Baixe 11 - Mechanisms e outras Notas de estudo em PDF para Engenharia Mecânica, somente na Docsity!
Mechanism design in Pro Engineer is very comprehensive. You can assemble components of the mechanism together using joints which accurately reflect their movements, you can animate the movement creating movie files which can be displayed on any PC and you can ask Pro Engineer to calculate the forces and movements of a mechanism to enable you to determine the suitability of the mechanism.
Degrees of Freedom
Understanding degrees of freedom is critical to selecting the appropriate joints for your mechanism. In mechanical systems, the number of degrees of freedom (DOF) represents the number of independent parameters required to specify the position or motion of every body in the system. A completely unconstrained body has six degrees of freedom, three translational and three rotational. Joint connections act as constraints, or restrictions, on the motion of bodies relative to each other, reducing the total possible degrees of freedom of the system. If you apply a pin joint to a body, you restrict the body's movement to rotation around the pin joint, and the degrees of freedom for the body reduce from six to one. Below is a table describing the joint connections you can create in Mechanism Design and the degrees of freedom corresponding to each joint.
No. of DOF
Joint
Diagram
Rotate Translate
Description
Pin
1 0 Rotates about an axis.
Slider
0 1 Translates along an axis.
Cylinder
1 1 Translation along and rotation about a specific axis.
Planar
1 2 Bodies connected by a planar joint move in a plane with respect to each other. Rotation is about an axis perpendicular to the plane.
Ball
3 0 A "ball-in-spherical-cup" joint allows rotation in any direction.
Bearing
(^3 1) Combination of a ball joint and a slider joint.
Weld
(^0 0) Glues two parts together.
Rigid
0 0 Glues two parts together while changing the underlying body definition. Parts constrained by a rigid connection constitute a single body.
Figure 1 : Degrees of Freedom with Joints
Mechanism Design
It will be valuable if you already understand the assembly process in Pro Engineer and have completed the assembly tutorial. We will use the pair of mole grips as a basis for explaining how to assemble mechanisms together. Mole grips are like a pair of pliers – with one additional function. They have an over centre mechanism so that when they are squeezed to
grip a bar they stay locked until they are released by the user. To achieve this they must be correctly adjusted to the size of the bar using the adjuster screw on the end of the handle.
Figure 2 : Mole Grips
To design a mechanism in Pro Engineer you must first design the individual parts of the mechanism. The part files for this model can downloaded from the internet and can be found at http://www.staffs.ac.uk/~entdgc/WildfireDocs/tutorials.htm Save all 8 parts to your working directory.
Use FILE > NEW to create a new assembly called mole_grips using the Empty template.
Choose INSERT > COMPONENT > ASSEMBLE and choose the part mole_main. The main handle of the grips should be placed in the graphics window (If the component placement dialog appears you did not choose the Empty template for the assembly – start again!).
Figure 3 : Main Part Added
Choose INSERT > COMPONENT > ASSEMBLE and choose the part mole_jaws. The component placement dialog should appear this time. In this dialog click on the connect tab (see Figure 4) to show an alternative dialog which must be used for assemblies involving mechanisms.
Figure 4 : Mechanism Connections You should notice that the default joint type is PIN. This type of joint constrains a pin into a hole aligning their axes and also mates 2 faces. This is the correct type of joint in this case. Pick the cylindrical surfaces of the two matching holes on each part (see Figure 5).
Figure 5 : Pin Join First Step
Drag the parts so the strut is in a more realistic position (See Figure 10).
The final part in the mechanism is mole_screw. This is the adjuster screw. As you turn the screw it moves in and out. The strut touches the end of the screw so that as the screw moves it has the effect of altering the distance between the jaws. Let’s try and simulate this now so that you can understand it better.
