Page 272 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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9.4.5 Design Procedure and Guidelines
The design procedure for wormgear drives is similar to that of gear drives. Here, we list
guidelines particular to wormgear drive design. A design case is given next to illustrate
the design procedure.
9.4.5.1 The Arrangement of Wormgear Drive and Selection of Accuracy Grade Levels
When a wormgear drive is lubricated by bath lubrication, the worm is to be placed over
the wormgear if the pitch line velocity of worm is greater than 5 m s −1 and below or
by the side of wormgear if pitch line velocity less than 10 m s −1 [3]. There is no spe-
cific arrangement requirement for spay lubrication. For general application, worms are
usually selected at 6–8 accuracy grade levels and one grade lower for wormgears.

9.4.5.2 The Selection of the Number of Worm Threads z and the Number
1
of Wormgear Teeth z 2
Considering transmission efficiency and fabrication capability, the commonly used
numbers of threads are limited to 1, 2, 4 and 6. The number of teeth of meshing gear
are obtained by z = uz . To avoid undercut, as well to improve transmission stability,
2
1
the number of wormgear teeth should be greater than 28. Under constant reference
diameter of the wormgear, increasing the number of wormgear teeth consequently
reduces the module, which eventually reduces the bending strength of the wormgear
teeth. On the other hand, if the module keeps constant, the increase in the number of
wormgear teeth will produce a large reference diameter, increasing the span between

two bearings supporting the worm and ultimately reducing worm rigidity. In summary,
the number of wormgear teeth should be within the range of 28–80.

9.4.5.3 Design by Wormgear Strength
After initial design variables have been selected, a wormgear drive can be designed in
either order to satisfy both contact fatigue strength and bending fatigue strength. The
2
final design variables are selected from Table 9.1 by the calculated m d according to Eq.
(9.16) or Eq. (9.19).

9.4.6 Design Cases

Example Problem 9.1
Design a wormgear drive to transmit a power of 9 kW at a worm speed of 1460 rpm.
The desired speed ratio is i = 20. The wormgear drive works steadily with a life
of L = 10 000 hours.
h

Steps Computation Results Units
1. Selection (1) Select Archimedean worms (ZA). ZA worm
of worm type
and materials
(2) Select quenched medium carbon steel with hardness
of 45–55 HRC for the worm.
Select cast tin bronze ZCuSn10Pl for the wormgear.