Bearing / Sliding Mounting
In manual transmissions, many gears rotating at different speeds share a common shaft. This reduces the size and complexity of the transmission, but it also means that the gears cannot be locked to the shaft. The gears have a precisely machined bore that runs on a ground or polished shaft, lubricated with oil. With the exception of idler gears, which simple change the direction of rotation, all gears that spin freely on the mounting shaft have some sort of feature that interfaces them with another gear or the shaft they are on. If a gear is made with a sintered powered metal or injection molding process, another gear can be added to the face of the gear. "Dog" teeth can also be used to selectively interface the gear with a splined coupling or another gear.
Bearings
More information can be found about bearings in this section of the Wiki : Bearings. The dynamic load rating on the bearing, free play, and sealing should be evaluated carefully to make sure that they are appropriate. If a helical gear is used, it will apply axial forces to the bearing, which could cause a plain roller bearing to fail.
Bushings
Bushing Selection should also be done carefully, with the same considerations as bearing selection. If possible, they can be lubricated directly at the contact area using a hole down the centre of the shaft and cross holes which allow oil or grease to flow down the shaft and into the contact area.
Sliding Gears and Dog Teeth | Sintered Metal Gear with Bushing Bore |
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A hole down the centre of this shaft allows oil to flow into the interface, as indicated. |
Spline Shaft Mounting
Splines are an excellent way to transmit torque and centre a gear for high power applications. They are used in automatic transmissions, in couplings between engines and gearboxes, and in machine tools where it is important to have concentricity and low amounts of slop in the system. Spline shafts can be made with many of the same processes as gears, including milling, shaping, grinding, and rolling. They can also be injection molded.
Involute Splines
Involute splines use the same shape as an involute gear tooth, which has a few key advantages. First, the angled sides of the teeth centre the spline shaft in the mating part, improving the accuracy of the joint. Second, the smooth profile and radii of the involute form reduce the stress concentrations that parallel key splines experience. Involute splines are an excellent choice in high speed or low noise applications.
Parallel Key Splines
Parallel key splines have radial flanks and circular faces at the top and bottom of the spline tooth. This means that they do not self-centre and if the corners are not relieved, they will have higher stress concentrations than involute splines.
Ball Splines
To minimize friction as the internal spline coupler moves along the shaft, ball splines use recirculating balls as the contact elements in a similar fashion to ball bearings.
Common Spline Profiles | Ball Spline- allows smooth linear movement | |
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Splines used to transmit power to the spindle, which moves axially, in a Bridgeport milling machine. |
Keyed Shaft Mounting
Keys are one common way to mount gears onto shafts- manufacturing is possible with common machine tools, the locking geometry is simple, and they can be repaired easily with hand tools if damaged. Typically, the keys are square or rectangular, so the keyway in the gear or pulley has sharp internal corners. A broach or shaper (using a single point tool to cut material along straight lines) is used to form the keyway in the pulley. This article, Keys and Keyways, from Engineering Product Design.
Examples of keyed shafts for power transmission and gear mounting. [ 9 ] |