Locking Assembly

What is a Locking Assembly

A locking assembly is a mechanical element that fastens a hub to a shaft purely by friction, with no key, spline, weld, or adhesive. It consists of one or more pairs of conical thrust rings and a set of high-strength clamping screws. When the screws are tightened, the rings ride up a shallow cone and are forced apart radially: the inner surface presses outward onto the shaft and the outer surface presses inward against the hub bore. The contact pressure generated this way is high enough that static friction alone resists the full working torque, axial thrust, and bending moment. Loosen the screws and the self-releasing tapers spring back, so the joint can be removed and reused.

The defining advantage over a parallel key is the elimination of backlash and the keyseat stress raiser. A key transmits torque through a small line of contact and a machined slot that locally weakens the shaft, concentrates stress, and allows a few hundredths of a millimetre of rotational play. That play is harmless on a one-direction conveyor but destructive on a reversing servo axis, where it fretts the keyway and eventually shears the key. A locking assembly grips the full circumference, so the load is spread over the entire interface, there is no slot to crack from, and concentricity is preserved. For these reasons keyless friction connections dominate modern gearbox output shafts, large rolls, fan and pump impellers, crane drums, wind-turbine drivetrains, and any reversing or shock-loaded drive.

The principle is old. Frictional shaft-hub joints descend from the classic shrink fit, where a hub is heated, slid onto a shaft, and grips as it cools. The shrink fit is permanent and hard to remove, so engineers developed mechanical interference fits that use a cone and a screw to create the same pressure on demand. The German DIN 7190 family of standards formalised the calculation of these interference fits, with DIN 7190-1 covering cylindrical fits and DIN 7190-2 (2017) extending the method to conical, self-locking interference fits, which is exactly the geometry inside a tapered locking assembly. Today specialist makers such as RINGFEDER (Germany), Stüwe, BIKON, Rexnord Tollok (Italy), KTR, Fenner, and Lovejoy supply standardised ranges that bolt onto shafts from roughly 6 mm to over 1000 mm in diameter.

The terminology is not consistent across catalogues, which trips up first-time buyers. RINGFEDER calls the internal element a Locking Assembly (series RfN 70xx) and the external flange variant a Shrink Disc (series RfN 40xx). Tollok labels both as TLK locking assemblies with different series numbers. Lovejoy markets them as Shaft Locking Devices (SLD). The functional split that actually matters is two-fold: whether the device clamps from the inside (internal, sits in the load path between shaft and hub) or from the outside (external shrink disc, clamps a hollow hub but is itself outside the torque path), and whether it is self-centering. Chapter 2 maps this taxonomy.

Four engineering metrics decide whether a given assembly is fit for a drive: the rated transmissible torque T, the surface pressure it imposes on shaft and hub (pW and pN), the screw tightening torque TA needed to develop that pressure, and whether the device self-centers. Get these four right and the joint will run maintenance-free for the life of the machine; get them wrong and the hub either slips under load or yields under clamping pressure.

Types and Classification

Locking assemblies are classified along three axes: clamping direction (internal versus external shrink disc), taper count (single-taper versus double-taper), and centering behaviour (self-centering versus non-self-centering). These choices determine the radial and axial envelope, the achievable torque density, the runout you can expect, and the price. The table below summarises the four practical families that cover almost every drive.

Single-taper internal elements are the simplest and most compact. A single conical ring pair clamps the shaft and bore over a short axial length, so the device occupies little radial space and several can be stacked in series to raise capacity. Because the cone is short and steep, a single-taper element is generally not self-centering: the hub can shift radially as it clamps, so a pre-centering shoulder is wise. The Lovejoy SLD 350 is a representative low-torque single-taper element, covering roughly 6 to 150 mm bore with transmissible torque from about 2.3 to 12,300 N.m.

