uninformed wrote:thanks john now for a simple question why are they quieter and stronger????????
serg
When designing gears it is necessary to evaluate for both wear and strength (2 similar but different formulae).
The strength formula determines the fatigue strength of the gear tooth, in bending as a cantilever. But it depends upon how many teeth are in mesh at ant one time, as to how much of the tangential load is applied to a gear tooth.
Many spur gears (depending on number of teeth in the gear and the pinion) will have 2 teeth in mesh. The tangential tooth load for helical gears is less than spur gears, because more teeth share the tangential load at any time.
The pressure angle of the gear teeth is an important factor for tooth strength. Pressure angle is the angle between the tangent line at the pitch circle and the direction of the normal to the tooth surface where the mating teeth contact. Generally PA is 20 deg these days, but 25 is not uncommon for high strength.
As the pressure angle increases, the tooth becomes thicker at the root and thinner at the tip. This thickness at the root is an important factor for too strength.
Helical teeth cut using 20 deg normal PA cutters will have a 20 deg PA normal to the helix angle, but greater than 20 deg in the direction of the tangential tooth load.
Most gear noise is a result of inaccuracies in the tooth profile and the pitch, resulting in loss of congugate action (if 1 is rotating at constant speed the mating gear is continually speeding up, the slowing down). When teeth wear, this gets worse.
Helical teeth are quieter because more teeth in mesh at the same time,improves the congugate action.
