NATIONAL INSTITUTE OF
INDUSTRIAL ENGINEERING
PGDIE-42
Industrial Engineering
Assignment on Research
Paper:-
A ONE-PIECE COMPLIANT
STAPLER
Presented By: -
PANKAJ KUMAR MOHLI
PGDIE 42
Roll NO. 56
Introduction and Motivation
Today's
competitive market demands continued consideration of alternative designs to
improve quality, economy, and safety, for the commercial success of any
product, big or small. Motivated by this challenge, this project aims to
improve upon the design of one of the most common pieces of stationery-a
stapler. Typically, a stapler is comprised of at least four separate rigid
parts and a spring assembled together to serve the various sub functions. In
general, designing a product with fewer components, and eliminating the
component assembly results in a decrease of its production cost considerably.
In order to economize the cost of production of a stapler, we present a novel
design for a stapler that consists of only one component and performs as good
as a conventional stapler. In the design proposed, the desired functionality is
achieved entirely through the compliance, i.e., elastic deformation in a
suitably shaped, single piece of flexible material. This feature accounts for
the use of the word compliant stapler to describe the new design which falls
under the class of compliant mechanisms. The principal advantage of this new
design is an enormous savings in its manufacturing cost besides light weight
and aesthetic looks.
The Concept of a Compliant Mechanism
A compliant mechanism represents a class of mechanical
systems that gain all or part of their mobility from the relative mobility of
deformable elements in their design, as opposed to only the rigid body members
[Midha1992]. Significant advantages offered by a compliant mechanism, such as
need for fewer parts, less wear, friction, noise and backlash due to clearances, make it a
superior choice of design over a rigid-body mechanism performing similar
functions for a given function in many cases. Researchers in the field of
compliant mechanisms believe that this field is important, and is expected to
continue to grow as materials with superior properties are developed. This emerging concept of compliant
mechanisms, especially the fully compliant, one-piece construction leads to a
new design paradigm-Integrated Design for Eliminating Assembly, IDEA. The
proposed design for a stapler serves as a good example of this concept.
The Design Approach
Due to lack of a systematic method, the design of compliant
mechanisms continues to rely upon the intuition and experience of the designer.
True to this statement, the creation of a compliant stapler too was largely a
creative effort derived from the intuitive understanding of the rigid-body mechanism
behavior and the elastic behavior of a continuum under specified loads and
boundary conditions.
The design of
compliant mechanisms are obtained, generally, by suitably replacing the rigid joints and
rigid links by either a fully compliant
entity of material or a structure
with discrete localized compliant
segments. The first logical step in the design of such mechanisms,
therefore, is to identify the basic sub functions and the parts of the mechanism
that accomplish those sub functions.
The next step would be to conceptualize the distribution of
compliance or the localized compliant segments which can perform similar sub
functions. The final step in the design is to obtain the suitable dimensions
for all of the segments of the compliant mechanism. The next section presents the first two steps
applied to the stapler design and the final design step is explained in the
section following the next.
Functional Description
and Conceptualization
The four main sub functions in a stapler can be identified
as: (i) holding the staples securely in a slot,
(ii) loading and unloading
(if necessary) of the staples, (iii) plunging a rigid member on to a staple and
(iv) stapling (i.e., piercing a staple through
a stack of papers and folding back of its sides).At least four rigid
parts and two springs accomplish these functions.
In order to meet the primary goal of this project-to design and
fabricate a stapler out of a single piece-the design is conceptualized as a
distributed compliant structure with a few discrete highly flexible segments. Although
the design resembles a conventional stapler, it can be seen that the major
ideas incorporated in the new design are: the two single-axis flexure hinges to
serve the purpose of the rigid pin joint and a compliant curved beam to serve
the purpose of the spring holding the staples. The flexure hinges are formed by
circular cutouts on both sides of the blank to from necked-down sections as
shown in figure. The compliant
curved-beam, whose one end is attached
to the middle part and the other connected to a slider, remains almost flat with a large
radius of curvature when staples
are unloaded, and bends
into an arch with a
smaller radius of curvature when the
staples are loaded.
