Geared Four-Bar Kinematic-Chain
Contents
Geared Four-Bar Mechanisms
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 gear is usually fixed to the Coupler, at the distal (moving) end of the Crank or the Rocker |
| • | The centre of the driven Geared-Rocker is usually at the grounded Joint of the Crank or the Rocker |
Geared four-bar mechanisms are often used as 'Function Generators'. Function-Generators provide an input to output motion relationship.
Summary of Geared Four-bar Mechanisms
 Gears - Four-bar Regressive |
Four-Bar Regressive Gearing. Output is co-axial with Crank centre. |
Four-Bar with Irresgressive Gears |
Four-Bar Ir-Regressive Gearing Output is co-axial with Rocker Pivot |
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Four-Bar Function-Generation with Two Gear-Pairs. Output is co-axial with Rocker Pivot |
One Gear-Pair
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[Remember: Gear-Pair Rule: The two Parts must be joined with Pin-Joints at each end of a Line. The Line is the Line-of-Centres. Thus when you add this Part:
See Select Elements dialog-box |
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Select these CAD-Lines, in order, along the centre of the:
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Add a Graph. Add a Measurement FB to measure the motion of the New Geared Rocker. |
'Ir-regressive' Geared Four-bar
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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. |
Two Gear-Pairs
Four-bars with two Gear-Pairs have one idler. Usually, the two 'centre gears' are edited so that they have the same module and number-of-teeth. Although, this is not necessary. There is a small complexity that you must consider when we add the second Gear-Pair.
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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 Coupler and not the Rocker. See Select Elements dialog-box |
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Select the:
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Add 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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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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