Geared Four-bar Mechanisms are usually built with:
• | A four-bar mechanism, with at least one Part that rotates continuously |
• | One or two Gear-Pairs |
• | The input 'drive' gear is fixed to the Coupler, at the distal end of the Crank or Rocker |
• | The output gear is usually at the ground joint of the respective Crank or Rocker |
'Geared Four-Bar Mechanisms' are often used as 'Function-Generators'. Function-Generators provide an input to output motion relationship.
Typically, the input to these mechanisms is a continuously rotating Crank.
Regressive and Ir-regressive Gears
I have no idea why they are called 'regressive' and 'ir-regressive' gears!
Regressive gears are at each end of the Crank,
Ir-regressive gears are at each end of the Rocker.
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Remember: The Parts must be joined with Pin-Joints to the Points at each end of the Line-of-centres. Make sure the Point at the end of the new Part is joined to the Crank, and not the Base-Part. See Select Elements dialog-box |
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Select the the CAD-Lines that are in:
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Add a Graph. Add a Measurement FB to measure the motion of the New Geared Rocker. |
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Remember: The Parts must be joined with Pin-Joints to the Points at each end of the Line-of-centres. Make sure the Point at the end of the new Part is joined to the Rocker, and not the Base-Part. See Select Elements dialog-box |
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Select the the CAD-Lines that are in:
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Add a Graph. Add a Measurement FB to measure the motion of the New Geared Rocker. |
Usually, the two gears on the same shaft at the are edited so that they have the same module and number-of-teeth. Although, this is not necessary.
There is a small complexity in MechDesigner (special rule) that means you must use a 'trick' when you add the second Gear-Pair.
Gear Pair 1 |
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We will add the first Gear-Pair to the Coupler. The input Gear will be at the end of the Crank. The motion of the input gear will be the relative motion between the Crank and the Coupler. |
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Remember: The Parts must be joined with Pin-Joints to the Points at each end of the Line-of-centres. Make sure the Point at the end of the new Part is joined to the end of the Coupler and not the end of the Rocker. See Select Elements dialog-box |
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Select the:
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Gear-Pair 2 This step requires a good understanding of the Add Gear-Pair command. |
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Make sure it is joined to the Rocker and not the Base-Part. |
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We want to add the 'Gear-Pair 2'. The Parts with the Gears must be joined to the end of the Line that will be the Line-of-centres. The Line-of-centres will be the CAD-Line along the Rocker - the output Part of the Four-bar Mechanism. The Blue Part is joined to the Rocker. However, the Green Idler Gear Part at the distal end of the Rocker is joined to the Coupler - not the Rocker. So, if we tried to add a Gear-Pair, we would not succeed. We must add a new Part to the distal end of the Rocker The new Part must:
To do this we:
See Select Elements dialog-box |
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The Parts at each end of the CAD-Line along the Rocker or Blue. One must be Green. |
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The Part is now Green We must move it at the same speed as the Idler Gear. To do this we measure the Idler Gear Part relative to the Rocker with a Measurement FB. |
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Now we can know the motion of the idler relative to the Rocker. |
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This makes the Motion-Dimension, and Motion-Part, move with the same motion-values given by the Measurement FB. The Motion-Part will ave the same motion as the existing Geared Rocker. Therefore, when we add the new Gear-Pair, the Parts move in the same way. |
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The two Parts at the end of the Rocker are now together. Also, it makes the Gears appear as Simple Gear-Train. You may need to edit the Gear Teeth so that the four-bar makes a complete rotation. |
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