Gears

Hey im currently working on the designs for more gearing system. I keep hearing people say ratios like 8:1. What does that exactly mean? does it mean there is 8 gear cogs? or does it mean the first cog is 8 times bigger or smaller than the second one?

8:1 is a ratio. You're closest with your second guess. The first gear (the driver) is 8x smaller than the second one (the driven gear).

It means 8 full rotations of the first gear make for a single full rotation of the large gear. The result is 8x less speed but 8x more torque on the big gear's shaft.

It so happens that 8:1 is a lot to pack into a single set of gears (you'd need a very small first gear and a very big second), so you would usually do it with a "2 stage" reduction. For example, a 4:1 first stage and a 2:1 second stage (where the first big gear is fixed to the second stage's small one), to achieve the final total ratio of 8:1.

You will grasp it soon enough, very simple once it clicks!

thanks man all makes sense now

one last question I assume the way u make that ratio is by number of teeth

Number of teeth for gears yes. Diameters/circumference is for smooth pulleys.

When you are designing the gearbox, make sure you pay attention to the gears sizes and Pitch Circle Diameter (PCD) and you know what Module, or MOD, of gear you are using. (Google those terms of you need to, they are fairly easy to get your head around)

Linked below is a typical data sheet for gears, and it tells you the MOD, PCD and the Outer Diameter, as well as width and bore size.

http://www.hpcgears.com/pdf_c33/23.50-23.55.pdf

When I started designing my first gearbox it drove me crazy making sure I was using the PCD not the OD or that there was sufficient material to put in the bore I wanted to. Also, don't go with anything less than 12 teeth on a gear. While you can get 9T, 10T and 11T gears they are not considered good practice and tend to be expensive specialist parts depending on who you buy them from.

PS: You may have known all this, but given how much of a pain I found my first gear box, there is some advice from one gearbox noob to anther :-)

well im planning on going scrapyard diving for parts. so im planning on grabbing a load of gears n just mess around with em until it all makes perfect sense to me.

and yes I will have to google some of those terms :P

Thats probably not going to work too well.

Gears arent expensive, just buy them and you'll have a much happier time

no I mean to practice with. il buy proper ones for my bot. but as its my first time gearing im just going to mess about and make sure I know how they work properly. would hate to buy gears n find out ive

Eh, I've scrapyard dived for gears before; the spur gears for Richie's lifter mechanism came from an old motorbike gearbox I found at the local scrap heap.. They're tough and already weight saved, and they work fine :L

Just buy a cheap Lego Technic set that has some gearing involved. I'd wager that it's how a lot of people who are interested in engineering got introduced to gears :P.