Taper bush jacking holes and tightening holes are two distinct tapped features on the same bushing flange, and they act in opposite directions on the same 8° tapered interface [S2]. Tightening holes pull the bush into the hub to generate radial clamping force on the shaft; jacking holes, also called extraction holes, push the bush back out of the hub when the drive component has to come off [S1][S4].
Identifying which hole is which is a one-second test: tapped (threaded) holes accept a screw finger-tight and are the installation holes, while clearance or partially drilled holes are the jacking feature [S8]. On a small 1008-series bush there are typically two tightening holes and one jacking hole; on a larger 2517 or 4040 series the count rises to three tightening and two jacking holes [S2].
Anatomy of the 8° taper and the two hole families
The taper lock system is a wedging mechanism with an 8° external taper on the bush that mates to an 8° internal taper in the hub of the pulley, sprocket, or coupling [S2]. Each through-hole in the bush flange is half-drilled into the bush and half-drilled into the hub, but only one side carries a thread; the other side is a clearance or partial-depth hole that the screw tip bottoms out against [S2][S7].
When a cap screw is run into a tightening hole, the screw tip reacts against the blind clearance side and pulls the bush axially into the hub. Because the bush also carries a radial split, the axial pull converts into radial contraction of the inner bore, generating the friction grip on the shaft [S3][S4]. On QD (Quick Detachable) flanged bushings the principle is the same, but the flange bolts are the tightening fasteners and the same bolts are relocated to jacking holes for removal [S1][S4].
Why the two hole types must not be swapped
Using a jacking hole as a tightening hole, or vice versa, inverts the axial load direction and breaks the wedging logic of the 8° interface [S2]. Field data from refrigeration and air-conditioning service technicians shows that taper bushes cracked in half are almost always the result of a hammer being used to force a stuck bush off, or of screws being run into the wrong holes during installation [S2]. The resulting misalignment of the taper faces is what produces pulley wobble, vibration, premature bearing wear, and in the worst case a catastrophic bushing fracture.
On a Taper Lock bush, the hub carries the thread and the screw tip reacts against the bush; the reverse geometry applies on a QD flange, where the screw thread is in the flange and the screw tip reacts against the hub face [S1][S2]. That is why a single instruction sheet cannot cover both systems: the fastener that pulls the QD bushing in is the same fastener that pushes the Taper Lock bush out.
Step-by-step field procedure: install and remove without damage
Installation sequence for a Taper Lock bush: clean the shaft and hub bore, insert the bush into the hub, align the half-drilled holes so the threaded halves sit in the hub, start both grub screws by hand, then torque them down gradually and evenly in an alternating pattern until the bushing is fully drawn in and the specified torque is reached [S2][S5][S7]. A torque wrench is the correct tool; air tools over-torque and crack the flange before the taper has seated [S3].
Removal sequence for the same bush: slacken and remove the grub screws completely, then thread them into the jacking holes only [S1][S2][S6]. As the screws are tightened the load reverses and the bush is pushed out of the hub; the radial split allows the bush to expand and slide off the shaft without a puller [S2]. Smaller bushes commonly have one jacking hole; larger bushes (2517, 3020, 3525, 4040, 4545, 5050) have two, and a 1/2" to 5" bore range is covered by that series family [S1][S2].
A common diagnostic shortcut when bushings are seized after years of service: thread one or two cap screws into the jacking holes and tighten gradually; the bush should walk out of the hub within a few turns [S1]. If it does not, the bore is corroded or the screws have bottomed in the wrong clearance, and the bush should be cut off rather than hammered, to avoid the cracked-in-half failure mode [S2].
Comparison matrix: tightening holes vs jacking holes on Taper Lock and QD
Decision criteria for technicians selecting or troubleshooting a tapered bushing system: hole function, fastener direction, number of holes, typical series coverage, and failure mode if misapplied. The table below is drawn directly from the supplier and service sources [S1][S2][S4][S8].
On a Taper Lock bush, the tightening holes are threaded in the hub and pull the bush into the 8° taper; the jacking holes are the partial-depth clearance holes in the bush, and the same grub screws are transferred to them to eject the bush [S2][S6]. On a QD bushing, the flange carries the threaded mounting holes that pull the bush in, and the flange also carries separate jacking holes into which the same bolts are moved to push the bush out, which is the defining feature of the QD system versus the flangeless Taper Lock design [S1][S4].
Hole count scales with bushing size: small 1008 to 1210 series use two tightening and one jacking hole; mid-range 1610 to 2525 uses two to three tightening and one to two jacking; large 3020 to 5050 uses three tightening and two jacking holes [S1][S2]. Misapplying a jacking hole for tightening produces an axial load in the wrong direction, which will not generate radial clamping and typically damages the flange or splits the bush [S2].
When a taper bush should be inspected or scrapped before reuse
A taper bush that has been removed cleanly and shows no cracking, no flange deformation, and intact threads in all of its holes can be reinstalled with the correct torque sequence; one that has been hammered off, shows brinelling on the outer taper, or has stripped jacking or tightening threads should be scrapped [S2]. For a structured inspection checklist covering flange cracks, taper scoring, keyway wear, and screw thread condition, see the field guide Reuse or Scrap: Inspecting a Taper Bush Before It Goes on a New Pulley. On QD systems the same rule applies, but inspection must also cover the flange jacking holes because stripped QD jacking holes are the single most common reason a QD bush has to be destroyed during removal [S1][S4].
Two trackable signals for the next maintenance window: log the torque value actually used on each installation (a QD SH requires roughly half the torque of a QD F or J), and mark each removed bush with the date and the drive it came off so a reused bush is not pushed past its design cycle. For related material on mechanical fastening on adjacent drive components, the DIN 127 Spring Washer Materials, Grades, and Sizing Reference covers the lock-washer side of the same fastener stack. For background on the bushing family itself, the encyclopedia entry on taper bush lays out the 1008 to 5050 series range and the imperial-to-metric bore coverage.
For component-level specifications, see function generator, and pressure transmitter.