PULL-PULL CABLE SAW SYSTEM FOR CARDBOARD CUTTING
SUPERIOR STUDENT SOLUTIONS - Ohio University, Mechanical Engineering:
Ryan Bartemes, Natalie Durnwald, Ryan Gabaree, Justin Guinther, Jonathan Kochis, Brian Rohm,
Steven Satava, Ryan Shanley
ABSTRACT
Superior Student Solutions from Ohio University has partnered with the cardboard cutting unit at
SW Resources (a NISH-affiliated non-profit agency in West Virginia) to design and manufacture a
pull-pull cable and saw carriage system to open up a new job opportunity for individuals with
physical disabilities. The current cardboard cutting operation is too physically demanding for
persons with disabilities and presents safety hazards to the workers. System testing showed
increased productivity and safety, while maintaining a meaningful task for the workers. SW
Resources is excited to implement this improved process into the cardboard unit.
BACKGROUND
Superior Student Solutions (SSS) is a team of eight mechanical engineers from Ohio
University, in Athens, OH. SSS was formed in September 2007 for a year-long capstone design
experience centered on the NISH Workplace Innovation Competition. SSS used the NPA Locator
on the NISH website to find their customer, SW Resources (SW), in Parkersburg WV. After
consultation with SW and observing their employees with disabilities perform a wide variety of
tasks, SSS selected a project in the Cardboard Cutting Unit (CCU). The CCU cuts down large
cardboard boxes to panels of a specific size used by their customers as inserts in packaging and
shipping operations. The process involves cutting the large boxes into four separate panels using a
circular saw that is mounted onto a carriage. The carriage currently slides on a track made from
steel angle iron (Appendix A, Figure 1). The task is complete when the workers stack the
individual panels, bind them together, and load them into a truck for shipping.
SSS observed that cutting boxes with the circular saw demands significant physical ability.
Consequently, SW is unable to hire any persons with physical disabilities into this unit. They are
limited to hiring people with anger management problems or similar non-physical disabilities. SSS
also observed binding due to inadequate clearance between the cardboard and the angle iron rails in
the current system. When the binding occurs, the operator must use additional time (upwards of
two minutes) to free the cardboard, resulting in a loss of productivity. Lastly, SSS observed that the
cardboard’s width of 40”, along with the 10” width of the saw carriage, causes the operator to
stretch to 50” in order to complete a single cut. This long stretch causes some of the employees to
lean forward onto one foot in order to make a complete cut. This long stretch in a dusty
environment with slippery floors causes a safety concern.
STATEMENT OF THE PROBLEM
There is a need for an assistive technology device that allows persons with physical
disabilities to safely perform the process of cutting cardboard in the CCU of SW at a productivity
rate greater than or equal to the current process.
PULL‐PULL CABLE SAW SYSTEM RATIONALE
SSS approached this problem with the main goal of reducing physical demand to a level
accessible to persons with physical disabilities. SSS designed the system to enable a specific SW
employee with physical disabilities, the opportunity to work in a position that he had previously
been unable to work. The team decided that automating the saw was the best way to effectively
remove the physical requirement of the task. The team researched patents and existing systems that
could be helpful in automating the current system in the CCU. After considering alternatives such
as a cardboard shear, band saw cutting, and reciprocating saw and applying design tools such as
scoring matrices and concept screenings, SSS decided to use a circular saw driven by a pull-pull
cable system driven by a single motor that turns a winding drum, similar to US Patent #5724769
(Cripe, 1996) (Appendix A, Figure 5). SSS also used a panel saw located at Lowe’s to help aid in
their design of system components (Appendix A, Figure 6).
To implement the new cutting system, SSS investigated the options of the current rail
system and came to the conclusion that it was necessary to redesign the rail system. The primary
reason being the friction in the current angle iron system added an additional 7 lbs. of rolling
resistance. The additional resistance necessitates a higher torque motor, increasing the cost of the
system. In addition, the low clearance due to the current rail system causes binding and decreases
productivity.
DESIGN
The design is based around simplicity, efficiency, and the ability to allow the customer to
still use the system manually if the drive system should fail. The design specifications for this
project were: 1) remove the physical requirement of the task 2) maintain or exceed current
productivity levels 3) mitigate the risk of injury to the operator through design practices. This
design is different from other common systems that are available on the market because it is
designed specifically for the current system that SW uses. In addition, this system contains a motor
and is completely automated when performing the cutting. Other available systems use a similar
rail and carriage system (i.e. panel saw at Lowe’s), yet they still require manual operation to move
the saw. The redesigned system is marketable because it helps create job opportunities to persons
with disabilities. Moreover, the system does decrease down productivity and it creates a safer and
more ergonomic working environment.
SSS was also able to eliminate other annoyances of the current system in the redesign. The
old system required modification to the saw used in order to account for a larger than standard saw
blade. The new system does not require this saw modification making maintenance and assembly
easier for the customer. SSS also designed the new system to be adaptable to use different saws.
