Bilateral Tolerance

Sarah
DeGuzman

Bilateral tolerance is one of the primary three tolerances that occur most often on working drawings. A bilateral tolerance exists when a variation from the desired dimension is shown as occurring in both the positive and negative directions. In this article, you will learn the purpose of bilateral tolerance, examples, and differences vs. unilateral tolerance.

Measuring tool for tolerance
Courtesy: SANS Machining

Purpose

It is economically impractical to produce closely matched parts without tolerances. Doing so would lead to a high rejection rate and an increase in the cost of manufactured parts. As a result, engineering tolerance is employed to provide the following benefits:

  • Provision of flexibility to the manufacturer.
  • For smooth product assembly even if the part produces in the worst possible scenario
  • Reduction of part rejection rate
  • Interchangeability of manufactured parts
Positive and negative deviation
Courtesy: Slide to Doc

A plus or minus tolerance (+/-) on a dimension constitutes a bilateral tolerance. It allows for both positive and negative deviations from the desired size. In most circumstances, bilateral tolerance defines as the same +/- number in both directions.

However, this isn’t always the case, and a bilateral tolerance doesn’t have to be equal in terms of positive and negative tolerances. A bilateral tolerance (for example: 10.0mm +0.1mm/-0.3mm) would also be acceptable. In such a case, the effects of an oversized part would be less desirable than an undersized part.

In most cases, engineering drawings do not show a tolerance on the dimension itself. Rather, an engineer adds a note somewhere on the drawing that dictates a standard tolerance in the absence of a specific notation. This note typically appears in the title block or note section.

Examples

Equal Bilateral Tolerance

An equal bilateral tolerance will have similar plus and minus tolerances such as 1.500 +/- 0.002.

Unequal Bilateral Tolerance

The plus and negative tolerances of an uneven bilateral tolerance are not the same, and neither is zero, for example, +.0005 and -.0003

Bilateral vs. Unilateral

Unilateral Tolerance

When the tolerance distribution is only on one side of the primary size, it defines as a unilateral tolerance. In other words, tolerance limits lie wholly on one side of the primary dimension, either above or below it. Unilateral tolerance employs when requiring precision fits during assembly. 

This type of tolerance usually indicates the same operator also machined the mating parts. Also, unilateral tolerance sees frequent use in the drilling process where hole dimensions will most likely deviate in one direction only. That is, the gap is always oversized rather than undersized.

The unilateral system brings advantages because the actual size is utilized for the GO limit gauge, especially when it requires precision fits. After all, it is easy and simple to determine deviations. This helps in the standardization of the GO gauge, as holes and shafts of different grades will have the same lower and upper limits, respectively. Changes in the magnitude of the tolerance affect only the size of the other gauge dimension, the NOT GO gauge size.

Difference of Unilateral vs. Bilateral Tolerance

Unilateral Tolerance

  • Make only direct variations from the nominal or primary dimension
  • Without modifying the nominal size of the shaft or hole, tolerances for the same allowances or kind of fit can be change.
  • They can be use in any form of industry.
  • Allows for standardization of GO gauges for holes and NOGO gauges for shafts.

Bilateral Tolerance

  • Variation is made in both directions from the nominal or basic dimension.
  • The tolerances for the same allowance or type of fit can be changed by changing the nominal size of the shaft or hole.
  • These are suitable for mass production.
  • It is not possible to standardize GO/NO GO gauges