OK to "short stroke" hydraulic cylinder?

[ColonelMonk]

Howdy ALL:

I'm making some modifications to an existing mast/boom design.

We want to lower the pivot of the mast so that in the retracted or stowed position, the overall height is closer to the ground. I have accomplished this by lowering the pivot about a foot, give or take...

It is desirable to keep the existing cylinder, so that we only need to stock 1 instead of 2, but I have a concern in that, the new geometry of the cylinder doesn't allow the cylinder to "bottom out" on the full retraction of the cylinder. The pivot arm would come to rest on its hard stop with the cylinder 4" short of full retraction. The stop is on the very end of the pivot arm, creating a beam supported at both ends about 9 feet apart and the load from the cylinder about 38" from one end.

To further complicate it, this "beam" isn't straight. From the pivot, it goes about 40" straight (just past cylinder mount) and then doglegs to the side 30 degrees. So the cylinder would be pulling against the beam near the welded joint.

So before we get into more detail, is it a well-known "no no" to short stroke a hydraulic cylinder? Seems that this situation would arise sometimes out of the desire to use a standard available cylinder stroke, but without the freedom in your geometry to use the entire stroke.

Sorry to say, in my many years of designing equipment I never once designed anything using hydraulics or something capable of this kind of force, so I'm feeling a little out of my element. I'm learning fast, but don't want to do anything silly.

Thanks,

CM

 

[zdas04]

I don't think that I've ever worked with any hydraulic designs that were intended for a high number of cycles that DID hit a hard stop. I've modified the design of a couple of downhole pumps that hat beaten themselves to death with hard tags on both ends. Something that cycles a few times a day can tolerate tagging, but high frequency (hundreds of strokes/hour kind of numbers) cant use the whole cylinder without shortening the component life. I'd expect that as long as your process was not relying on a latch at one end (or both) of the travel, then short stroking wouldn't be a big deal.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[ColonelMonk]

David,

Thanks for your reply. Let me see if I understand you correctly:

You are saying that it's "unusual" to use the cylinder itself as a hard stop, or? This alone would be a great piece of information. Whoever originally designed this system (which was probably fabricated to tell the truth, and not engineered)spec'd the cylinder to stop at max extension/retraction. Movement of the pivot arm in question is 90, from horizontal(retracted) to vertical(extended). Let me correct myself slightly - I'm told that sometimes the pivot arm will go beyond the vertical, but with the geometry I'm seeing that would be due to mfg tolerance most likely...

I see where you say you can't use the whole cylinder as a stop for high frequency use, but I wasn't exactly sure if you meant hard stops as in the cylinder's max/min stroke, or something external to the pivot arm causing it to stop.

A little more information - this mast and boom are not something that would be cycled with anything but occasional frequency. Sounds silly doesn't it? But it's the truth. Think of it as a retractable sign post or something... It gets set up, stays there for day/weeks/months, then is stowed and moved to another location. It's probably overbuilt in the extreme for what it is, we are actually looking at reducing the material bulk....

Having said that, I'm worried about bending/twisting the beam and/or buckling the wall of the beam.

Bending/twisting because it's not tremendously built like an excavator or crane would be, and buckling because the tubing where the cyl mount is, is 5" sq, thinner wall, and the cyl mount itself only 2". So the force from the cylinder mount is not directly to the walls of the tubing in the same plane as the motion, but instead against the thin wall perpendicular to the plane of motion.

With a 2" diameter cylinder and a guesstimate of 2500 psi working pressure, Looking at around 7000-plus lbf from that cylinder correct? I'm still trying to get specs on the pump we use (supplier did not provide it). So if I don't bottom out the cylinder, then I'm pulling a component of that large force.

I haven't analyzed the structure yet for what sort of force is required to lift this, but I suspect that we're well overkill. This is why I'm worried about the damage if I don't stop on the cylinder.

Am I being silly or what?

CM

 

[ColonelMonk]

Furthermore, consider an excavator arm or backhoe.....

Is the maximum travel governed by the limits of cylinder travel, or do they rely on hard stops on the structure to limit travel?

It's really basic, but helps provide a frame of reference for the intended use of the hydraulic cylinder.

I know that there are many applications where the cylinder is used for the hard stop. Looking at my snowplow, as well as the front end loader on our tractor, and the cylinders are the limiting factor in all cases.

