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A pinion shaft is the main drive member of most gear train assemblies. The shaft transfers the input of a drive shaft or crank to the rest of the gears to produce the work for which the gear train is designed. A pinion shaft can transfer this drive motion to a linear gear assembly or to a right angle bevel gear set, such as a vehicle differential. Pinion shafts and their associated drive gears are typically one-piece assemblies machined from a single piece of steel stock. Pinion shafts may pass through several supporting bearings along their routes and will generally feature an exit seal in applications involving oil-filled gear boxes.
Most gear train assemblies transfer a rotational mechanical input into an output of increased or decreased velocity or torque. They also allow for the direction of the input to be adjusted to drive elements at right angles. Most input drives are supplied by a drive shaft or a crank mechanism. Common examples of these inputs include the drive shaft and differential of an automobile and the drive mechanism of a fishing reel. Both take the input from the drive shaft or reel crank and pass the rotational motion to a series of gears via a pinion shaft. The pinion shaft, therefore, forms the first stage of the gear train.
Pinion shafts are usually equipped with one or more gears that transfer the drive input to the rest of the gear assembly. Most pinion shafts and gears are one-piece parts machined from a single steel billet. This increases the strength of the shaft and its resultant ability to absorb the considerable torque applied to it during operation. Unfortunately, it usually means that a worn or broken pinion gear requires the replacement of both gear and shaft.
These shafts are typically fairly short, only spanning the gap between the gearbox entry and the first gears in the train. In some gear train designs, the shaft will pass through a bearing or bushing that supports the shaft and reduces friction. In sealed gear boxes that operate in an oil bath, the pinion shaft will be fitted with an oil seal where it exits the gear box. Simpler open drives, such as manual door openers and pump cranks, feature exposed pinion shaft-and-gear assemblies that rely on manual spot lubrication. In some gear trains, the pinion shaft-and-gear assembly will be made of a harder material than the gears it drives, as gear-box ratios mean they experience higher gear-tooth contact rates.