Pillow bearing massive failure

[john.k]

There was one of these things on U tube,the pivot bearing seats were simple U shapes,anyhoo,the 4x4 drove up,over ,came crashing down ,the steel deck flipped over the 4x4 and ended up as a roof rack......there is a lot of 4x4 funnies on this video ,favourite is the swb jeep stopped on a 45deg slope .....everytime he lets out the clutch,the front comes up,he tries to back down,the front comes up when he touches the brakes .......he bails out ,and the jeep runs back ,rolls 100 times ,bits fly off,including the engine.

 

[EPAIII]

Make that "chunk" a cube with the hole somewhat BELOW the center. And run two or three grade 8 or higher cap screws all the way from the TOP on EACH side to secure it. Make the hole a sloppy fit for any misalignment and use a heavy dose of some good grease (molly?).

A chunk of steel same size as the pillow block with a 2-5/8" hole would work fine. Assemble with antiseize.

Anything precise will bind up. No need for bronze or oilite. Plastic would be fine, but why? Steel and be done.

 

[scott-ak]

Dang that is massive overkill. The one I designed and built for our course is mostly 1" x 1/8 wall square tube and sees 6000# rigs all the time. I didn't use a shaft. just a V block and most of the time the frame just sits on the ground. I'll have to look for a picture

 

[Scottl]

I wonder if you could just use a replacement pillow block with a steel strap added to reinforce the top. The strap would have a cutout that would press on the cast iron top of the pillow block. Longer mounting bolts and shim washers so the strap ends would seat about a half turn of the bolts after the strap contacts the top of the pillow block.

That way would keep the spherical feature without elaborate machining on a new steel pillow block.

 

[gbent]

We are told that after the bearing had failed, the shaft would lift up about 3" when the T-T tipped forward and hit the bumpstops.

That sentence gives you the only data point you need to properly redesign the pivot bearing. It tells you how much uplift force the bearing needs to withstand.

 

[tylersteez]

That pillow block just does not have the strength and beefiness for any of that, a cast iron housing with a less than 1/2" (being optimistic) wall thickness being subjected to sharp spikes in load are your issue here.

I'd get two chunks of steel and bore them out for beefy bearings, cut out the stubs that are too misaligned for a tighter bearing fit, run a new one piece bar thru each bearing/bearing block and mount them in place, ensure it spins freely, then attach the drive on section to the bar.

If i was to save this design i would either machine steel blocks out for spherical bearings or just do bronze sleeve bearings if the stubs are accurate enough to not bind up.

 

[Overland]

There is rotation inertia as this tips.
When it hits the stop, sometimes quite violently, this inertia will continue and becomes a force pivoting about the stop and trying to lift the frame, but the top of the bearing is trying to prevent that, -- and it can't.
The argument about beam deflection doesn't work for me. There would need to be massive deflection to cause rotation of the spherical bearing, in the housing, for it to fail.

So, we're going to make a couple of steel pillow block housings, with a bore of 2-9/16".
We'll make them 2-1/2" thick. There is misalignment for sure in the bearings, so hopefully this will be enough clearance - but we can take a die grinder if we need it.

We will use some old tires as suggested to try to absorb some of the energy, but this is a high-end resort location, so aesthetics are apparently a concern. Make of that statement what you will, but don't go public please.

Thanks everyone for sharing your thoughts. I'll share progress.
Bob

 

[michiganbuck]

Overland said:
We are told that after the bearing had failed, the shaft would lift up about 3" when the T-T tipped forward and hit the bumpstops.

That would happen because the ground under the appraise would change, so at times would become a lever to lift in the pivot location. The pivot location has to be way stronger than a calculated number.

 

[Joe Gwinn]

There is rotation inertia as this tips.
When it hits the stop, sometimes quite violently, this inertia will continue and becomes a force pivoting about the stop and trying to lift the frame, but the top of the bearing is trying to prevent that, -- and it can't.
The argument about beam deflection doesn't work for me. There would need to be massive deflection to cause rotation of the spherical bearing, in the housing, for it to fail.

