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Mechanics question concerning rails. Rack and pinion type.

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fuzzybabybunny

fuzzybabybunny

Moderator<br>Digital & Video Cameras
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In this rail system the rack (strip of teeth) is on one side of the sliding plate:

http://i15.ebayimg.com/03/i/001/2b/88/6326_1_b.JPG

In other rail systems the rack is down the center of the sliding plate.

Are there any usability implications to having the teeth strip and the gear that propels it being offset vs. centered? Would the one that's centered have a smoother action?

I want to say no, because the gear + teeth don't seem like they'd produce any kind of directional forces other than back and forth, but would this still be the case if helical gears are used? From the wiki article it says that with helical gears, a resultant thrust is encountered along the axis of the gear... but perhaps this extra force isn't a big deal with a simple camera rail, or is it?

http://en.wikipedia.org/wiki/Gear#Helical_gears
 
It's called "Rack and pinion"
The tooth strip is the "rack"

helical will put a force on the side, but since you're talking about almost no load, and almost no use, it doesn't matter.
I suppose with a real load, you'd want some bearings in the rail, and no slop, to keep things straight.
 
Is this for your panorama machine? I wouldn't worry about the side loads of helical gears, the loads are small and the gears are not going to be moved for long periods of time or at high speeds. I'm not sure about the position of the rack on the sliding plate though, but intuitively it seems like it wouldn't be too important.
 
Originally posted by: GodlessAstronomer
Is this for your panorama machine? I wouldn't worry about the side loads of helical gears, the loads are small and the gears are not going to be moved for long periods of time or at high speeds. I'm not sure about the position of the rack on the sliding plate though, but intuitively it seems like it wouldn't be too important.
Click to expand...

Yeah, it's for the modular camera bracket thing. It has posed some interesting design problems for me, but it's fun! But I often wish that I could just go poof and have a prototype in my hands so that I could actually feel how it moves and stuff. Like if I could just have all the parts on hand and build it like Legos. Designing all this in CAD leaves a lot of questions in the air that I'm forced to just make intuition judgment calls on. But I think my intuition is generally pretty good.

Just intuitively, I *think* there will be a difference in feel between a center rack and a off center rack. No matter the physics on paper, the real world is not a perfect system and there will be slop in the system (in fact, there must be slop for the plate to slide back and forth and for a locking mechanism as well). The answer I'm having a hard time deducing is how much of a difference in feel will there be? I want to say also that there won't be a lot as well... but was wondering if other people had opinions.

I'm having to weigh a number of factors: I want the system to be lightweight and have a slim profile. I also want it to have simple geometry for easy machining. The reason I ask about the center vs. off center positioning is because I want to do an off center positioning of the rack so that the center region can be cut out for guide posts and/or a locking mechanism.
 
If you're that concerned - put a rack on both sides. still have the center clear, and it will stay aligned for sure..

more $$ though.
 
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