Sure, will do it over the weekend and report back.
Thanks, while the spreadsheet works even if it’s numbers are wrong (comparing
known-good configs with what you are considering), if it is giving significantly
incorrect results, it would be good to figure out why and fix it.
Also It may be worth mentioning that the actual pull of the cord (9lbs) or
spring is half (4.5lbs) because it does not pull directly on the chain but
from the pulleys.
isn’t it that the pull on the chain is half (so 8 pounds of force from the
spring/weight/cord becomes 4 pounds of pull on the chain)
Yes, if the spreadsheet is factoring in all the variables (which i’m sure it does) then the error should be on the human factor.
Correct, 9lbs pull on the cord/spring becomes 4.5 on the chain.
Off the top of my head i know the sled should be pulling from both chains more than its own weight at any position on the board - will check the spreadsheet over the weekend to check exact numbers.
Maybe custom ordering springs could be a good idea, they are not expensive when ordered in large quantities.
Yes, if the spreadsheet is factoring in all the variables (which i’m sure it does) then the error should be on the human factor.
I’m less confident when the expected numbers are so far off from the measured
values
Off the top of my head i know the sled should be pulling from both chains more than its own weight at any position on the board - will check the spreadsheet over the weekend to check exact numbers.
Yes, the spreadsheet should show this
Maybe custom ordering springs could be a good idea, they are not expensive when ordered in large quantities.
unless you go with constant force springs (pricy and complex to use), you still
have the problem that springs provide more force as they are
stretched/compressed from their resting length, so they:
provide the least tension when you need the most (large amount of slack, sled
near the motor, high tension on the sled side of the motor so you can have high
tension on the slack side too)
provide the most tension when you need the least (small amount of slack, sled
far from the motor, low tension on the sled side of the motor so you don’t want
high tension on the slack side)
People have done weights with a bunch of pulleys to redirect the lines and
weights around things. You don’t have to get that fancy, simple pipe sweeps
(large radius plastic pipe, check the electrical section of your hardware store)
work quite well, and while the weights don’t provide different tension in
different places, they at least don’t add force at the wrong time.
I think the differences between the two are due to unrealistic values entered into the spreadsheet
4lbs of tension at the bottom corner is with the router at the very tip the board, unless someone is trying to skim off 0.125 out of a 4x8 plywood edge, this scenario is very unlikely to happen (and not possible without a custom skirt around it) However, it’s still technically possible (with skirt) so let’s look at the numbers
motorSep = 119.20
motorHt = 17.00
workWidth = 96.00
workHeight = 48.00
sledWeight = 22
Chain tension = 4.24
Chain tension measured at the sprocket (sled side) = 5.20lbs
Even in this unlikely situation, the chain tension at the sled side is still higher than the cord/spring side, so no possible backlash issues (and sled tension only goes up from this point on).
Knowing the cord/spring provides enough tension for the slack side without adding backlash, I think it is the best practical approach. It makes for an easier installation (no need for extra parts) and cleaner looking setup.
a thought I just had, the spreadsheet doesn’t consider the weight of the chain
between the motor and the sled, could you use a string (or similar lightweight
thing) and measure the force at the sled?
There are two things that we’ve been combineing here
the force available to move the sled towards the outside edge
the force that should not be exceeded on the slack to avoid backlash issues.
for the second, the weight of the chain is in the right direction and adds to
the force from the sled’s weight (and should probably be measured at the motor,
not the sled for best accuracy)
for the first, the force of the weight of the chain is in the wrong direction
and therefor subtracts from the force from the sled’s weight.
The difference between the tension at the sled (4.24) and at the sprocket (5.20) is just that. Before testing i thought it would be 3/4 chains or .8lbs but is more than that, almost all the chain is feed out (just ~9in at the slack side)
The spreadsheet does a good job on estimating chain tension, but as aforesaid it does not account for chain weight and sprocket radius
