doubt about technical design tolerances. . .

[filippo4825]

good afternoon to all, I return to write on this beautiful forum that until now has been regarding me a manna from the sky.

I have a problem with the subject of tolerances, not so much generality, calculations and standards of representation etc., more for the practical side. In the sense I had to make a design of a mechanical piece of my choice, a piston found in the garage. I have not had any major problems measuring it and representing it in projections, sections etc., but I was told by the prof that it should be completed with tolerances.

Now I do not ask clearly that you tell me exactly what to add, because I want to understand how to proceed as it might happen to me also to the exam, but can you kindly tell me how to proceed practically? I know that dimensional tolerances must be chosen according to the functionality of the piece, and clearly being a cylinder piston coupling there must obviously be a minimum game.

How do I proceed? after finding in the table the row corresponding to the piston diamentro how do I choose the maximum amount of play that I can tolerate?

 

[cacciatorino]

How do I proceed? after finding in the table the row corresponding to the piston diamentro how do I choose the maximum amount of play that I can tolerate?

It's a question that can only respond to that that by trade he designs blast engines, or who makes adjustments.

that between piston and cylinder is in fact a special coupling, which for example works hot but is measured cold. It is clear from this that, for example, the cold game will be different if the cylinder is made of aluminium or cast iron (the first material is heated more than the second). Another problem is that the piston is not axial-symmetric, so it will dilate differently along the diameter of the pin and along the other orthogonal diameter at the first: not by chance the pistons look cylindrical but actually should be slightly oval.

In short, I don't know how to give it, if not the suggestion to go to see the engine workshop manual from which you recovered the piston, and to evaluate what it says about the permitted wear, from which perhaps you could get some indication on the manufacturing tolerance.

 

[filippo4825]

Thank you for the answer.

I tried to look for info about the engine in question but I found nothing, no technical data in particular, or project, being a simple thermal group for a 50ino, besides of course the measure, the displacement, the material that is aluminum, that are found in the various markets. and then in the various forums you can find for example discussions on tolerances but in case of grinding the cylinder, I need it for the cylinder out of the factory.

I obviously don't have to really do this, so I think the prof only wants a demonstration that I understood the correct use of tolerances and tables with a nominal diameter. so all theoretical. flying over the specialist part, thermal dilations, materials, more or less expensive labors.

according to you what can I insert to complete the design, more or less acceptable?

 

[paulpaul]

Bye!
I would simply hypothesize the piston as a hollow cylinder, to be able to shape it at the elementary level. I would then consider a working temperature of 400-500 °C, which is the average temperature that gross is found in the hottest points of the piston of a two-stroke eight (and for simplicity the hypotensive distributed over the entire surface of the piston). knowing the coefficient of expansion of the material calculates "the hot deformed" (even with a fem in the case) and therefore the increase of external diameter. do the same with the cylinder and make sure that the diametral piston/canna game is - for example - between 0.05 and 0.1 mm, and then calculate the cold tolerances (I don't know the exact values of the game for these engines, but I think your prof wants to evaluate the reasoning).
Actually the t varies and quite depending on the surface of the piston we are considering: the t said before you have them on the sky of the plunger, while on the cape are even less than half. the piston comes then to assume cold a shape about truncated, as it told you hunter, so that it becomes as cylindrical as possible!

 

[goccia]

I think you've chosen an unsuitable job. I realize something that works at the temperature of 20 degrees, that the one with which the tables are realized.
a mere mechanism not?
some texts such as the backincasa report the degrees of tolerance and fundamental deviations depending on the type of processing.

 

[filippo4825]

Bye!
I would simply hypothesize the piston as a hollow cylinder, to be able to shape it at the elementary level. I would then consider a working temperature of 400-500 °C, which is the average temperature that gross is found in the hottest points of the piston of a two-stroke eight (and for simplicity the hypotensive distributed over the entire surface of the piston). knowing the coefficient of expansion of the material calculates "the hot deformed" (even with a fem in the case) and therefore the increase of external diameter. do the same with the cylinder and make sure that the diametral piston/canna game is - for example - between 0.05 and 0.1 mm, and then calculate the cold tolerances (I don't know the exact values of the game for these engines, but I think your prof wants to evaluate the reasoning).
Actually the t varies and quite depending on the surface of the piston we are considering: the t said before you have them on the sky of the plunger, while on the cape are even less than half. the piston comes then to assume cold a shape about truncated, as it told you hunter, so that it becomes as cylindrical as possible!

I think you've chosen an unsuitable job. I realize something that works at the temperature of 20 degrees, that the one with which the tables are realized.
a mere mechanism not?
some texts such as the backincasa report the degrees of tolerance and fundamental deviations depending on the type of processing.

thanks to the answers and suggestions, but I spoke directly with the prof who told me that clearly not knowing how to design it and not having to dimension it (what will be done in depth in construction of machines, applied mechanics etc.), suggested me for completeness to put only the general quotas in the cartiglio (I chose the averages) and to choose a specific tolerance in coupling spinotto/piston hole, that I chose (with hint of the bookin back . .

 

[goccia]

and how does the piston move along the cylinder if there is interference?

 

[filippo4825]

and how does the piston move along the cylinder if there is interference?

no drop interference in coupling between pin / piston hole. the rest of the tolerances all general average according to norm I then reported in the cartilage. clearly at the design level is not correct, but as mentioned above I do not have to realize it, it is only for educational purposes, because I should take into account everything that has said paul at least, knowledge that I have not...

 

[meccanicamg]

a few pistons for 2t engines I disassembled them and reassembled them but sincerely with the knob just as soon, so h7/r6 is too tight.

 

[filippo4825]

was the least of the recommended spinotti/hole couplings on the return house, the other 2 required the "hot" mounting, really too exaggerated.

 

[meccanicamg]

I'll come home every once in a while if you make things a little exaggerated. keeping in mind that the spindle is completely cylindrical and that it goes inside a roll case, provided the tolerances are infinite, I do not know if it is true that the male spindle is in r6 .... I think it's a few cents less like js6 max k5. the hole not in h7... unless it is cold. axially held by two seeger...the spinotto obviously

 

[filippo4825]

but in any case the design part, the working cycle (and therefore the detailed choice of tolerances) etc., goes beyond the matter in question. In my specific case the prof looks more than anything with attention, respect for the various rules related to the drawing, the schematic representation of mechanical pieces (section, projections assonometrics, as well as the correct quotation), the organization of the sheet and the cartiglio etc. cleaning and order, in short the basics. the tolerances here are only "acceptions". the prof cares more than anything else that I have understood the generalities of tolerances (to what they serve, where to put them in the drawing) without entering the details. for this I avoided looking for more precise tolerances, because also I pushed various hand plugs inside the hole, or at the max with the help of a rubber hammer, I certainly did not need a press (as is indicated in the description of the tolerance in question). but it had to be only "plausible" to complete the table (a tolerance with game surely would count it as error and would lower my vote).