Parts to build a linear slide

[GladiatorsCoach]

Hello, I am sure this belongs somewhere else and has been asked a thousand times and for that I apologize, but my team and I are looking for some help in building a linear slide. Up to now we have been using the Tetrix linear slide kit with decent results, but we are also convinced there are better options. We have purchased 14 inch drawer slides from Home Depot and have a basic understanding of how to string them and power them, but where we are lost is what hardware to use. Does anyone have any suggestions on wire/cable/string that works best? Also, I am assuming that we want v-groove bearings as the point of contact on the slide. What suggestions can anyone offer to size or brand?

[3805Mentor]

I've seen lots of different solutions. Our team has used 200 lb 0.75 mm Spectra (also marketed as polyethylene PE) braided fishing line, and 2 mm Dyneema Robline.
We have Rev Robotics pulleys that have been used with the 15 mm Rev extrusion linear kit. I just now saw that Rev also offers 1.2 mm cord UHMWPE.

http://www.revrobotics.com/rev-41-1368/

Dyneema, Spectra, PE, UHMWPE are all different names for the same stronger than steel for its weight rope.

[FTC11411]

We use OpenBuilds parts for the linear slide components. The pulleys we use are the metal V wheel kits ( http://openbuildspartstore.com/metal-v-wheel-kit/ ). For string, we use paracord from Michaels ( http://www.michaels.com/parachute-co...M10323232.html ) which has worked out well.

[3805Mentor]

We use OpenBuilds parts for the linear slide components. The pulleys we use are the metal V wheel kits ( http://openbuildspartstore.com/metal-v-wheel-kit/ ). For string, we use paracord from Michaels ( http://www.michaels.com/parachute-co...M10323232.html ) which has worked out well.

You probably are not putting much load on the string, so the following doesn't apply for many cases. Paracord stretches. Our team ran into problems with the Rev linear slide binding when they tried to use regular string from a hardware store which stretched allowing the slides to advance unevenly. The only spec I could find for Paracord 95 was that it has 100 lb breaking strength and 30% stretch. I couldn't find at what load. From OutdoorBunker - " This cord also stretches up to 30% for comfort when worn"
Companies like Samson seem to indicate normal working loads at around 10% of breaking strength. The Robline I posted has specs of 0.2% stretch at 20% of breaking strength, breaking strength 880 lbs.
For a string 18" long, the Robline with 180 lbs of load will stretch 0.04". Assuming the 30% stretch for the Paracord is at 20% breaking strength, 18" long with 20 lbs of load will stretch 5".

If stretch isn't a concern for your application, or maybe you want stretch, no problem. Stick with Spectra if you don't want stretch.

Can anyone tell my other hobby is sailing?

[3805Mentor]

So I've been stewing about do these numbers make sense. Shock cord stretches about 50%. I found a site that says paracord 95 is a braided outer layer nylon with a single straight nylon core. Looking at Samson a similar nylon construction stretches around 3% to 5%. So that would be 1/2 to 1" of stretch. Not sure whether to trust the advertised spec or not. Still significant stretch on something you don't want changing lengths.

[gof]

We're using Kevlar string from McMaster. Thin enough to easily roll over the REV v-groove pulleys already mentioned above. Holds up very well to abuse.
https://www.mcmaster.com/#8800k43/=1bniv16

Even with string with no stretch, you should make some sort of pulley caps to keep the string from falling off. We also use a spring in the system to ensure constant tension.

[GladiatorsCoach]

Thanks to all that commented! I think we have figured it out! Now we have another question.... we want to incorporate VEX 393 motors as servos and understand that they are legal. The question is if the VEX square shafts and gears are legal?

[5294-jjkd]

Yes they are, we've used them for several years in various mechanisms.