Double-taper internal elements use two cone pairs working in opposition, which doubles the clamping surface and roughly doubles the torque for the same bore. They also bridge a larger fit clearance, so installation tolerances are relaxed. A short double-taper element such as the RINGFEDER RfN 7012 is not self-centering and needs a centering register, while a long, flat-cone double-taper element such as the RfN 7014 or Tollok TLK 130 develops enough guide length to center itself. The RfN 7012 spans 19 to 1000 mm bore and is the workhorse general-purpose locking assembly.

Self-centering elements trade some radial compactness for the ability to hold the hub concentric and square to the shaft during clamping, without a separate machined register on the part. They are the right choice for gears, timing pulleys, balanced rotors, and anything where runout and dynamic balance matter. The Tollok TLK 130 (self-centering, high torque, 18 to 240 mm bore, up to about 133,700 N.m) and the RINGFEDER RfN 7014 are the common references. The penalty is a longer hub and a slightly higher price.

Shrink discs invert the geometry: they clamp from the outside onto a hollow shaft or a thin-walled hub, squeezing it onto the inner shaft. The disc sits outside the torque path, so the joint can carry very high torque with no element wear, and it is the standard way to mount a gearbox onto a driven shaft, or a roll onto a hollow journal. A three-part shrink disc such as the RINGFEDER RfN 4051 is used for thin hubs and hollow shafts where an internal element would not fit. Because the contact is between the actual shaft and hub, the shrink disc is forgiving of hub wall thickness only down to a limit; below that, the hub must be reinforced.

Clamping Principles and Series

Every locking assembly works on the inclined-plane principle: a screw applies a modest axial force, and a shallow cone multiplies it into a large radial force. The smaller the cone angle, the greater the force multiplication, but the steeper the cone, the easier the device is to release. Manufacturers tune the cone angle so the taper is self-releasing (it springs back on loosening) rather than self-locking (it stays jammed and needs a puller). This trade-off, plus the number of cones and the guide length, is what separates the series in a catalogue. The table below compares representative series from three major makers so the same shaft can be cross-specified.

Internal expansion (locking assembly). The device is slid into the hub bore, the shaft is inserted, and the screws are tightened. The inner cone pushes outward on the shaft while the outer cone pushes outward on the hub bore, locking the three parts into one rigid body. The device carries the full torque between shaft and hub, so its own material strength and the hub wall strength both matter. This arrangement suits solid hubs with a generous bore, such as sprockets, sheaves, and gear blanks.

External contraction (shrink disc). The disc is fitted over a hollow hub that has already been slid onto the shaft. Tightening the screws contracts the outer thrust rings, which squeeze the hub onto the shaft. Torque then passes directly from the shaft, across the shaft-to-hub interface, into the hub, and the shrink disc only supplies the squeeze. This is why shrink discs reach very high torque without wearing: the friction surface is the customer's own shaft and hub, sized as large as the application allows. The RINGFEDER RfN 4051 three-part shrink disc is the canonical example for hollow gearbox output shafts.

Self-releasing versus self-locking tapers. Almost all standard catalogue assemblies use self-releasing tapers (typically several degrees of half-angle), so loosening the screws frees the device. A few heavy or steep designs are self-locking and need forcing screws or a hydraulic puller to break loose; these are chosen only when space forbids a longer self-releasing cone. When you read a datasheet, "self-releasing taper" signals easy field service, while "press-off" or "forcing screw" hints at a self-locking design.

Installation discipline determines whether the rated torque is real. Manufacturers state that the published torque, axial force, and surface pressure are valid only when the screws are tightened to the catalogue value TA, in several gradual passes, in a crossed diametric sequence, with lightly oiled threads. Tightening in one pass, or to the wrong torque, or with dry or grease-packed threads, changes the developed preload and the friction coefficient, and the connection will not reach its rated torque. The lowest allowable screw torque for the RINGFEDER RfN 7012, for example, is 0.5 times the catalogue TA; below that the joint is not certified to hold.

