Design and Analysis of an Oscillating Mechanism for Applications in a Bone Saw.
DeVore, Sara.
2014
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Abstract: An
oscillating bone saw is the primary surgical power tool used for resection of bone
during total joint replacement of the knee and hip. For several decades, a pistol-grip
configuration of the saw has prevailed, where the motor is positioned orthogonal to the
oscillating mechanism. This design evolved as a compromise between using the power head
as either a drill or a saw, thus ... read moreeliminating the need for a second tool during surgery.
However, after direct observation of bone sawing and through feedback from orthopedic
surgeons, it became apparent that the preferred ergonomics for drilling were
significantly different from sawing. Furthermore, studies of similar oscillating saws
used in woodworking applications revealed that in contemporary designs the motor and
mechanism are aligned along the same axis, creating a body-grip configuration, rather
than a pistol-grip. The aim of this thesis is to design an in-line oscillating mechanism
to accommodate a body-grip design for improved ergonomic handling of a surgical bone
saw. A computer model of an in-line mechanism is first developed by reverse engineering
a commercially available woodworking tool. Kinematics of the computer generated solid
model are validated by two methods: (1) a motion study is performed where blade velocity
as a function of motor speed is recorded with a laser vibrometer, and (2) an analytical
model is developed by using a vector loop method. Components of the virtual assembly are
then refined and mass properties are added such that a motion study with the computer
model generates results that compare reasonably well with both vector kinematics and
experimental measurements. After validating the computer model for the existing
mechanism, the design is modified to accommodate specifications for an in-line surgical
bone saw. The updated model is then used to perform a computational study of the
mechanism kinetics to determine resultant forces on the oscillating components and
related bearings. Simulations are run for a common surgical blade oscillating through a
5$^{\circ}$ angle at 10,000 cycles per minute, which is representative of a typical
surgical bone saw. To reduce peak loads and to minimize tool vibration, the
computational model is used to counterbalance the new in-line oscillating mechanism. At
full speed prior to counterbalancing, oscillating components generate a peak load that
varies between 9 N and 91 N at the front motor shaft bearing. While it is not possible
to completely balance an oscillating load with a rotating mass, the peak and alternating
load is reduced by adding an offset mass to the motor shaft, resulting in a more
balanced load that varies between 48 N and 53 N. A natural frequency analysis is
performed to confirm that the operating frequency of the saw does not excite any natural
frequencies of the mechanism. Forces generated by the counterbalanced mechanism are used
to size the bearings, achieving an end of life criterion that exceeded 8000 hours. In
addition, the peak force is used to conduct a linear elastic finite element analysis on
the cam fork, which is the primary component responsible for converting rotary motion of
the motor cam into oscillating motion of the blade shaft. Considering a ductile yield
criterion, the fork is designed such that the maximum stress results in a factor of
safety of at least 2.0. Mechanism grease is specified to lubricate contact points.
Finally, a push button mechanism is designed to provide a means of quickly changing
blades without secondary tools while wearing surgical gloves. Using a Goodman failure
criterion, the push button spring is designed to have infinite life. Following mechanism
design and analysis, a gear case is designed and a demonstration prototype of the new
in-line system is developed.
Thesis (M.S.)--Tufts University, 2014.
Submitted to the Dept. of Mechanical Engineering.
Advisor: Thomas James.
Committee: Anil Saigal, Dan Hannon, and Eric Smith.
Keywords: Mechanical engineering, Mechanics, and Design.read less - ID:
- w9505b77k
- Component ID:
- tufts:21397
- To Cite:
- TARC Citation Guide EndNote
