Page 268 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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The rate of heat generated by friction during worm operation is
H = 1000P(1 − )
1
The rate of heat removed by dissipation from the free surface of housing is
H = S(t − t )
a
o
2
s
To keep thermal balance, the amount of heat developed by the continuous operation
of wormgear drive should be equal to the heat removed by dissipation from the free
surface of housing within the same time, that is, H = H ,wehave
1
2
1000P(1 − ) ∘
t = t + ≤ 80 C (9.25)
o
a
S
s
The required area for conventional housing design may be roughly estimated from
1000P(1 − ) 2
S = m (9.26)
(t − t )
s o a
where
−2 ∘
s – heat transfer coefficient of housing surface, = (8.15–17.45) W m C[4].
s
A larger value is for good ventilation;
∘
∘
t – oil operating temperature, t ≤ 80 C, normally t = 60–70 C;
o o o
∘
t – ambient air temperature, t = 20 C;
a a
t − t – temperature rise.
o a
∘
If the oil operating temperature exceeds 80 C, or the dissipated surface area is
insufficient, various means of cooling the housing or the lubricant oil may be used.
For example, cooling fins can be incorporated to increase housing surface area; or fans
and water coils can be used together to achieve cooling by increasing heat transfer
coefficients.
9.4 Design of Wormgear Drives
9.4.1 Introduction
The task and objective of wormgear drive design are similar to those of gear drives. The
provided design specifications include transmitted power, the rotational speed of worm
and wormgear or gear ratio and operating conditions.
The design of a wormgear drive needs to determine worm types, accuracy grade levels
and the arrangement of worm and wormgear. It also involves the selection of tentative
material candidates, heat treatments, as well as initial variables like the number of worm
threads. These initially selected design variables must satisfy both strength and thermal
criteria.
Like the design of other machine elements, iteration in wormgear drive design is also
unavoidable as information provided is scarce for solving unknowns directly. Besides,
the values of some variables must be assumed and a trial solution be performed.

9.4.2 Materials and Heat Treatments
In a wormgear drive, sliding is far more extensive than in spur gears, and wormgears
are more vulnerable to sliding friction. Materials, therefore, are required to have low