Chapter 2 Review of Forces and Moments - Brown University
Chapter 2 Review of Forces and Moments - Brown University
Chapter 2 Review of Forces and Moments - Brown University
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
2.3.4 Measuring <strong>Moments</strong><br />
Just as you can buy a force transducer to<br />
measure forces, you can buy a force<br />
transducer that measures moments. We<br />
showed an example <strong>of</strong> a forcetransducer<br />
attached to the wheel <strong>of</strong> a car<br />
during our earlier discussion <strong>of</strong> force<br />
transducers.<br />
Another common moment-measuring<br />
system is a torque-wrench. (So then is<br />
Oprah a talk wench?) When you tighten<br />
the bolts on a precision machine, it’s<br />
important to torque them correctly. If<br />
you apply too much torque, you will<br />
strip the thread. If you don’t apply<br />
enough, the bolt will work itself loose<br />
during service.<br />
You can buy a tool that measures the<br />
moment that you apply to a bolt while<br />
tightening it. The device may be<br />
mechanical, or electronic. An example<br />
(see www.mac.ie/whatwedo/<br />
torquestory.asp ) is shown below.<br />
2.3.5 Engineering systems that exert torques<br />
There are many practical examples <strong>of</strong> moments, or torques, in engineering systems. For example,<br />
(i)<br />
(i)<br />
(ii)<br />
(iii)<br />
(iv)<br />
The driving axle on your car turns the wheels by exerting a moment on them.<br />
The drive shaft <strong>of</strong> any motor exerts a torque on whatever it’s connected to. In fact, motors are<br />
usually rated by their torque capacity.<br />
The purpose <strong>of</strong> a gearbox is to amplify or attenuate torque. You apply a torque to the input shaft,<br />
<strong>and</strong> get a bigger or smaller torque from the output shaft. To do this, the input <strong>and</strong> output shafts<br />
have to rotate at different speeds. There are also some clever gearboxes that allow you to add<br />
torques together – they are used in split-power variable speed transmissions, for example.<br />
A torque converter serves a similar purpose to a gearbox. Unlike a gearbox, however, the input<br />
<strong>and</strong> output shafts don’t rotate at the same speed. The output shaft can be stationary, exerting a<br />
large torque, while the input shaft rotates quickly under a modest torque. It is used as part <strong>of</strong> an<br />
automatic transmission system in a car.<br />
<strong>Moments</strong> also appear as reaction forces. For example, the resistance you feel to turning the<br />
steering wheel <strong>of</strong> your car is caused by moments acting on the wheels where they touch the<br />
ground. The rolling resistance you feel when you ride your bike over s<strong>of</strong>t ground or grass is also<br />
due to a moment acting where the wheel touches the ground.