attrite evaluation in the articulated quadrilateral

[AngeloB]

I got stuck on a problem, allego pdf.

1) I move a hatch with an articulated parallelgram.
2) calculation of the forces that the members of the parallogram exchange, graphically (or use the cad, in pdf I illustrated the graphic calculation. )
3) everything works if I do not consider friction.

If I want to assess the effect of friction, I have some difficulty.
how do you evaluate friction in a graphical calculation?

Thank you for your help.
Hello everyone

 

[meccanicamg]

rotational frictions are calculated as a couple of friction, so you will have pairs.

 

[AngeloB]

the axis is attached to the hatch

 

[gtemp]

the axis is attached to the hatch

But I'm stupid!! Sorry about the question.
However I'm doing the calculations and I'm not with the values of the forces at the point
C
I'm sure I'm wrong but I'm 116.86 and 69.7

 

[Onda]

I don't remember how the frictions are calculated with a graphic pattern.
I would proceed like this.
Once you calculate the forces and notice the friction coefficients to the pins I can determine the friction forces. ♪
by friction forces multiplying by the pin radius I find the moment of friction. ♪
I then consider a finished displacement of the piston and determine the work done by the piston. for example with a shift of 10mm the work is 778.30nm (l_pistone).
I redesign the cinematism with the stretched piston of 10mm and evaluate the rotation occurred in each pin. alpha_i rotation is different for each pin
the product of the moment of friction for the rotation occurred is the work of friction. _
I deduct from the work of the piston all the work in the pins and get the net work to the axis of rotation of the door. l_netto=l_pistone-somma(l_i)
I determine the overall efficiency by dividing the net work to the axis of rotation of the door for work due to the rotation of the door caused by the movement of 10mm of the piston.
I divide the force to the piston for the efficiency found and determine the new force to the piston, necessary to win the frictions.
attention that, given the type of cinematism, the friction forces may vary according to the point of work, it is not said that all this should not be repeated for other corners of the door and the cinematism itself

 

[AngeloB]

Thank you

I recalculated with your system, only in the initial position, get an efficiency of 0.8%.
Sounds reasonable.
now I try to repeat the calculation graphically.
the calculation with the cinematic simulator of the cad is instead completely high.
I'll try to get things together.

Hi.

 

[gtemp]

Sorry, angelb.
but the calculation you had initially considered the frictions or not?
where the fc force of 77830 n.
then as I wrote you in the previous post according to me the forces on the 2 pins of the biella are different from your 109.6 and 78.6.
I thank you in advance for the clarification.

 

[AngeloB]

hi g temp.
the calculation, chart, initial not without calculating the frictions previews 78.4 kn of cylinder strength. 77.8 kn is corresponding value that gives me the cad.
on the biella motrice hò: 186.2; 109.3; 78.4 kn

Hi.

 

[gtemp]

hi g temp.
the calculation, chart, initial not without calculating the frictions previews 78.4 kn of cylinder strength. 77.8 kn is corresponding value that gives me the cad.
on the biella motrice hò: 186.2; 109.3; 78.4 kn

Hi.

Thank you.
then I make a mistake in calculating reactions on the biella.
to calculate the moments of the 2 reactions compared to the lower point do I have f1 I used as arms 620 and 370 but the accounts do not return to me.
Sorry about insistence. .

 

[AngeloB]

I managed to repeat the calculation also with the cad.

I did not know why, but I had to manually set for each bond a pair of constant value friction, equal to mi=fi*f*raggio perno (nmm)
in my mechanism the thing goes in favor of safety as the fis are maximum in the starting position.

the result was an increase of the cylinder force of about 9% instead of 19% as in the fully manual calculation.

Hi.

 

[Onda]

I didn't check the accounts, but the "hand" system seems correct. If you can't verify exactly what he does.

 

[Onda]

I forgot . . . . efficiency is 80% not 0.8%

 

[AngeloB]

I agree, I always do the calculations to "hand", but I'm learning to trust the results of the fem and the cae of inventor.

 

[Onda]

In fact, since the hand calculation seems correct to me, before calculating the piston with 10% less thrust I would try to understand why that result comes out from the cinema module.