The current system had to be modified on site to be used with different brands of saw. This will not
longer be the case because the carriage has universal clamping capabilities.
The system is comprised of three main components: the rails, the saw and carriage, and the
motor and winding system. The circular saw that is used to cut the cardboard sits in a carriage
(Appendix A, Figure 9) made from stock angled aluminum. The saw is attached to the carriage
using aluminum clamps. The carriage glides along two stainless steel guide rails aided by nylon
roller bearings (Appendix A, Figure 3). The bearings consist of u-bolts with nylon spacers placed
around the diameter of the rails. The rails are set on aluminum supports and bolted into angle iron
2 PULL‐PULL CABLE SAW SYSTEM using u-bolts as clamps. The saw carriage is pulled forward and back by two cables which are
attached to the carriage using modified eye bolts, cable thimbles and clamps. The eye bolts also act
as cable tensioning devices for the system. Each cable is routed around two pulleys and attached to
the winding drum. The winding drum is turned using a DC motor (Appendix A, Figures 3,4,7,8),
which is controlled by the electronic system activated by a pair of dependent start buttons. There
are two limit switches located at the extremes of the carriage travel and controls the direction of
travel. When activated, the first switch reverses the direction of travel and cuts power to the saw.
After a delay governed by the circuit, the saw is returned to its starting position. The second limit
switch stops the saw in its original position and resets the system. At any point during system
(Appendix A, Figure 3) operation an emergency stop button can be activated to almost
instantaneously stop the saw in its current position using dynamic braking.
DEVELOPMENT
Rail Subsystem
SSS approached the design of the rails in great detail in order to prevent complete system
failure. If the rails were to fail, the possibility of reverting to manual use of the saw would be
removed. Therefore, the rails were designed for infinite life with a Factor of Safety of 2. The
primary failure mode for the rails would be due to bending stress over the length of the rails, with
maximum stress occurring at the center of the rails. In addition to bending, SSS designed the rails
for infinite fatigue life by using a steel rail with a wall thickness great enough to withstand the
cycled stress on the rails. Due to the volume of cardboard that the CCU processes, the rails must be
able to withstand continuous loading for a high number of cycles. Therefore, SSS needed to select a
material that had a high fatigue limit. In addition to bending stress and fatigue, other factors
considered were corrosion, cost, and wear resistance. SSS decided to use 1’’ 304 stainless steel
tubes due to the material’s high strength, high resistance to wear, and corrosion resistant properties.
Carriage and Pulleys Subsystem
The main concern when designing the saw carriage was that the saw blade must be able to
cut through the entire thickness of the cardboard when the saw is seated on the carriage. In order to
design for this, the saw must sit below the rails. In addition, to avoid binding the deflection of the
carriage must not exceed the .25’’ clearance that exists between the top of the cardboard and the
bottom on the carriage. SSS considered aluminum and steel using two different designs. The first
design was to have the bottom of the saw carriage as a full plate that would have a slit cut into it for
the saw’s blade to rotate through. The alternative design was to have angle iron stock across the
front and back of the carriage to act as the supports of the saw. SSS concluded that 6061-T6
aluminum would be a better choice than steel due to its low-cost, high strength to weight ratio, and
minimal deflection (1.3e-7 inches) resulting from the 10.1 lb saw. Furthermore, SSS performed
cost analysis between the two designs of the carriage and determined that the angle iron support
carriage would be best due to the lower cost and ease of manufacturability.
The pulleys were designed to be used with a 7x7 cable with a diameter of 1/16’’. According
to a standard pulley to cable ratio chart, SSS determined that the tread diameter of the pulley should
be a 1.875’’, a 30:1 ratio of pulley to cable. The main mode of failure of the pulley would occur at
the pin holding the pulley to the mount and the mounting holes. Using the 20 lb. tension of the
3 PULL‐PULL CABLE SAW SYSTEM cable (force to pull carriage with FOS of 1.6), the bearing stress for the pin was calculated at 164 psi
with the bearing stress for each mounting hole would be 71 psi. These calculations were based on a
worst case situation for tension in the cable. The team decided on a 2’’ diameter bronze plated
pulley with 1.875’’ tread diameter attached to a zinc plated mount.
Motor and Winding Subsystem
The motor and winding subsystem is composed of the motor, winding drum, cable, and
cable accessories. A 7x7 1/16” galvanized steel cable was chosen mainly because of its resistance
to corrosion and 120 lb breaking strength. The cable is attached to winding drum similar to
industrial winches. The winding drum material is 416 stainless steel. This material was chosen for
its corrosion resistance, wear characteristics, and ease of machining. The groove in the drum
prevents the cable from binding and kinking, ultimately preventing system failure. For safety
reasons the drum is out of reach and placed under the roller bed to prevent injuries. The team
chose a DC motor to perform the desired task. Initially, the team wanted to use a less expensive AC
motor, but value engineering and cost analysis suggested the DC motor to be superior for our
application. The chosen motor is a parallel shaft gear motor with a built in gear ratio of 15:1. The
gearing was needed to reduce the output RPM to the desired speed and to increase torque available
when winding. The electronic circuit is the driving force behind the operation of the system. The
system consists of two dependent, momentary start buttons that power a series of electromechanical
relays. A twist-to-release emergency stop button acts as the main power switch to the system. The
indicator light functions as a visible indication of when the system is powered on. To operate the
system the user will have to press both start buttons simultaneously. The simultaneous start buttons
were an added safety feature to prevent accidental system starts.
