Have you ever purchased something that cautioned, “Some assembly required” on the packaging, only to learn later that even with a master’s degree in mechanical engineering the parts don’t seem to assemble in the same fashion as the instructions describe? This is typically an issue with the tolerances of the components in the assembly. When working with gears, the clearances between the bore and shaft and the key and keyway and the location of the set screws can all contribute to an easy assembly or a nightmare.
When working with metric gearing, the tolerance for the bore is detailed as an alphanumeric callout. Typically, the callout selected is either H6, H7, or H8. These bore tolerance bands reflect a tight oversized hole with H6, a moderately oversized hole with H7 and a looser fit oversized hole with H8. The accompanying table details the band range for each tolerance, which is dependent on the bore size.
The corresponding shaft tolerance needs to be determined based on whether the fit between the gear and shaft is to be an interference fit, a transition fit, or a clearance fit.
When selecting a keyway tolerance, there are two common selections in the metric system. The first is Js9. This is a ± band clearance where the value of the tolerance is equally oversized or undersized. The second is a P9 tolerance. This is an undersized clearance. The advantage of the Js9 tolerance is that the key can be inserted and the gear manipulated without much difficulty, whereas the P9 tolerance is a press fit tolerance. Once the key is inserted into the keyway, it is not going to move.
The final consideration with sizing is the thread size and location of the set screw. For many applications, just one set screw, positioned at 90 degrees to the keyway, is sufficient. For other applications, a second set screw is considered for just above the keyway. Although not as common, a more effective position for two set screws is to have each one offset 120 degrees from the keyway in a triangle-type pattern. This positioning helps to balance the gear during high speed rotation but is more difficult to set up.
Set screws and keys are not the only methods for attaching gears to shafts. In future columns, I will detail some of the other methods. Just remember — when designing your gear train, take into account the clearances needed to assemble all of the components.