This sounds correct.

So, we're going to make a couple of steel pillow block housings, with a bore of 2-9/16".
We'll make them 2-1/2" thick. There is misalignment for sure in the bearings, so hopefully this will be enough clearance - but we can take a die grinder if we need it.

We will use some old tires as suggested to try to absorb some of the energy, but this is a high-end resort location, so aesthetics are apparently a concern. Make of that statement what you will, but don't go public please.

Can you put sand traps at both ends, so teeter-totter rotation energy is absorbed churning sand?

A steel block with a waxed rock maple bearing sleeve pressed in should work. If the steel block can be installed by sliding over the stub axle and then bolted down, the block and sleeve need not be split.

 

[Garwood]

Dang that is massive overkill. The one I designed and built for our course is mostly 1" x 1/8 wall square tube and sees 6000# rigs all the time. I didn't use a shaft. just a V block and most of the time the frame just sits on the ground. I'll have to look for a pictureView attachment 371118

That looks like good engineering sir. You could have done the truss webs in an equilateral triangle fashion for higher strength, but your job looks great!

Congrats to the engineering team involved in your design!

 

[michiganbuck]

Certainly a lot of design ideas out there.
My boss of years ago told of a whope de doo that was a series of hill that a car could go air born. People got too reckless and some cars rolled over so they were discontinued.

auto teeter-totter for an off-road course - Bing images

www.bing.com

 

[Overland]

I'll tell you a funny story - which could have been a problem.

My son figured 20 feet long would be good.
Than a friend of mine asked the dumb question "how long is the wheelbase of the SUV's ?"

Ah, good question. More then 10 feet and it would start to tip before back wheels were on.
Sometimes you're lucky.

Bob

 

[neilho]

I'm not understanding the two main arguments for failure.

I get that there's the inertia of the trailing teeter-totter as the leading side hits its stop, but isn't there a several ton vehicle on one or both sides of the teeter-totter? That'd tend to hold it down. Not completely, obviously, just tends to complicate the calculations.

The spherical connection running out of rotation? I assembled one last week that could be turned 360 degrees in its bore. It's spherical, ain't it?

Given the distance from the job and the desire to fix it and fix it once, I agree with the block of greased steel approach. Easy to replace, too.

 

[Scottl]

There is rotation inertia as this tips.
When it hits the stop, sometimes quite violently, this inertia will continue and becomes a force pivoting about the stop and trying to lift the frame, but the top of the bearing is trying to prevent that, -- and it can't.
The argument about beam deflection doesn't work for me. There would need to be massive deflection to cause rotation of the spherical bearing, in the housing, for it to fail.

So, we're going to make a couple of steel pillow block housings, with a bore of 2-9/16".
We'll make them 2-1/2" thick. There is misalignment for sure in the bearings, so hopefully this will be enough clearance - but we can take a die grinder if we need it.

We will use some old tires as suggested to try to absorb some of the energy, but this is a high-end resort location, so aesthetics are apparently a concern. Make of that statement what you will, but don't go public please.

Thanks everyone for sharing your thoughts. I'll share progress.
Bob

Perhaps heavy duty dock bumpers might do the job while being more attractive than old tires?

 

[jccaclimber]

I'm not understanding the two main arguments for failure.

I get that there's the inertia of the trailing teeter-totter as the leading side hits its stop, but isn't there a several ton vehicle on one or both sides of the teeter-totter? That'd tend to hold it down. Not completely, obviously, just tends to complicate the calculations.

The spherical connection running out of rotation? I assembled one last week that could be turned 360 degrees in its bore. It's spherical, ain't it?

Given the distance from the job and the desire to fix it and fix it once, I agree with the block of greased steel approach. Easy to replace, too.

