Posted by Ken in CO on Thursday, September 27, 2001 at 2:04PM :
In Reply to: Re: If I remember correctly... posted by Dan Shockley on Wednesday, September 26, 2001 at 5:44PM :
Dan is right in that it doesn't matter the difference in tooth count between side and spider gears. This is due to the translation of the spider gear in same direction of the side gear as it rotates. The amount of rotation(translation of the spider gear) of the carrier is always midpoint(average) between the rotation of the two side gears. Thus when one side gear is stationary, the rotation of the carrier is half of the rotation of the side gear that is rotating.
Hopefully this will explain it:
- an 18 tooth side gear,
- an 18 tooth spider
thus a tooth occupies 10 degrees, a 'valley' occupies 10 degrees.
Starting with a side gear tooth(1) in the vertical, 0 degree position, rotate the side gear 20 degrees, so that the next tooth(2) is in the vertical position.
Now, with the carrier held static(trans in gear, both tires off ground), the opposite side gear will have moved 20 degrees in the opposite direction of rotation. We've all seen that happen right? And the spider gear will have rotated 20 degrees in relation to the side gear, while it's centerline will have translated 0 degrees(the average of 20 and -20).
Now, with the opposite side gear held static(the tire is on the ground, trans in neutral) it gets a little more complicated due to the translation in the same direction of the side gear rotation of the carrier(thus translating the centerline of the spider). So now the spider gear is spinning with the side gear but it's also moving with the carrier. Repeat the above process turning the side gear 20 degrees. This time the spider will rotate 10 degrees,and translate 10 degrees. The side gear tooth(2) that is now vertical is not engaged fully with the spider gear because the spider translated 10 degrees in the same direction as the 20 degrees of side gear rotation. So, note the numbers? That's where you get the 50% difference in rotation figure between driveshaft and wheel when turning the assembly with one tire on the ground.
And this factor works in opposite 'appearance', rotate the wheel while counting the drive shaft rotation and you'll think you have ultra low gearing, turn the driveshaft while watching the wheel and you'll think your gearing is a little anemic.
Ken
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