Materials, Surfaces, and Standards

A locking assembly is a precision friction component, so the material of the rings, the surface condition of the mating shaft and bore, and the governing standards together decide whether the rated torque is achievable and repeatable. The rings themselves are usually made of high-strength alloy steel (commonly a quenched-and-tempered steel in the 42CrMo4 family or similar), hardened where the cones bear, and the clamping screws are property class 12.9, the strongest common bolt grade, because the entire clamping force comes from screw preload. For corrosive or hygienic duty the same designs are offered in stainless steel, for example the stainless RfN 7012, at reduced surface-pressure ratings because stainless is softer and the friction coefficient differs.

Surface finish. The friction joint needs surfaces that are smooth and round but deliberately not polished. The standard requirement is a roughness of Ra 3.2 micrometres or better on both the shaft and the hub bore. A mirror finish actually lowers the achievable friction coefficient and can reduce transmissible torque, while a rough or scored surface concentrates pressure and cuts the real contact area. The surfaces must be clean, free of paint and rust, and lightly oiled before assembly. High-pressure greases or molybdenum-disulfide pastes must not be used, because they drop the friction coefficient well below the catalogue assumption (around 0.12 oiled) and the joint will slip.

Tolerances. Because the device bridges the gap between shaft and bore, the fits are not as tight as a press fit, but they are specified. For the RINGFEDER RfN 7012 the recommended shaft tolerance is k9 to h9 and the hub-bore tolerance is N9 to H9, with maximum admissible tolerances of k11 to h11 and N11 to H11. Looser tolerances than these leave the rings unable to bridge the clearance; tighter ones make the device hard to install. The table below collects the practical material, finish, and tolerance reference values an engineer needs at the drawing stage.

Standards. The calculation of the interference fit follows DIN 7190: part 1 for cylindrical fits and part 2 (2017) for the conical self-locking fits inside a tapered assembly. The screw strength follows ISO 898-1 for property class 12.9, and the bolted-joint preload and tightening torque follow VDI 2230. Shaft and bore tolerances follow ISO 286. Manufacturers do the DIN 7190 calculation in advance and publish the resulting rated torque, axial force, and surface pressure with a safety margin already built in, so the buyer normally selects from the table rather than recomputing the contact mechanics. For crane, hoist, and lifting drivetrains the relevant machinery and FEM design rules add further service factors on top of the catalogue rating.

Key Specification Parameters

A locking-assembly datasheet looks dense, but only a handful of parameters drive the selection: the bore diameter d, the outer diameter D and length, the rated transmissible torque T, the surface pressure on shaft pW and on hub pN, the screw tightening torque TA, the transmissible axial force, and whether the device is self-centering. The table below shows real catalogue rows for the RINGFEDER RfN 7012 so the relationships are concrete, then each parameter is decoded.

Rated transmissible torque (T). This is the steady torque the connection can carry by friction with the manufacturer's safety margin already applied, assuming the screws are tightened to TA on properly finished, lightly oiled surfaces. As the table shows, torque rises steeply with bore because both the contact area and the lever arm (the shaft radius) grow with diameter: the RfN 7012 climbs from about 306 N.m at 19 mm to roughly 178,553 N.m at 300 mm. Always compare your design torque, not nominal torque, against this number.

Surface pressure on shaft (pW) and hub (pN). These are the contact pressures the device imposes on the shaft outer surface and the hub bore at full TA, typically in the range of 100 to 280 N/mm2. pW and pN are the most overlooked numbers on the sheet, yet they decide whether the hub survives. A thin-walled hub may yield under pN long before the joint reaches rated torque, so the hub outer diameter must be large enough (manufacturers give a minimum hub-OD-to-bore ratio) to contain the pressure. If the hub cannot take pN, switch to a larger outer diameter, a lower-pressure series, or an external shrink disc that loads the hub in a different way.

