Taper bushings use a four-digit numbering convention for Taper Lock (e.g. 1008, 1210, 3020, 5050) and a letter code for QD (SH, SDS, SD, SK, SF, E, F, J, M, N, P, W, S), and the sequence is non-optional: the bushing number is dictated by the hub it has to seat in, while the bore inside that bushing is the variable that matches the shaft [S1][S7].
For Taper Lock, the smallest stock series is 1008 with a bore window of 1/2 to 1 in; the largest cataloged inch series in general industrial use is 5050 at 1-3/4 to 5 in, with 7060 cited as the upper-end envelope by industry references [S1][S7]. QD runs from SH at 3/8 to 1-1/8 in up to P/W/S above 12 in+, giving QD a materially wider bore envelope than Taper Lock [S1].
Taper Lock series, ordered smallest to largest bore window
Sorting the standard Taper Lock series by their maximum bore produces the ordering 1008 (1/2 to 1 in), 1108 (1/2 to 1-1/8 in), 1210 (1/2 to 1-1/2 in), 1610 (1/2 to 1-5/8 in), 2012 (1/2 to 2 in), 2517 (5/8 to 2-1/2 in), 3020 (3/4 to 3 in), 3525 (1 to 3-1/2 in), 4040 (1-1/4 to 4 in), 4545 (1-1/2 to 4-1/2 in), 5050 (1-3/4 to 5 in) [S1][S3]. The numeric pattern is not arbitrary: the first two digits encode the maximum bore in sixteenths of an inch (1008 = 10/16 = 0.625 in barrel reference), and the last two digits encode axial length in inches times 100, so a 2517 is a 2-1/2 in bore class bushing that is 1.7 in long, which is the kind of number-crunching that lets you sanity-check a catalog entry at a glance [S3][S6].
Inside each series, bore sizes are not continuous; they step in fixed increments tied to standard shaft/keyseat combinations. For a 1008 the available bores are 1/2 to 9/16, 5/8 to 7/8, and 15/16 to 1 in, with keyseats scaling from 1/8 x 1/16 up to 1/4 x 1/16 [S3][S6]. A 3020 expands to seven bore groups from 3/4 in up to 3 in, each with a matching shaft keyseat from 3/16 x 3/16 to 3/4 x 1/4 [S3]. Catalog weight scales with size, roughly 0.2 lb for a 1008 and several pounds for a 5050, so shipping and lifting constraints quietly enter the selection at the top of the range [S3].
QD series, ordered smallest to largest bore window
QD bushings sort by letter code SH, SDS, SD, SK, SF, E, F, J, M, N, then P/W/S at the top end [S1]. The published bore windows are SH 3/8 to 1-1/8 in, SDS 1/2 to 1-5/8 in, SD 1/2 to 2-1/8 in, SK 3/4 to 2-7/16 in, SF 3/4 to 3-3/16 in, E 1 to 3-15/16 in, F 1-3/16 to 4-15/16 in, J 1-7/16 to 5-15/16 in, M 2-1/16 to 7-7/16 in, N 2-15/16 to 8-15/16 in, and P/W/S above 12 in [S1].
QD is the system you reach for when the shaft is large or the hub is heavy, because the P/W/S envelope clears 12 in and the flange cap-screw design handles higher torque than the typical Taper Lock draw-screw arrangement [S1]. A 2025-published metric QD chart (SH size) lists 15 to 38 mm bores, which lines up with the imperial SH range and confirms that metric and imperial are stocked as parallel offerings rather than afterthoughts [S8]. A practical rule: if your shaft is above 5 in or your drive is in the tens of horsepower range, default to QD unless the hub is already Taper Lock.
Selection logic: hub first, bore second, material third
The single most common mistake is choosing a bushing by shaft diameter alone. The bushing number is set by the hub it has to drop into, so the workflow is: read the hub (it will be marked, or check the sheave/sprocket part number), look up which bushing series that hub accepts, then pick the bore inside that series that matches the shaft [S1][S7]. A QD bushing will not seat in a Taper Lock hub and vice versa: the taper angles, bolt patterns, and mounting geometries are different and there is no adapter [S1].
