Calculating Knurled Inteference Fits

[BlaineW]

My company has various shaft/hub press fits of varying diameters where the shaft is knurled.

Shaft has a bore through it, steel, in some cases induction hardened.

Hub is a powdered metal.

In some cases the hubs occasionally crack, and in some other cases, the shaft bore closes down and needs to be honed to size.

I've been trying to find some way of showing the relationshipo of the knurled press fit dimensions to torque to try and optimize the designs and see if we can avoid the two above conditions. I'm familiar with the press fit design but I get stuck when knurling comes into the picture.

If anyone can point me to some theory behind how a knurl will effect a press fit so I can create a excel forumala sheet I would greatly appreciate it.

Thanks

 

[Cockroach]

Your process begs the question, why are you knurling? Are you looking for the knurled pattern to increase hold capacity or are you building up the shaft OD?

Hubs cracking are a classical problem with over sized interference fits. My suspicion is that knurling the shaft OD is unnecessarily over sizing it for the Hub. You cannot control the knurling process so that the final OD of the shaft is uncontrolled. What I mean is that you either knurl or you don't, the process natually imparts compressive forces to the surface resulting in an uncontrolled increase in diameter.

The interference fit is typically 0.0005/0.0010 inches on diameter, only in extreme cases would it be 0.002 inches. Then the force required to press fit the Hub to the Shaft dramatically increase, so damage to the Hub is a real possibility here. An option would be heat shrink the Hub to the Shaft, this would remove the necessity to press the pieces together.

You want to model the knurling process? Not likely. Why don't you simply QA the shaft OD and then apply the Von Mises-Hencky model? Alternatively, go to an online website dedicated to Interference Fits and run their calculator to see your press and holding forces, torque resistance to the mating part.

But I would never knurl the shaft in a press fit UNLESS the machinist over cut the diameter and I was desperate to build it back up to a required diameter.

I mean in my humble opinion, and about thirty years experience.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[MikeHalloran]

Cockroach is right; knurling is not a controlled process.

There is a limit to the torque capacity of a simple press fit, round shaft in round hole.

If you need more than that, the next step up is not knurling, it's rolling a spline on the shaft. The tooling is different, more expensive, faster, and produces controlled geometry. You'll probably have to roll the splines while the shaft is solid and bore the center out later; spline rollers apply large radial forces during the process.

With a powder-metal hub, you get the internal spline almost for free; just subsitute a splined core for a round one.



Mike Halloran
Pembroke Pines, FL, USA

 

[CorBlimeyLimey]

Knurling for a press fit ???

You would do better with an undersized shaft and a Loctite adhesive.

http://www.loctite.com/int_henkel/loctite_us/index.cfm?&pageid=19&layout=3

 

[Cockroach]

Yeah Cor, I've seen a knurl being used to build up a shaft size when it has mistakenly been over turned. We're talking a few thousands here, not a deep cut across the OD of the shaft. It saves making a brand new piece and scrapping what would otherwise make for an expensive mistake. Obviously it is used out of despiration!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[CorBlimeyLimey]

It's one thing to use a knurl to increase a diameter in 'desperate' circumstances (I've even used centre punch marks); but quite another to attempt to use it as a production feature. As I stated before, one of the Loctite Retaining Compounds would more likely be better.