The coupling bore — the precision-machined hole through which the coupling hub connects to its shaft — is the most technically consequential dimension on a coupling drawing and the specification detail most likely to be wrong when a coupling arrives at installation. An incorrectly specified bore does not announce itself visually; it reveals itself when the coupling is forced onto a shaft that is slightly too large, when a shaft rotates eccentrically inside a bore that is too large, or when a coupling that appeared to seat correctly in the workshop fails in service because the fit that transmits torque was inadequate for the operating load.
Understanding bore specification — the tolerance system that governs fit between bore and shaft, the keyway geometry that supplements friction torque transmission, and the alternative connection systems that eliminate some of these challenges entirely — is practical engineering knowledge that prevents the coupling specification errors that cause installation problems, premature failures, and the expensive downtime that accompanies them.
The Tolerance System — H7/h6 and What It Actually Means
Shaft and bore dimensions in industrial drive systems are controlled by the ISO tolerance system — a standardised framework that assigns letter and number codes to dimensional tolerances, with the letter indicating the position of the tolerance relative to the nominal dimension and the number indicating the magnitude of the tolerance band. For coupling bores and their mating shafts, the H7/h6 pairing is the standard specification for a clearance fit — one in which the bore is consistently larger than the shaft across the full range of manufacturing variation, so that the hub can be assembled onto the shaft without force.
H7 for the bore means that the bore tolerance is positioned entirely above the nominal dimension — for a 50 millimetre nominal bore, H7 specifies a range from 50.000 to 50.025 millimetres. The bore is never smaller than nominal and is never more than 0.025 millimetres larger for this diameter. h6 for the shaft means that the shaft tolerance is positioned entirely below the nominal dimension — for a 50 millimetre nominal shaft, h6 specifies a range from 49.984 to 50.000 millimetres. The shaft is never larger than nominal and is never more than 0.016 millimetres smaller.
The combination of H7 bore and h6 shaft guarantees a clearance between 0.000 and 0.041 millimetres across the full tolerance range of both components — the bore is always at least as large as the shaft, and never more than 0.041 millimetres larger. This clearance is small enough that the bore-shaft interface, when combined with a key or set screw, transmits torque reliably in most general industrial coupling applications. It is large enough that assembly and disassembly can be performed without heating or pressing equipment under normal conditions.
The H7/h6 clearance fit is appropriate for general industrial coupling applications where torque is transmitted through a key, where the coupling must be removable for maintenance without special equipment, and where the shaft speed is moderate. It is not appropriate where the coupling must transmit torque without a key — relying solely on bore-shaft friction — because the clearance reduces the normal force at the interface and consequently the friction torque capacity to levels that are inadequate for most drive applications without a key.
Interference Fits — When Friction Must Transmit the Torque
An interference fit — also called a press fit or shrink fit — reverses the relationship between bore and shaft: the bore is smaller than the shaft by a controlled amount, so that assembly requires force, heating, or cooling to bring the components to the required dimensional relationship. After assembly, the elastic compression of the hub around the shaft generates a normal pressure at the interface whose magnitude determines the friction torque capacity of the connection.
The H7/p6 pairing is a common interference fit specification for coupling applications requiring keyless torque transmission. For a 50 millimetre nominal diameter, p6 shaft specifies a range from 50.026 to 50.042 millimetres — the shaft is always larger than the H7 bore's maximum of 50.025 millimetres, guaranteeing interference across the full tolerance range. The actual interference ranges from 0.001 to 0.058 millimetres depending on where within their respective tolerance ranges the bore and shaft dimensions fall.
The friction torque capacity of an interference fit is calculated from the interface pressure, the coefficient of friction between the hub bore and shaft surface materials, the interface length, and the nominal diameter. For aluminium coupling hubs on steel shafts, the coefficient of friction is typically 0.12 to 0.15 for dry surfaces, and the interface pressure from a correctly calculated H7/p6 interference on a 50 millimetre diameter aluminium hub produces a friction torque capacity of several hundred newton-metres — adequate for most general coupling applications in the torque range where aluminium hubs are appropriate.
The assembly and disassembly requirements of interference fit couplings are the primary practical constraint on their use. Assembly requires either hydraulic pressing — pushing the hub onto the shaft with controlled force using a press fixture that loads only the hub, not the coupling element — or thermal expansion, heating the hub to expand its bore before sliding it onto the shaft and allowing it to cool and contract. Disassembly requires the reverse process — either hydraulic extraction with a correctly designed puller or thermal cycling. The requirement for this equipment and the risk of hub damage during incorrect disassembly makes interference fits less practical than clearance fits in maintenance-intensive applications where coupling removal is frequent.
