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09/10/2005
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A keyless taper bushing mechanism has been developed for shaft mounted gearboxes and pulleys that, in tests, transmits 300% more torque than traditional friction-based locking bush designs.
Fenner's new 'Taper-Grip' keyless bushing uses a taper mechanism similar to the company's 'Taper-Lock' bush. However, the Taper-Grip is designed in the form of a spiral or thread where an inner bushing engages with the outer sleeve (hub section) and exerts a clamping force when axial pressure is applied against the spiral taper.
The angle of the taper is finely engineered to allow one face to ride up upon the opposing face and squeeze the inner sleeve along its complete length. This results in a uniform and very high clamping pressure along the entire length of the assembled bushing and a very easy removal process - simply a tap in the opposite direction.
The inner sleeve is slotted to allow for the radial compression and when released, the metal springs back away from the shaft. Slots also allow for a wide shaft tolerance (up to h11). Axial pressure exerted against the thread is applied using cap head screws mounted in a collar at one end of the bushing.
The inner bushing is screwed into the hub (only finger pressure required) while the assembly is positioned and locking force then applied by tightening the screws against a hardened collar. In order to apply the huge clamping force available from Taper-Grip, the cap head screws are tightened to the recommended setting using a torque wrench.
The device is designed primarily for mounting Fenner's high power shaft mounted gearboxes in applications such as conveyors and quarrying equipment. According to Fenner, the bushings were developed to compliment the company's 'SMSR Power Plus' shaft mounted gearboxes that transmit 50% more torque (up to 280kW) than competitive designs for the same design envelope. Apparently, the increased power required a locking system that could ensure the new higher power levels could be delivered to the driven shaft and yet maintain the ease and practicality of a tapered bushing design.
Taper-Grip offers advantages when removing the gearbox. In some environments, there is a tendency for shaft mounted reducers can often suffer from atmospheric or fretting corrosion and can therefore seize solidly onto the shaft. This can make removal difficult and time consuming.
But Taper-Grip eliminates this problem. The bush is manufactured from high grade steel to enable high screw tightening torques that, in turn, generate a huge radial force to increase the grip on the driven shaft. When the bush screws are loosened and the tapers released, there is sufficient clearance within the assembly to permit easy removal. Because the bush is screwed into the hub, it is inherently safe. Even if the locking screws completely removed, it cannot inadvertently fall out during handling.
The new locking mechanism offers another advantage. After the bush is screwed into the hub during initial assembly, a gearbox can be conveniently positioned on the driven shaft by simply sliding the unit along the shaft. The device can also be fitted to shafts that have pre-cut keyways.
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Author
Tom Shelley
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