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dynamics_bike-example - Dynamic Mechanism Design It is...

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Dynamic Mechanism Design By D Cheshire Page 1 of 5 It is essential that you have completed the previous two tutorials on mechanisms from this series before starting this one. The part and assembly files for this model can downloaded from the internet and can be found at http://www.staffs.ac.uk/~entdgc/WildfireDocs in the Dynamics section. Save all 7 parts and 1 assembly to your working directory. Open the assembly and you will see that the mountain bike has already been assembled for you and operates correctly as a mechanism. Check this out now by using the DRAG command. Some points worth noting are:- 1. The method used to model the frame of the bicycle. 2. The front wheel is the first part assembled and is effectively locked to ground so it doesn’t move. 3. The rear wheel is assembled using a planar joint so it is free to move backward and forward as the suspension flexes since the wheelbase will change. Figure 1 : The Bike Review all these features now and ensure you understand how they were achieved by referring to the earlier mechanism tutorials. You may even prefer to create your own assembly file and assemble your own cycle to ensure you fully understand the process. Joint Definitions As you drag this mechanism you will probably have noticed that the damper joint moves too far. The parts can actually move so they pass through each other. This means the suspension travel is much too large. Any joint in Pro Engineer can have it movements restricted – here’s how. First use the DRAG tool to position the mechanism such that the damper is near one extreme of its travel as shown in Figure 2 (you may want to switch to hidden line display to see this). Then choose MECHANISM > JOINT AXIS SETTINGS and pick the SLIDER joint to define its settings. Press the MAKE ZERO button to define the current position as the zero location. Next click on the Properties tab in this dialog and tick Enable Limits and enter the values for maximum and minimum travel. This damper has a range of 50mm and the travel is in the opposite direction to the joint so the maximum value will be 0 and the minimum -50. Figure 2 : Joint Limits Now if you DRAG the model the movement should reflect a real bike frame. Springs Although the bike moves correctly there are some elements of the bike suspension that are missing. The first of these is the spring. It is possible to model a spring using the HELICAL SWEEP command and this would look like a spring. For the analysis of mechanisms we need a spring that reacts correctly like a spring applying forces as it is compressed. This type of spring can be defined in mechanisms. Make sure you are in mechanism analysis mode (APPLICATION > MECHANISM) then choose The SLIDER joint
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Dynamic Mechanism Design By D Cheshire Page 2 of 5 MECHANISM > SPRINGS and create a NEW spring. From the dialog box
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