Spinning things want to keep spinning. In this case that's mainly the platform
When the front slams into the ground and can't go any deeper that point tries to become the pivot so it can keep rotating.
The vehicle pushing down isn't enough to *instantly* stop the rotation.
I'd bet the vehicle has some temporary increase in front/rear axle loading at that instant in time as well, but at least it's already headed downward and has more suspension to keep the rubber side planted.

Additional food for thought, you can make it easier to balance by lowering the platform relative to the pivot, or harder to balance such as with Scott's example. The OP mentioned that being able to balance at the tipping point was part of the goal.

The OP's comments about engineering sound a lot like "We considered the static case, we did not consider the dynamic case".

I'm going to disagree on principle with "The pivot location has to be way stronger than a calculated number." A little safety factor goes a long way in allowing simpler calculations, but if you're "way off" then that just means the calculations were not done correctly.

 

[72bwhite]

You don’t really need great bearings
get 2 steel blocks and bore a hole for a greased bushing
then add full length stops like that truss style one spread the load
you aren’t dealing with a spinning shaft

 

[Scottl]

Not to derail the thread but last night I saw a different type of teeter-totter used on a program about marine salvage. This one was used to retrieve a vehicle (Jeep pickup) that had fallen through the ice. After a diver found the vehicle the hole in the ice was enlarged and the teeter-totter slid down in front of it. As the Jeep was winched up the weight shifted and it wound up with its wheels on the (14" thick) ice. I always wondered how they got them out without crashing through the ice. The ice was considered still too thin for a tow truck so a small tractor did the positioning of the teeter-totter and winching.

 

[Overland]

Neilho,
The error we made was not considering that when it tipped forward and hit the stop, it would create a significant lifting force at the pivot, due to the rotational inertia.
I believe the reason the SUV doesn't help to resist this force is because the SUV is impacted by these same forces, and tends to lift at the rear, taking weight off the pivot.

I believe the discussion regarding the deflection in the axle/cross-member is not the reason for failure.

Well we've sent a new bearing to the location as a "fix"; and I'm waiting for a CAD file I can send to the steel shop for new bearing carriers.

It occurs to me that I can machine out the new bearing carrier on my manual lathe by machining some facets that approximate the curve of the spherical housing. With some coordinates from a CAD file, my taper turning attachment, and DRO on carriage and crosslide, with some care I think this might work. Obviously the support for the bearing will be somewhat compromised, but if I do narrow facets, and can get some accuracy, it "might could work" !
So I've ordered another bearing so I can measure it and do some drawings. If I put a good surface on the A36 blank I should be able to reference to the "edges" of the facets. The bore will be about 5" diameter, so at least I'll be able to see in there.

Maybe I have too much time on my hands, but this could be an interesting challenge. Now if anyone near Greenville, SC, has a CNC lathe with a 4-jaw chuck......

We'll see how this drama plays out.
Bob

 

[Freedommachine]

Most teeter totters are built like the one Scott-ak posted.

Another alternative is to mount the articulating section using an old mobile home axle. They use #42 spindles which are 2.25" in diameter often found in 7000 lb axles. Best of all, mobile home axles are dirt cheap.

Weld the spring shackles to the articulating section and affix bump stops directly above the axle. Choose the proper spring rate so that a vehicle coming onto the ramp will easily compress the springs - setting the ramp against the bump stops. Move your f350 shocks and mount them to the axle.

When the CG of the vehicle crosses the pivot of the ramp and the whole thing totters, the shocks will absorb the shock load. The leaf springs only act to hold it all together.

Spring rate matters though. It's a teeter totter, not a catapult

You could also weld the mobile home axle tube directly under the pivot point of your ramp but this does nothing to help with the shock load.

You are an engineer trying to work through a problem best solved with hillbilly ingenuity.

 

[Scottl]

Overland,
Instead of machining a sort of spherical bore in the steel I wonder if it would be possible to mill a radius on the outside of a cast iron pillow block and use it as a bushing in a steel replacement. I'm thinking a close fit plus red Loctite might give the best of both worlds.