A Ø25 mm shaft gets a fixed hub with no axial sliding. What keyway dimensions and width fit belong to it?
Size follows from the shaft diameter
DIN 6885-1 gives key width b, key height h and the seating depths t1 (shaft) and t2 (hub) per shaft-diameter range, not as a smooth formula. For Ø25 mm the shaft falls in the >22 to 30 mm range: b = 8 mm, h = 7 mm, t1 = 4.0 mm and t2 = 3.3 mm, with a +0.2/0 mm depth tolerance. Read the table from the actual shaft diameter, not a rounded CAD dimension.
Width sets the fit, not the depth
The fit follows from the tolerances of both key and slot. P9, N9 and JS9 describe the slot width; none alone guarantees interference or sliding. Use the actual limit dimensions and specify the key tolerance as well. H9/D10 are workshop conventions, not a universal prescription for a sliding fit.
Worked example: P9 on b = 8 mm
At b = 8 mm the P9 slot ranges from 7.949 to 7.985 mm. Explicitly assume an h9 key: 7.964 to 8.000 mm. The extremes give 0.021 mm clearance or 0.051 mm interference: a transition fit, not guaranteed clamping. An H9 slot from 8.000 to 8.036 mm gives 0 to 0.072 mm clearance with this h9 key.
Assumptions and applicability limits
The table covers parallel keyways up to Ø110 mm; spline and Woodruff joints fall outside it. No shear or bearing strength of the key itself is calculated, and no stress concentration at the slot corners is assessed. Check that loading separately for high torque or shock loads.