Box Truck U-Bolts Working Loose - Will Lock Washers Help?

[George7941]

On our Freightliner box trucks, the 19 inch U Bolts cannot maintain an adequate preload for more than 3 or 4 years. Not that the nuts are walking out. I use threadlocker on assembly. Here in the rust belt, the nuts become hard to turn after a few months and inpossible to turn after about 20 months unless heated red hot.

The U blots are stressed heavily when the truck box is fully loaded. Mainly the 1/2 in thick tie plates bend and perhaps the bolt stretches. The U bolt is threaded 5/8-18. The flat washer supplied witht he U bolt is not a Belleville washer. One side is perfectly flat while the other side is domed as seen in pic 3. The thickness fo the washer is the same at its ID and OD but the washer is thicker midway between ID and OD.. I don't see that this dome shape helps maintain preload.

Will split lock washers or Belleville washers help maintain preload? Can a split lock washer, flattened at least partly on tightening, maintain its spring tension and spring back to take up any slack that develops? Would Belleville (disc) washers be better?

Buyers Products includes split lock washers with their U bolt kits
https://www.buyersproducts.com/product/square-bend-u-bolt-kit-745

Amazon has Grade 8 split lock washers and they are not expensive
https://www.amazon.ca/gp/product/B000BD8M4M/ref=ox_sc_act_title_1?smid=A3DWYIK6Y9EEQB&psc=1

 

[jhellwig]

If the nuts aren't loosening the bolts are stretching. Lock washers aren't going to fix that.

Bellville washers are typically used in applications where heat cycling can loosen joints.

The "dome" on your flat washers are just a result of the production process. They are punched out of a sheet of metal.

 

[supton]

Can the u bolts go up a size? It sounds like they may be undersized for the application, if they are stretching out.

That said, what's the cost here? Every 3 years, torch them out, replace. If not every 3, then whenever they are found to be loose during inspection.

Question: if you add a lock washer, is there any liability concern about you changing a design that you were not qualified (lettered, professional engineer, whatever) to say could be done? If you make a change to a commercial truck, and a box falls off and kills a busload of nuns, could you be held responsible? [Yes I'm going hyperbolic. Just testing the extremes here.]

 

[oldhp]

U-bolts look too small diameter wise for that length.

 

[oldcodger]

When you replace the ubolts see if you can get self locking nuts. They seem to do better than split lock washers.

 

[George7941]

5/8 dia is the standard size for box U bolts. that is what the box manufacturers use. All of our trucks came with 5/8 dia U bolts when we bought the trucks.

I have been adding extra U bolts to cope with the stress. Typically the box manufacturer uses 5 or 6 U bolts per side.

U-bolts look too small diameter wise for that length.

 

[IndyIan]

So the plates at one end of the u-bolt are bending? Allowing the u-bolt to lose tension and clamping force? Or is someone stretching the u-bolt on installation with too much torque?
I think 1st you have to have contact points that don't deform(double plates up or higher strength steel), then 2nd figure out the tension/stretch you need from the bolts to keep everything tight. So you need the bolt grade/strength and thread pitch, to look up what torque is needed to get the tension you need. Enough to hold everything tight, but not to much to stretch bolts or crush their mounting points.
Maybe the bolts are undersized? Seems bit odd to need to have to be so precise on such a critical item, when there's no penalty for heavier ones? Maybe they just supply cheap lower grade ones for air hauling(boxes of stuff) and your application needs stronger ones?

 

[Lubener]

If there's enough thread , you can try double nutting it.

 

[JimPghPA]

If it were mine I would seriously upsize to a much thicker diameter, and be sure there is enough thread so I could double nut each. Tighten the second while stopping the first from turning.

 

[Exhaustgases]

Loctite and jam nuts, agree with others here studs are too skinny.

 

[Trav]

Try Nord-Lock washers, if they ever loosen again then it definitely stretch or something is bending. Nord-Lock wedge washers are the only type of lock washer that actually works.

https://www.amazon.com/Pair-16mm-Vi...-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

 

[shortyb]

Have you contacted the box manufacturer about this? Seems like this may happen to other customers and their engineering dept. should have a solution.

@supton, busload of nuns LOL. Thats so funny because I use the exact same example when giving classes in driver training about careless response. Always gets a laugh and the comment about what I have against Catholics .

 

[ka9mnx]

Contact the manufacturer like was said. What's the length of the box? If I remember correctly, it's been a long time, u-bolts are usually spaced about four feet. If no response from the man, add more bolts to relieve stress from the existing bolts.

 

[eljefino]

Use a paint stick or similar to match mark the nut against the threads, then make it part of the pre-trip inspection. I bet like others say you aren't actually loosening the fastener but stretching the u-bolt or deforming something else.

 

[George7941]

One of the trucks we bought new, with a 24 foot box, had a sticker from the box manufacturer, stating that the torque on the U bolts should be checked after a year. It had the 5/8 in dia U bolts. The truck frame consists of two C channels, with cross members. The sticker specified 90 ft lbs on the web side of the C channel and 60 ft lbs on the open side. Here is where it gets interesting - the tie bar begins to bend when I reach 50 ft lbs on the open side and I have to stop.

I think the U bolt is getting loose because the tie bar is bending. I have seen tie bars made out of channels and not flat stock like the ones we have. I am going to check with some suppliers and see what is available.

At the very least, if nothing else, I will try using two tie bars as some one had recommended earlier, and see how that works out.

 

[George7941]

I just checked with the truck spring shop who supplies me with the U bolts. He said that it is normal for the tie plates to bend and that is when you stop tightening. Also they require periodic tightening. He said, being in the rust belt,they require replacement every few years.

 

[tnt_motorsports]

You are dealing with a soft joint here. The tie plate is deflecting leading to a loss of clamp load and loose nuts. Either get in the habit of replacing them every couple of years or look to fix the joint.

You could consider a long bushing you slide over the u-bolt prior to putting the tie plate and nut on. When you tighten the nut the joint becomes hard because the tie plate contacts the bushing. The length of the bushing will need to be pretty accurate to ensure that the total joint has enough clamp load to keep the box on the frame.

Just my $0.02

 

[Exhaustgases]

I forgot about the tie bar bending deal, and how no one in manufacturing knows much anymore. I'm guessing pretty thin tie bars to match the skinny U bolts? If they were 5/8 or more thick they should not bend. Then you have to worry about the thin frame I suppose.
I'd want to make internal supports that attach to hopefully existing holes in the frame to keep the frame from crushing.
If they would have done it correct in the first place there would be no issues. What made now in the automotive world doesn't have issues?

 

[paulo57509]

In general, split lock washers are just feel good devices. They do not help maintain fastener tightness.

Fastener tightness is achieved by creating stress in the threads. Split washers flatten out way before thread stress is reached; they essentially become flat washers at this point.

There are much better methods of thread locking available. Yeah, I use split lock washers if there's not enough clearance for a flat washer.

 

[George7941]

Split washers flatten out way before thread stress is reached; they essentially become flat washers at this point.

Yes, I tested that out. Bought some Hillman Grade 8 split lock washers and they flattened out at less than 15 ft lbs. Stacked eight of them and the stack flattened out at about 50 ft lbs, before my desired torque of 60 ft lbs.

Gave up on the lock washers and just used flat washers. I applied anti seize so that I can retighten as necessary for a few years until it seizes in place from rust. Planning on checking torque every six months.