So I'm thinking, you meant that it would be a bad idea in the case of extreme high cycling but otherwise it's common.

Still bring me back to whether or not I should use this cylinder or spec another one where the min postion matches my stop point. Obviously overthinking it, but isn't that what we engineers do for a living?

 

[JayMaechtlen]

Well, for low-cycle use-
If you need a travel stop, and it's not convenient to provide it some other way, then it would seem reasonable to arrange the cylinder so that one or both of its travel limits provide that stop.
If that meant that the cylinder then had too much leverage on the device, you could limit the applied force through the hydraulic pump/pressure relief setting.

fwiw
Jay


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[ColonelMonk]

Jay, thanks. I did consider that as an option. Our supplier of this hydraulic power pack is a dolt, I've never seen any supplier so unwilling to supply basic specifications for a unit WE are buying. He didn't want to send me schematics of the valving, or the working pressure and flowrate - oddest thing I've ever seen. We are trying to work that stuff out, no resolution just yet.

So yeah, there is an adjustable relief on it far as I can tell, and once I know what force is required I could limit it if I was concerned about being able to bend the pivot arm.

That was where the cyl stop seemed like a no brainer, it's certainly the strongest link in the chain.

For another idea, what about using a hard stop right on the pivot itself? The force wouldn't be pulling from so far out on the angled beam then, it would be stopped close up, which also has a lower velocity.

CM

 

[zdas04]

For a non-latching application, you are relying on hydraulic pressure to hold the arm in position. Increasing pressure on one side of the piston or the other repositions the arm. Think about your track hoe example. The hoe spends nearly all of its time working with all the hydraulics in an intermediate position, and you add pressure above or below the piston to move the arm to where you want it next. That is what you are doing with your mast, isn't it? I don't think your short-cycling concerns should really be concerns.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[ColonelMonk]

David:

Correct, there's no latch, we are in fact relying on the hydraulic pressure to hold the mast up.

I do get your track hoe example of course, very elementary.... but not just what I was asking.... of course you can stop any sort of equipment without going to the travel limits.

Where I think my example is different is the possibility that the hydraulic system is stronger than my beam material, which would allow a careless user to hold the power on after the beam reached the horizontal and bend something. Can calculate all that, and it's probably what I should do if I want to be quite sure that won't happen.

Other possibility is to just spec another cylinder. The one we have is already custom and not too expensive, just trying to do what the boss wants - that's not always the "right" thing to do...
Thanks for input so far

 

[dhengr]

ColonelMonk:
I don’t have the vaguest idea what you are really doing, or what the geometry of your crane/boom/mast (whatever) looks like, and I’ve seen a few of them. Maybe it’s just my old age dementia acting up again, but a sketch of the system, both structural and hydraulic is a must for a meaningful discussion. You’ve kinda given us some description of what you have, but nobody but you can see it, we can’t see it from here, and have no way of knowing how to put these pieces together, to really make sense out of them. How did it look in the first place and how does it look now; and when you start moving a pivot point at the foot of a mast around, how does that effect the rest of the structure and motions of the system? You really have to look carefully to determine all the things that might be effected, from what otherwise looks like a simple change. If I need a hard stop on a boom or a mast I would much rather that that stop be a structural part of the system rather than having one end of the cylinder stroke or the other doing that duty. I can then put a pressure relief valve on the cylinder for when the stop is encountered. I can then fit the structural stops as one of the last assembly steps, so a hundred other tolerances don’t influence and final results. As Zdas04 suggests, hitting the hard stops is really hard on the cylinder and its end support details. In fact, if the cylinder often goes to full stroke, I like cushioned stops on these units. The structure, the geometry through the full operation range, and the cylinder, must all be worked out together. Until you can fiddle with the geometry, know the dimensions and the structural components, know the cylinder dimensions, operating pressures, pressure relief, etc. it is tough to know what will finally control the design. But, I don’t think I would let the full stroke length of the cylinders dictate the design. Once you get the mast in position, you can use a pilot operated check valve to hold things in place until you want to readjust the mast again.

 

[zdas04]

Sounds like putting a mechanical stop on the mast would be the least expensive option if you are concerned about some gorilla trying to take the cylinder to full travel.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[IFRs]

When using cylinders to their stops repeatedly or by design, I use cushioned cylinders so they don't beat themselves to death.