Question on designing using #35 Chains

[waterbott]

When using websites such as http://www.botlanta.org/converters/d.../sprocket.html, they usually tell you the exact amount of chains or the exact distance that you will hypothetically need between two sprockets. How well does this translate to real life? Should I use the exact amount recommended or should I always give room for error? If so, how much error?

For context, I'm part of the CAD team, but I have little experience with actual robot assembly.

[ebarker]

We use the calculators.

do the 1st pass with "chain length calc"

then do a 2nd pass with the "sprocket center dist calc" using the number of integer number of links (rounded up or down) calculated on the 1st pass.

when you assemble the machine, use a good chain tensionerr to draw the chain together.

and you will be good to go.

[nuclearnerd]

Remember that chains don't wrap around the pitch circle of sprockets, but instead form a polygon. The difference in circumference of these shapes can change the chain tension as the sprockets rotate (especially with small sprockets), so you always have to leave a bit of slack on the chain (but not too much or you'll jump teeth!).

Also note that the chain will grow with use (as the individual bushings wear). If you have a mounting that doesn't let you adjust tension, you may have to replace chains at some point (no idea how often in an FRC context).

Tsubaki has a good write up on page 15 of their chain guide here: http://www.ustsubaki.com/pdf/iandm_guide.pdf

[cxcad]

Hey, I know that team. Just go exact distance and don't bother with tensioners. That's what we did Las year on the drive train and we had no problem

[Munchskull]

If you are planing to run center to center with out tensioners then check out this thread.

To summarize it, all the parts you use have some play in them. compounded that play will add up to the point where you do not have enough tension. to fix this you can add 0.012" to the center to center distance for #35 chain and 0.018" for #25 chain.

[jman4747]

I dug out my 29th edition of Machinery's Handbook and fond that the calculators the OP is utilizing are the equations on page 2553 of said handbook. The calculator isn't for computing an exact distance based on just the pitch but on an old formula for a "tight" chain. You are supposed to use the bottom calculator first then with the decimal output that it gives for "Links" use the top calculator while replacing the decimal value you got for step one with a close integer value. The result of the top calculator is a "true" exact value and you aren't usually supposed to add anything to it.

See attached equation

Edit: Do what Ed said. That's what this calculator was meant for.

[Nate Laverdure]

Note that if the sprockets are same size (most of the time in a drivetrain), the eqn simplifies to something quite manageable without the help of an online calculator.

c = P*(L-N)/2

[marshall]

Originally Posted by jman4747

Do what Ed said.

Yep.

[Jay O'Donnell]

Originally Posted by Nate Laverdure

Note that if the sprockets are same size (most of the time in a drivetrain), the eqn simplifies to something quite manageable without the help of an online calculator.

c = P*(L-N)/2

What are the variables equal to?

[Alan Anderson]

Originally Posted by Jay O'Donnell

What are the variables equal to?

If you can't see the image Josh posted, here's the text of the relevant paragraph:

Center Distance for a Given Chain Length.--When the distance between the driving and driven sprockets can be varied to suit the length of the chain, this center distance for a tight chain may be determined by the following formula, in which c = center-to-center distance in inches; L = chain length in pitches; P = pitch of chain; N = number of teeth in large sprocket; n = number of teeth in small sprocket.

Nate just simplified it for the case where N = n.