Internal thread as sort of a knurl

[rabtrfld]

We have a chronic problem with pressing an axle into a wheel hub.
It goes in with clearance, then gets hydraulically pressed to swell and lock the pin.
But such is the tolerance, that the clearance varies by 5:1. The combination of small pin with large hole doesn't always lock.
And unfortunately the pin must press flush with the surface of the hub, so making the pin longer is not an option. The combination of large pin with small hole would protrude.

So I had the idea of grooving the inside of the hole, but it's only 5/16 in. dia. so boring grooves would be a pain.
But why not thread the hole with a tap? Maintaining the same through bore. It would be relatively cheap and easy, and would provide extra hidden volume inside (not visible to the customer) to "absorb" the extra length of a longer pin.

This is a big customer and I'm putting my neck on the line to suggest it, so I'm asking you experienced folks if it sounds practical.

 

[carlherrnstein]

It better be made to the customers requirements. They are the one paying for it and expect to get what they ask for remember they also pay for the failed parts, that should be built into the price of the good parts.

That being said it's possible suggest changes that make something easier to make but, the customer has to ok it. Think GM ignition switch recall or takata airbag recall.

 

[Erich]

The thread removes a lot of the material resisting the "press" fit. The tops of the threads will crush at much lower forces.
In the end you will have an assembly that is not as tight as you think. It can probably be made to work, but it will require analysis or testing of the extreme limits to figure out what really meets the (usually unstated) requirements.
Either propose the whole effort to the customer OR tighten control of pin diameter or hole diameter. Just because the customer gives you a wide tolerance you don't have to use it all. Run at one end of the diameter spec and you can probably make them work without involving the customer.

 

[CalG]

Use a form tap. No lost material that way. Plan on reaming to size after roll forming.

 

[john.k]

it used to be common to push a splined drift through wide tolerance hubs ,so that a shaft would be a pressfit.......Similar to thread ,but running through ,rather than round and round.,and a lot quicker to do ....tooling a lot more long wearing too.

 

[Milland]

As mentioned a thread or other internal material removal process will likely compromise press hoop stress too much when the tolerances work against you (large wheel bore, large axle bore (threaded, grooved, etc.), small pin).

If you can afford the extra time/process for tapping, you can afford a facing op that cleans off stickout of a pin that can't go in far enough when the tolerances stack to the tight side. This would give you the functional requirement of no excess pin while staying with the customers numbers.

But yes, I'd like to see things tightened up a bit with the tolerance range to reduce or eliminate the variability.

 

[rabtrfld]

I misstated the problem... parts that are in tolerance actually work. The problem is about one part in a thousand that is out of tolerance.

Facing is out of the question. Threading the hubs could be done on the same machine at the same time the hubs are made. Facing would have to happen after the hubs are hardened and ground, assembled into ball bearings, and pressed on a large asymmetric assembly. They would have to be milled in some kind of fixture, even if you could mill flush to a hardened, ground surface. Then the bearings would have to be meticulously cleaned in an extra operation. And the assemblies are not precisely alike after assembly, so presenting the work face accurately to the tool would be a real challenge, And the assembly would have to be turned over to do both sides. Easier to just thread the holes.

remember they also pay for the failed parts, that should be built into the price of the good parts.

Its not that kind of situation, its a roller conveyor chain, and one point of failure sends thousands of feet and pounds of grimy chain back to our plant. This has happened. The cost of a failure is not proportional to the cost of a part. And if we built in ten percent for failures, we wouldn't get ten good feet of chain out the door.

And somebody explain how tightening the tolerance on the print will prevent one part in a thousand from going rogue? Our supplier can't even hold the existing tolerance. Actually, one in a thousand wouldn't be bad, if they were roofing nails or something, and you could just toss the ones with the head on the wrong end . I was actually hoping to fool-proof the design, effectively increasing the tolerance. And without adding another assembly step.

I cant help thinking a grooved bore would hold more positively than a smooth bore. After all, a nut can't slide straight off a shaft, even if it rotates freely.

Well, now I sound like Socrates' wife, as if my only motive is to argue everything you say! Actually very good advice here, much appreciated, I am more prepared for the inevitable meetings. In the end, I don't think we can even afford a simple threading step. The parts are made & assembled overseas & we have no actual control of the manufacturing. And we can't do 100% inspection on the parts before assembly. And we get most of our parts from one supplier, so we can't change that. And the problem has happened before and heads rolled and nothing changed. So I prefer a mechanical solution over diplomacy or specsmanship.

The last time we had this problem, we gave some workers little pry bars from the hardware store and had them hand-pry 100% of the wheels before packing the chain. The force was not calibrated and tended to decrease over the work shift, but that is what we are going to do again.

