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Wednesday, July 1, 2015

Indexers with single dwell (1-dwell) VS double dwell (2-dwell)

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To configure a rotary indexer, there are several parameters to decide such as number of stops (S), indexing angle (bm), maximum output torque, etc. In this post, we will show one more parameter which may affect the design if misunderstood. It's a number of dwell. In most cases, we use 1-dwell (single dwell) for the application. The indexer performs single index within the designated indexing angle and wait (dwell) until cycle complete.

However, for some indexer models, there will be the option to select 2-dwell cam type (double dwell, double indexes). Or there may be only 2-dwell version especially for the models which have large number of stops and long indexing angle e.g. S=20, bm = 210 deg.

2-dwell indexer will perform differently from 1-dwell indexer though they're both having the same S and bm. The 1-dwell indexer will index only one once per each full revolution of the input cam shaft. But the 2-dwell indexer will index 2 times and also stop 2 times per each revolution of the input cam shaft as can be seen in the following displacement diagram.
diagram explaining difference between 1-dwell and 2-dwell rotary indexers
Displacement diagram of both 1-dwell indexer and 2-dwell indexer
The 2-dwell indexer divides displacement into 2 halves. The first half take half indexing angle (bm/2) to rotate the output shaft to next station. The dwell angle is also divided by 2. It has 2 times displacement compared with the displacement of the 1-dwell cam as can be seen in the red line on the above chart.

In this post, we use indexers with following parameters for comparison.
They both have ...
  • Number of stops, S = 12 stops. So the displacement (hm) becomes hm = 360/12 = 30 deg.
  • Indexing angle, bm = 210 deg.
  • Same input shaft speed (w)
1-dwell and 2-dwell rotary indexers
1-dwell and 2-dwell rotary indexers with S = 12 and bm = 210 deg.
According to the explanation of the 2-dwell cam motion, the second indexer will perform 2 indexes and 2 dwells per 1 turn of the input shaft. The actual indexing angle will become bm/2 = 210/2 = 105 deg. Therefore, the 2-dwell indexer takes 105 degrees to complete the first index with output shaft displacement (hm) of 30 deg. Then it waits (dwell) until the input shaft angle reaches 180 deg. and it restart the next indexing from 180 deg. to 180 + 105 = 285 deg. After that, it waits until the input shaft complete its turn. Then the next cycle starts...

Here is the animation of how both indexers move.
animated gif: how 1-dwell and 2-dwell rotary indexers move
How 1-dwell indexer and 2-dwell indexer move
Watch the following video for the animation made with Unigraphics NX4 motion simulation.



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