Engineering fit

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During assembly of two mating parts, there may be either tightness or looseness between them. The degree of tightness or looseness between them is termed as fit. Manufactured parts are very frequently required to mate with one another. They may be designed to slide freely against one another or they may be designed to bind together to form a single unit.

There are three general categories of fits:

  1. Clearance fits for when it may be desirable for the shaft to rotate or slide freely within the hole.
  2. Transition fits for when it is desirable that the shaft to be held precisely, yet not so tightly that it cannot be disassembled, this is usually referred to as a Location or Transition fit.
  3. Interference fits, for when it is desirable for the shaft to be securely held within the hole and it is acceptable that some force be necessary for assembly.

Within each category of fit there are several classes ranging from high precision and narrow tolerance (allowance) to lower precision and wider tolerance. The choice of fit is dictated first by the use and secondly by the manufacturability of the parts. Fits may be specified according to the standards of geometric dimensioning and tolerancing.

Fit Classes[edit]

Interference fits[edit]

Main article: Interference fit

Interference Fits, also known as Press Fits or Friction Fits, are fastenings between two parts in which the inner component is larger than the outer component. Achieving an interference fit requires applying force during assembly. After the parts are joined, the mating surfaces will feel pressure due to friction, and deformation of the completed assembly will be observed.

Force fits[edit]

Force Fits are designed to maintain a controlled pressure between mating parts, and are used where forces or torques are being transmitted through the joining point. Like interference fits, force fits are achieved by applying a force during component assembly.[1]

For more information, see Interference fit#Force.

FN 1 to FN 5

Shrink fits[edit]

Shrink fits serve the same purpose as force fits, but are achieved by heating one member to expand it while the other remains cool. The parts can then be easily put together with little applied force, but after cooling and contraction, the same dimensional interference exists as for a force fit. Like force fits, shrink fits range from FN 1 to FN 5.[2]

Location fits[edit]

Location fits are for parts that do not normally move relative to each other.

Location interference fit[edit]

LN 1 to LN 3 ( or LT 7 to LT 21?[citation needed] )

Location Transition fit[edit]

LT 1 to LT 6

Location Clearance fit[edit]

LC 1 to LC 11

RC fits[edit]

The smaller RC numbers have smaller clearances for tighter fits, the larger numbers have larger clearances for looser fits.[3]

RC1: Close Sliding Fits[edit]

Fits of this kind are intended for the accurate location of parts which must assemble without noticeable play.

RC2: Sliding Fits[edit]

Fits of this kind are intended for the accurate location but with greater maximum clearance than class RC1. Parts made to this fit turn and move easily. This type is not designed for free run. Sliding fits in larger sizes may seize with small temperature changes due to little allowance for thermal expansion or contraction.

RC3: Precision Running Fits[edit]

Fits of this kind are about the closest fits which can be expected to run freely. Precision fits are intended for precision work at low speed, low bearing pressures, and light journal pressures. RC3 is not suitable where noticeable temperature differences occur.

RC4: Close Running Fits[edit]

Fits of this kind are mostly for running fits on accurate machinery with moderate surface speed, bearing pressures, and journal pressures where accurate location and minimum play are desired. Fits of this kind also can be described as smaller clearances with higher requirements for precision fit.

RC5 and R6: Medium Running Fits[edit]

Fits of this kind are designed for machines running at higher running speeds, considerable bearing pressures, and heavy journal pressure. Fits of this kind also can be described with greater clearances with common requirements for fit precision.

RC7: Free Running Fits[edit]

Fits of this kind are intended to use where accuracy is not essential. It is suitable for great temperature variations. This fit is suitable to use without any special requirements for precise guiding of shafts.

RC8 and RC9: Loose Running Fits[edit]

This kind of fit are intended for use where wide commercial tolerances may be required on the shaft. With this fits, the parts with great clearances with having great tolerances. Loose running fits exposed to effects of corrosion, contamination by dust and thermal or mechanical deformations.

ISO Metric fits[edit]

ISO-R286 and ANSI B4.14831

Fitting[edit]

The task of fitting is making skilled cuts to parts, on a cut-and-try basis (cut, try; cut more, try again), so that they will fit together with the desired degree of fit. Prior to the advent of interchangeable manufacture, fitting was the only way to create precise assemblies, such as the locks of firearms, and it was done manually, one assembly at a time, with tools such as files and laps. The person who did the fitting could be a craftsman such as a gunsmith, or a factory worker at a bench (called a fitter), or a toolmaker or toolfitter. When interchangeable manufacture began, its first form was one that did not obviate fitting, but simply shifted its focus, from the fitting of mating parts to each other, to the fitting of identical parts to a jig, template, or gauge ("filing to jig", "filing to gauge"). Gradually, manufacturing technology transitioned ever more toward processes in which parts were machined in such a precise, repeatable way that they required no fitting at all. For example, a pistol's hammer could be milled with a milling machine directly to gauge size, rather than being rough-milled to nearly that size followed by hand filing to gauge.

See also[edit]

References[edit]

  1. ^ Mott, Robert. Machine Elements in Mechanical Design (Fifth ed.). Pearson. p. 495. 
  2. ^ Mott, Robert. Machine Elements in Mechanical Design (Fifth ed.). Pearson. p. 495. 
  3. ^ "ANSI Standard Limits and Fits (ANSI B4.1-1967,R1974)". Retrieved 9 September 2013.