A taper bush removed from a conveyor, fan, or pump drive should be quarantined for inspection before it is mated to a replacement hub, with measurable acceptance limits rather than a visual gut feel: bore roundness under 0.05 mm, taper surface roughness under Ra 1.6 µm, and a keyway free of wallow or burrs are the criteria that separate a reusable bush from scrap [S1][S2].
The taper lock system uses a self-locking 1:12 geometry (around 4.76° included angle) to clamp the hub onto the shaft through a split, tapered sleeve driven by cap screws; the radial clamping force that holds the assembly together depends on the bush bore, key, and taper face all being within original machining tolerance, so any of those surfaces that have moved, corroded, or deformed will directly cost torque capacity [S1].
Symptom Check: Why Bushes Come Out of Service
Most taper bushes that come back to the bench for evaluation were removed for one of three root causes: pulley slippage on the shaft, excessive vibration, or routine hub swap during a scheduled overhaul, and each of those failure modes leaves a different tell on the bush that the inspector can read before reassembly [S2].
A slipped pulley leaves polished, shiny fretting tracks on the bore opposite the keyway and burnished bands on the shaft, and a pulley that has been running hot will often show blueing on the bush flange or socket-head capscrews, all of which disqualify the bush for reuse because the bore metallurgy and hardness have been compromised by friction heat [S2][S6]. Vibration-driven failures instead show as wallowed keyways, rounded key corners, and a split that has opened more than the original clearance, because the loose fit has hammered the assembly every revolution [S2][S3]. A bush removed during a planned change-out with no slip or heat history is the only category that should enter the reuse pipeline at all, and even then only after a full dimensional check [S3].
Cleaning and Disassembly Before the Gauge
Before any measurement, the bush needs to be fully stripped of any anti-seize, light oil, or surface rust film that could mask a defect, and the standard sequence is degrease, dry, then a light solvent wipe of the bore and the taper face so the inspector is looking at bare metal [S1][S6]. The shaft surface where the bush sat should also be cleaned and inspected at the same time, because paint, burrs, tool marks, or residual coating on the shaft will be transferred into the new joint if the same shaft is being reused [S1][S4].
For the cleaning pass itself, a soft brass wire wheel on the taper face is acceptable, but file work, emery cloth on the bore, or any abrasive on the keyway is not, because those operations remove material from surfaces that are finish-machined to tight tolerance and cannot be re-machined in the field [S1][S3]. After cleaning, the bush should be left to acclimatise for a few minutes so any solvent film flashes off and the metal returns to ambient before bore and taper measurements are taken, since thermal expansion of even a few degrees will push a borderline reading across the rejection line on small-bore sizes [S3].
Dimensional Inspection: Bore, Taper, and Split
The bore check is the first quantitative test, and the goal is roundness plus diameter within the manufacturer's published tolerance, typically held to around h7 on the small sizes and h8 on the larger 5050-class bushes; an out-of-round reading above 0.05 mm at any axial plane means the split has yielded and the bush should be scrapped rather than reused, because once the split has deformed the radial clamping geometry is permanently changed [S1][S3].
The taper face is checked with a flat reference and a feeler stock or a dial indicator on a surface plate, looking for both corrosion pitting and contact pattern: a healthy taper shows continuous contact across at least 85% of the face area, and any pitting deeper than about 0.1 mm or any single pit cluster larger than 2 mm² is grounds for rejection, because the localised high spots will carry the entire clamping load and gall into the hub bore on the next tightening [S1][S2]. The split line itself is the third visual, and a bush whose gap has opened, or whose edges have rounded or peened over, has lost the spring tension that drives the radial squeeze, so it goes in the scrap bin regardless of how clean the rest of the surfaces look [S1][S3].
Keyway and Key Condition: The Torque Path
Torque is transmitted through a parallel key seated in matching keyways cut in both the bush bore and the shaft, and the bush bore keyway is finish-machined to a tolerance that lets the key bear on both faces simultaneously, so any wallow, burr, or radius change at the keyway corners means the bush is no longer a torque-carrying part [S1][S3].
Inspection uses a new key of the correct BS 4235 Part 2 size as a go/no-go gauge: the key should slide in with light hand pressure, seat on the bottom of the keyway across its full length, and show no daylight at the top corners [S1][S2]. A key that rocks, sits proud, or only contacts at the ends indicates either a wallowed keyway or a sprung keyway wall, both of which are scrap conditions because they convert the keyed joint into a hammering joint that will fret the shaft within hours of restart [S2]. For corrosion-resistant drives, stainless shaft key sizing for washdown service matters for the replacement key selection even when the bush itself is standard carbon steel. For high-cycle reversing drives, a tangential key pair geometry and failure mode reference is worth a read because the inspection criteria for a tangential key seat differ from a single parallel key.
Threads, Screws, and the Removal Holes
Each bush carries two distinct sets of tapped holes, the fastening holes threaded in the hub and the removal holes threaded in the bush itself, and the inspector needs to confirm that both sets are clean, undamaged, and that the right screw size still engages to full depth [S4][S6]. A stripped or corroded fastening hole means the cap screw cannot develop the rated pre-load, so the clamping force that drives the 1:12 taper home is reduced and the bush will slip on the shaft, while a damaged removal hole means the next teardown will require a press instead of the standard screw-into-bush extraction sequence [S4].
The socket-head cap screws themselves should be replaced at every disassembly as a matter of course, because grade-8.8 or higher cap screws lose a portion of their yield strength every time they are torqued past yield, and the taper lock sizing and selection practice is built around the screws being at full rated pre-load when the bush is seated [S7]. Reusing old screws to save a few dollars is one of the most common ways a "reused" bush fails on its first duty cycle, because the under-torqued screws let the taper seat walk back during the initial run-in hours [S7].
Re-Torque Schedule After the New Pulley Is Mounted
Once the inspected bush has been accepted and mated to the new pulley, the assembly should be re-torqued after the first 2 to 4 hours of operation, because the taper seats further during initial run-in and the screws lose a measurable portion of their pre-load as the joint settles, which is true for both Taper Lock and QD systems [S7].
The re-torque pass uses the same cross or alternating pattern as the initial tightening, the same manufacturer-rated torque value (not a guess), and the same screw grade that was fitted originally, and any bush that requires a third re-torque pass within the first 24 hours is showing a symptom of a deeper problem: either the bore was never round, the taper face never seated, or the key was never bottoming in the keyway, all of which point back to a failed inspection rather than a tightening issue [S3][S7]. For the broader question of taper bush and timing pulley interchangeability when swapping a pulley model alongside the bush, the same inspection criteria apply, and any pulley that shows groove wear past the belt-maker's profile limits is itself a scrap decision independent of the bush condition [S2].
Trackable signals for the next maintenance window: log bore diameter, bore roundness, taper-face contact percentage, and keyway fit feel for every reused bush, then compare against the as-new baseline; a downward drift on more than one of those parameters across consecutive overhauls on the same drive is the early warning that the drive duty is exceeding the bush's design envelope and a redesign (different series, QD, or keyless) should be on the next shutdown scope.
For component-level specifications, see powder new material.