Tolerances and Fits

In this tutorial, you will understand the importance of Fits and Tolerances in modern manufacturing. You will be exposed to the concept of tolerance inclusion in a design, explore the different types of fits, and the process of selecting the best fit for an application.

Contents:

  1. What is Tolerance?
  2. What are the types of Fits?
  3. BIS System of Fits and Tolerances
  4. Selection of Fits
  5. Tolerance in Manufacturing
  6. Tolerance in Bolt Spacing
  7. What is Selective Assembly?
  8. What is Surface Roughness?

What is Tolerance?

Due to the inaccuracy of manufacturing methods, it is not possible to machine a component to a given dimension. The components are so manufactured that their dimensions lie between two limits—maximum and minimum.

  • The basic dimension is called the normal or basic size, while the difference between the two limits is called permissible tolerance.
  • Tolerance is defined as the permissible variation in the dimensions of the component
  • The two limits are sometimes called the upper and lower deviations. The following figure shows the basic tolerances in a hole-shaft pair.
    Hole and Shaft Tolerances
  • There are two systems of specification for tolerances, namely, unilateral and bilateral. In the unilateral system, one tolerance is zero, while the other takes care of all permissible variations in basic size.
  • In the case of bilateral tolerances, the variations are given in both directions from normal size. The upper limit in this case is the basic size plus non-zero positive tolerance, and the lower limit is the basic size plus non-zero negative tolerance.

What are the types of Fits?

When two parts are to be assembled, the relationship resulting from the difference between their sizes before assembly is called a fit. Depending upon the limits of the shaft and the hole, fits are broadly classified into three groups—clearance fit, transition fit, and interference fit. The following figure shows the types of Fits.

Types of Fits
  • A clearance fit is a fit that always provides a positive clearance between the hole and the shaft over the entire range of tolerances. In this case, the tolerance zone of the hole is entirely above that of the shaft.
  • An interference fit is a fit that always provides a positive interference over the whole range of tolerances. In this case, the tolerance zone of the hole is completely below that of the shaft.
  • A transition fit is a fit that may provide either a clearance or interference, depending upon the actual values of the individual tolerances of the mating components. In this case, the tolerance zones of the hole and the shaft overlap.
  • There are two basic systems for giving tolerances to the shaft and the hole, namely, the hole-basis system and the shaft-basis, system.
  • In the hole-basis system, the different clearances and interferences are obtained by associating various shafts with a single hole, whose lower deviation is zero.
  • This system has an advantage over the shaft-basis system because holes are machined by standard drills or reamers having fixed dimensions, while the shafts can be turned or ground to any given dimension.
  • In the shaft-basis system, the different clearances or interferences are obtained by associating various holes with a single shaft, whose upper deviation is zero.
  • The shaft-basis system is popular in industries using semi-finished or finished shafting, such as bright bars, as raw material.

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BIS System of Fits and Tolerances

According to a system recommended by the Bureau of Indian Standards, tolerance is specified by an alphabet, capital or small, followed by a number. The description of tolerance consists of two parts—fundamental deviation and magnitude of tolerance.

  • The magnitude of tolerance is designated by a number, called the grade. The grade of tolerance is defined as a group of tolerances, which are considered to have the same level of accuracy for all basic sizes.
  • There are eighteen grades of tolerances with designations IT1, IT2, …, IT17, and IT18. The letters of the symbol IT stand for ‘International Tolerance’ grade.
  • The tolerance for a shaft of 50 mm diameter as the basic size, with the fundamental deviation denoted by the letter ‘g’ and the tolerance of grade 7 is written as 50g7.

Selection of Fits

The selection of interference fit depends upon several factors, such as materials, diameters, surface finish, and machining methods. The guidelines for the selection of clearance fits are as follows:

  • The fits H7-d8, H8-d9, and H11-d11 are loose running fits and are used for plumber-block bearings and loose pulleys.
  • The fits H6-e7, H7-e8, and H8-e8 are loose clearance fits and are used for properly lubricated bearings, requiring appreciable clearances. The finer grades are used for heavy-duty, high-speed bearings and large electric motors.
  • The fits H6-f6, H7-f7, and H8-f8 are normal running fits, widely used for grease or oil lubricated bearings having low-temperature rise. They are also used for shafts of gearboxes, small electric motors, and pumps.
  • The fits H6-g5, H7-g6, and H8-g7 are expensive from manufacturing considerations. They are used in precision equipment, pistons, slide valves, and bearings of accurate link mechanisms.
  • The fit H7-p6 or H7-p7 results in interference, which is not excessive but sufficient to give non-ferrous parts a light press fit. Such parts can be dismantled easily as and when required, e.g., fitting a brass bush in the gear.
  • The fit H6-r5 or H7-r6 is a medium drive fit on ferrous parts, which can be easily dismantled.
  • The fits H6-s5, H7-s6, and H8-s7 are used for permanent and semi-permanent assemblies of steel and cast iron parts. The amount of interference in these fits is sufficiently large to provide a considerable gripping force.

