clarifications for verification on pins of a lifting arm mmt

[Rufy]

Good morning to all, I am proceeding to analyses concerning the pins of the lifting arm of a ground moving machine.

I have determined various states of critical stress for the machine on which static analyses have already been made on the structures and on which I would also perform static analysis of the pins.

the problem survives not so much in the procedure of fatigue testing, as in determining the actual forces on which to work: I'll explain.
It is obvious that there are peak forces due to obstacles or working conditions at the limit, but I do not think that these are the conditions to which to make a check to fatigue, because the pin would be too oversized! what forces then consider? also the fatigue cycle of what type is it? will never be symmetrical alternation, but will tend more to be asymmetrical alternation in traction and/or compression. but there is a difference, for example, regarding the pins of the bucket fulcrum or bucket cylinders and the pins of the lifting cylinders, which at the limit have a cycle also button from the zero (they are always pushed, unlike the others who have load cycles and discharge).
this according to my knowledge, but the intent of my questions is the comparison with someone more experienced, with more experience than me. maybe have some "solid" indication to refer to, so be sure of the assumptions adopted, or if there is any official document to rely on.

a final clarification concerns the pattern of the moment in the pin, schematized as a beam on two supports. in the external bushings there is a triangular pattern, while on the bushings in the center perno the trend is rectangular. is present then the game between external bushings and internal bushing. I uploaded an image to clarify

image.jpg


between we leave the pattern and we refer only to the lengths lb1 lbb and g. according to the scheme the arm would be given by lb1/3+g+lbb/2, but I was told that considering this arm (which represents the arm of the ideal scheme presumed) the pins are too oversized; the experience suggests to represent the lbb bushing as a perfect fit and limit the arm to lb1/3+g. Does anyone confirm this hypothesis?

My doubt arises from these considerations: why not consider as arm lbb/2+g and recess on lb1? under these assumptions should not be lb1/3+g = lbb/2 + g??? Moreover if it is true that the pin wrapped by a bushing does not flet, then should not flet only where there is the game between the bushings?
In short, the simplifications that have suggested that I do not seem too clear.

Thank you.

 

[gerod]

Bye.
earth motion machines are standardized and therefore you should find the indications for the calculation of mechanical organs (I do not have them under hand, but I think there is something).
Therefore, before proceeding, give an eye to these norms (take the last publication of the harmonized standards of the machinery directive and look for machines earth movement).
In my opinion lb1 and lb2 are not inconstricted but are constraints in which you only have rotation (they are supports).
I would not worry about the internal part (lb1 and lb2) but the concentration just outside lb1 and lb2.

for fatigue you must first identify the cycles and then count. Perhaps on the norms there is written how to make calculations! !
Hi.

 

[Rufy]

Unfortunately in those rules, which I have already consulted there is nothing.. the rules to which you refer should be the en 474, in particular for the blades 474:3, but speaks more than other general performance requirements and safety in general. nothing specific, especially on the type of fatigue cycle.
the problem, said earth earth is this: I dimensioned, in addition to these pins with double support, a swing pin and I get a diameter. in an old car, same performance, this pin is much smaller. apart from the geometric variable of the structure, I think they have not had problems because first of all they are also checked, while fatigue is not said that, despite the performance of the machine is of a certain level, this is actually exploited (maybe it is exploited by 1 out of 20!). Moreover the machine is quite recent as production. to me is not verified, with their diameter, not even with a force equal to half of that considered, so there is some discrepancy. . .
even because the pins with double support come to me like theirs, while the ones with swings, since the forces that pass there are buyable, I get bigger, to them even smaller!

I add that I consider, as you say, lb1 and lb2 as you stand, the question was whether it is a sense to consider lbb as an ink. Ideally not, but I think it's something related to someone's experience, which dimensionalizes by understanding the oversized handcuff. I already am not considering it for the purposes of calculating the moment, if I consider that also the difference between my verification and that of the other pin mentioned above goes further away.
for external of lb1 and lb2 do you mean some sort of leverage effect? I cannot understand, because the force is applied to the center, if lb1 and lb2 are the two supports externally the pin is discharged. I'm not considering the surface pressure test right now, which I'll do later.

Thank you very much for the surgery

 

[magara]

Bye.

comparing me with some technicians / colleagues, for static verification of the pins in the mechanical field, as arm for the calculation of the moment generally consider the game + a part of the external bushing that in general can be placed about 1/3 of the diameter of the pin.

 

[Rufy]

Bye.

comparing me with some technicians / colleagues, for static verification of the pins in the mechanical field, as arm for the calculation of the moment generally consider the game + a part of the external bushing that in general can be placed about 1/3 of the diameter of the pin.

Thank you for your contribution.. I only saw the answer now.

actually for the calculation I used just the arm you say.. I think it is a simplification dictated by experience, not to oversize.

I remain a little doubtful about the forces to be used for a possible fatigue test. . in fact if for a static dimensioning I use the most critical load condition, how do I behave for the fatigue, wanting to check in endless cycle?

I would like to find a link for the forces to be used hard, starting from the most critical configurations that are analyzed statically. .
Now, for example, I've been dimensionalizing transmissions and I have the same problem: I know the maximum critical loads, but for example if I have to dimensional the wheel hub with infinite fatigue, as it changes its admissible sigma, will also change the forces to consider in fatigue.

for pins for example I have considered statically the worst of all conditions, with relative peaks in maximum valve, while for fatigue only that relative to a central/frontal load, at nominal plant pressure. the machine will never always work in those conditions, but oversize 5-10mm a pin costs nothing.. oversize a whole wheel hub instead is less elegant.

Hi.