Questions About Interference Fits

[Wesley souza]

In mechanics, there are several ways of attaching a part to another and what catches my attention the most are the parts that are joined under the pressure fitting. I keep imagining what dimensions and clearances the pieces must have so that they fit together and then don't come apart. Could someone point me to a reading that addresses the topic?

I'm making a small edit to the original text. please i don't speak english very well and if anyone has any doubts about any term used wrong, please let me know. I didn't want to sound impatient, but I've already looked for such knowledge and nowhere have I found an answer.

 

[FEAnalyst]

Check for example "Machine Design. An Integrated Approach" by Norton, it has a chapter about interference fits.

 

[Baluncore]

The search term is "interference fit".

 

[Wesley souza]

Thanks for your help

 

[Wesley souza]

Veja por exemplo "Design de Máquina. Uma Abordagem Integrada" da Norton, tem um capítulo sobre ajustes de interferência.

Thanks for your help

 

[Wesley souza]

O termo de pesquisa é "ajuste de interferência".

Thanks for your help

 

[dlgoff]

In mechanics, there are several ways of attaching a part to another and what catches my attention the most are the parts that are joined under the pressure fitting. I keep imagining what dimensions and clearances the pieces must have so that they fit together and then don't come apart. Could someone point me to a reading that addresses the topic?

I'm making a small edit to the original text. please i don't speak english very well and if anyone has any doubts about any term used wrong, please let me know. I didn't want to sound impatient, but I've already looked for such knowledge and nowhere have I found an answer.

I have this .pdf file that may be helpful. See the attached file.

 

[Wesley souza]

Thanks

 

[dlgoff]

Thanks

You are welcome. I hope it helps a little.

 

[Baluncore]

The topic appears to be joining solid parts by pressing them together with a specified crush.
Where temperatures are changed, the term would be "shrink fit".
I think an alternative close term would be "esmagamento de interferência".

 

[john.phillip]

I want my comment back, so I can translate it to English for everyone's benefit.

 

[Baluncore]

I want my comment back, so I can translate it to English for everyone's benefit.

Your message was in my email notifications. Which is translated here by google to;
"Wesley, what you seem to be looking for is the IT ABNT/ISO quality table for shaft-hole part. See the attachment, especially page 27."
But only the one line at the bottom of the imaged table, (which is in Portugese), on that page was relevant.

 

[john.phillip]

Many thanks, that's accurate enough but special attention to the following technical term in the foreign language "Qualidade IT" must be noted, as it's the main keyword in the subject and it refers to the relationship between tolerancing, assembly/maintenance difficulty, quality of fit and machinery limitations, as normalised in the technical standard published locally by ABNT.

"But only the one line at the bottom of the imaged table, (which is in Portugese), on that page was relevant."

The entire pdf is relevant and mostly self-explanatory, as it contains workable examples throughout.

1) Page 27 is the starting point for the answer, as it shows the relationship between the type of fit and the standard tolerance needed to achieve it. The last row in Page 27 is directly related to the question and gives two options: either H6 p5 (Extra high mechanical precision), or H7 p6 (High mechanical precision), for a shrink-fit/press-fit. Once those two options are noted, it's possible to browse the pdf for the actual tolerances for the shaft and hole components as a function of the nominal diameter and the chosen precision, where the achievable precision is a function of the available tools and skills.

2) Next stop is page 16, where mechanical precision work is defined in the range IT01 to IT3 for both shaft and hole components. It's recommended to start with the most generous tolerance first due to economics; thus, IT3 should be picked together with H7 p6 (high precision, instead of H6 p5 for extra high precision);

3) Then, on page 28, the tolerance can be read for the nominal diameter in question. Let's say it's 30mm nominal diameter and IT3, thus a standard relative tolerance of 4µm is provided;

4) Now, on page 17, let's say that the manufacturing operations are machining and boring. For 30mm nominal diameter (1.18"), machining provides a tolerance of ±0.0015" (±38.1µm) and boring ±0.002" (±50.8µm). However, both tolerances are quite wide for IT3 requirement of 4µm, thus an additional finishing step should be used for both shaft and hole components. From the table, grinding gives +0"/-0.0005" (+0µm/-12.7µm) for the shaft, and precision boring gives +0.0005"/-0" (+12.7µm/-0µm), which is much closer to 4µm for IT3 and will potentially reduce rejection rates;

5) On page 35, for the hypothetical nominal diameter of 30mm, the column for letter H (as per page 27) defines the upper bound tolerance as 0µm for the hole component and IT3;

6) On page 37, for the hypothetical nominal diameter of 30mm, the column for letter p (as per page 27) defines the lower bound tolerance as 26µm for the shaft component and IT3;

7) On Page 22, the reference system is provided as either shaft-based or hole-based and recommends the use of hole-based system, as it's usually easier to adjust a shaft to a hole, than the other way around. Also holes might be produced with drill bits, which potentially provide tolerances H6 and H7 straight away. The pictures on the top of the page show the 3 possible outcomes from left to right, for hole-based systems: loose fit, tight or transitional fit, and interference fit, being the latter related to the question (28mm H7 p6 in the picture);

8) With hole-based system chosen, the hole is produced first and then the shaft is adjusted to the hole to produce quality fit IT3. As the tolerances are tight, a micrometer caliper is a recommended tool for the shaft. For the hole, there are different solutions that depend on how large and accessible the hole is;

9) On page 23, for 30mm nominal diameter, hole-based system, and H7 p6 (the capital letter on the hole tolerance indicates it's hole-based), the bilateral tolerances are given as -0µm/+25µm for the hole component and +42µm/+26µm for the shaft component, and in accordance with IT3;

10) With the provided tolerances from the previous step, the minimum diameter of the hole would be 30.000mm and the maximum diameter 30.025mm. For the shaft, the maximum diameter would be 30.042mm and the minimum 30.026mm. But those dimensions for shaft and hole are not final, as an adjustment is needed to meet the required quality level;

11) A final adjustment is made to the dimensions of the previous step, as per pages 29-32, in order to incorporate IT3. On page 30, the hole is on top, letter "H", and the shaft is below, letter "p". Thus, variable "EI" is used for the hole and "ei" for the shaft. Page 29 provides an overview of a single tolerance bar for clarity. Page 31 is for the hole component. Page 32 is for the shaft component and shows equation ei = es - IT;

12) Applying the provided information gives the following: For the hole "H" on page 31, no further calculation is needed, as from the figure, EI = 0; For the shaft "p", last example on page 32 (valid from "m" to "zc"), the equation is ei = es - IT = 42µm - 4µm = 38µm;

13) With the calculations from the previous step, the final tolerances for the hole are unchanged: 30mm -0µm/+25µm => minimum diameter 30.000mm and maximum 30.025mm. For the shaft, the bilateral tolerances are now 30mm +42µm/+38µm => thus, the maximum diameter is 30.042mm and the minimum diameter is 30.038mm;

14) The maximum and minimum amount of interference becomes (negative sign indicates interference): 30.000mm - 30.042mm = -42µm; 30.025mm - 30.038mm = -13µm;

15) Although 42µm might seem a small interference at first glance, it can induce very high stresses after shrinkage, thus a stress calculation or analysis is highly recommended to confirm or otherwise the suitability of the design for both shaft and hole components.