PRODUCT DESIGN

Cold-Working Processes

By cold working is meant the forming of the metal while at a low temperature (usually room temperature). In contrast to parts produced by hot working, cold-worked parts have a bright new finish, are more accurate, and require less machining.

Cold-finished bars and shafts are produced by rolling, drawing, turning, grinding, and polishing. Of these methods, by far the largest percentage of products are made by the cold-rolling and cold-drawing processes. Cold rolling is now used mostly for the

production of wide flats and sheets. Practically all cold-finished bars are made by cold drawing but even so are sometimes mistakenly called “cold-rolled bars.” In the drawing process, the hot-rolled bars are first cleaned of scale and then drawn by pulling them through a die that reduces the size about 1 32 to 1 16 in. This process does not remove material from the bar but reduces, or “draws” down, the size. Many different shapes of hot-rolled bars may be used for cold drawing.

Cold rolling and cold drawing have the same effect upon the mechanical properties. The cold-working process does not change the grain size but merely distorts it. Cold working results in a large increase in yield strength, an increase in ultimate strength and hardness, and a decrease in ductility. In Fig. 2–12 the properties of a colddrawn bar are compared with those of a hot-rolled bar of the same material.

Heading is a cold-working process in which the metal is gathered, or upset. This operation is commonly used to make screw and rivet heads and is capable of producing a wide variety of shapes. Roll threading is the process of rolling threads by squeezing and rolling a blank between two serrated dies. Spinning is the operation of working sheet material around a rotating form into a circular shape. Stamping is the term used to

describe punch-press operations such as blanking, coining, forming, and shallow drawing.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

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Shell Molding

The shell-molding process employs a heated metal pattern, usually made of cast iron,

aluminum, or brass, which is placed in a shell-molding machine containing a mixture

of dry sand and thermosetting resin. The hot pattern melts the plastic, which, together with the sand, forms a shell about 5 to 10 mm thick around the pattern. The shell is then baked at from 400 to 700°F for a short time while still on the pattern. It is then stripped from the pattern and placed in storage for use in casting.

In the next step the shells are assembled by clamping, bolting, or pasting; they are placed in a backup material, such as steel shot; and the molten metal is poured into the cavity. The thin shell permits the heat to be conducted away so that solidification takes place rapidly. As solidification takes place, the plastic bond is burned and the mold collapses.

The permeability of the backup material allows the gases to escape and the casting to air-cool. All this aids in obtaining a fine-grain, stress-free casting. Shell-mold castings feature a smooth surface, a draft that is quite small, and close tolerances. In general, the rules governing sand casting also apply to shell-mold casting.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

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Sand Casting

Sand casting is a basic low-cost process, and it lends itself to economical production in large quantities with practically no limit to the size, shape, or complexity of the part produced.

In sand casting, the casting is made by pouring molten metal into sand molds. A

pattern, constructed of metal or wood, is used to form the cavity into which the molten metal is poured. Recesses or holes in the casting are produced by sand cores introduced into the mold. The designer should make an effort to visualize the pattern and casting in the mold. In this way the problems of core setting, pattern removal, draft, and solidification can be studied. Castings to be used as test bars of cast iron are cast separately and properties may vary.

Steel castings are the most difficult of all to produce, because steel has the highest melting temperature of all materials normally used for casting. This high temperature aggravates all casting problems.

The following rules will be found quite useful in the design of any sand casting:

1 All sections should be designed with a uniform thickness.

2 The casting should be designed so as to produce a gradual change from section to section where this is necessary.

3 Adjoining sections should be designed with generous fillets or radii.

4 A complicated part should be designed as two or more simple castings to be assembled by fasteners or by welding.

Steel, gray iron, brass, bronze, and aluminum are most often used in castings. The minimum wall thickness for any of these materials is about 5 mm, though with particular care, thinner sections can be obtained with some materials.

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

Budynas−Nisbett

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Power Transmission Case Study Specifications

A case study incorporating the many facets of the design process for a power transmission speed reducer will be considered throughout this textbook. The problem will be introduced here with the definition and specification for the product to be designed.

Further details and component analysis will be presented in subsequent chapters.

Chapter 18 provides an overview of the entire process, focusing on the design sequence, the interaction between the component designs, and other details pertinent to transmission of power. It also contains a complete case study of the power transmission speed reducer introduced here.

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

Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

Budynas−Nisbett

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