Design Calculations
The dimensions for the new design were chosen to suit one of
the standard sizes of the staples available in market. The important dimensions
to be calculated were the cross sections of the flexural pivots and the
compliant curved beam that serves as a spring.
The proper dimensions of these flexural pivots is critical
because they must be long and thin enough to travel through the required angle
of rotation without becoming over stressed and must also be thick enough to
withstand the required amount of fatigue loading. In order to estimate the
cross-section of the flexural pivot for the top portion, it was assumed to bend
through an angle e in a circular arc of radius L under a force F required to
staple through a few papers, which is applied at the end of the top portion.
e= Lid
where Lis the length of the stapler and dis the distance through which the top part moves
through to push a staple down. The force required for stapling was estimated to
be about 50
N. The corresponding moment M on the flexural pivot will be:
M = F x L
Using the values of moment and the angle of deflection, the
arc radius R for the necking of the flexure was obtained by using the
expressions derived by Paros and Weisbord [1965]:
¢ 9nR112
M z ""2Ebt512
Adopting the following values for the design parameters (see
Figure 3) t = 5 mm, b = 13 mm,
d = 37 mm, L = 115 mm, E = 700 MPa, R was computed to be 20
em.
The compliant curved beam was so dimensioned to have a spring
constant equal to that of the linear spring used in a conventional stapler of
an equivalent size. For the prototype size the required spring constant was
estimated to be 0.5 NIem. The corresponding cross-section dimensions calculated
using the formula in (Roark and Young, 1975) was determined to be 13 mm X 1 mm.
The details are given in the Appendix. The
dimensions of the sharp
edge that pushes
staples and the
shape of the indentation on the
base were selected to suit the standard
size of staples chosen. All other dimensions of the stapler were
selected to coordinate the motions of the edgeL and the staple stack for accomplishing
the desired functionality of stapling, and also to enable the stapler
fabrication out of a single-piece.
Manufacturability and the Prototype
The concept of one-piece IDEA results in a considerable
decrease in its production cost when compared with the production cost of a
rigid-body mechanism for similar function in several ways. It eliminates the
need for manual
labor or automation equipment in component assembly, reduces the inventory
and the variety of manufacturing equipment required, decreases the amount of material handling involved, decreases the number of manufacturing operations, reduces
the overall turn-out time, and
maintains a cleaner factory
environment. The suitable manufacturing processes for such designs are molding,
casting, extruding, and the like.
The proposed design can be manufactured in a single stage by
injection molding of a suitable plastic material. Injection molding process is
suitable for a large scale production. The sharp edge and the forming base
could be made of steel and used as inserts in the injection mold. However, for
a prototype of the proposed design, it is economically not viable to make a
mold. Hence, it was decided to fabricate a low-budget prototype by cutting
process using a milling tool.
Discussion and Closure
A novel one-piece design for a stapler has been presented.
The new design is representative of compliant mechanisms, an emerging class of
mechanisms which derive their mobility through elastic deformations in a
flexible material as opposed to the rigid-body motion of the conventional
mechanisms. The proposed design can be manufactured in a single stage by
injection molding a suitable plastic material with appropriate properties. The
new design with performance quality as good as a conventional one can result in
a significant decrease in its production cost when produced on a mass scale. It
also makes a stapler lighter and more amenable to improve upon ergonomic and
aesthetic aspects than a conventional stapler. A proof of the concept prototype
has been fabricated. Plate 3 shows a comparison of disassembled parts of a
conventional stapler and the prototype of the one-piece compliant stapler. The
conventional stapler shown in the Plate has 20 separate parts whereas the new
design has only one part.
Acknowledgments
The authors would like
to thank Professor Sridhar Kota for his suggestions and encouragement,
and Messers Steve Erskine, Tim Kuebler, and John Mears for their help in the
machine shop.
References
Midha, A.., 1993, "Elastic
Mechanisms", Modern Kinematics-Developments in the last forty years, Ed. A. G. Erdman,
John Wiley and Sons Inc., New York, pp. 422-428.
Paros and Weisbord, 1965, "How
to Design Flexure Hinges," Machine Design, Nov. 25, p. 151. Roark, R. J.
and Young, W. C., 1975, "Curved Beams", Formulas for Stress and
Stain, McGraw
Hill Book Company, pp. 239-247.
