ENGG1960: Introduction to Biomedical Engineering Tutorial – Linear Fits and Tolerances

As a junior engineer, you are required to work out the necessary limits of size for a simplified knee joint implant. Various kinematic joints are readily available and readily manufactured. Each has its advantages and disadvantages. You may like to research each in your time outside of this Tutorial.

Types of Kinematic Joints After a brainstorming session, you have chosen to select a revolute joint as potentially the most appropriate. In order to minimize the complexity and cost of knee replacements, the marketing division believes that there is a potential market in low cost, reliable, lightweight knee joints that allow only motion along one rotational axis.

Revolute Joint Image You may like to research each of the joints noted and their particular strengths and weaknesses.

A basic concept sketch is drafted that identifies the construction of the knee joint. The various components used are noted as is their basic function and materials.

Initial Conceptual Sketch of Knee Joint using a Revolute Joint In order to minimize the complexity and cost of knee replacements, the marketing division believe that there is a potential market in low cost, reliable, lightweight knee joints that allow only motion along one rotational axis.

Layout Drawing of a Simplified Knee Joint using a Revolute Joint

The Layout Drawing of a Simplified Knee Joint describes a design which is made up of; a rod to represent the Tibia, a fork to represent the Femur and a pin to provide rotational movement between the two. The pin has a head and a collar with a pin to prevent the assembly from coming loose.

The pin is 10mm in diameter. The rod has a nominal width of 20mm. The rod is to have an average location fit (H7/h6) into the fork. The pin has a fit in the fork and rod designated by a precision running, sliding fit (G7/h6).

1. What are the values of the maximum and minimum clearances for the fit of the rod into the fork?

2. What are the limits of size for the pin and the pin holes in the rod and fork?

3. What are the maximum and minimum amounts of relative lengthwise movement between the fork and rod resulting from the tolerances for the pin and its associated holes?

4. Determine an appropriate surface finish and manufacturing process for the pin, and the hole in fork and the internal surfaces between the rod and fork if;

Cost is a concern. Unnecessarily high finishing costs are not encouraged. Medium type loads at slow to medium speeds are to be applied to the revolute joint in its service life.

Lubrication cannot be depended on.

The machinery available for manufacture is reasonably precise with operators with a slightly above average skill level.

(Hint: You may like to refer to the standard roughness values overleaf as a guide and sketch the surface finishes required on the Layout sketch below.)

Layout Drawing of a Simplified Knee Joint using a Revolute Joint

Standard Roughness Values

Roughness Ranges for Common Production Processes Once you have completed the Tutorial and you have roughly 45mins to spare, please view the 4 Corners program, “The Walking Wounded” televised on Channel 2 26/05/2014 located at http://www.abc.net.au/4corners/stories/2014/05/26/4010089.htm You should view the program to appreciate the importance that an Engineer must place on; the appropriate selection of materials, correct dimensioning and most importantly, ethics.

Paul Briozzo 24/05/2015