Process for producing a strong "fastener" 2

[sm96785]

Hello, first post.

I have a question about a part I am designing which involves some knowledge about material selection, heat treating and coating application. To be honest I am not very familiar with such metallurgical processes to be able to confidently call it out on my own.

The part that I am designing is used in place of an OEM lower ball joint of an automotive spindle. In place of the original ball joint I am basically bolting a spherical bearing to the spindle via the use of a custom tapered stud. The part will have tensile loads on it as the shock is mounted to the lower control arm and will want to pull the stud out of the spindle every time a bump in the road is encountered. It will also see a shearing load during cornering as the grip of the tires to the road will be trying to shear the joint, although I am not as concerned about shearing forces, moreso about the tensile fatigue type of failure. This is used on the rear of the vehicle, which has 60% of the weight on the rear, which for our purpose would be estimated at 1200lbs or 600 lbs static load per wheel. Part will not be subjected to any extreme environments, at worst it will see some water (not salt water), operating temps ranging from 50-110F. So now you have a rough idea of the application.

Attached is a screenshot that shows the joint cross section. The tapered stud is torqued 1st with the upper nut and then the lower castle nut is "snugged" and cotter pin inserted. The OD of the shaft has a slight interference fit with the ID of the spherical bearing.

So my question comes down to this. To achieve a stud that will not fail in this application, what are the materials and heat treating processes and corrosion protection that would be appropriate given that I only need two made (meaning some operations are not cost effective, unfortunately I will probably not be able to have rolled threads)?

So far I am considering the following:
Material: 4130, 4140, 8620
Heat treating prior to machining: ?
Max surface roughness callout: 32micro inches
Heat treating (quenching/tempering) after machining: ?
Case Hardening: is that a bad combination for cut threads (stress risers)
Desired Rc Harndness of final part: 30-35?
Coating: Zinc (concerns for hydro embrittlement?), Phosphate, Others?

Any suggestions or thoughts would be greatly appreciated.
Thanks

 

[desertfox]

A couple of questions why can you not have rolled threads and whilst selecting a high strength material for the bolt what about the strength of the Aluminium casting, little point in pre-loading a high strength bolt if that pre-load causes the alumium in the joint you're proposing to exceed its yield stress.

 

[Tmoose]

To reduce the risk of the cylindrical part of the stud being subjected to cyclic bending loads and fretting I would want to tighten the castle nut hard. But the clamping force will be in opposition to the upper nut's (likely much smaller) clamping force. A shoulder could be added to the stud for the castle to clamp the "sphere" against, but it would have to end well before the taper enters the control arm, and the result would be the tapered portion would be subjected to the cyclic bending.

I have the feeling "protecting" the stud and control arm taper from bending may be why you chose the arrangement you did.

 

[sm96785]

desertfox - Although I am yet to find out, my assumption is that having parts with rolled threads requires a minimum lot size in the hundreds. I wouldnt be worried about overstressing the aluminum in the tapered portion, as the is what this joint is originally designed for. But yes if I tighten the larger castle nut too much, it could defeat the tapered fit, so I was planning on not tightening the castle nut that much. So that leaves the potential for fretting to occur between the spherical bearing and aluminum, and/or a bending stress in the 3/4" section, but I would prefer it to be bending there rather than in the smaller tapered portion. So it is apparent that this isn't the most robust joint design, but it's currently what I am up against at the moment and trying to make-do.

Tmoose- Correct, I want to keep the taper portions locked together.

After a bit more reading it seems like I am heading down the path of 4140 steel hardened and tempered to about 35 Rc. Any thoughts on a coating? I would prefer something that wont add to the dimension of the part.

Thanks for the discussion.

 

[sm96785]

I thought I would share this article I found about phosphate coatings used on cyclic stressed test specimens.

http://www.dtic.mil/dtic/tr/fulltext/u2/a230871.pdf

Since the nature of phosphate coating is to etch the metal (porus, stress risers), it decrease the fatigue life by a measurable amount. So it seems like I won't go that direction. I think I am going to lean towards a standard zinc coating and make sure they can bake out the hydrogen. Although this will add thickness to my part, I will just have to take it into account.

Let me know if you have any other thoughts. Thanks

 

[tbuelna]

The tapered fit between the stud and spindle is the correct approach, as long as the upper nut is properly torqued. The taper fit removes any clearance and prevents fretting at this interface. Unfortunately, the same is not the case for the fit between the spherical ball bore and stud. There also appears to be a potential issue with the limited contact area between the spherical ball end face and the aluminum spindle. A steel hat bushing in the aluminum spindle would be worth considering here.

 

[CoryPad]

4140 steel at 33-39 HRC would be typical for fasteners, although studs like these sometimes have a carburized or induction hardened case to improve bending fatigue resistance. Parts like these usually are formed rather than machined, but I guess your quantities are small. You can roll threads with an attachment on a lathe which is suitable for small quantities. A good coating would be Magni 565. This coating is used widely for these parts, outperforms zinc electroplating, is compatible with aluminium, and prevents hydrogen embrittlement.

 

[desertfox]

Hi

There are many materials and processes that can be used on fasteners to improve fatigue life, as others have posted.
For me until you understand the loads, stresses and frequency of cyclic loading its meaningless, for example when preloading the tapered fastener to some pre determined figure and at which this figure causes the joint (aluminium in this case) material to yield, then the bolt no longer retains the initial preload you required.
Further the correct preload is the key to avoiding fatigue in bolted joints see this article for reference:-

http://books.google.co.uk/books?id=...eload for bolts under fatigue loading&f=false

 

[tbuelna]

..The part will have tensile loads on it as the shock is mounted to the lower control arm and will want to pull the stud out of the spindle every time a bump in the road is encountered...

I was re-reading the OP and this sentence caught my eye. If the vertical loads being transferred thru the spherical bearing are indeed high, then the axial load capacity of the spherical bearing itself may be a bigger concern than shear in the stud. Most spherical bearings only have an axial capacity of around 15-20% of radial capacity. So you might want take a second look at your spherical bearing sizing.

Lastly, one change I would suggest to the design of the stud is to add a shoulder for the upper face of the spherical bearing ball to bear against. This will provide a steel-to-steel contact at this limited area interface, and it will also allow the spherical bearing ball to be tightly clamped without affecting the axial preload at the tapered stud interface.

Hope that helps.
Terry