I understand better: I use the fem every time I want to have a more precise result than that obtained by a hand account. typical example is a hyperstatic structure, where performing a hand account is long, complicated and often useless.

Now, in the cinematic analysis you made, I don't see errors (I have reviewed the accounts, but only verified the method) so a difference of 50% in friction losses seems remarkable. could mean some wrong input data (can be both in the accounts by hand and in the cinematic analysis).
Now, since a 50% difference in friction results in a 10% difference in piston thrust, I see in front of two roads: the first is to dimensional the piston on the accounts by hand, the second is to understand why there is a difference and therefore understand what is the right account.

 

[AngeloB]

at the moment, I consider that the cylinder should provide a boost greater than 20% than the theoretical calculation (without friction).
the hydraulic pressures of work (in different load conditions) I had already calculated them, now the greater than 20%.

the calculation by hand of the frictions is certainly excessive, as I abounded on the rotation values (in radiants). I have considered the radial forces in the pins as fixed to the maximum value, while instead they decrease with the rotation of the hatch.

Also in the cad calculation I manually set the values of friction pairs, (considering the maximum radial force) but probably the cad has calculated the correct angle rotations, therefore the obtained value is less than that by hand, but higher than the "real" value.

always that a coefficient of friction equal to 0.2 is correct.

probably the calculation of frictions (in the cad) does not work well, force there is a kiss.
the latita manual and it is not easy to explain with the autodesk assistance.
I sent an email with the calculations and espionage of the problem to wasim younis (an expert autodesk of the simulation) hopefully answer.

In the meantime, I can try to do some counter tests in simpler cases or repeat calculations made by you.

Do you have any mechanism to re-verify?

 

[AngeloB]

p.s.
I also have another recurring problem which requires validation of the method.
the fem verification of the crane bases.
to understand the movements of the bridge, subject to a concentrated load.
are verifications that I do habitually but I never have feedback from the yard.

 

[Onda]

at the moment, I consider that the cylinder should provide a boost greater than 20% than the theoretical calculation (without friction).
the hydraulic pressures of work (in different load conditions) I had already calculated them, now the greater than 20%.

the calculation by hand of the frictions is certainly excessive, as I abounded on the rotation values (in radiants). I have considered the radial forces in the pins as fixed to the maximum value, while instead they decrease with the rotation of the hatch.

Also in the cad calculation I manually set the values of friction pairs, (considering the maximum radial force) but probably the cad has calculated the correct angle rotations, therefore the obtained value is less than that by hand, but higher than the "real" value.

always that a coefficient of friction equal to 0.2 is correct.

probably the calculation of frictions (in the cad) does not work well, force there is a kiss.
the latita manual and it is not easy to explain with the autodesk assistance.
I sent an email with the calculations and espionage of the problem to wasim younis (an expert autodesk of the simulation) hopefully answer.

In the meantime, I can try to do some counter tests in simpler cases or repeat calculations made by you.

Do you have any mechanism to re-verify?

I'm sorry, but I don't have mechanisms at the moment, but the calculation method is correct.
I think the friction coefficient 0.2 is conservative, considering a bronze bushing and a steel pin, lubricated.
Of course, the system must be able to be operated even from a standstill in its maximum load point, so you have to win a static friction (first detachment) that is normally higher than a dynamic friction.
Basically, in my opinion, such a system should also be sized for lack of lubrication, as it is not said that the maintenance of the system is perfect. So, at the end of the day, I think the size is fine. Obviously initially you will not use full cylinder pressure, but a reserve is useful.
you must then evaluate how correct the weight of the door is. often between design and construction make us several kilos of difference, stucco, paint, teak etc.
in this link you will find a pdf of one of the many producers of bushings, explaining the general rules, such as rating of friction coefficient, evaluation of pv, selection of the type of bushing, tolerances of the branches of the bushing and the tree. etc.http://www.tehburo.ru/catalog/sfb_boccole-bushes.pdf

 

[Onda]

p.s.
I also have another recurring problem which requires validation of the method.
the fem verification of the crane bases.
to understand the movements of the bridge, subject to a concentrated load.
are verifications that I do habitually but I never have feedback from the yard.

I normally in these cases model support for a certain area so as to consider the correct stiffness.
if you place something I see how I can help