Keyway Size Calculator

A parallel key is a deliberately weak link that carries torque between a shaft and a hub through two distinct failure paths. The key can shear across the plane of the shaft surface, resisted by the width times the length, or it can crush against the keyseat walls, resisted by roughly half the key height times the length. Both act at the shaft radius, so torque capacity scales with radius as well as key section. Because both modes must be checked and the smaller answer governs, key sizes are not derived from the torque at all in normal practice: they are read from a table indexed on shaft diameter, and the designer adjusts length to reach the required capacity. ANSI B17.1 and DIN 6885 give nearly identical sections over the common metric range, with an 8 by 7 mm key for a 22 to 30 mm shaft and a 20 by 12 mm key for a 65 to 75 mm shaft. Key length is customarily set by the hub width, with about 1.5 times shaft diameter a reasonable starting point. Typical allowable values for a mild steel key such as AISI 1018 are on the order of 60 MPa in shear and 90 MPa in bearing, divided by a service factor of 2 to 3 depending on whether loading is steady, intermittent or shock. The main limitation is the shaft itself: a keyseat is a sharp notch that concentrates bending stress by a factor of roughly 2 to 3, so fatigue cracks usually start at the keyway root rather than in the key, which is why corner fillet radii of about 0.25 to 0.5 mm are specified.

Keyed shaft connections predate any formal standard. Wooden and iron keys were driven into slots to lock wheels, pulleys and millstones onto shafts long before the stresses involved could be calculated, and early industrial practice used tapered gib head keys hammered into place, relying on friction and wedging as much as on shear. The problem this created was interchangeability: every shop cut keyways to its own proportions, so a replacement pulley from a different maker rarely fit an existing shaft. Systematic key sizing followed the same standardization wave that produced screw thread and limits and fits standards. American practice was consolidated in ANSI B17.1, Keys and Keyseats, issued in 1967 and reaffirmed repeatedly since, which fixed key width and height against shaft diameter bands together with tolerance classes for clearance, transition and interference fits. The German DIN 6885 series, whose first sheet covers parallel keys and their keyways, performed the same role for metric practice and was later paralleled by ISO documents. Because the two systems were developed against similar shafting and similar steel strengths, the resulting sections largely converged, which is why the ANSI and DIN tables in this calculator agree across most of the range. The Woodruff key, a semicircular form that self aligns on tapered shaft ends, entered American machine shop practice in the late nineteenth century and was standardized alongside parallel keys.