Page 271 - Analysis and Design of Machine Elements

250 Wormgear Drives 249

Allowable contact stresses of cast grey iron and cast
200
aluminium-iron bronze, [σ H ], MPa 150

100

Steel worm with hardness >45HRC, cast aluminium-iron bronze wormgear
50
Steel worm with hardness >45HRC, grey cast iron wormgear
Steel worm with hardness <45HRC, grey cast iron wormgear

0
0 0.5 1 1.5 2 2.5 3 3.5 4
Sliding velocity v , m/s
s

Figure 9.7 Allowable contact stresses of grey cast iron and cast aluminium–iron bronze [6].

7
5
7
5
When N > 25 × 10 ,use N = 25 × 10 and when N < 10 ,use N = 10 . The basic allow-
6
′
able bending stresses [ ] at N = 10 canbeobtained fromTable9.2.
F
9.4.4 Design Criteria
In general, the load carrying capacities of most wormgear drives are limited by pitting,
wear and thermal capacity. Since the strength of worm teeth is stronger than that of a
wormgear, failures often happen on wormgear surfaces. Thus, the calculation of working
capacity applies only to wormgear teeth. The computed value of both contact stress and
tooth bending stress are compared with fatigue strength of wormgear materials to eval-
uate strength. Furthermore, the continuous rated capacity of wormgear drive is often
∘
limited by temperature rise and oil temperature must not exceed 80 C for satisfactory
operation.
Therefore, enclosed wormgear drives are designed by surface contact strength, that is,
≤ [ ], checked by bending strength, that is, ≤ [ ], and oil temperature is limited
F
F
H
H
∘
to t ≤ 80 C by thermal capacity analysis. For open wormgear drives, wear caused by
o
significant sliding velocity is the primary failure mode. Open wormgear drives are thus
designed by bending strength ≤ [ ], with an assumed number of wormgear teeth
F
F
greater than 80.