transmission relationships

[wiz]

Bye to all,
I soon have the practical test of the state examination (mechanical examination) and carrying out the examination themes of the previous sessions I am encapsulated several times in a problem that I do not know how to solve: when I have a multi-stage gearbox to design, how can I share the total transmission ratio between the various stages? Is there any criteria to follow?
to be clearer I have attached a couple of examples, in which it is asked to calculate the transmission ratios of the cinematisms of the various stages knowing only the value of the interassis and the global transmission ratio. with so few data I don't really know how to do it. I have no problem with the other points.

 

[volaff]

Hi, I'm sorry if I interfere... Can't you pass me or can you tell me where you got the material to take the state exam?

I'd be very interested.

Thank you and good Sunday!

 

[wiz]

Hi, I'm sorry if I interfere... Can't you pass me or can you tell me where you got the material to take the state exam?

I'd be very interested.

Thank you and good Sunday!

If you mean the texts of past tests, I took them from the site of the torino polytechnic, in the session dedicated to state examination, others you can find them at the sites of many universities. for the rest I studied on my old books and notes used for the various exams and a little on the "manual of the mechanical engineer" [Hoepli]

 

[meccanicamg]

Bye to all,
I soon have the practical test of the state examination (mechanical examination) and carrying out the examination themes of the previous sessions I am encapsulated several times in a problem that I do not know how to solve: when I have a multi-stage gearbox to design, how can I share the total transmission ratio between the various stages? Is there any criteria to follow?
to be clearer I have attached a couple of examples, in which it is asked to calculate the transmission ratios of the cinematisms of the various stages knowing only the value of the interassis and the global transmission ratio. with so few data I don't really know how to do it. I have no problem with the other points.

the choice of the subdivision of the transmission ratio has not to say the true a great criterion of subdivision (we have already spoken in other posts).

substantially depends on the size of the gearbox. if you do a high reduction ratio at the first stage you will get something compact in the next two stages. if you keep the reduction ratio low to the primary you will get quite large and bulky cascade wheels.
the reduction ratio with cylindrical or conical helical gears we say that tends to go up to 5 ~ 7 with adequate correction of the teeth since you will have a pinion with a few teeth and a wheel conducted with so many teeth that cause the intermittent step effect (i.e. tends to scale the tooth of one on the other).

other thing not to forget is to divide the gear ratios using as the first attempt first and not divisible numbers between them in order to optimize the wear life and the effects of noise due to unprecise processing if using unrectified gears.

another smart thing is to use a spreadsheet where you can make attempts to get to convergence first.

 

[wiz]

the choice of the subdivision of the transmission ratio has not to say the true a great criterion of subdivision

It was what I thought too, my doubt was linked more than anything to the fact that he told me to "calculate" transmission reports, as if I had to use particular formulas or procedures, while it is more than "choose" them, no? It will mean that if I get an exercise similar to the exam I will do the subdivision without making too many problems.

the spreadsheets unfortunately cannot be used at the exam, and the time available is what it is, so you can not make many attempts

 

[athlon]

if you have a programmable calculator you can make a small program that given the total reduction ratio and indicated the number of stages gives you various plausible options that approach the demands, from them to you choose the most elegant solution by favoring precisely relationships between dense wheels that maximize the duration and favoring the reduction in the first stages in order to reduce the size of the dense wheels that need to manage more torque and that then are more '

 

[wiz]

excuse athlon what is the criterion for maximizing duration?

 

[athlon]

excuse athlon what is the criterion for maximizing duration?

avoid pinions with a not too small number of teeth
do not exaggerate with the reduction ratio in the single stage
teeth ratio possibly not multiple
calculate the stress on the various dentate wheels of the whole gearbox so
that long-term wear is more or less equal on all stages so that the total life of the reducer is as long as possible

 

[wiz]

ok thanks:smile:
only another question concerning the attached texts: Since there are few data available, for the dimensioning of the toothed wheels, given the transmission ratio, it is right to hypothesize the number of teeth of the smaller wheel as the starting date and then calculate all the rest (with the formula lewis) and then perform the due checks? or is it better to make calculations by assuming a starting value of the module m?

 

[meccanicamg]

ok thanks:smile:
only another question concerning the attached texts: Since there are few data available, for the dimensioning of the toothed wheels, given the transmission ratio, it is right to hypothesize the number of teeth of the smaller wheel as the starting date and then calculate all the rest (with the formula lewis) and then perform the due checks? or is it better to make calculations by assuming a starting value of the module m?

I would say that you can impose z because of the relationship you want and impose m in order to find b or impose z e b to determine m but then you have to iter because m is the one between the standards.
to tell the truth usually I leave from z known and m and in the end, I value them. from there I calculate the wheel width knowing the torque to be transmitted. If you are looking in the forum you will find my useful curves for the calculation of gears using the specific pair.

for simplicity I attach the standard kit.

the first serves to know the torque in transit given the number of turns and the power in kw
c [N*m] = w [kW] * cp [N*m/kW]the second according to the gear material calculates the band width
b [mm] = c [N*m] / clip [N*m/mm]if you use the formulas of lewiss you will get the same system calculated to the comma, but to have a fast iterative way you need the chart....what you hardly use....maybe it does not come to mind to anyone or maybe use the calculation programs or maybe go to spans