Once the series is fixed, the bore must hit the shaft. Taper Lock bores are stocked in stepped increments with a matching keyseat, and a 3020 will not accept a 2-9/16 in shaft because that size simply is not in the 3020 bore table; you would step up to 3525 [S3]. Taper Lock stainless is the standard choice for food, pharma, and washdown service, and QD is the practical answer when bores above 5 in are required [S1]. For deep dives on the related keyed-failure problem, flywheel keying under reversing load covers the geometric side, and the taper bush reference is the catalog-level anchor.
Taper Lock vs QD side by side
On bore range, Taper Lock tops out near 5 in (5050) while QD clears 12 in+ in the P/W/S sizes, so QD wins any shaft above 5 in [S1][S7]. On installation, Taper Lock uses a visible hub split with draw screws that pull the bushing into a tapered bore, while QD uses a separate flanged bushing with 2 or 3 cap screws arranged around a smooth tapered bore opening; both install in minutes once torqued correctly [S1][S3]. On interchangeability, neither system cross-fits the other, so a brownfield replacement is locked to whatever the hub already accepts [S1]. On metric availability, both ship in metric bores; the SH metric QD chart for instance runs 15 to 38 mm [S8]. On standards, the relevant safety reference is ANSI B15.1 for guarding of rotating power transmission drives, which the manufacturer's installation sheet calls out explicitly [S3].
For belt-drive context, the T5 vs T10 timing pulley reference is the right companion read when a taper bush is being ordered into a small timing pulley, because a 1008 or 1108 is typically the upper practical limit on those pitch sizes.
Shaft tolerances, keyseat choices, and failure modes
Taper Lock is designed for an h9 shaft, with one manufacturer (Fenner) stating an accommodation window of +0.051 / -0.127 mm on the nominal bore; any shaft oversize makes the bushing tight to slide on and reduces the effective clamp force once torqued [S5]. A common field practice when a bush feels too tight on assembly is to open the slot with a flat screwdriver or fox wedge just enough to clear the shaft, then torque normally with no grease on the tapers or threads [S5].
Keyseat must match between bushing and shaft. A 1008 uses 1/8 x 1/16, 3/16 x 3/32, or 1/4 x 1/16 depending on bore group; a 3020 steps up to 3/16 x 3/16, 1/4 x 1/4, 3/8 x 3/16, 1/2 x 1/4, and 3/4 x 1/4 across its bore groups [S3]. Mismatched keyseat is one of the standard failure initiators because the key rides proud of the shaft and lifts the bushing off-center, which then loads the cap screws asymmetrically. The other classic failure is hammering the small end of a 3535 or larger bushing directly; the manufacturer's instructions require a block, sleeve, or drift against the large end only [S3]. Installation torque must be re-checked after the seating hammer blow, alternating screws until the wrench reading stops changing [S3].
Bore-numbering, hub material, and metric options
The two-digit prefix of a Taper Lock number is the maximum bore in sixteenths of an inch, and the two-digit suffix is the axial length in inches multiplied by 100, which is why a 2517 is barrel-class 2-1/2 in and 1.7 in long, and why a 4040 is barrel-class 4 in and 4.0 in long [S3][S6]. The scheme lets you reject a mislabeled bushing without measuring: if the prefix says 30 but the bore is bored to 3-1/4 in, the part is non-conformant because cataloged bushings cannot be bored larger than the largest bore listed for that series [S3].
Stainless is the standard material upgrade for washdown, food, and pharma, and is normally stocked in the common Taper Lock sizes rather than the QD envelope [S1]. Metric QD is a stocked range, not a custom run, with the SH size covering 15 to 38 mm bores and higher metric QD sizes scaling similarly [S8]. When the application is reversing or heavily shock-loaded, the bushing by itself is rarely the limit; the keyed joint is, and the geometry-side reference is tangential key pairs under reversing flywheel load.
Trackable signals to watch: (1) bore increments and keyseat tables in the next Martin, Dodge, or Fenner catalog revision, since the stepped bore groups are where most field mismatches originate [S3]; (2) metric QD stocking depth in M and N sizes, where 2 to 8 in shaft range is the most common request for fractional-horsepower industrial fans and agitators [S1][S8].
Spec-level background on the components involved: pressure transmitter, and flow meter.