned. Temps are rarely uniform in machines. It may be the uniformity is greater when it's warmer. It can warp a small amount, but still provide enough force to bind the mechanism.

I would put a pressure gauge on the return side of the cylinder to verify that there is pressure as expected and check the flow restrictions to see if they are obstructed. If the pressure in the cylinder goes to 0 before the cylinder extends, then there a mechanical problem. If it doesn't, there is a flow problem.

 

[DarrenTBros]

Hi Everyone,

Thank you for your comments.

Having reviewed soon afterwards what we initially posted, the circuit as initially drawn was clearly wrong. It has taken a little while to fix the schematic and in the meantime we have several responses.

Please find attached the corrected schematic for our hydraulic system (which should make more sense).

The weights are well balanced and guided centrally into the cradle. The cradle assembly is on its own set of robust linear bearings and shafts. The cylinder and its mounts appear to have sufficient 'free play' to self align throughout lowering and raising (but will will recheck).

We expect lots of hot weather here leading into Christmas which will provide an opportunity to do some more observing and take some measurements. I have only been brought into the picture now but apparently this station has been causing grief for some years. If the problem was flow related we too would have expected to see more problems when we hit the colder weather, not when it warmed up. The hydraulic fluid grade we use is ISO VG68.

Some things on our checklist now are (when it jams) :
. See if adding an additional load to the cradle allows it to finish lowering.
. Disable Valve X (bypass Flow Restrictor X) to see if reducing the resistance in the return path allows it to finish lowering.
. Check if we have some mechanical interference in the cradle linear bearing guides (or elsewhere) or binding in the cylinder arising from misalignment.
. Add pressure gauges in the return line (either side of the series One Way Flow Restrictors Y and Z) to see what that tells us.
. Open up (right up) the series One Way Flow Restrictors Y and Z to see if lowering finishes - this may point to there not being enough pressure to overcome the check valve.

 

[HPost]

The change in the schematic doesn't change anything. The principle of operation is the same.

It will be worth checking the oil that the cylinder will be pulling in on the down stroke. There will be more oil needed to fill the head end than there is coming out of the rod end. The system system will need to pull in some make up oil. If it can't, the cylinder will stop when the head end sees a vacuum.

Perhaps you could try loosening a fitting on the tank line or on the head side. If there is a vacuum there, releasing it will allow the cradle to drop. Lower the cradle until it stops, then loosen the fittings. There will be no pressure in the line on the down stroke.