Repeated key failure on a coastal pump coupling
A coastal water-treatment plant near Port Harcourt has a horizontal centrifugal pump whose 45 mm AISI 304 stainless shaft drives through a keyed coupling. The key is 12 x 8 x 70 mm and has failed three times in eight months, each time with a twisted shear fracture near the motor end. The coupling half is cast aluminium alloy. The pump runs at 1,450 rpm with frequent starts, and the atmosphere is humid with chloride-laden air. The maintenance fitter proposes replacing the key with a high-carbon steel key, filing it to fit, and tightening the coupling with a hammer. The store holds EN8 bright drawn bar and a length of 316 stainless bar. The manufacturer's manual specifies a parallel key, an interference-fit coupling and maximum shaft runout of 0.05 mm.
1. Analyse why the key fails repeatedly at the motor end and explain the roles of fit, material and environment.
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A twisted shear fracture means the key is being loaded in torsion, but three failures in eight months means the key is a symptom, not the root cause. A filed-to-fit key sits loose in the keyway, so load is concentrated on a narrow contact strip and the key hammers under every start until it shears. The motor end is where start-up torque and any misalignment are most severe, so check coupling alignment, soft foot, shaft runout against the 0.05 mm limit, and wear in the coupling bore and keyway.
The material choice matters, but a harder key is not automatically better: a fully hardened high-carbon key is brittle and notch-sensitive, and if it survives it may damage the shaft keyway. The environment adds a third mechanism: aluminium coupling, 304 shaft and a chloride-laden atmosphere form a galvanic couple, and corrosion products in the keyway can jack the fit loose and create pits that initiate cracking. The correct diagnosis therefore covers fit, alignment, runout, keyway wear and corrosion, not only key strength.
2. Recommend the key material and condition, and justify your choice against the EN8 and 316 alternatives in store.
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I would use the EN8 bright drawn bar, machined to the correct parallel key dimensions with chamfered ends, in a normalised or lightly hardened and tempered condition rather than fully hardened. EN8 is a medium-carbon steel with useful strength and enough ductility to deform before brittle fracture, and it is the closest of the two store items to a conventional key steel. It should be a tight side fit in the shaft and coupling keyways with a small clearance on top, not filed to a sloppy fit.
A high-carbon, fully hardened key would be stronger in shear but brittle and less forgiving of shock starts; it also risks damaging the shaft keyway and removing the key's role as the sacrificial element. The 316 stainless bar has lower strength than EN8, galls easily and would not solve the root cause; its corrosion resistance is an advantage, but a key that is mechanically wrong is not fixed by corrosion resistance. Before fitting either, I would check the transmitted torque against the key shear area and confirm that the coupling and shaft, not the key, are the real problem.
3. Set out the fitting and corrosion-control procedure you would enforce when the pump is reassembled.
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Clean and inspect both keyways for burrs, wear and corrosion pits; measure width and depth and reject the coupling if the bore or keyway is worn beyond limits. Fit the key with a light press or tap fit on the sides, never by filing to a loose fit, and check that the coupling pulls up to its correct axial position. Press or heat the coupling into place with controlled heat or a puller; hammering burrs the shaft, damages the bearing and does not achieve the specified interference fit.
Check alignment and shaft runout after assembly, against the 0.05 mm limit, and recheck after the first run-in. For corrosion, coat the shaft and keyway with a suitable barrier compound, keep the coupling bore dry and drained, and consider replacing the aluminium coupling with a steel or coated coupling, or introducing a barrier between dissimilar metals. Record key dimensions, fit, alignment readings and any parts replaced. Then review the maintenance interval, because repeated starts and a coastal atmosphere will reproduce any remaining fault.