Hydraulic Cylinder Loading Conditions

[George Zucas]

Hello,

I am working on a hydraulic cylinder application that will lift some weight that is not aligned with the axis of the cylinder. Take a look at the picture and imagine that instead of ropes there is a hydraulic cylinder fixed to the beam doing the lifting. The load creates moment since it is not aligned with the cylinder. The thing is, hydraulic cylinder catalogs always mention load capacity, which is probably assumed to be aligned load. Since the load create tension, not compression, I think buckling check is not appropriate as well. What would be the criteria in this case?

Thank you for any help.

 

[Baluncore]

Firstly, a cylinder is “stronger” when extending because it has more effective piston area. That is because the rod area is subtracted from piston area on the rod side. When used in a simple tension application you should select a cylinder with a relatively small diameter rod.

The connection at the end of the cylinder and of the rod is usually an eye to prevent misalignment or bending of the rod.
Are you proposing to rigidly fix the rod to the load ?

If so, you will need a thick rod that will never exceed the elastic limit. You may also have problems with the rod and piston seals due to side pressure. When fully extended the direction of the rod may not be well defined.

 

[George Zucas]

Yeah, actually I used quite a bit of cylinders in this application so I studied all that. For example, for this one, the cylinder I'm thinking of selecting can carry 2.1 times more load when extending (piston area data is from a manufacturer's catalog). For the ones that are compressed I checked the buckling case.

This is a bit of a different case, it will neither extend or retract when loaded, it will basically be a structural element. I drew a very basic representation of the system, basically the load will be on the middle of the smaller horizontal tubes. The big tube slides in wall holes. The piston will carry all of that in the shown vertical position. The entire system moves vertically (including the support of the cylinder so it won't extend, it will also move with the rest). When in position, the piston will extend itself and get rid of the load. Hopefully it is clear :).

Sorry I didn't mean to rigidly fix it, there will be an eye.

I'd think that the given load capacity is for a load directly in the axis of the cylinder. In this one , loads create a moment. Should I consider that or should I only consider the vertical force (which is weight of the system and the load)?

Also I think that since the cylinder is trying to pull back the system, instead of pushing, this is a more critical case so capacity is much less ( I am using pulling load capacity).

Thank you for the answer.

 

[Baluncore]

I'd think that the given load capacity is for a load directly in the axis of the cylinder. In this one , loads create a moment. Should I consider that or should I only consider the vertical force (which is weight of the system and the load)?

Load capacity of the cylinder is specified along the cylinder–rod axis, assuming "ball" type end joints. You know the maximum fluid pressure specification and the area of the piston, (less rod area), so you can compute the load capacity under tension. If the cylinder does not remain vertical then the force along the cylinder may be greater than the weight supported.
If the ends of the cylinder are free to align themselves, then there can be no moment applied to the cylinder through the eyes.

I think you should prepare a diagram that shows the cylinder in the surrounding structure that positions the cylinder.
What keeps the prongs in your picture horizontal? They should hang down due to the lower pin joint.

 

[George Zucas]

I try to provide only representative drawings so that I won't get into trouble with my company. The picture is a bit different from the actual case. Sorry, english isn't my first language so I have trouble explaining everything clearly. There are actually two big connected tubes like the one in the picture and they are placed in rectangle holes (well not holes but I don't know what to call it, maybe extrusion), they have wheels on each side. So they are driven in those extrusions. The prongs are on the bottom tube. Since there are four supports (wheels) it cannot rotate as you said. The hydraulic cylinder keep this system from going down.

Think of it like this, it is a car on a completely vertical road, hood (prongs) is open and there is load on it. The cylinder is attached to the back of the car, preventing it from going down.

But I see your point. I'll try to prepare a better visual after work, previous one was a rush job.

 

[George Zucas]

Now I see what you mean, stupid me. The moment created by the load should completely be supported by the wheels since the connections are eye type. That is the whole point of it. The hydraulic cylinder will only pull the weight of the system, the rest will be dissipated by the wheels which are very strong.

Thanks a lot.

 

[Baluncore]

Thanks a lot.

That is OK. Getting the question right is usually the biggest challenge. It is a free service we offer.