Limits, Fits and Tolerances

We often see different items in our day-to-day life where the item is joined with another object with the help of a special technique that prevents the joint from loosening or opening. There are numerous types of techniques but limits and fits are some of the most commonly used techniques. 

What are the limits?

Here limits do not mean the mathematical definition of the limit of a function. Here it means the mechanical mating parts that need to work together perfectly. A perfect combination of mating parts will have a high tolerance level. Limits refer to the highest and the lowest tolerance level of both the mating parts. 

What are fits and clearance?

Fit refers to the way the tolerance level of both mating parts works when they are joined together. Therefore, fit can be defined as the combination of two limits. Therefore, fits and tolerances go hand in hand. Fits have different types, clearance, interference, and transitional. Clearance of both mating parts is known as clearance. A fit is chosen in mechanical engineering by observing the movement of one part concerning another when it is a clearance fit. In the case of a tight interference fit, it is observed whether the movement takes place as a whole. 

There are different sizes of mating parts, however, the concept of limits and fits remain the same. The concept is used to improve the performance of the shaft and the hole which are the two mating parts. 

Fits have a standardized size as per the classes made by ANSI and ISO. Clearance, interference, and transitional are the three different classes determined as per ISO and ANSI. Each of the classes possesses a plethora of different options that are made available to help in choosing the correct one for one certain purpose. 

More about limits and fits

In terms of the application of limits and fits in mechanical engineering, the concept helps in determining the tolerance level of the different parts so that the proper working and the longevity of the machine can be ensured. A fit must be chosen depending on the specific conditions and requirements of the work.

The main subtypes of fits are clearance fit, transitional fit, and interference fit. These three subtypes of fits are further classified into various other subtypes. However, more detailed usage of a subtype as per the requirements and the conditions will result in an increased cost of making a machine. It is so because, with more categorized usage of fits, the fit of the machine will be more accurate. Thus, the machine accuracy will increase but the difficulty of fixing the part on a machine will increase as well. Therefore, the cost of applying the fit on a machine increases as we use a more classified version of a part. 

A brief description of all the types of fits

A clearance fit is a subtype of fit in which a lot of space is left between two different mating parts. However, in the case of a transitional fit, the fit neither has a lot of space nor is it tightly fixed. This type of fit is somewhere between the interference type and a clearance type. This type can be further transformed to be any of the two types by leaving adequate space or not leaving any space at all. An interference type of fit refers to a certain kind of fit that leaves no space. Unlike the clearance fit, this fit is placed tightly. Therefore, to create an interference fit, a lot of force is required along with the assistance of different other techniques that aid in the process. 

Conclusion

Fits and limits are an important part of mechanical engineering. It helps in connecting different parts that further make a complete machine. The fits and the different types of fits help in enhancing the accuracy of a machine. With the use of a proper fit, an accurate and good machine can be made.