Proper torque transmission in a hydraulic pump depends heavily on the precise fit between the drive shaft and the key. Selecting the correct ISO tolerance class for the key and keyway is crucial to prevent fretting, backlash, and premature pump failure.

The most common key fit classes for hydraulic pump shafts are P9, N9, and H9. The uppercase letter indicates the keyway tolerance in the hub (pump rotor), while the number defines the tolerance grade. For typical high-torque applications, an N9 fit for the key width on the shaft and a P9 fit for the hub keyway is standard. This combination ensures a light interference fit, meaning the key is slightly snug. This prevents the key from moving under load, reducing wear.

A looser H9 fit is sometimes used for sliding applications where the hub must move axially. However, in fixed axial pumps, an H9 fit can lead to"play" and micro-movement, causing impact loads on the key edges. Over time, this produces metal debris that contaminates the hydraulic fluid.

The most critical dimension is the key width (b). Engineers must verify the actual shaft keyway width and the hub keyway width against the nominal dimension specified in ISO 286. For example, a 10 mm key with a shaft tolerance of N9 results in a width range of 9.988 mm to 9.964 mm. The mating hub slot, with a P9 tolerance, ranges from 9.964 mm to 9.940 mm. This small overlap creates a press fit of 0 to 0.024 mm, ensuring zero clearance.

Key fit failure often manifests as shaft scoring, key shearing, or loose vibration. To avoid this, always use a new key when replacing a pump, measure actual keyway sizes with a micrometer, and never reuse a worn key that has deformed edges. Understanding and applying the correct key fit tolerance class ensures a reliable, long-lasting hydraulic pump assembly.