Rear axle/drum question

Bob from Northport said:
Can anyone advise if there is a way to check whether a rear axle shaft or the drum is worn if the drum hub ?? slides onto the axle too far??
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Not really. Many hubs that I see are worn.

I have an aluminum plug that slips into the bearing area of the hub that when you place your thumb over the hole at the outboard portion of the hub and pull the plug out quickly, it will make a 'pop' if it is in spec. If the 'pop' is not very pronounced, that means the bearing has worn the hub to an oversize which allows the drum to not be in the centerline of the shoes and housing plate. At that point, it is cheaper to replace the hub. Also, if you use a bore scope probe on your cell phone, you can look inside of the hub. Typically if I see scoring which is indicative of an axle key shearing and the axle spinning inside the hub, then the taper of the hub is generally compromised.
 
If the hub fits too loosely on the shaft it will mount too far in and the drum will rub on the shoes or backing plate. This is after putting some torque on the nut. Lots of people, me included, use the spacers available from the parts suppliers, see below. Brent has a method of checking the diameter where the bearing rides which is a different thing. Having the drum rub against the backing plate or shoes is a sure way to know that the taper at the hub and/or axle is worn.

The taper on the hub and axle should be without deep scratches and fit together tightly. The keyways have to be tight on the keys and the keys should be excellent or new. No chips on the corners of the keyways. The torque on the nut should be about 100 foot-pounds and should be checked after about 100 miles and every year thereafter. Do not drive the car with a loose hub.

As far as the drum is concerned, if it is pressed steel, throw it away and get a cast iron one. If it is cast iron, measure the ID and if it is 0.06 over 11 inches, throw it away and get a new one.

https://www.snydersantiqueauto.com/product.htm?pid=978129&cat=41627
 
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Torquing the rear drum onto the axle over 50 lbs-ft will drive the drum too far onto the axle & deform the hub's taper.
 
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Bob Bidonde said:
Torquing the teat drum onto the axle over 50 lbs-ft will drive the drum too far onto the axle & deform the hub's taper.
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I.e. otherwise known as "stretch." And since the hubs are forged steel, the stretch can be considerable, can exceed the yield point of the metal, and can become "permanent."

Which is why they invented "shims."

A better solution might be to choose this time to buy cast iron brake drums MOUNTED on new hubs - which will conform to original dimensions.

Not that you can't stretch these too...

80 years is a long time. Ask anyone who has lived it.

Joe K
 
Ditto JoeK. It's unfortunate that many Model "A" rear hubs have been deformed by torquing the axle nut way too much. There are many publications specifying torques of 100 lbs-ft plus on the basis of the thread size and a SAE chart. They have no idea how high the hoop stress in the hubs gets at these torque levels. I doubt that the factory production torque was anywhere near 100 lbs-ft.
 
“Hoop Stress”. Been out of college a long time. Interesting question.

Having said that. I have been using 100 lbs-ft for ever. Every time I have done any research on the subject. I have always found a range of 100 to 125 lbs of torque.

Is Hoop Stress a real problem in this application. Very interesting approach to question.

Enjoy.
 
One of the solutions mentioned to me in "recycling" a rear axle that the thread had become damaged was to "turn it down to 9/16" and re-thread."

This I did IN PLACE on the car using a grinder and a Little Giant diestock minus the handles. It was a precarious exercise only aided by the fact that grinding using the engine to rotate the axle actually yields a pretty uniform and concentric beginning for the diestock.

Now what do you torque this? I looked at the torque recommendations then available which are more or less repeated today online.

One source: https://www.boltdepot.com/fastener-information/bolts/US-Recommended-Torque.aspx which yields 91ft-lbs for 9/16" fine thread on a grade 5 bolt. My thinking was that the axle was likely better material than today's grade 5 but possibly not to the equal of today's grade 8 (129ft-lbs) So I went to 100 ft-lbs.

That axle continued in service until the entire rear axle was removed and replaced in the current re-work of the car. (When you have no less than four spare axles - you use the best one available.) I think the axle shaft is still around here somewhere...

Meanwhile grade 5 torque for a 5/8" fine thread is 128 ft-lbs and grade 8 is 180 ft-lbs. Either of these I consider too high given this is not a matter like common bolting which relies only on the "stretch" of the tension member (bolt) to do the actual restraint.

WHN above: In the case of the rear axle nut, the "stretch" is a combination of the tensile stretching of the segment of the axle beyond the taper AND movement counter to friction of the hub on its taper which tends to relieve the tension. The two opposing actions find a balance in torquing of the nut.

Another way to look at this. From contact on a bolted joint to full torque tension, it may take two turns to reach the target tension. From contact on a bolted hub to full tension may take FIVE turns to reach the target tension - three of these turns simply "expand" the hub on its taper and find it's "friction point."

The problem and the issue which can lead to stretching the hub - the bolt yield is considerably stronger than the yield of the hub.

Dropping back to 9/16" lowers the bolt yield point appropriately to match the yield point of the hub.

The oversize axle thread a disadvantage AND advantage of Ford "overdesign."

