All gears slide into mesh, even with computer generated involute forms. The necessity for a bit of running clearance and lubrication will cause an impact when the tooth forms mesh. The only gear application that eliminates that cyclical contact is the worm and wheel. It is always in constant mesh, however it is not a convenient form for most applications.
Helical gears generate side thrust unless cut in a herringbone pattern where a right and left hand helix angle are cut on the same blank, This is common for extremely high torque loading as in the turbine reduction gears of large turbine drive ships.
The helical gears are quieter as the contact is gradually spread over the entire width. And the next tooth is already approaching engagement as the previous tooth is leaving.
It is unlike the spur gear which has a full contact immediately, even if the involute tooth form tends to roll into the mesh. The engagement is more intermittent compared to the helical gear. That is the why of the singing you hear on spur gear tooth forms at higher rpm. The spur gear has a greater tooth contact area at all times compared to the helical gear for the same diametral pitch.
High loading is the spur gears’ forte.
A few years ago, one of the hot tips for racing motorcycles was to replace the helical gearing between the crankshaft and the clutch basket with spur gears. This eliminated the small HP loss to the greater friction caused by the side thrust of the helical gears. You could hear, really hear, the difference. Honda split their spur gears in two, axially, with one fixed to the shaft and the other slightly floating, spring-loaded to take up the few thousandths of the normal mesh clearance under light to normal loading. Under heavy load, it made full contact. It was an expensive solution, later discarded.
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