request explanation on topics of construction of machines

[Liquirizia28]

Hey, guys, I'm studying to take the machine construction test. I would have questions to ask you.

(1) study life fatigueIn case I find myself having to check the fatigue life of a tree on which elements such as dense wheels are cast, how do I face the study? My problem arises once you find all the tensions, various diagrams of moments and cut, etc. what I don't understand is like a voltage coming from one or more dense wheels, this can have an average or alternating component.
If for example you have variable loads over time within a maximum and a minimum, clearly the alternating component and the average are calculated. but in cases where there are no variable loads like a transmission through dense wheels as I address the thing? Moreover if you want to apply the relationships on the fatigue study (as goodman) I need both components.
I was thinking now that since the load is constant, there is no alternating component but only an average component.
In this case, for example, goodman's relationship would be simplified to:

average sigma = breaking sigma / safety coefficient

but then how can I establish the number of cycles endured by the tree? It's just a rupture check!

(2) studio bullonatureI have to study a bolted joint with 6 screws (note: m12, 8.8) and understand what is the maximum torque moment applicable to the joint, considering also the elasticity of the flange and a known external axial load (ne). I did not understand if this time required is equal to the moment of tightening the screw itself!
I for now have confined myself to the study of tensions between vine and flange using the triangular diagram, where I placed as known the precarious ns (given from the tables following m12, 8.8) and the axial outer load in it. found the increment that occurs on the vine, I added to this also the precarious ns and in the end I found the total axial load on the vine.
at this point the moment of tightening is found with the formula:

clamping time = 0.2 * ns(total) * d(vine diameter)


Sorry about the stupidity of questions...and thank you in advance:finger:

 

[meccanicamg]

Hey, guys, I'm studying to take the machine construction test. I would have questions to ask you.

(1) study life fatigueIn case I find myself having to check the fatigue life of a tree on which elements such as dense wheels are cast, how do I face the study? My problem arises once you find all the tensions, various diagrams of moments and cut, etc. what I don't understand is like a voltage coming from one or more dense wheels, this can have an average or alternating component.
If for example you have variable loads over time within a maximum and a minimum, clearly the alternating component and the average are calculated. but in cases where there are no variable loads like a transmission through dense wheels as I address the thing? Moreover if you want to apply the relationships on the fatigue study (as goodman) I need both components.
I was thinking now that since the load is constant, there is no alternating component but only an average component.
In this case, for example, goodman's relationship would be simplified to:

average sigma = breaking sigma / safety coefficient

but then how can I establish the number of cycles endured by the tree? It's just a rupture check!

if the shaft is the object of fatigue testing it is necessary that the forces of the connected mechanical components move on it. First of all it is not the tension state of a gear that moves on the tree but a force that acts on the gear in a button way that evaluated on the tree generates a tension on the tree. fatigue testing, as the term says is an oscillating or variable action verification on the mechanical component. static analysis is at the base of everything and especially if there are no variable loads such as torsion, bending or torsion you can not have fatigue analysis. if you have a carving you must see if it will yield in a static way (if there is no oscillating force) or if it is considered in fatigue testing.
I'll attach you a wohler diagram in which you see the variables you need. substantially is the most complete method using gaugh pollard and all the various calculation refinements for real piece evaluation.

2) studio bullonatureI have to study a bolted joint with 6 screws (note: m12, 8.8) and understand what is the maximum torque moment applicable to the joint, considering also the elasticity of the flange and a known external axial load (ne). I did not understand if this time required is equal to the moment of tightening the screw itself!
I for now have confined myself to the study of tensions between vine and flange using the triangular diagram, where I placed as known the precarious ns (given from the tables following m12, 8.8) and the axial outer load in it. found the increment that occurs on the vine, I added to this also the precarious ns and in the end I found the total axial load on the vine.
at this point the moment of tightening is found with the formula:

clamping time = 0.2 * ns(total) * d(vine diameter)


Sorry about the stupidity of questions...and thank you in advance:finger:

I'd say that for how you described it at first, it's nothing to do with what you calculated. If you have two screwed flanges and you have to calculate the maximum transmissible torque moment means that you take the two extremes and twists (perpendicular to the screws). disk joints are together for friction, so it is what you have to calculate and the maximum torque is given by the maximum feasible friction and consequently also by the pair of screws but..... but the screws do not work to cut! the axial load that you have goes to remove the closing load of the vines then to decrease the friction force, then less torque transmission. the cutting joint (a pioli) works to cut and has no tightening screws. I'll set you up

 

[ceschi1959]

Hey, guys, I'm studying to take the machine construction test. I would have questions to ask you.

(1) study life fatigueIn case I find myself having to check the fatigue life of a tree on which elements such as dense wheels are cast, how do I face the study? My problem arises once you find all the tensions, various diagrams of moments and cut, etc. what I don't understand is like a voltage coming from one or more dense wheels, this can have an average or alternating component.

suppose to press with a finger with constant force on a tree. if this stand still you will have constant tensions and deformations, but if this turns, the force is constant but it will apply in different points of the tree (which turns) so a point that before was in tension after alpha degrees of rotation will be in compression. That's why the trees are calculated hard. Did I explain?

