Constant-velocity telescopic cylindersùdesign fundamentals 3

[aboctok]

I am having difficulty finding good-quality resources that explain the workings of constant-velocity telescopic hydraulic cylinders. I need to develop some understanding, not just learn to copy a mysterious schematic. This doesn't need to go into great depth, but must account in some detail for the pressure distribution across cylinder segments. Thanks in advance for any advice!

 

[israelkk]

To my best understanding, cylinder velocity has no relevance without the load and inertia it pushes, pulls or rotates. You have to solve the complete dynamics of the cylinder fluid filling and the load and inertia properties.

 

[aboctok]

No, I'm not talking about solving for cylinder velocity or any other parameter. This is a particular TYPE of hydraulic cylinder called a constant-velocity telescopic cylinder (or, obviously, any similar or equivalent name). It is called that because a normal telescopic cylinder extends and retracts section-by-section, due to the differences in effective area between the cylinder segments. The sections of a constant-velocity cylinder move simultaneously. I am seeking a clear explanation of the fundamental design, in terms of fluid flow and pressure distribution during extension.

 

[MikeHalloran]

It's unusual for Google to serve up just one hit:

http://forums.hydraulicspneumatics.com/eve/forums/a/tpc/f/8641063911/m/161101861

... which makes it appear that _perhaps_ someone has figured out how to make what you want, but they may not have found a market for it.

Just to attract the attention of any manufacturers who might consider entering the business, how many thousands of these devices are you prepared to buy?



Mike Halloran
Pembroke Pines, FL, USA

 

[aboctok]

Hi Mike, yes that's what I thought (surprised at the scarcity of info). Thanks for the link; I did see that one as I did the rounds in my original searches (before posting here).

I'm also surprised to see that most of what little traffic there is on this topic involves issues of novel design (eg: special projects). But the stark fact is that these devices already exist and are in service; we know this because

(a) the manufacturers list them, and
(b) we are troubleshooting an existing machine

So we are not talking about exotic design challenges or special equipment, just machines that are not ubiquitous. Of course it's as obvious as night follows day that in the end, we may have to consult the manufacturer. However, before doing that, I naturally would like to acquire a basic knowledge of the workings of the device! After all, when the technician asks my opinion, it would be pretty lame of me to just shrug my shoulders and palm it back to the manufacturer. The point of posting here is to try to educate myself in the basics of this particular type—not brand or model—of cylinder. I have a general idea of the internal geometry, but I am after a basic explanation of its operation, in the same way that we have a generic understanding of the workings of NON-CONSTANT-SPEED telescopic cylinders. In those models (which are very common), the stages extend in order of decreasing effective piston area. In the constant-speed type, internal ports pass fluid to the side of each stage—until it is fully extended—into a volume that is equal to the annular area of the preceding stage. It's an interesting design!

 

[MikeHalloran]

The wikipedia page:

http://en.wikipedia.org/wiki/Telescopic_cylinder

parrots the text of the magazine article it references for constant thrust telescopic cylinders. Neither includes a section, or design rules.



Mike Halloran
Pembroke Pines, FL, USA

 

[aboctok]

Mike

That would put us on track—a nice clear section and some design rules. Yes, the Wikipedia article gave me that old circular feeling too.

A Swedish uni page has one interesting section (p7) that compares constant-thrust and regular telescopics:

http://www.iei.liu.se/flumes/tmhp02...1.124218/Lec02_HydraulicSystems_Cylinders.pdf

But I can't quite make enough of that to work out the design rules myself.

 

[MikeHalloran]

I would have preferred the long background story you chose to omit.

Eventually I came across the Parker (Commercial) catalog, which has some useful sections, and separately in several places a suggestion that constant thrust telescopics may be designed and built in some unmentioned magical way, OR just by jiggering the effective areas of all the sections so they are equal. Perhaps that is sufficient.





Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

aboctok, good link.
The check valves in the pistons force the annular flows into the stage piston areas. Each annular area equals the ajacent stage area. The veloctiy of the rod end 'p' is the sum of all stage velocities. By operating area equallity, each stage velocity equals v1. So the velocity of 'p', vp = v1*n. v1 = q1/a1.
It would seem then that if any check valve malfunctions open the stage above the valve would not move at v1. Flow from the annulus area would flow back to the previous stage. It would act like a standard telescoping cylinder. Or some confused motion if the check valves chattered.
Make sense? Interesting. I have never had occasion to look into this type cylinder.

Ted