Tolerance in Manufacturing

The specification of the machining method for a given component depends upon the grade of tolerances specified by the designer. The approximate relationship between the grade of tolerance and the manufacturing method with desirable accuracy under normal working conditions is shown in the table below.

Grade Processes
Grade 16 Sand Casting, Flame Cutting
Grade 15 Stamping
Grade 14 Die Casting, Molding
Grade 11 Drilling, Turning, Boring
Grade 10 Milling, Planning, Rolling
Grade 9 Horizontal and Vertical Boring on Automatic lathes
Grade 8 Turning, Boring, Reaming on center, capstan, and turret lathes
Grade 7 High Precision Turning, Broaching, Honing
Grade 6 Grinding, Fine Honing
Grade 5 Lapping, Fine Grinding, Diamond Boring
Grade 4 Lapping

Finer grades of tolerances result in costly manufacturing methods. They are to be specified only on grounds of functional requirements. Coarse grades of tolerances reduce the cost of production. The designer should always take Finer grades of tolerances result in costly manufacturing methods. They are to be specified only on grounds of functional requirements. Coarse grades of tolerances reduce the cost of production. The designer should always take into consideration the available manufacturing facilities and their cost competitiveness before specifying the grade of tolerances.

Tolerance in Bolt Spacing

When two or more components are assembled using bolts, it is often required to specify tolerances for the center-to-center distance between bolts. The following figure shows the assembly of two plates.

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Assembly of Plates

Consider the worst geometric situation when the following conditions exist:
(i) The holes are of the smallest size, Dmin,
(ii) The bolts are of the largest size, dmax,
(iii) The plate A has a minimum spacing (a – x)
(iv) The plate B has maximum spacing (a + x)
The tolerance on-center distance is given by (a ± x)
In case there are several holes, one hole is considered as the master or reference hole and the location of other holes should be provided with the required tolerance.

What is Selective Assembly?

The selective assembly is a process of sorting the manufactured components into different groups according to their sizes and then assembling the components of one group to the corresponding components of a matching group.

  • In this method, larger shafts are assembled with larger holes and smaller shafts with smaller holes. This results in closer clearance or interference without involving costly machining methods.
  • Selective assembly results in finer clearance or interference, even with large manufacturing tolerances.
  • The main drawback of this method is that a hundred percent inspection is required for components and there is an additional operation of sorting the components.
  • Interchangeability is affected and servicing or replacement of worn-out components becomes difficult.
  • The method can be used only when many components are manufactured, otherwise, some groups may not contain a sufficient number of components.
  • Selective assembly is particularly useful in case of interference fits where tight control over the range of interference is essential to avoid loosening of mating components.

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What is Surface Roughness?

The surface roughness or surface finish plays an important role in the performance of certain machine elements. Friction and wear increase with surface roughness, adversely affecting the performance of bearings.

  • Rough surfaces have a reduced contact area in interference fits, which reduces the holding capacity of the joints.
  • The endurance strength of the component is greatly reduced due to poor surface finish.
  • The corrosion resistance is adversely affected by the poor surface finish.
  • The designer must specify an optimum surface finish from the considerations of functional requirements and the cost of manufacture.

Key Points to Remember

Here is the list of key points we need to remember about “Tolerances and Fits”.

  • Tolerance is defined as the permissible variation in the dimensions of the component.
  • When two parts are to be assembled, the relationship resulting from the difference between their sizes before assembly is called a fit.
  • A clearance fit is a fit that always provides a positive clearance between the hole and the shaft over the entire range of tolerances.
  • An interference fit is a fit that always provides a positive interference over the whole range of tolerances.
  • A transition fit is a fit that may provide either a clearance or interference, depending upon the actual values of the individual tolerances of the mating components.
  • The grade of tolerance is defined as a group of tolerances, which are considered to have the same level of accuracy for all basic sizes.
  • There are two basic systems for giving tolerances, the hole-basis system and the shaft-basis, system.
  • There are two basic systems for giving tolerances to the shaft and the hole, namely, the hole-basis system and the shaft-basis, system.
  • The surface roughness or surface finish plays an important role in the performance of certain machine elements.

If you find any mistake above, kindly email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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