 

[Liquirizia28]

if the shaft is the object of fatigue testing it is necessary that the forces of the connected mechanical components move on it. First of all it is not the tension state of a gear that moves on the tree but a force that acts on the gear in a button way that evaluated on the tree generates a tension on the tree. fatigue testing, as the term says is an oscillating or variable action verification on the mechanical component. static analysis is at the base of everything and especially if there are no variable loads such as torsion, bending or torsion you can not have fatigue analysis. if you have a carving you must see if it will yield in a static way (if there is no oscillating force) or if it is considered in fatigue testing.
I'll attach you a wohler diagram in which you see the variables you need. substantially is the most complete method using gaugh pollard and all the various calculation refinements for real piece evaluation.

suppose to press with a finger with constant force on a tree. if this stand still you will have constant tensions and deformations, but if this turns, the force is constant but it will apply in different points of the tree (which turns) so a point that before was in tension after alpha degrees of rotation will be in compression. That's why the trees are calculated hard. Did I explain?

Perfect! Clear!
That's what I didn't understand. Since there was not explicitly a variable load (I don't know for example a cam that interacts with a spring that compresses/decompresses) I didn't know how to interpret static forces discharged from elements such as dentate wheels or pulleys. At first, when looking at an exercise I had noticed that the alternating component of tension was present (derived as mentioned by mechanicalmg - and as I did - by an analysis of the wheel forces that discharges on the shaft, on which tensions arise) and not the average component. then by virtue of this I had understood that it was a speech of equal forces of traction and compression but I could not focus it on practical level, so there was something that actually escaped me. but now thanks to the example of the finger I understand.
For example, in the case of flender moment and its relative tension, I will place the value as maximum in traction and minimum in compression, to find the alternating sigma.

but if I had to reason with the tangential efforts of torque and cutting moment? because in reality I believe that in the torque moment there is not this phenomenon precisely because of the tangential tensions that are formed circumferentially throughout the tree and therefore would be present only as constant efforts, right?
(by the way, correct me if I'm wrong: the torque moment would be that given by the tangential component of the force exchanged between the wheels, for the radius of the wheel, no!? )

I'd say that for how you described it at first, it's nothing to do with what you calculated. If you have two screwed flanges and you have to calculate the maximum transmissible torque moment means that you take the two extremes and twists (perpendicular to the screws). disk joints are together for friction, so it is what you have to calculate and the maximum torque is given by the maximum feasible friction and consequently also by the pair of screws but..... but the screws do not work to cut! the axial load that you have goes to remove the closing load of the vines then to decrease the friction force, then less torque transmission. the cutting joint (a pioli) works to cut and has no tightening screws. I'll set you up

therefore moment of tightening of the screw and torque moment of the joint are not related!
then having to study the cut would be correct to reason in these terms?
from the tables I echo the preload of a life m12 8.8 and through the formula (vf,o = ns*f attrito*n°superfici involved / 1.25) I echo what in the photos you attach would be the vf, or, that is, the maximum permissible effort to avoid the slide (and therefore have the force of friction!? )
then for the presence of an external axial load, to be divided between the bolts, I find a v'f,o=vf,o(1 - external load / ns).
then multiplying it by the number of screws I get the maximum cutting effort bearable by the set of screws.
and at this point the torque moment is simply the moment given by the cut force found (v'f, or x 6) for the circumference radius where the bolts are arranged!?
but how should I use then the elasticity of the flange?

 

[meccanicamg]

for fatigue testing on torsion you can have constant static torsion or button twist or alternating twist.

for the joint, it has that the torsion pair of the screws (closing pairs) is related to the torque transmissible for friction from the surfaces of the two discs and depends on the coefficient of friction and the axial force that exists on the piece.

As for the elasticity of the joint I do not know what to tell you why the joint is considered infinitely rigid and if instead behaves as elastic you have to consider bolting with seals ....see the theory of rubber seal under the head of the screws (theory you find on all books). you will have more or less linear behavior of each screw and the clamping contribution will be different than having the elastic element.

 

[Liquirizia28]

for the torsion I formulated badly: I meant that given by the tangential f of the wheel (so usual dilemma of constant force but fatigue study) that I think it is time to consider as a medium voltage (thinking methods to combine tensions in multiaxial fatigues, like the sines method)

for the joints unfortunately are in difficulty, the topic seals I have not faced it in the course! I will now see to read it myself in the book or possibly on the internet.. But I don't think it's attributable to this. I had not accidentally thought of the triangular diagram between screw and flange in which the rigidities are studied and therefore the increases of force on screw and flange given by the external load.

 

[meccanicamg]

only in case the friction force is not enough go to download the torque moment of the joint as a cutting effect to the screws, because the two joints slip:
t=2•m/(nb•dv)where nb=number bolts, dv=diameter bolts, m= torque motion joint. if they combine all the screws together.

since the partially smooth thread stem results in a nominal diameter you will have a resistant area of:
air=π•d2/4and therefore a tangential tension of the circular section results:
tau=(4/3)•t/ar which must be less or equal to
tau,adm=0.58•rm/gr=sigma,adm•0.58 of the material of the screws.

 

[meccanicamg]

only in case the friction force is not enough go to download the torque moment of the joint as a cutting effect to the screws, because the two joints slip:
t=2•m/(nb•dv)where nb=number bolts, dv=diameter bolts, m= torque motion joint. if they combine all the screws together.

since the partially smooth thread stem results in a nominal diameter you will have a resistant area of:
air=π•d2/4and therefore a tangential tension of the circular section results:
tau=(4/3)•t/ar which must be less or equal to
tau,adm=0.58•rm/gr=sigma,adm•0.58 of the material of the screws.

was the answer of another post.....discussed