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What is the difference between a plain bore and a finished bore?
Posted on Oct 11, 2013
Plain bores, often referred to as minimum plain bores or MPB, are simply untapered bores drilled through the center of a pulley, gear, sprocket, or sheave. Sometimes component part numbers use MPB to designate the plain bore style.
Finished bores are plain bores with the addition of either a keyway, set screws, or both. Timing pulleys and roller chain sprockets often use a F in the part number to illustrate a finished bore.
For example: 25BF14x1/2 is a roller chain sprocket for #25 chain, with a hub (B style), with a Finished bore, with 14 teeth, bored for a 1/2” shaft.
22-8MXF12X1 is an example of an exclusive B&B Manufacturing pulley with 22 teeth, 8 mm Powerhouse MX™ pitch, with a Finished bore, designed for 12 mm wide belts, and bored for a 1“shaft.
A keyway is a slot cut into a pulley, gear, sprocket, or sheave to accept a key that engages with a similar slot on a shaft to prohibit the relative motion of the two components. Keys connecting shafts to pulley hubs are commonly used to achieve reliable no-slip power transmission in belt drive systems
A set screw is a screw through the pulley, gear, sprocket, or sheave used to tighten the component to the shaft and limit slippage. Typically, set screws are used on larger diameter components as they typically transmit higher loads.
There are some advantages to having a plain bore. With a plain bore, it is easier to customize how the component is affixed to the shaft. A plain bore can be opened to a larger bore diameter, can be made into a finished bore, or can even be customized in other ways such as incorporating pins or screws.
There are also advantages to having a finished bore. The most noteworthy advantage is the increased prevention of shaft slippage. It allows for the component to move larger loads with increased accuracy. A noteworthy disadvantage is in the food and beverage industry where set screw holes will at times fill with debris depending on the application.
Another common way to affix components to shafts is through the use of bushings such as QD bushings and TL bushings. There are many advantages and disadvantages to this style of power transmission component, but bushings and their advantages and disadvantages will be detailed in a future blog post.