Screw tightening torque (TA). This is the wrench torque applied to each clamping screw, and it is the single field-controllable parameter that sets the whole connection. Every other rated value assumes TA is reached on all screws. TA ranges from a few N.m on small assemblies to several hundred N.m on large ones (17 N.m at 19 mm, 485 N.m at 300 mm for the RfN 7012). A calibrated torque wrench is mandatory; the lowest allowable torque is 0.5 times TA, and over-torquing risks yielding the hub.

Transmissible axial force and bending. Many assemblies also carry a rated axial force and a bending moment, because the same friction grip resists thrust and tilting, not only rotation. For mixers, vertical pumps, and any shaft seeing thrust, check the axial rating in addition to torque. There is an approximately linear relationship between TA, the resulting axial force, the surface pressures pW and pN, and the rated torque T, so all the rated capacities scale together with the screw preload.

Bore d, outer D, and length. The bore must match the shaft; the outer diameter and axial length set the hub bore and minimum hub length. A device that meets the torque but does not fit the radial or axial envelope of the part is useless, so the geometry and the torque must be selected together. Self-centering versus non-self-centering, finally, is the parameter that decides whether the hub needs a separate machined register and whether the runout will satisfy a balanced rotor.

Selection Decision Factors

To turn the preceding chapters into a specific part number, work through the decision sequence below. Most selection errors come not from one wrong step but from deciding too early, for instance fixing the radial envelope before checking the surface pressure the hub can survive. These steps double as a fixed RFQ template.

1. Design torque, not nominal torque: take the worst-case torque and apply a service factor for the duty, roughly 1.5 to 2.0 for smooth continuous drives, 2.0 to 3.0 for reversing or moderate shock, and 3 or more for crushers and impact loads. Add the effect of any axial thrust or bending moment.
2. Shaft diameter and bore fit: match the assembly bore d to the shaft, and machine the shaft and hub bore to the recommended tolerances (k9 to h9 shaft, N9 to H9 bore for the RfN 7012 reference) at Ra 3.2 micrometres or better.
3. Internal element or external shrink disc: choose an internal locking assembly for a solid hub with a generous bore; choose an external shrink disc for a hollow shaft or a thin-walled hub such as a gearbox output. The hub wall must withstand pN either way.
4. Self-centering or not: require a self-centering type (RfN 7014, TLK 130) for gears, pulleys, and balanced rotors where runout matters; a non-self-centering type (RfN 7012, TLK 200) is acceptable for slow drums and conveyor pulleys with a pre-centering shoulder.
5. Surface pressure versus hub strength: confirm the hub outer diameter is large enough that pN does not yield it; check the manufacturer's minimum hub-OD-to-bore ratio. If the hub is too thin, go to a larger outer diameter, a lower-pressure series, or a shrink disc.
6. Radial and axial envelope: verify the device outer diameter D and length fit the part, that there is wrench access to torque every clamping screw, and that there is clearance to slide the device on and off for service.
7. Material and environment: select steel for general duty, stainless for corrosive or hygienic service (at the reduced rating), and confirm the operating temperature, since heat changes both the steel properties and the friction coefficient.
8. Installation and torque control: specify a calibrated torque wrench, the catalogue TA, a crossed multi-pass tightening sequence, lightly oiled threads, and no high-pressure grease, because the rated torque is only valid when these are followed.

One last dimension is often overlooked: serviceability and reuse. Self-releasing tapers let a locking assembly be loosened, repositioned, and reused many times without damaging the shaft, which is a real advantage for maintenance-heavy machinery and for hubs that must be re-indexed. Plan for it by leaving removal clearance, keeping spare 12.9 screws on the shelf (a corroded or stretched screw will not reach rated preload), and inspecting the tapers for galling at each rebuild. Established makers such as RINGFEDER, Stüwe, BIKON, Rexnord Tollok, KTR, Fenner, and Lovejoy publish full catalogues with rated torque, surface pressure, and TA already derived to DIN 7190, and cross-reference tables let you substitute one maker's series for another on the same shaft.