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Typical Force on front LBJ Stud...

Typical Force on front LBJ Stud...

Typical Force on front LBJ Stud...

(OP)
Hey...I'm just trying to find out what some of you guys see as a range for the static forces on the lower ball joint stud...

Here's what I have...  the static loaded weight if each front wheel is 1320 lbf.  Using a 3/4/5 rule for y/x/z.  I have 3960 lbf(x-axis), 5280 lbf (y-axis), and 6600 lbf (z-axis) at the tire contact patch.

Now that I have this I set up some FBD to calculate the reaction forces at the lower ball joint to determine the loads each of the lower arms carry...  This is also done with various steering angle to determine the max loads each of the arm would see whether in tension or compression...

So based on the geometry of the suspension, I have calculated the following loads carried by each of the links that make up the lower control arm for the max loaded condition...  8512 lbf(Tension) and 13519 lbf(compression),  these loads don't raise any red flags, but when I do analysis on the ball joint studs...I am getting ridiculous amount of strain in the material...

For example...the outer lower ball joint I am receiving  0.0229 in/in of strain which based on typical steel MOE of 30E6 this comes to 687 ksi...that's out of this world...and this is based on the factory ball joint design...So either my loads are assumed to be way out there, or there is something that I am overlooking...

So that is why I'm wanting to see what you guys have seen as typical design loads that the lower ball joints sees, which based on the above loads of 8512/13519 is a resultant load of 15976 lbf...

RE: Typical Force on front LBJ Stud...

>> Using a 3/4/5 rule for y/x/z. <<

That's where you lost me.  What rule?



 

Mike Halloran
Pembroke Pines, FL, USA

RE: Typical Force on front LBJ Stud...

(OP)
3G's in the lateral direction
4G's in the longitudinal direction
5G's in the normal direction...

so for the a corner weight of 1320lbs...this give the forces of the x,y,z axis as a multiple of each...

RE: Typical Force on front LBJ Stud...

Doesn't the tire's friction coefficient limit two of those forces?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Typical Force on front LBJ Stud...

(OP)
Yes it would limit the longitudinal and lateral forces, but I'm using this to help build into the design some added SF that would otherwise account for impact loads, fatigue...etc.  Besides, steel is allot cheaper the lawyers...lol  Not all of us can afford a dynamic simulation software.  So I have to do it old school...  which means I need to build in a bit of uncertainty in the designed loads..  Think of it this way...after all said and done it comes out to a SF of about 4 give or take....so there is quite a bit of margin in there...

My next option is to attach some strain gages to the ball joint stud...and measure the max strain....but I'm hoping before I do all that...that there would be someone that has done enough analysis on suspensions to remember some of the typical loads the LBJ saw for a typical car with a GVW of 4500lbs produced...

RE: Typical Force on front LBJ Stud...

Hi entx

Why don't you upload your FBD then we can see better what your problems are.

desertfox

RE: Typical Force on front LBJ Stud...

I know exactly where the 3/4/5 comes from, and you are right, it accounts for real world impact loads. Non aero racing cars are usually designed to 1/2/3.

The exact loads are very dependent on your suspension architecture and geometry.

Sounds to me like you are probably on the right track, albeit a tad on the high side. What size ball joint stud are you looking at?

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

I also think you're in the ballpark but a bit high.  It's been years, but I used to strain gage ball joints, calibrate them in the lab and then collect road data.  Pot hole and curb strikes were the most severe events.  

I'd gage the joints to pick up bending from lateral and longitudinal loads. The calibrations were done on multiaxial load simulators where I could put known (measured by calibrated load cells) loads into the vehicle suspension and measure the output of the strain gaged joints.  Lateral calibration loads were input to the suspension at the static loaded tire radius.  Longitudinal calibration loads were applied at the spindle centerline.

As I recall, peak loads were between 2000 lbf and 3000 lbf for the pot holes and curb strikes.  Of course, those are highly dynamic events so the static measurements used for the calibration process aren't really valid.  All I could really say was that a during a highly dynamic event a strain was produced on the joint that equalled the strain produced by a static known load applied to the wheel adapter in the lab.

RE: Typical Force on front LBJ Stud...

And here was me wondering what Pythagoras's theorem had to do with ball joint loads.

I am trying to envisage a strain gauge attached to a working ball joint stud.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: Typical Force on front LBJ Stud...

(OP)
Thanks for the responses guys....

First thing...after looking over my calcs over and over again...I may have not carried the correct sign convention on one of my results, so this hopefully will reduce my estimated loads...also I didn't really account for the z-axis loads...and after some thoughts on this and a rough calculations with this...it appears that the z-axis will further help reduce the load on the lower ball joint by counteracting some of the loads induced from the x and y axis, but I won't know this for sure until I start over with some fresh paper and a clean slate....so I need to clear my desk of all the clutter so I can start over...haha

Once I do this I'll post them on here to get y'alls input...we can post pdf's here can't we?  I see the link to post on engineering.com, but not sure what formats we can post...


