Page 50 - Shigley's Mechanical Engineering Design

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Introduction to Mechanical Engineering Design 25
determined a need for one of these speed reducers to satisfy the following customer
requirements.

Design Requirements
Power to be delivered: 20 hp
Input speed: 1750 rev/min
Output speed: 85 rev/min
Targeted for uniformly loaded applications, such as conveyor belts, blowers,
and generators
Output shaft and input shaft in-line
Base mounted with 4 bolts
Continuous operation
6-year life, with 8 hours/day, 5 days/wk
Low maintenance
Competitive cost
Nominal operating conditions of industrialized locations
Input and output shafts standard size for typical couplings
In reality, the company would likely design for a whole range of speed ratios for
each power capacity, obtainable by interchanging gear sizes within the same overall
design. For simplicity, in this case study only one speed ratio will be considered.
Notice that the list of customer requirements includes some numerical specifics, but
also includes some generalized requirements, e.g., low maintenance and competitive cost.
These general requirements give some guidance on what needs to be considered in the
design process, but are difficult to achieve with any certainty. In order to pin down these
nebulous requirements, it is best to further develop the customer requirements into a set of
product specifications that are measurable. This task is usually achieved through the work
of a team including engineering, marketing, management, and customers. Various tools
may be used (see footnote 1, p. 7) to prioritize the requirements, determine suitable
metrics to be achieved, and to establish target values for each metric. The goal of this
process is to obtain a product specification that identifies precisely what the product must
satisfy. The following product specifications provide an appropriate framework for this
design task.

Design Specifications
Power to be delivered: 20 hp
Power efficiency: >95%
Steady state input speed: 1750 rev/min
Maximum input speed: 2400 rev/min
Steady-state output speed: 82–88 rev/min
Usually low shock levels, occasional moderate shock
Input and output shafts extend 4 in outside gearbox
Input and output shaft diameter tolerance: ±0.001 in
Input and output shafts in-line: concentricity ±0.005 in, alignment ±0.001 rad
Maximum allowable loads on input shaft: axial, 50 lbf; transverse, 100 lbf
Maximum allowable loads on output shaft: axial, 50 lbf; transverse, 500 lbf
Maximum gearbox size: 14-in 14-in base, 22-in height
Base mounted with 4 bolts
Mounting orientation only with base on bottom
100% duty cycle