Which type of tolerance allows for variation only in one direction from the specified dimension?

Unidirectional tolerance specifically allows for variation in only one direction from a specified dimension, ensuring that parts remain within acceptable limits without allowing deviations in the opposite direction. This type of tolerance is particularly useful in scenarios where a deviation in only one way is critical for functionality or assembly.

For example, if a part needs to fit into a limited space, unidirectional tolerance could specify that a dimension may be larger but not smaller, effectively ensuring that the part does not interfere with surrounding components. This precision can be crucial in manufacturing processes where maintaining certain measurements is essential for the product's performance and compatibility.

The other types of tolerances have different characteristics that do not fit this specific criterion—bilateral tolerance allows for variation in both directions, fixed tolerance generally refers to a set measurement that cannot be altered, and variable tolerance permits changes within a range, but without a restriction to only one direction.