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# Gear Center Distance Variation

## Gear Center Distance Variation

(OP)
I would like to use a pair of equal diameter spur gears to connect opposing friction drive rolls ("pinch rolls"). The hardened steel rolls will be about 3 inches in diameter (I can specify this) and the strip will be between .035 and .058 inches thick, therefore the center distance will need to vary by about .030". The rolls will be pinched together with a pneumatic cylinder and linear guide. One roll (gear) will be driven by a (gear)motor with the other roll driven by the first one. Will a standard stock spur gears (such as those available from Boston Gear or others) work in this application? The drive and load direction will always be in one direction, and the gears will be open with grease lube. Should I go with the largest diametral pitch? Should the load factor be de-rated? The speed is very low - less than 60 rpm. I understand that the contact ratio should not be allowed to go below 1.2 - but how do I calculate that. Thanks in advance.

### RE: Gear Center Distance Variation

Look in MIL-HDBK-400, page 95B for calculating contact ratio. You will have to work backwards from that page to get the inputs to the formula. You should be able to skip the hob related calcs because you are not fabricating the gears.

This: https://www.sdp-si.com/D805/D805_PDFS/Technical/80... has similar formulas.

Larger pitches are more tolerant of variations in center distance, but have lower contact ratios.

### RE: Gear Center Distance Variation

Depending on what you're doing exactly, you may want to avoid shifting the center distance of the gears- it's possible that you can get some sinusoidal speed shift in the output gear as it moves, which you may not want.

There's more than one way to drive a system like this- you can use a roller chain, or you can build a 'swing gear' arrangement which allows the drive and output gears to move without any variation in CD or backlash in the gear train:

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