Keyway Specification — The Detail That Determines Whether the Key Works
The keyway — the slot in the shaft and the corresponding slot in the hub bore that together receive the key — is the primary torque transmission element in most clearance-fit coupling connections. The key, typically a rectangular parallel key to DIN 6885 or IS 2048, sits in matching slots in the shaft and bore and transmits torque by direct shear between the key and the keyway side walls. The fit between the key and each keyway — not just the presence of the key — determines whether this torque transmission mechanism works correctly.
The key width relative to the shaft keyway width specifies a close sliding fit — the key should slide into the shaft keyway without measurable play but without requiring force. The key width relative to the hub keyway width specifies a slightly looser fit — the key should be free to slide axially into the hub keyway during assembly. These two fit requirements are different because the shaft keyway transmits the full torque through the key side walls, and clearance in the shaft keyway allows the key to rock under alternating torque — a rocking motion that progressively wears the keyway walls and produces the fretting and keyway elongation that eventually causes coupling failure. The hub keyway operates differently — it accommodates the key with a slight clearance that facilitates assembly without transmitting the torque-rocking motion that damages the shaft keyway.
Key height specification — the dimension that determines whether the key projects above the shaft surface into the hub bore — must provide adequate engagement depth in both keyways without the key top surface contacting the opposite side of the hub bore. A key that contacts the bore wall on its top surface is being loaded in compression rather than shear, and aluminium hub materials are not rated for this loading mode at the stress levels that misspecified key height generates. The standard key height for a given shaft diameter is specified in DIN 6885 and IS 2048, and should be followed rather than improvised — the key height standards encode the engineering analysis of appropriate engagement depth and stress distribution for each diameter range.
Taper Lock Bushings — Eliminating the Fit Specification Problem
The taper lock bushing — a split-taper sleeve that fits between the shaft and the coupling hub — provides a practical alternative to direct bore-to-shaft fits that eliminates many of the specification and assembly challenges of conventional bore-keyway connections. The bushing's outer taper engages with a matching taper bore in the coupling hub, and tightening the flange bolts drives the bushing deeper into the hub, compressing its split body onto the shaft and simultaneously expanding the hub bore to create a firm, concentric connection without the precise bore-to-shaft tolerance relationship that direct fits require.
The commercial advantage of taper lock couplings for maintenance-intensive applications is the ability to mount and remove the coupling with standard tools — the flange bolts that tighten for installation are reversed and used as jack screws for removal, releasing the taper engagement without requiring a puller or heating equipment. This tool-free removal capability significantly reduces the time and risk associated with coupling maintenance in the field, and it allows the same coupling hub to be remounted on a shaft of a different diameter simply by replacing the bushing — a flexibility that direct-bore hubs do not provide.
For aluminium coupling hubs specified for taper lock connection, the hub bore is machined to the taper lock standard rather than to a shaft diameter — a standardised taper bore that accepts the range of bushing sizes available for that hub size. The taper lock system effectively separates the hub design from the shaft specification, allowing the hub to be catalogued as a standard product whose bore configuration adapts to the installation rather than being machined to a customer-specific shaft diameter. This is the approach that makes aluminium coupling hubs commercially viable as catalogue products rather than requiring every hub to be custom-bored to the specific shaft it will be installed on.
Common Specification Errors and How to Avoid Them
The coupling bore specification errors that most frequently cause installation and service problems fall into three categories. First, specifying the bore diameter without specifying the tolerance — writing 50mm bore on a purchase order without specifying H7 or the equivalent tolerance class leaves the manufacturer to apply their default tolerance, which may not match the shaft tolerance that the installer will present at assembly. Always specify bore diameter and tolerance class together.
Second, specifying keyway dimensions that do not match the DIN 6885 or IS 2048 standard for the shaft diameter — either because the designer used a non-standard key size for convenience or because the keyway was copied from a drawing without verifying its standard compliance. Non-standard keyways require custom broaching tools that are not held in stock, significantly extending lead times and increasing cost. Always verify that keyway dimensions match the appropriate standard before issuing a coupling drawing.
Third, specifying interference fits on aluminium coupling hubs without verifying that the hub wall thickness at the bore is adequate to withstand the hoop stress that the interference generates. An aluminium hub with a thin wall section between the bore and an external feature — a flange bolt hole or a coupling jaw pocket — may crack during interference fit assembly if the wall section cannot contain the induced stress. This is a design check that must be performed before specifying the interference magnitude, not after the first hub cracks during assembly.