pic: Team 3647 West Coast Drive

[Mk.32]

Someone asked for a photo of the bearing block, here it is.
Two 500 Hex Bearings press fit into 1/4 alum plates.
Bolt holes are pocketed so it slides about .25inch to allow for tensioning.

[JamesCH95]

Originally Posted by Ninja_Bait

I looked it up: Pop Rivets vs. Bolts

These are two kind of arbitrary sources, but it shows that while rivets are not quite as strong, they're comparable. The steel 1/4" rivet (shear: 2750 lbs.) beats Grade 2 1/4" bolts (shear:~2200 lbs.). However, the higher grades outmatch the rivets by a lot.

I assume they did a straight shear-strength calculation, which is good, but doesn't describe the whole situation.

Friction matters a lot, especially when clamping aluminum to aluminum. The static coefficient of friction is generally over 1, sometimes well over. You'd be looking at (to use my own numbers) 75%*1750lbf (proof load GR2 1/4-20) * 1.2 (coefficient of friction Al-Al) = 1575lb, so it would require 1575 lb of force to overcome the friction generated by the bolt's clamping force before the bolt sees a single pound of shear force, assuming the bolt was properly torqued.

Edit: I looked at the bearing strength of aluminum, assuming 6061 T6 it is 56ksi. Doing the math on an 1/4" diameter hole in an 1/8" thick plate says that anything over 1750lbf will start to yield the material around the fastener, so the excess strength of the rivet over a grade 2 bolt is meaningless because that will not be the mode of failure. The bolt's clamping force now makes it a clear winner.

[craigboez]

Originally Posted by Mk.32

Someone asked for a photo of the bearing block, here it is.
Two 500 Hex Bearings press fit into 1/4 alum plates.
Bolt holes are pocketed so it slides about .25inch to allow for tensioning.

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Neat design. How do you keep the two plates aligned with each other and the shaft perpendicular to the frame?

[Mk.32]

With 2 bolts and a shaft going though the blocks, they will be aligned when the bolts are tightened. I have seen this type of design before and it seems to work pretty well.

[AdamHeard]

Originally Posted by Mk.32

With 2 bolts and a shaft going though the blocks, they will be aligned when the bolts are tightened. I have seen this type of design before and it seems to work pretty well.

I've got my doubts about the two separate plates being able to adequately support the shaft. It would probably work, but it's going to put unnecessary excess load on the bearings and possibly deflect more.

The bearings shouldn't count on the shaft for alignment.

I know 254, 1538, 968 and our bearing blocks all are complete and rigid assemblies.

[Chris is me]

Originally Posted by AdamHeard

I've got my doubts about the two separate plates being able to adequately support the shaft. It would probably work, but it's going to put unnecessary excess load on the bearings and possibly deflect more.

We take a weight penalty by doing this, but this is why we just drill through a 1/8" extrusion and mount our bearings in a stationary manner. Precision milling multiple solid parts like the Poofs do and making them concentric is harder for us with our resources, so maybe it would be easier to do that and add floating tensioners or idlers.

To make up for the inefficiency of not necessarily being able to get a perfect chain tension, we switched to belts. This line of thought ultimately led to how we decided to make our "west coast" (east coast?) drive.

[AdamHeard]

Originally Posted by Chris is me

We take a weight penalty by doing this, but this is why we just drill through a 1/8" extrusion and mount our bearings in a stationary manner. Precision milling multiple solid parts like the Poofs do and making them concentric is harder for us with our resources, so maybe it would be easier to do that and add floating tensioners or idlers.

To make up for the inefficiency of not necessarily being able to get a perfect chain tension, we switched to belts. This line of thought ultimately led to how we decided to make our "west coast" (east coast?) drive.

The assumption that the bearing blocks must be difficult to make is flawed. They can be dlne really simple and easy.

[Mr. Van]

Originally Posted by AdamHeard

The assumption that the bearing blocks must be difficult to make is flawed. They can be dlne really simple and easy.

If I recall, this bearing block assembly has been used by numerous teams (254, 4, 968, 1868, 60, and probably others) for many years - going back to perhaps 2004 or earlier. There must be complete drawings of the part available somewhere...

We have developed an alternative (which may be heavier, I don't know). In the past, we've taken two bearing plates and milled a grove in them above and below the bearing on the "inside". A top and bottom plate was bolted into this groove making a "box" that surrounds the chassis 1" x 2". This keeps everything square. We actually make this "oversized" to allow for adjusting the heights of the wheels as well as their fore/aft position.

- Mr. Van
Coach, Robodox

[Andrew Schreiber]

Originally Posted by AdamHeard

The assumption that the bearing blocks must be difficult to make is flawed. They can be done really simple and easy.

How? < legit question

[Marc S.]

Originally Posted by Andrew Schreiber

How? < legit question

I don't want to sound like a smart @ss, but a bearing block is just that, a block. Add a 1.125" hole through the middle(for the bearings), a couple of threaded holes to hold it all together and you basically have a bearing block. We add a .125" chamfer to fit in the filleted slot, and make it .010" under the width of the frame tubing to make sure it clamps. Check out our cad and you'll see what i'm talking about. Our bearing blocks can be machined on a standard mill no problem.
This is also how 60, 254,968, 1538, and many other WCD's are setup.

[Mk.32]

Newb question but hat does the block add over the plates I used?
Isn't the bearings taking all the weight either way?

[AdamHeard]

Originally Posted by Marc S.

I don't want to sound like a smart @ss, but a bearing block is just that, a block. Add a 1.125" hole through the middle(for the bearings), a couple of threaded holes to hold it all together and you basically have a bearing block. We add a .125" chamfer to fit in the filleted slot, and make it .010" under the width of the frame tubing to make sure it clamps. Check out our cad and you'll see what i'm talking about. Our bearing blocks can be machined on a standard mill no problem.
This is also how 60, 254,968, 1538, and many other WCD's are setup.

254, etc.... use a much different design that is similar in functionality, but different in execution.

Our blocks are pretty much a block with the bearing hole for both bearings. This makes each face plate effectively a large, special shaped washer.

[AdamHeard]

Originally Posted by Mk.32

Newb question but hat does the block add over the plates I used?
Isn't the bearings taking all the weight either way?

There isn't anything holding the bearings concentric and in plane with each other very well. This could cause the bearings to get loaded nonradially and become less efficient.

It also could be a negligable amount.

Check out one of our bases to see an example of our block, which is very simple to make.

[Madison]

Our experience has been that separate plates on either side of the rail on this type of drive do not work well. The tension in the chain tends to pull the axles at each end toward the center, creating toe on the wheels.

I'd argue that the bearing block design is the most important (and most overlooked) element in this sort of design. Plates alone probably won't cut it.

[Mk.32]

Well taking the feed back from above, here is take 2.



Literally a bearing block. With a pocket cut into the rail that allows it to slide about .25inch for tensioning. And two 1/8 plates that hold it in place.