Posts Tagged ‘tools’

Mechanical Advantage of a Compound Pulley

Thursday, September 29th, 2016

    In this blog series on pulleys we’ve gone from discussing the simple pulley to the improved simple pulley to an introduction to the complex world of compound pulleys, where we began with a static representation.   We’ve used the engineering tool of a free body diagram to help us understand things along the way, and today we’ll introduce another tool to prepare us for our later analysis of dynamic compound pulleys.   The tool we’re introducing today is the engineering concept of mechanical advantage, MA, as it applies to a compound pulley scenario.

    The term mechanical advantage is used to describe the measure of force amplification achieved when humans use tools such as crowbars, pliers and the like to make the work of prying, lifting, pulling, bending, and cutting things easier.   Let’s see how it comes into play in our lifting scenario.

    During our previous analysis of the simple pulley, we discovered that in order to keep the urn suspended, Mr. Toga had to employ personal effort, or force, equal to the entire weight of the urn.

F = W                                    (1)

    By comparison, our earlier discussion on the static compound pulley revealed that our Grecian friend need only exert an amount of personal force equal to 1/2 the suspended urn’s weight to keep it in its mid-air position.   The use of a compound pulley had effectively improved his ability to suspend the urn by a factor of 2.   Mathematically, this relationship is demonstrated by,

F = W ÷ 2                              (2)

    The factor of 2 in equation (2) represents the mechanical advantage Mr. Toga realizes by making use of a compound pulley.   It’s the ratio of the urn’s weight force, W, to the employed force, F.   This is represented mathematically as,

MA = W ÷                           (3)

    Substituting equation (2) into equation (3) we arrive at the mechanical advantage he enjoys by making use of a compound pulley,

MA = W ÷ (W ÷ 2) = 2           (4)

Mechanical Advantage of a Compound Pulley

Mechanical Advantage of  a Compound Pulley

    Next time we’ll apply what we’ve learned about mechanical advantage to a compound pulley used in a dynamic lifting scenario.





 Copyright 2016 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog



Systems Engineering In Medical Device Design – Introduction

Monday, November 26th, 2012
     I once worked with a medical device design engineer who, although talented enough, was not adept in the subtle yet indispensable skill of verbal communication.  He lacked a concise, organized approach to his projects, and his problem solving skills were unilateral and obtuse, that is to say, his only aim was to satisfy his personal requirements, what he felt was important.  Creative problem solving and brainstorming with customers as to what they desired did not fall within his repertoire.  As a result customer complaints and a long string of product failures eventually led to him losing his position.

     Where specifically had he failed?  The net result of his approach was that he designed devices that did not deliver the desired customer results.  They also had a varying tendency to be either unnecessarily expensive to produce, unreliable to operate, or difficult to service.  All concerned with the product were often dissatisfied, from customers to service technicians.  This caused the company we worked for to incur considerable expense to rectify his design errors.  The company also lost some of their customer base to competitors.  Sadly, none of this would have happened if my coworker had used a systems engineering approach in designing his projects.

     Before we get any further into a discussion on systems engineering, let’s get a handle on what is meant by a system.  In a nutshell, a system is a combination of interacting components that are organized to achieve one or more specific purposes.  The components can be tools, machine parts, electronics, people, or any combination thereof.  For example, hundreds of parts can be combined by a manufacturer into a system to form a medical device such as an x-ray film developing machine, the end result of which is to produce a film of diagnostic quality. 

      The system part of Systems Engineering stays true to this definition. It is an interdisciplinary approach to complex engineering projects which guides all activities during the course of a product’s life cycle, from conception to production. While doing so it will integrate and monitor work processes between all departments involved, with a constant eye towards optimization of processes and reduction of costs in order to satisfy stakeholder requirements.

     A key objective of systems engineering is to produce systems that satisfy stakeholder needs by producing reliable, cost effective, and safe products capable of performing tasks as designated by the customer.  Within the medical device arena stakeholders include patients, nurses, doctors, the US Food and Drug Administration (FDA), device service technicians, device dealers, as well as the device manufacturer.

     Next time we’ll begin our exploration of how systems engineering addresses the medical device design process with a discussion on the first of its five stages, known as Concept.


 Systems Engineering In Medical Device Design