Keyless Locking Devices Keep a Firm Grip on Shafts

Oct. 22, 1999
The power-transmission industry has always struggled with the problems of connecting things to shafts

The power-transmission industry has always struggled with the problems of connecting things to shafts. Although keyed mounts are an industry standard, the devices have several limitations. In keyed connections, the clearances between the component hub, shaft, keyway, and key allow for metal-to-metal contact. This leads to fretting and corrosion. A poor fit also allows backlash. In addition, machining the keyway into the shaft is tedious, permanent, and expensive. It also weakens the shaft and reduces its torque potential.

Keyless shaft-to-hub locking devices solve the problems associated with keyed mounts. They allow mounting components more securely and conveniently than keyways, set screws, or interference fits. They are easy to install and remove and allow for precise axial or radial positioning of components on shafts. They also provide infinite positioning, which is critical in applications that require synchronization between components. Overall cost is often less because the devices do not require extensive machining. Some keyless locking devices also allow generous tolerances on shafts and mating hubs.

Typical keyless locking devices have an inner, collet-like sleeve with a tapered OD and an outer sleeve with a tapered ID. The tapers are identical, but opposite to each other. The inner sleeve fits around the shaft while the outer sleeve fits inside a bore in the hub of the component to be mounted, such as a pulley, gear, or sprocket. A single nut or bolt squeezes the inner sleeve onto the shaft and expands the outer sleeve against the component hub bore. This mechanical shrink fit resists shock and torque reversals, eliminating key wallowing, backlash, and fretting corrosion.

The gripping stress with a keyless connection distributes evenly around the shaft and component hub bore, instead of being concentrated at a key and keyway. This lets shafts transmit more torque than if they had a keyway, letting designers downsize the shaft and peripheral components.

Some keyless locking devices can be used directly over flats or empty keyways to repair worn or damaged connections. But consult the manufacturer before attempting such a retrofit.

Most keyless locking devices have straight ODs and therefore require machining to size the mating hub of the mounted component. This can be time consuming if the components were previously taper bored.

Recently, one manufacturer has introduced a quick-change keyless bushing with a tapered outer section. This design is directly interchangeable with widely used quick-detachable bushing systems with industry-standard designations, such as JA, SH, SD, and SDS. This design does not require machining a tapered mating hub, thereby easing installation. Mounted components can be easily positioned or removed for service. Many power-transmission components, such as roller-chain sprockets, V-belt sheaves, and timing-belt pulleys are manufactured to accept this “QD” system.

Be careful of damaging hubs when mounting keyless locking devices because they exert pressure between the component hub and shaft. A hub diameter that is too small could burst under the high forces exerted by the locking device. Manufacturers publish tables and provide formulas to determine a minimum hub diameter. If hollow shafting, tubing, or a pipe is to be used, minimum recommended wall thickness should be checked, or contact the manufacturer.

This information supplied by Fenner Drives, Manheim, Pa.
For more information, Circle 625.

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