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Wasteaway

What are these rears rated for?

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I have a dilemma. 

I purchased an RD688s with the belief that it has 53,000# rears. This number seemed odd to me and yes, shame on me for not making damn sure before pulling the trigger. I did call a local dealer and without any hints, he stated 53Ks, among other things such as the engine, rear axle ratio (4.42) and front axle rating (20K) which I can confirm accurate. This info was from the vin (Mo57706). After getting the truck home from some 600 miles I can find no evidence of 53K rears. I do have the build sheet on the truck which does have some discrepancies, it shows Mack 38000 - SB38 with a PID code of 268-1003. The truck was purchased new through McNeilus as a mixer and two lift axles were installed at the time.

Now things get fuzzy. The dealer stated he could not determine the axle rating because of the Hendrickson Walking Beam suspension. Hendrickson lists the HN462 as 46,000# springs which is what the build sheet shows. So it seems the truck has 38k pound rears over 46k# springs. Here is the big question........are the Mack SB38s the same axles (axle housings....wall thickness, etc) as the Mack 44,000 pound rears? Is it the same axle with the springs determining the capacity or are the axle housings different on the heavier axles? I can live with 38s if they are the same as the 44s (design and strength) with the Hendrickson suspension but I am going to have a tough time with 38k axles if they are pretty light duty. I would think, as a mixer, this truck saw some pretty heavy loads off road every day and I am certain the axles are original as I have every service record from day one.

 

Sorry for the long winded post

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Easiest way is to send the VIN to the MACK Museum with a few bucks and request the build sheet. It will take a bit of time but you'll get the fulls specs on it.   Paul

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my experience with the 38k rears is they are about bullet proof, you may break a axle shaft but that's it, we dump trucked with them with very good success, wheel hopping in sand would blow an axle but never failed a housing or drop in.  

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The matter of difference between 38 and 44K axle housings was discussed on here a couple of times in the distant past.

I myself is still "in a smoke" on that though.

The general idea was 44's are cast banjo's and 38's are stamped steel/fabricated ones.

I have them both setups in my junk but can't determine sharp having no knowledge on decoding the marks.

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I copied spec sheets for you. They do not give housing thickness, but they do show minor differences, so they are not the same. Don't know where that 53 number came from. From what I've seen, unless you need certification for some reason, or if you are going to do some extreme heavy haul, I don't see a problem. (Now for the fine print, just my humble opinion, and you know what an opinion is worth ---- whatever you paid for it) 

doc075.pdf

doc076.pdf

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never heard of a 53. most older style 44 and 38 had the cone hub cap which means it had a''full foating'' axle shaft. (the axle floated between the hub cap and the hogs head) back in the day 38'swere indestructible. most noticeable difference between 38 and 44 housings is the 44's have a flat ''web' welded on the butt welded flange between the top and bottom half for added strength.

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I have seen sums freaky arrangements with springs tronion  greasable  with hollow  tube or solid with rubber I had a 93 RB it had 44 springs spread with 5 lug Dayton hubs factory 77-DM 6-lug cone axle small 38k springs  but they all break at some point they all have 1 1/4 u-bolts & 7/8 spring cap bolts ive seen both break and also hollow truioun tubes snap off.

Edited by blackdog2

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Thanks for the replies. Thanks Dave for the files....good stuff. I can't help but wonder why the 38s would have larger bearings than the 44s. I'm sure I will pour over this info at a later time. At least you guys reassure me that these rears aren't weak by any or most standards. As an owner/operator I seldom have any truck issues, much less any abuse related ones. Looks like a lift axle will be in order for my application.

Quote

 

 

 

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SB38 is 11mm fab steel housing approved to 38 k GAWR (40K GAWR for low mileage Municipality applications) with HMX, HN type bar-pin suspensions. The axle housings are sturdy and tested at much heavier loads but the cracks will start at the bar pin bracket weld-seams when loaded excessively. When loaded appropriately, at rated capacity, the axle-housings will last 500,000 miles with 99% reliability and much longer at slightly reduced reliability. If you intend to load this at 46k GAWR  (100% of the time) then expect  a failure much faster. I also strongly recommend against doing this as it would be a safety hazard.

Edited by kt_Engineer
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On 12/30/2016 at 0:33 PM, kt_Engineer said:

 

 

Edited by Wasteaway
duplicate post

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57 minutes ago, Wasteaway said:

Exactly the type of thing I want to avoid. Basically, what I got is what I got and I am looking for some reassurance that these 38s are not fragile. IDK where you get your information Engineer, but it is very informative and I thank you. The intended use of this truck is as a septic pumper and as such it obviously is gradually loaded from stop to stop until the last where it is completely filled and then, 99% of the time, immediately trucked to the sewer plant to empty. I WILL be adding a lift axle (this I wanted to avoid) and the rears will only be overloaded "on site" or when backing as the lift axle will be up for those brief times. Estimated weight to the rears is about 45-46000.

I am NOT a official spokes person but all I can tell you is my knowledge on development/testing and manufacturing of MACK axle and Mack T300 transmission is extensive and I can provide technical details like no other. If you are  adding lift axle and overloading rears only "onsite", you should be fine. The creep rating (i.e driving on grades no greater than 3% and at speeds no greater than 5 MPH) for SB38 is 50,000 lbs. You should be fine loading upto 50k lbs on-site.

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What model and year Mack do you have? Is it Camelback? 

Rd688S is 44k rear tandem.

Rd688ST is 38k rear tandem.

Rd688SX is 55k rear tandem.

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take some pics of what you have. that will help see what set up you got and give us a better clue

 

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