Unilateral and Bilateral Tolerance in Engineering Drawing

Tolerance in Engineering Design

Let us begin with a general introduction to engineering tolerances. Tolerancing is a system that defines how much a part’s dimensions can vary within an allowable limit.

For example, consider a shaft that goes inside a bearing. Its diameter needs to be within a certain range for the assembly to function properly. If the shaft is too big (above its tolerance limit), the fit will be too tight, which can damage the bearing. Contrarily, if it is too small (below its tolerance limit), it will fit loosely inside the bearing, which is again a problem.

Generally, tolerances and allowances of parts are communicated between different teams via engineering drawings. There are several ways to express desired tolerances, which can change depending on the type of tolerance, its application, and design intent.

Unilateral tolerance and bilateral tolerance are two of the most common ways to express engineering tolerances.

What is Unilateral Tolerance?

The first question to investigate is: what is unilateral tolerance? Unilateral tolerancing is the type of tolerance definition in which the dimension can deviate in only one direction. In other words, the dimension can be either smaller or higher than its nominal value, but not both.

Unilateral tolerance is useful when one-sided variation is preferable for functional reasons. For instance, in certain applications, it is acceptable for a component to be slightly oversized but being undersized is unacceptable.

Unilateral Tolerance in Manufacturing

A pin-pinhole joint presents one such scenario. Pins are commonly used to align parts with each other without achieving a permanent joint. For example, injection molding dies have pins on one half and pinholes on the other. This allows them to maintain proper alignment through thousands of injection molding cycles.

To achieve a functional engineering fit between the pin and pinhole, the pin cannot be larger than the corresponding pinhole’s diameter, otherwise, it will get stuck and, thus, lose its functionality. There is, however, a limit to how much lower the pin diameter can be to avoid an overly loose pin-pinhole joint. Thus, a unilateral tolerance is useful here.

By defining an appropriate unilateral profile tolerance, the designer basically simplifies the manufacturing processes by conveying design intent to the machinist. Moreover, unilateral tolerancing also adds clarity to the inspection and quality control procedures, saving time and resources.

Unilateral Tolerance Example

It is important to know how a unilateral tolerance appears in an engineering drawing. The unilateral tolerance example below shows that the nominal dimension is 10.5 while the allowable deviation is -0.1. This being a unilateral tolerance, the upper deviation is 0.

Therefore, the range in which the final dimension can vary is 10.4-10.5.

What is Bilateral Tolerance?

Now, let us move on to the question of what is bilateral tolerance. As the name suggests, bilateral tolerancing allows dimensions to deviate on both sides. The allowable deviations on both sides may be equal or unequal, depending on the design requirements.

Bilateral tolerance is the most common and intuitive form of engineering tolerance, as it is common knowledge that a dimension can vary from its nominal value in any direction during a manufacturing process.

Bilateral Tolerance in Manufacturing

A practical of bilateral tolerancing comes from the manufacturing of pistons for automotive engines. The piston must fit inside the cylinder with a very precise clearance for a smooth, sliding motion and to prevent problems such as excessive surface wear or oil leakage. To achieve this fit, a bilateral tolerance is necessary as it prevents cases of both oversizing (high friction, rugged motion) and undersizing (oil leakage, unwanted non-linear motion).

Bilateral tolerancing offers higher flexibility in production to maintain a balance between precision and manufacturability by conveying to the manufacturing team that it is allowed a certain margin of error. Moreover, bilateral tolerances are also helpful for quality personnel as they are used to compute tolerance stacks.

Equal Bilateral Tolerance Example

Let us look at an equal bilateral tolerance example. In this case, the tolerance zone is symmetric about the nominal dimension value. The figure below shows that the nominal dimension is 1.5 while the equal bilateral tolerance is ± 0.0002.

The ‘±’ here indicates a bilateral tolerance. In this example, the allowable range for this dimension is 1.4998-1.5002.

Unequally Disposed Tolerance Example

The other type of bilateral tolerance is unequally disposed bilateral tolerance, where the ‘plus‘ and ‘minus’ are different values. In other words, the tolerance zone is non-symmetric about the nominal value.

The unequally disposed bilateral tolerance example shown below demonstrates an unequally disposed bilateral profile tolerance. The notation means that the total tolerance zone is 4 units wide, with 3 units above the nominal surface profile and 1 unit below.

In other words, the surface is allowed to have craters no more than 1 unit deeper and protrusions, or ridges, no more than 3 units higher at any point along the ideal profile.

This example is a good demonstration of how the professional use of GD&T symbols and proper unilateral tolerance and bilateral tolerance can convey exact manufacturing requirements.

Difference Between Unilateral and Bilateral Tolerance

The differences between unilateral tolerance and bilateral tolerance are not too many but are important to understand for design engineers to create professional engineering drawings. The table below summarizes the main pointers on how unilateral tolerancing and bilateral tolerancing differ.

Other Types of Engineering Tolerance

Unilateral tolerance and bilateral tolerance are quite the most common forms of engineering tolerances. However, there are various other methods to define the tolerance limits of a part dimension.

Depending on the intent of the designers and manufacturing requirements, an engineer may choose to use a different type of tolerance definition.

A list of numerous other types of engineering tolerances is as follows:

• Limit Tolerance: Defines the upper and lower limits for the dimension. For example, 49.5 mm – 50.5 mm. This is considered less restrictive as it appears as a hard limit on the extreme deviations rather than a dimension to be ‘maintained’ around an ideal nominal value.
• Hole/Shaft Basis: A fairly popular way of defining tolerance limits for shaft-hole engineering fits. The desired fit between a hole and a shaft can vary widely between tight fits, transition fits, or loose fits. The hole/shaft basis standard allows engineers to convey both the preferred fit type and tolerance limits at the same time. Moreover, they can choose between hole or shaft basis systems to set either the hole as a reference or the shaft.
• Form Tolerance: A form tolerance is imposed on geometric entities like roundness, runout, or profile rather than a numerical dimension value. Form tolerances are equally important as dimensional tolerances as they ensure part functionality and allow to conveying of design intent to the manufacturing team.