pic: Bearing block idea
 

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cxcad,

This tensioning method relies on the clamping force (essentially friction) to hold the bearing block in place. Under load there is a high likelyhood of the bearing block slipping and relieving the tension on the chain. I would highly recommend looking into some sort of locking CAM mechanism to hold the block in place after it is tensioned.

There are many methods of tensioning drive systems in FRC that are very similar to this that you might be able to use for inspiration (look at some of the West Coast Drive white papers)

Looks like a good start,
-Dan

Re: pic: Bearing block idea
 

I like the idea, but like Dan said this isn't too reliable of a method. What makes this any better than the standard WCD?

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We used something similar for our shooter to eliminate the use of half links and such...not sure if it'd hold up for a drivetrain

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WCD is far from "standardized". There are 100's of variants.

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We've been doing this for six years now and have had no issues with it. It's simple to machine and light weight.
http://i.imgur.com/i7UTytH.png

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Do you have bolts going on both on the top and bottom?
I designed this for it to be easy to machine and honestly I really don't understand how 254's system works. Can anyone shed light on that?

Re: pic: Bearing block idea
 

There are no screws on the bottom to tension it. We are using 1x1.5 tubing though, if you are planning on using sheet metal I would mount it with screws on both sides. http://i.imgur.com/kGi1xuv.png

Unfortunately I can't comment on how 254 tensions their belts. (Do they use belts?)

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A true WCD does have standards it generally follows. There are variants inspired by the West Coast Drive, but they don't follow the same categorization.

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254 doesn't use belts on their drive. 25 chain is what they've run since 2004.

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That's what I had thought but I haven't seen their robot in person this year so I was unsure. thanks :)

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Standards imply widely-used best practices are usually published in some coherent form or another. Maybe there are traditions or recommendations for manufacturing capability, yet I haven't read anything about a standard WCD.

(You poked a badger with a spoon... ;))

1885 used friction to tension our live-axle wheel blocks through a 2x1 rail in 2013. It worked very well through 3 official 2 offseason events.

Re: pic: Bearing block idea
 

I don't quite understand the use of friction to tension. Can you provide an image?

Re: pic: Bearing block idea
 

Do something on the upper and lower surfaces of the block to increase friction. Likewise for the inside surface of the channel so that squeezing with your locking screw will have more effect. groove patterns of some sort like the "teeth" on a zip-tie.

Re: pic: Bearing block idea
 

This picture should help. The way I understand it, the bearing block is being pulled to the left by the left by the chain (not installed) until it contacts the side of the cam, as the cam it turned counter clockwise it pushes the bearing block to the right, tensioning the chain.

Very interesting that in practice the friction of 4 bolt heads on the drive rail is enough to keep tension where theory says thats probably a bad idea. I will keep this in mind, thank you.

I guess now is a good a time as any to post what MOE uses. This is the first year we ventured into the realm of cantilevered drives after the unmitigated disaster that was mecanum in 2013 (If I called this a WCD someone might shoot me :p ).
We wanted a belt tensioner that did not rely on friction and we came up with a few ideas. (My favorite was a roller spool that we could change the diameter of to increase tension.) Ultimately we used what is shown in these pictures. There is a curved slot in the drive rail that the shoulder bolt can move through. As you tighten the bolt sticking out of the top of the section of U channel it raises the shoulder bolt and the roller on it pushing the belt up and tightening it. The countersunk 1/4-20 is the pivot point.

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We use belts in our Drive Train and use exact center to center distances. That allows us to skip the tensioner all together. Although in the real world exact C to C is not recommended it is not a problem for FRC robots. We have been using this style of drive train for the last three seasons and we have never had a belt fail or slip. I highly recommend it to all teams.

If you are going to use belts, I would suggest using a 5mm HDT belt that is 15mm wide. We typically use 24 tooth pulleys, you might be able to go a bit smaller, but I cannot say that we have tried it. The WCP center to center calculator works excellent. http://www.wcproducts.net/how-to-belts/

Before you machine any parts, order your belts and pulleys. If they are out of stock you will be kicking yourself in the end, we know from experience.

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This was the first year we dipped into the realm of a custom drivebase with chain spacing in mind! We calculated the center to center spacing needed between the wheels then sent the plates off to be machined. We had a fall back plan which was inspired by something 2168 did which was to take round delrin and drill and off-center hole in the middle and you turn the spacer to tension as needed.

We assembled both bases in January and in that time they have gone through a lot of testing and about 30 matches without any significant stretch in the chain. When we assembled we used brand new chains so they were tight and since have worn in as they should developing a tiny bit to slack but it is still tight.

Our plan is if they become an issue later on in the season we will just replace all of the chains with brand new stuff and dry run the base for a few minutes to help break them in quickly.

I would highly recommend you look into something like this as it can save a lot of time and hassle over designing bearing block tensioners. It works great for our team because we have limited access to milling machines (either CNC or manual) so a simple plate drive with c-c wheels was far easier for us and saves some tiny bit of weight. IIRC team 359 used to put sliding bearing blocks in their drivebases up until around 2010 when they calculated c-c and never needed to adjust the tension that year and have since gotten rid of them completely.

It has been nice not having to worry about the chains for once!