material pulleys

[Sfiligoy]

I am a student of mechanical ing. I introduced myself a few months ago, but I followed you and followed you for a long time. from this community there is a lot to learn.

do you know what is why the pulleys with many teeth (and therefore high diameters) are produced only in cast iron or not in steel? among all the catalogs I have consulted (avoid to name), I have noticed the passage to the cast iron for medium-high diameters.

having to dimension a tab (pair 76 nm on a diameter of 20 mm), the average pressure on the hips is higher than the admissible pressure. for the cast iron I have the value of 50 mpa, which I will go down with a proper safety coeff. since that value of couple is not so reliable.
discarding the passage to the grooves, they are looking for a pulley (280 mm in diameter) in steel, so as to have a higher admissible pressure, but not found.

Thanks and good weekend.

 

[meccanicamg]

Have you thought of making two small tongues at 120 degrees? or a grooved coupling? the contact pressure decreases considerably.
clearly the pulley in its geometry must be correctly dimensioned.
The small pulleys are probably made directly from aluminium and steel rounds. For the large size it would be too material and above all are not measures of frequent use, therefore at the manufacturing level the use of cast iron is justified.

 

[meccanicamg]

another alternative is to mount in the cast iron pulley the castor or hole insert with steel tongue as normally done. very rarely there is the direct hole in the cast iron.http://www.mateg.it/2016/wp-content/gallery/cinghie-e-pulegge/puleggia-ghisa.jpg

 

[meccanicamg]

Are you sure you're not using cast iron panties with numbers?
a 20 mm diameter shaft with only 76nm twist with s235jr steel has an oversized factor just above 1.25 with about 225mpa of minimum guaranteed actual rs.
the poorest cast iron used for pulleys is en gjl-250 with rp0,2 equal to 200mpa.
to well look in your case you have a tab 6x6 uni 6604 rounded total length 24mm.
pair to transmit 76nm.

the tongue with rs = 500mpa is very much oversized, being in c45/c50 drawn cold incruit.
the tree admitted that it is in s235jr is not ripped locally and does not tear the seat even if all has coefficients little greater than 1.
cast iron has practically the same consideration of steel for constructions, having chosen them with low characteristics both.
if the service factor is 1 and there are no abnormal behaviors of the load trend there are really no problems.

However I would do the tree in 39crnimo4 which has good resistance and also fatigue it behaves well if the load was variable. pulleys the cast iron dome with or without a calettator but with a thickness of 50mm or with a double tab.

below values considering 200mpa the minimum yield limit of the system.

 

[stefanobruno]

below values considering 200mpa the minimum yield limit of the system.
View attachment 46595

with thing generi questa tabella?

 

[meccanicamg]

with thing generi questa tabella?

with excel. there are simply the formulas that I have shared many times on the sizing tabs.

 

[stefanobruno]

with excel. there are simply the formulas that I have shared many times on the sizing tabs.

The tablel mie excel non sono così beautiful :wink:

 

[Sfiligoy]

I thank you for the countless solutions and for the answer to my question.
I answer only now because I rechecked everything a moment and noticed some inaccuracies.

in the considered point I have a flenching moment of 400 nm (value resulting from legislation, instead of inserting a safety coefficient of 3 or 4 the norm provides the strength value that the system must hold) and torque moment of 76 nm. (given by literature, therefore subject to variations, although contained. )
I chose a 25crmo4 steel, with 600 mpa sealing.
I applied von mises and got a diameter of 20mm, with a x=1.16 if not err. (I would have to increase it considering the variability albeit minimal of the torque moment). I should increase the tree now considering the presence of the quarry.

from the table you posted here I noticed that you put t1=3 mm while in the uni 6604, the value of t1 for the diameter of 20 mm is 3.5 mm if not erro. Maybe he threw it to make a quick calculation. :smile:

if I consider d=24mm, the choice falls on a tab 8x7 uni 6604 rounded total length 18mm, without having to insert two tabs.

 

[meccanicamg]

I did everything with the phone and it can be that t1 was the one of the.riga above or below looks as standard....it must be 3.5http://meccanicaecnc1.mastertopforum.com/download.php?id=3047the increase for the section because of the place for tab is always better to make it even if the carving should be checked.
I would say that if you have determined a higher diameter then 24 mm you definitely get an improved effect.