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 eliminat... read moreing 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:
- DCA Citation Guide EndNote
