Before installing industrial belt drive components, you need to start with a basic understanding of the various types of mounting devices used to attach pulleys to shafts. Knowing the advantages of these different devices, and how they operate, helps prevent installation errors that cause drive component damage and costly equipment downtime.

Several types of tapered bushings can be used to mount V-belt sheaves and synchronous belt sprockets, hereafter called pulleys, to drive shafts. These bushing types include keyed versions, for locking to the shaft, which are described in this article. Other types do not require keys (see box) and will be covered in a future article.

Keyed tapered bushings

A common device for attaching pulleys to motors, gearboxes, and shafts is a keyed tapered bushing, which is typically made of gray iron, steel, ductile iron, or sintered steel.

With these bushings, a tapered bore in the pulley fits over a mating tapered outside diameter on the bushing, Figure 1. To attach a pulley, a mechanic tightens mounting screws, which draw the tapered surfaces of the bushing and pulley hub together. Forcing the mating tapers together creates a wedging action between the bushing and pulley, and between the bushing and drive shaft. The bushing has a radial slot or saw cut, that lets it tightly grip the shaft under the wedging action. The gripping force between bushing and shaft is comparable to that of a press fit so that the bushing transmits torque and resists slippage. Correctly applied, the bushing eliminates pulley wobble as well as fretting corrosion on the shaft.

To remove a pulley, the mechanic tightens one or more removal, or “jackapart,” screws, which separates the pulley and bushing without damaging the shaft or drive components.

For a given bushing OD, manufacturers offer several different bore sizes to accommodate different shaft diameters. For example, flanged bushings with a 2-in OD are available with bore sizes ranging from ½ to 1³/₁₆ in. One bushing can also be used for pulleys of different widths, and pulleys grooved for different belt cross sections.

Some manufacturers offer bushings with unfinished small-diameter bores and without saw-cuts. This lets the end user rebore, then cut on-site, achieving quick turn-around when unusual bore and key sizes are needed.

Flanged bushings, Figure 1, are used in a wide range of heavy-duty V-belt and synchronous belt drive applications. Designed according to Mechanical Power Transmission Association (MPTA) QD1 Detachable Bushing and Mating Hub Guidelines, these bushings are well suited for high power drives subject to large pulsating or vibrating loads. Standard bore sizes range from ½ to 7-in. or more.

Flangeless, or flush-mounted, bushings, Figure 2, are commonly used on chain drives and most European belt drives. Those made from special materials, such as stainless steel, are well suited to the food industry because of their smooth, easy-to-clean surface. Again, bore sizes range from ½ to 7-in.

Shaft keys

During assembly, a key is inserted into machined keyways in the bushing and shaft to lock them together and prevent the shaft from rotating in the bushing, Figure 3. These keys are generally made from bar stock and come in square, rectangular, and tapered shapes. The Woodruff key, used primarily in machine and automotive applications, is shaped like a half-moon.

Shallow keys are used for bushings where the bore is especially large in comparison to the outside diameter. Here, the keyway in the bushing is shallower than normal to accommodate a larger diameter shaft. Manufacturers usually provide a shallow key with the bushing.

An improperly fitted key — either too tight or too loose — can cause key or hub failure. Therefore, to ensure a snug fit, width and height dimensions of both key and keyways must be held to tolerances specified by ANSI standard B17.1 for Keys and Keyseats.

In most flanged bushings, a setscrew in the flange, Figure 4, tightens against the key to prevent it from working out of the keyway. This feature is especially useful for applications subject to vibrating or pulsating loads and in vertical shaft applications. One type of bushing has an integral key that is formed as part of the bore, Figure 5, so there is no separate key to come loose. Both types of bushing are popular in vertical shaft installations.

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