Locking assemblies transmit torque between shaft and hub through radial expansion of tapered thrust rings, producing 360° frictional contact and zero backlash, replacing keyed connections in conveyor pulleys, crane drums, and bucket-wheel drives [S3].
The selection problem in material handling is not nominal torque alone but the combined envelope of torque, bending moment from belt tensions, shock loading, bore tolerance, and hub material yield strength; under-specifying on any one variable causes the burnt-clamping-set / deformed-end-disc failure mode documented in conveyor-pulley retrofits [S4]. For broader context on torque-misalignment trade-offs in adjacent driveline components, the engineering choices for gear couplings in agricultural service follow the same logic of contact-area versus point-load mechanics.
What a Locking Assembly Is and When It Replaces a Keyway
Frictional locking assemblies are two-part (inner/outer ring) or multi-piece devices whose tapered thrust surfaces are forced together by clamping screws, generating radial pressure that grips the shaft and the hub bore simultaneously [S3]. Because contact is distributed over the full circumference rather than concentrated at a keyseat, stress concentration factors drop and the assembly is effectively wear-free in steady torque service.
Keyways concentrate torque-transmission stresses along a single line of contact, and reciprocating or reversing loads initiate fretting and micro-slip at that line; the frictional design eliminates that wear path, and the parts are interchangeable because they ship as a standard unit requiring no hub machining [S3]. For OEM engineers reviewing the broader shaft-hub connection family, the locking assembly reference page sits alongside keyed and shrink-fit options in the material handling components encyclopedia.
Decision Criteria: Torque, Bore Pressure, Bending Moment, Surface Finish
Four variables govern correct sizing. First, transmissible torque must be calculated at the design shaft diameter with the manufacturer's tabulated axial clamping force per screw. Second, hub contact pressure must stay below roughly 60% of the hub material's yield strength to avoid permanent bore deformation; that is why the RINGFEDER selection tool asks for hub material grade (e.g. GG-25 cast iron, St 52, or 42CrMo4) rather than just diameter [S1].
Third, the bending moment from belt tensions on a conveyor pulley can shear the clamping set long before the torque limit is reached; wider locking assemblies with optimised bolt-hole count and grade-10.9 or 12.9 screws raise the moment capacity without oversizing the device [S3][S4]. Fourth, shaft and bore tolerance classes (typically h6/H7) and surface roughness (Ra ≤ 0.8 µm on the shaft, Ra ≤ 1.6 µm in the bore) directly set the slip-torque margin and must be specified on the drawing.
Main Types Compared on Four Selection Criteria
Three families dominate the conveyor and drum market. (a) Two-part tapered locking assemblies: simplest install, lowest cost, but limited bending-moment capacity, so they fit small-drum drives under roughly 100 kN·m. (b) Three- or four-piece heavy-duty locking assemblies: higher transmissible torque per unit length and markedly better bending-moment tolerance because contact length is longer and screw count higher, suiting long conveyor pulleys from 200 mm to over 1000 mm shaft diameter [S1][S4]. (c) Internal-external expanding sleeves (RINGFEDER-style with the inner ring pressing outward): the most compact axial footprint, useful where the hub is short or the shaft shoulder is close, but installation demands a clean jacking-screw sequence [S3].
On cost, the two-part design wins per kN·m. On bending-moment margin, the multi-piece design wins decisively. On installation time, all three install in minutes using quarter-turn screw increments rather than the hours needed to cut and fit a keyseat [S3]. On serviceability, all three are field-replaceable without hub rework, a major advantage over welded or shrunk-on connections. The detailed mechanical limits, including permissible bore pressure curves and temperature derating, are tabulated on the RINGFEDER locking assembly product page and should be cross-checked against the hub's certified yield-strength value.
Where Locking Assemblies Fit and Where They Do Not
Frictional locking assemblies are well matched to belt-conveyor head and tail pulleys, bucket-elevator drums, crane slewing rings, winch drums, and large fan or mixer shafts where the load is predominantly steady torque plus a predictable bending moment from belt or chain pull [S3][S4]. They are also the standard repair solution when a keyed connection has failed in service, because the hub does not need to be remachined; the locking assembly is simply slipped onto the prepared shaft and tightened.
They are the wrong choice for highly oscillating drives where micro-movement between bore and shaft would cause fretting corrosion (use a tapered shrink fit instead), for very high-speed shafts above roughly 20 m/s surface speed where imbalance and centrifugal relaxation of the clamping force become dominant, and for applications requiring axial sliding under load (use splines). Drives with extreme radial deflection, such as cardan-shaft intermediate supports, are also better served by flexible couplings covered separately in the material handling components encyclopedia.
Failure Modes the Selection Must Prevent
The three dominant failure modes reported in conveyor-pulley retrofits are burnt clamping sets (loss of preload through thermal cycling or insufficient re-torqueing), deformed end discs (hub bore yielded because contact pressure exceeded the cast-iron limit), and sheared screws (bending moment exceeded the screw shear plane) [S4]. Each is prevented by a different design action: torque-controlled re-tightening after the first duty cycle for the first, correct hub material and surface area for the second, and adequate screw count and grade for the third.
Installation discipline matters as much as sizing: locking screws must be tightened in quarter-turn increments in a diametrically opposed sequence to keep the runout within roughly 0.03 mm, and jacking screws must be backed off before final clamp-up to prevent pre-load asymmetry [S3]. Skipping this step is the most common root cause of premature slip in the field, even when the assembly is correctly sized on paper. Adjacent component choice, such as the universal joints in agricultural PTO drivelines, follows a similar pattern: torque rating is necessary but never sufficient, and installation sequence plus service interval finish the spec.
Sourcing, Standards, and Specification Checklist
OEM data sheets typically cite transmissible torque (kN·m), axial clamping force per screw (kN), permissible hub contact pressure (N/mm²), and the tightening torque per screw (Nm) for a given screw grade; the RINGFEDER online selection tool and case study both push the engineer to enter hub material and bending moment explicitly, rather than sizing on shaft diameter alone [S1][S4]. Material handling conveyor systems are often specified to ISO 5048 / DIN 22101 for belt-driven calculation of the belt tensions that feed the bending-moment input.
A workable spec line for procurement reads: "Frictional locking assembly, 3-piece heavy duty, size to suit Ø…mm shaft / Ø…mm hub bore, hub material St 52, transmissible torque ≥…kN·m, permissible bending moment ≥…kNm, screws grade 10.9, surface roughness Ra ≤ 0.8 µm on shaft, supplied with calibration certificate." Pair this with the manufacturer's installation manual and the inspection torque values, and the storage-handling and traceability guidance for the spare assemblies on site.
Trackable signals to watch: RINGFEDER's online selection tool periodically updates its hub-material library and bending-moment calculator, and the blog case-study series on belt-conveyor pulleys is the public record of which extreme-load retrofit cases the company has published engineering data for [S1][S4]. The next node is a published bending-moment versus bolt-count comparison for shafts above 600 mm, which remains a gap in publicly available conveyor-pulley case data.