Engineering fits

Engineering fits are a system of tolerances that defines how precisely two parts with the same nominal size mate, typically a hole and a shaft. The system is also known as limits and fits or ISO fits. The resulting fit, the relationship between the tolerance zones of hole and shaft, decides whether the joint moves, is only accurately centered or is firmly pressed together. Engineering fits are standardized in ISO 286 (Part 1 covers the basics, Part 2 the tables of limit deviations); US inch drawings often use the ANSI B4.1 classes (RC, LC, LT, LN, FN) instead.

A tolerance zone is written as a letter and a number. The letter sets the position of the zone relative to the nominal size (capital letters for holes, lowercase for shafts), and the number sets the IT tolerance grade. A 25 H7 hole measures 25.000 to 25.021 mm and a 25 g6 shaft 24.980 to 24.993 mm, so a 25 H7/g6 fit has a clearance of 0.007 to 0.041 mm. Mechanical engineering mostly uses the hole-basis system: the hole gets an H zone and the character of the fit is set by the shaft zone, because precise holes are made and inspected with standard reamers and plug gauges. The shaft-basis system is used, for example, with drawn bar stock onto which several parts with different fits are mounted.

The main types of fit:

A clearance fit guarantees that the shaft is never larger than the hole: H11/c11 for loose joints, H7/f7 for pivot pins and plain bushings, H7/g6 for precise guiding, H7/h6 for parts that slide but stay accurately centered. A transition fit (H7/k6, H7/n6) may give a small clearance or a small interference depending on the actual sizes and centers parts assembled with a mallet or a press, such as gear and pulley hubs on a keyed shaft. An interference fit (H7/p6, H7/s6 and tighter) transmits forces and torque through friction and is assembled by pressing, by heating the hub (a shrink fit) or by cooling the shaft.

Specify fits wherever the function depends on how two parts mate: pins and bushings, shafts in hubs, dowel pins, guides for sliding parts, bearing seats, or pressed joints that must transmit torque without additional fasteners. For other dimensions, general tolerances suffice.

Start from the function: should the part rotate or slide, only be centered, or carry a load? Consider the operating temperature (the different thermal expansion of aluminum and steel can close a clearance or loosen an interference), surface finishing, and how the parts are assembled and disassembled. Stick to commonly used fits for which shops have the tooling and gauges.

A common mistake is to write just "H7" on the drawing without relating it to the mating part. Always design a fit for the pair of parts and check the limit clearances and interferences in the ISO 286-2 tables. The specified zone must be manufacturable and measurable: an H7 zone in a small hole requires reaming or fine boring and inspection with a plug gauge, and a shaft in grade IT6 usually needs precision turning or grinding.

Surface roughness and surface finishing also affect a fit. Rough surfaces are partly flattened during pressing, so the actual interference drops, and an anodized or zinc-plated layer makes a hole smaller and a shaft larger. The drawing should therefore state whether a dimension applies before or after finishing. Rolling bearing rings have their own tolerances, and the shaft and housing fits are chosen according to the load and the bearing manufacturer's catalog.

For pressed joints, verify that the hub can withstand the interference (especially thin-walled and plastic parts). For replacement parts made from a sample, measure both parts of the joint, because wear may have changed the original fit.