I think the best advice was the reminder that the customer has to approve changes. As problems increase, customer relations deteriorate, making it more risky to even approach them with changes, so nothing is done, rinse & repeat. Somebody stop me from saying "pry, pry again."

 

[MattiJ]

As mentioned a thread or other internal material removal process will likely compromise press hoop stress too much when the tolerances work against you (large wheel bore, large axle bore (threaded, grooved, etc.), small pin).

If you can afford the extra time/process for tapping, you can afford a facing op that cleans off stickout of a pin that can't go in far enough when the tolerances stack to the tight side. This would give you the functional requirement of no excess pin while staying with the customers numbers.

But yes, I'd like to see things tightened up a bit with the tolerance range to reduce or eliminate the variability.

Dunno about weaker joint.. As I understood this is more like riveted joint than interference press fit. Some grooving could actually improve it by providing ”locking dents”?

 

[sfriedberg]

Other than finding another vendor who can hold the specified tolerance, how about 100% inspection of the incoming parts? Don't do it by hand! Invest in fixturing and automation. Ideally, you'd dump 10 pounds of parts into a hopper, a feed system would supply individual parts in a known orientation to a test fixture, out of tolerance parts get kicked into a reject bucket, good parts get moved on to a known-good bucket.

 

[Milland]

Other than finding another vendor who can hold the specified tolerance, how about 100% inspection of the incoming parts? Don't do it by hand! Invest in fixturing and automation. Ideally, you'd dump 10 pounds of parts into a hopper, a feed system would supply individual parts in a known orientation to a test fixture, out of tolerance parts get kicked into a reject bucket, good parts get moved on to a known-good bucket.

If I understand the follow-up information, some of the assembly is coming from cheap-ass overseas sources, so there's competitive $$ pressures that the OP has to deal with, and it's clear that throwing money at things isn't in the cards.

Given the process stuff, someone like Motion Guru would be a great resource, but we're at a point where it's in the "professional advise" stage where one should be paid for input. Eh, maybe he does freebies...

Given it's late (early) and my brain isn't at full-speed, I'm not sure how the entire process works, but if there's additional assembly at the OP's plant of imported smaller assemblies, I'm thinking some low-level instrumentation may help, like a pressure sensor on a cylinder to do force analysis during insertions that throws an alarm if that 1-1000 part comes along so it's more readily found before stuff goes out the door.

I might try reading the flow path again after I've gotten some sleep, or I might just slink out, whistling softly and hoping nobody noticed I was here...

 

[1953chevB]

We have a chronic problem with pressing an axle into a wheel hub.
It goes in with clearance, then gets hydraulically pressed to swell and lock the pin.
But such is the tolerance, that the clearance varies by 5:1. The combination of small pin with large hole doesn't always lock.
And unfortunately the pin must press flush with the surface of the hub, so making the pin longer is not an option. The combination of large pin with small hole would protrude.

So I had the idea of grooving the inside of the hole, but it's only 5/16 in. dia. so boring grooves would be a pain.
But why not thread the hole with a tap? Maintaining the same through bore. It would be relatively cheap and easy, and would provide extra hidden volume inside (not visible to the customer) to "absorb" the extra length of a longer pin.

This is a big customer and I'm putting my neck on the line to suggest it, so I'm asking you experienced folks if it sounds practical.

Dang sounds like a bad design, pin and axel really.
it is hard to give any advice with out see drawings, and proprietary stuff.
seems with the limited info, the hub should have a an internal spline with a slip fit adjacent dia.
the shaft with external spline with and adjacent diameter for location.
the hub would be easy to broach, the axel (has to be rolled or hobbed) would be tougher and more expensive.
both can be verified with gages, but this change would be more expensive and there is
a budget problem all ready.

 

[dian]

maybe try a valve guide knurl.

 

[boslab]

Suppose an internal knurled is just a left hand thread combined with a right hand, if you want diamond knurled anyway,
Mark

 

[Street]

If you cannot inspect the parts 100% before assy then you will always run the rework risk. especially if you source from Taiwan or china.They will sell you any old stuff.
Chain you will find is only as good as the weakest part that goes into it.
It is common to find rework in chain due to manufacturer not spending money where they should, even though they may be accredited to some fancy standard they use for sales.

That is if you have robust inspection and manage not to let things pass out to the customer, so you can identify rework required on that part.

Sometimes is just a plain bad design.

Loctite fits maybe a option if range of tolerances are ok and you have a tested procedure ie use high strength 680 and chain parts are cleaned and free of oil Also the chain must not have any heat exposure or the loctite will be of no use.
Testing of the final part may also be required to verify it.

If you want to continue with deformation of the pin, maybe recess the end bore a little and shorten the pin some so you have some tolerance for varying pin heights, which will sit below the surface by varying amounts. just a little counterbore

 

[EPAIII]

Loctite?

Permatex?