Choose INSERT > COMPONENT > ASSEMBLE to assemble the mole_screw. Don’t forget to click on the CONNECTION tab (see Figure 4). We need a different type of joint this time called a SLIDER so choose this now below the word TYPE. A slider joint allows movement along an axis so that we can move the screw in and out (we won’t bother simulating the screw rotation). The joint definition is similar to the previous PIN joint. First we need two axes to align so choose the hole in the end of the mole_main part and the cylindrical surface of the mole_screw. A slider also requires two planar surfaces to stop rotation – pick DTM3 in mole_screw and DTM in mole_main (of course strictly speaking our screw does rotate but it does not matter). DO NOT close the Component Placement dialog yet!
Figure 10 : Slider Joint
We now have to link the strut to the screw. This means we need an extra
joint so the screw will have two joints. Click in the Component Placement dialog to make a new joint. The type of joint we need here is a Cylinder so choose it now below Type. A cylinder joint is a PIN joint without the translation surfaces – it just needs two axes to align. If you look carefully you will see that the strut has an axis through the cylindrical surface at the end and the screw has an axis which has been especially created the same distance away from the end of the screw as the strut radius. They are both labeled A_2 and are highlighted in Figure 11. Pick these now to complete the joint. Don’t worry if the pin moves to the wrong position along the axis. Close the placement dialog and try dragging the pin to a more realistic position.
Figure 11 : Cylinder Joint As you are dragging parts in this mechanism you may notice that when dragging the handle the screw moves in and out. In real mole grips the screw position would not change when moving the handle. How can we make this work correctly? Go to APPLICATIONS > MECHANISMS and MECHANISM > DRAG and notice the DRAG dialog box. This has a tab called Constraints. Click on it
now and then click on the body-body lock constraint icon. This allows you to restrict movement of a joint. Click on the mole_main part then click on the mole_screw part and choose OK. These two parts will be locked in their current position. You should now be able to drag the handle without the screw moving. You can remove this lock by removing the tick next to the body-body lock in the constraints tab. The screw is then free to move again and can then be locked again in a new position by adding the tick.
Figure 12 : Joint Constraints
Mechanism Animation
You have seen how by dragging you can move the mechanism through its range of movement. This can be used to generate a sequence of movements for the mechanism that can be replayed within Pro Engineer whilst modifying some design parameters or can be saved as a video in one of the standard Windows video formats then replayed outside of Pro Engineer in Windows Media player or similar.
To define an animation choose APPLICATION > ANIMATION. The screen will change with the important addition of an area below the main graphics window. This is where the timeline editor will appear. A timeline defines what events happen and at what they start/stop. But what are these events and how do we define them? In simple terms an event is a mechanism position. The model can be dragged to different positions and a snapshot taken of the model in that position. Here is how…
First let’s set the mechanism up correctly. The command ANIMATION >
SNAPSHOT is used to drag the mechanism. Choose this now and in
the Constraints tab click on the body-body lock constraint icon. Click on the mole_main part then click on the mole_screw part and choose OK to lock the screw in place like before.
Make sure by dragging that the mole grips are in the closed position with the handle near horizontal. At the top of the drag dialog you will see
the camera icon. Press this now and Snapshot1 should be created in the snapshots tab. This has memorized the position of the mechanism as
it is now. Move the handle to the fully open position and press again. You should now have 2 snapshots. Go back to the Constraints tab and change the unlock (remove tick) the Body-Body lock. Move the screw to a new position then replace the tick to lock the screw movement. Take two more snapshots with handle closed (Snapshot3) and open (Snapshot4). Close the dialog. We have just defined 4 events that need to be turned into an animation.
Figure 13 : Key Frame Editor
Press the icon or choose ANIMATION > KEY FRAME SEQUENCE. Choose NEW and the Keyframe Sequence Editor dialog appears (see Figure 13). Type Mole in the name field. Below Keyframe Snapshot should be listed and time 0.000. In the dialog press to add this Snapshot1 to the animation. Then select Snapshot2 from the list and change the time to 2 and press again. Press OK. You should see the Mole animation appearing in the timeline.
Now in the animation toolbar press or ANIMATION > START to generate all the intermediate frames between Snapshot1 and Snapshot2. Once generated you can press or ANIMATION > PLAYBACK to play the sequence again using the Animate dialog which has controls like a video recorder.