EVALUATION
Testing and evaluation was performed by various members of SSS. Three separate
operators performed 10 cycles each with an average of 5 seconds per system cycle. Evaluation of
the SSS system was based solely on the comparison to the current system in use. The design
currently in use requires an operator to perform a task that is very physically oriented; requiring a
reach of over four feet, in an area where slipping and falling is easily foreseeable. Also, the current
system encounters a significant amount of binding, estimated at about 6% of all boxes cut, which
can cause delays in excess of one minute resulting in an average time of 6.5 seconds per cut for
every 100 cuts made. When no binding is present a single cut, down and back, can be completed in
approximately 3 seconds. The system built by SSS takes five seconds to complete a single cut but
the binding was completely eliminated during system redesign. The additional clearance improves
overall productivity in the CCU by approximately 23%. A video of the SSS design can be seen on
videos 1,2,3,4. When the elimination of the physical labor as well as the reduction in the possibility
of slipping is considered the design is a great improvement over the current system.
When implemented onsite it was determined that a variable speed switch would be
necessary for a speed adjustment between cutting one and two inch thick cardboard boxes. Whith
this modification the device was able to meet all initial productivity, endurance, and specification
testing. Further testing will resume with the employees themselves when the cardboard cutting unit
is again operational.
4 PULL‐PULL CABLE SAW SYSTEM DISCUSSION
The pull-pull cable system designed by SSS was developed for the individuals with physical
disabilities working at SW in order to increase job opportunity in the CCU. However, this system
can be adapted at any company and is readily transportable and can be easily fastened to the
supports of such systems. Upon delivery the designed system will be ready for immediate use by
SW. The intent of the design was to make jobs available to those who would not have previously
had the opportunity. This intent was achieved and is apparent in the amount of independence it will
provide SW. No longer will they need to look for workers who are physically capable. This device
can be used by a wide variety of persons with physical disabilities, including those who are
restricted to a wheel chair, walker, or crutches. The system is completely independent of the
operator once the dual start switches are engaged, and the movable control panel gives the operator
freedom of moving around if needed.
SW was involved in many aspects of the design process. The team not only worked closely
with their customer SW, but as well as their Mechanical Engineering Advisor Dr. Kremer.
Additionally, SSS met with a board of advisors on four different occasions during the design
process to get information and advice from engineers with real-world experience. SSS is confident
that the final prototype system is a good solution to the problems in the CCU, and they are excited
to implement it at SW.
The complete cost for the components of the system came to a total of $807. The itemized
material list is shown in Table 1. The cost of the motor, controller, and electrical components of the
circuit are the major contributors to the overall cost of this project. The manufacturing cost was
another significant contributor to the overall product cost. The most time consuming tasks of the
project were the complex machining of the winding drum and wiring of the electrical system. The
team spent approximately 120 man-hours assembling the prototype. Using the 2006 technical labor
rate of $15.00 for skilled laborers (USDL, May 2006) the total cost of the manufacturing process
was calculated to be $1800.00. Although if the product needed to be replicated the manufacturing
cost would be much lower due to the lessons learned during the assembly process. The team
estimates the manufacturing cost could be cut in half if a second prototype was needed. Including
both the component costs and the manufacturing cost the total system cost is estimated at $2600.
SSS believes that the cost of the system is far outweighed by the benefits of this system for SW.
When this pull-pull system is introduced into SW, then the CCU will permit the hiring of persons
with physical disabilities. In addition, this system will increase the productivity in the CCU leading
to an increase in profits for the unit. Finally, the proposed system will make a better working
environment in the CCU by reducing the possibility of slipping while operating the saw.
REFERENCES AND ACKNOWLEDGEMENTS
Cripe, Todd E., “Motor Vehicle Window Construction With Pull-Pull Cable System”, US Patent no.
5724769, March 1996.
US Department of Labor., “National Compensation Survey” May 2006.
<http://stats.bls.gov/ncs/home.htm>
5 PULL‐PULL CABLE SAW SYSTEM SSS would like to thank the non-profit agency SW Resources for their help and suggestions and
acknowledges their Senior Design professor Dr. Gregory Kremer for his suggestions and
constructive criticisms. In addition, SSS thanks ME Professor Dr. Israel Urieli, ME Lab
Coordinator Mr. Randy Mulford, and ME advisory board members Mr. Tom Wakeman, Ms.
Suzanne Tkach, and Mr. Tom Moehring.
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