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V. Ryan © 2001  
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Below is a question regarding 'compound gears'. Gears C and B represent a compound gear as they appear 'fixed' together. When drawn with a compass they have the same centre. Two gears 'fixed' together in this way rotate together and at the same RPM. When answering a question like this split it into two parts. Treat gears A and B as one question AND C and D as the second part. 

This is an example of a “compound gear train”. Gear A rotates in a clockwise direction at 30 revs/min. What is the output in revs/min at D and what is the direction of rotation ? 



First find revs/min at B;
B is smaller therefore it rotates faster and revs/min increase. C is fixed to B and therefore, rotates at the same speed. 90 REVS/MIN at C Next find revs/min at D D is smaller than C, therefore rotates faster (increased revs/min). A revolves in a clockwise direction, B is therefore anticlockwise, C is fixed to B and is also anticlockwise, which means D revolves in a clockwise direction. 

Try the following question:  

