Answer to Question #158920 in Mechanical Engineering for MANIKANDA RAJAN

Question

how compressive strength and the bending strength of gear tooth are considered for the design of speed reducer

Answer 1

As shown in the Figure , the tooth fillet breakage occurs at the root when applied force exceeds the limit. The following is the equation for bending stress

Allowable Tangential Force 

Fig. Lack of bending stress at root

How to encourage bending strength

To encourage bending strength, values in the equation for allowable tangential force (7.1) should be set to; The value of denominator should be small & The numerator should be large

• Use durable materials (Increase allowable bending stress at root)
• Enlarge the gear size (For large module / wide tooth width)
• Strengthen tooth profile (Decrease tooth profile factor)
• – Large pressure angle
• – Positive correction
• Increase contact ratio (Decrease load distribution factor)
• – Small pressure angle
• – Large tooth depth
• Improve accuracy

Tooth surface durability is calculated in accordance with the contact stress, to figure out the strength against the tooth surface damage (Pitting).

The allowable tangential force Ftlim for surface durability is;

Allowable Tangential Force (Ftlim)

Fig. Contact Stress

How to increase surface durability

• Use hard material treated by quenching ( Increase allowable hertz stress )
• Enlarge gear size (Large pitch diameter / Wide effective tooth width )
• Increase contact ratio (Decrease contact ratio factor )
• Improve accuracy