Many industrial applications require
machinery to be powered by engines or electric motors. The power
source usually runs most efficiently at a narrow range of rotational
speed. When the application requires power to be delivered at a
slower speed than supplied by the motor, a speed reducer is
introduced.
The speed reducer should transmit the
power from the motor to the application with as little energy loss as
practical, while reducing the speed and consequently increasing the
torque. For example, assume that a company wishes to provide
off-the-shelf speed reducers in various capacities and speed ratios
to sell to a wide variety of target applications. The marketing team
has 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) 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 shaft diameter
tolerance: ±0.001 in
Output shaft and input shaft 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
Base mounted with 4 bolts
Mounting orientation only with base on
bottom
100% duty cycle
Maintenance schedule: lubrication check
every 2000 hours; change of lubrication
every 8000 hours of operation; gears
and bearing life >12,000 hours;
infinite shaft life; gears, bearings,
and shafts replaceable
Access to check, drain, and refill
lubrication without disassembly or opening of
gasketed joints.
Manufacturing cost per unit: <$300
Production: 10,000 units per year
Operating temperature range: −10◦
to 120◦F
Sealed against water and dust from
typical weather
Noise: <85 dB from 1 meter
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