If this were a Volkswagen, the engineers would have cut down the size of the bolt/nut to conserve on materials. Stretch bolts (tensioning to beyond yield point - common to a fault on VW) bring this thought to full application.

Joe K
 
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Red book says 125 foot-pounds. There were no torque specifications when the Model A was manufactured but different wrenches were use for different applications, which in effect supplied the correct torque. The wrench for the rear axle nut was long.

The hub must be tight enough on the axle that it cannot move. If you are going to use 50 foot-pounds, I recommend some Loctite on the hub and axle.
 
nkaminar, I disagree with your view of the hub taper. The taper centers the axle shaft, and the shear key transmits the torque. If you doubt this, drive your car with the keys out and a high torque on the axle nuts. Then you only have friction transmitting the torque, and you will not get far.
 
Bob Bidonde said:
nkaminar, I disagree with your view of the hub taper. The taper centers the axle shaft, and the shear key transmits the torque. If you doubt this, drive your car with the keys out and a high torque on the axle nuts. Then you only have friction transmitting the torque, and you will not get far.
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Bob, not wanting to get between you and Neil, but something that I learned back in the B&S karting days was that when the fit between the tapers of the flywheel and the crankshaft were correct, the engine builders did not use a key. The reason they did not was the racers were using methanol for fuel and needed more ignition advance to take advantage of that fuel, ...which the flywheel key controlled the timing. Because they needed more advance in the timing, the taper of the flywheel and the nut that held the flywheel onto the shaft was all that was used after the flywheel was advanced on the shaft. As I understand it, when two tapers are correctly lapped, then there is no need for a spline or key. Now I agree that most Model-A hubs are worn/damaged/deformed and won't work well without a key installed, however (-at least in theory) it should be able to be driven without a axle key.
 
Bob from Northport said:
Can anyone advise if there is a way to check whether a rear axle shaft or the drum is worn if the drum slides onto the axle too far??
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Having the dimensions of the taper on the axle would help, like, the OD of both ends of the taper and the length of the taper. Can anyone supply these dimensions?
 
I had a west bend 580 on my racing cart but this was before the really fast carts.bi worked in a small engine shop and pulled countless flywheels off tapered shafts. This always required a proper puller and a big hammer. Back in those days I think when you have good mating tapers once the nut is tightened you could remove the nut and throw it away. Good tapers do not come apart on there own. I think there are very few good tapers left on model As today. Also for timing purposes I have seen offset keys.
 
I readily admit that I don’t know a fraction of what some of the earlier posters have to offer but I can tell you this, when I got my model A it had a couple of those shims on both sides. It was working fine when I got it and still is. A friend of mine (a really sharp guy) stripped the threads on the end of one of his axles at about 115 ft/lbs when going for 125. Now I learn from mistakes, I prefer to learn from somebody else’s mistakes. What really impresses me is that he fixed it with JB Weld (seriously)
 
Since I am installing my new drums for the first time, this thread has left me confused. Les’s book says 125 but that seems to be wrong from this thread. So please give me a recommended torque I should use for my new cast iron drums. Thanks
 
A taper fit shaft/hub mate would need no key in a perfect world but a rear axle is subject to more than just torsional loads. It gets thrust loads as well as rapid shock loads. A person should get good results with less than 100 Ft/Lbs of torque on the nut but it requires a recurrent re-torque to insure a tight joint. If it get loose at all then it will start fretting and damage the fit. This is why so many hubs go too deep on torque up. When the drum starts making contact with the backing plate then they have worn too far. If a new hub goes too deep on the old shaft then the shaft is history too.

Most torque applied to the axle by the drive train is taken by the tapered fit but a key helps to stabilize the joint. No lubricant of any kind should be used on a taper joint unless it is of an overkill size that can take much more torque than that applied. The model A axle/hub was not in an overkill design category so they have to be maintained clean and with a torque that won't damage the threads of the axle or it's retaining nut. This stuff isn't getting newer so fatigue from many years of use requires a lot of care in choosing a nut torque on an old 5/8" NFC thread. I'd consider 100 Ft/Lbs to be near max for old fatigued parts like this.

Mel Gross will mate a serviceable hub to a new drum for a good price. He turns the drums after mating them to insure concentricity of the hub/drum. His prices are good too.
 
I've been driving Model A's for 60 some odd years. In high school I learned if you abuse them, you shear axle keys. I use the shims if the axle surface warrant it. I torque to 80 lbs and then align the cotter key. Recheck after a tank of gas.
Dan
 
Bob Bidonde said:
nkaminar, I disagree with your view of the hub taper. The taper centers the axle shaft, and the shear key transmits the torque. If you doubt this, drive your car with the keys out and a high torque on the axle nuts. Then you only have friction transmitting the torque, and you will not get far.
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If you were to try a parallel shaft with only a key to drive. You won't get far.
Keys are usually used in conjunction with a taper when the orientation of the shaft and hub is important. When it is not, a key is rarely used. Have a look at the taper drive on your drill press or lathe - no keyway
All that said, both used together will give a stronger result than either on its own.
 
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