But you know it's funny...BobM3 mentioned he was measuring the dynamic loads to be in the range of 2000-3000lbf as when I was calculating the stresses based on assumed loads and got the way out there loads...I looked at the ball joint itself and worked from there and it wasn't until I was working in the 3500lbf range that the stresses started to become a bit more reasonable with about 180ksi if I recall....so I know I have to be way off...I just need to re-think my methodology...

Also this is in the future....but I will be making some new studs for the ball joints...and for me to get the yields strength needed, or at least I'm assuming I will need once I do the load calculations...I may be using 4340 QT to get the 200ksi range...but my question is can you easily localize heat treat a part...I assume there is some compound that can be applied to insulate area of the part to not receive heat treat....I don't want to heat treat the whole part, just the area that is highly stressed so I can retain ductility in the reaming part...I'm not sure if this would be done by induction heat treat or just in an oven...  I may just need to ask this question in the materials group section...

GregLocock

The ball joint's diameter where the highest stresses are is Ø.654", which is the point right below the tapered fit of the steering knuckle....and the distance to this point from the load center is about .8"

Patprimmer

It's not hard to attach strain gages to a ball joint...  you just won't get much life out of it....but its very doable...

RE: Typical Force on front LBJ Stud...

There's a good factor of safety on the ball joints.  Before gauging them we'd machine flats on them (the studs) to help locate the gages, drill several radial holes and drill a hole along the axis of the stud.  The radial holes not only allowed us to route the wires, they served as stress concentrations for the gages.  In my time at the company there were dozens of ball joints modified like this and I never saw one break.  We were hard on the cars during commissioning of the rigs.  The suspensions saw hundreds of pot hole strikes on the simulators.

RE: Typical Force on front LBJ Stud...

Bill did you mean to type 2000 and 3000 lbf? That seems very low to me.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

Greg -

You mean Bob?  It's been over 10 years and it's quite possible I'm not recalling the numbers too well.  The ball joints  did make good load transducers though (much better than control arms) so they must of had a decent amount of strain.

Entx -  I believe that the large lateral loads that occur during pot hole strikes are the result of the moment developed due to the the offset from the vertical input at the tire and the jounce bumper hit.  Calculate what lateral loads you would develop at the ball joints when you force the suspension up into the jounce bumper and create 30-40 g's of vertical acceleration.

 

RE: Typical Force on front LBJ Stud...

Sorry Bob, yes I meant you.

A 1.125" ball joint with a 5/8" thread is designed for a typical service load of 7364 lbf, I'd have no qualms about exceeding that if I could guarantee that it was sensibly loaded, for occasional impacts.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

Just thinking about it I suggest you reverse engineer some other designs, a 5/8 thread sounds marginal for a heavy car like you are proposing. Big hint.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

(OP)
This morning...I redid all the calculations...and as it turns out I did in fact drop a negative sign somewhere in the mixed which skewed my final answer...and also this time I accounted for the side load that the z axis introduces, even though is relatively small compared to the x and y forces..it still accounts for a small bit...

This time I am coming up with about 6242 lbf...  just want to check and see if this is in the range from what you guys see for your calculations...now remember, this figure accounts for the SF that I mentioned in the first post, which if I just used the loaded wheel weight for all three loads, about 1320 lbf...then the 6242 lbf would then workout to 1560 lbf...based on an average 4x SF that's built in already...

This is at least a bit more reasonable...though when I put this force, 6242 lbf, into the stock ball joint stud...the max strain is about .009 in/in which again based on typical steel MoE 30e6 this comes to about 270ksi for max bending stress...which is a bit more reasonable as we can have materials to this level...

Does anyone out there know what material is typically speced for ball joints...not sure if there is a common material speced for this or if it is speced for the designed loads...I may get PMI and tensile testing to see what the original material is so I can feel a bit more confident in my hand calculations and my assumptions on my loads...

RE: Typical Force on front LBJ Stud...

(OP)
Bob,

when you mentioned the 30-40 G's I assume you would mean that don't account for any longitudinal or lateral loads...right?  Only account for the vertical load...right?  Also is this what you guys saw for the momentary strain, when you guys measured them??  That's alot higher than I would have ever looked at...

RE: Typical Force on front LBJ Stud...

I suggest you look at the Aurora rod-end catalog, it gives you thread sizes ball sizes and max radial loads. And I really suggest you check what size rod ends people use in similar applications. Big hint repeated.

I don't understand your comment about safety factors, 3/4/5 is based on measured loads, it has NO factor of safety (apart from a bit of rounding up) in it at all.

I'm a little dubious that a lower ball joint would see a max force less than that applied at the CP.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

(OP)
Greg,

Is there some resource from ball joint manufacturers which has the intended loads for given ball joints???  I tried looking at some ball joint manufacturers that I can think of...Moog, TRW and lemforder are the only one that come to mind...

Also this may be a bit of a dumb question...cause I have never been able to find info on ball joints...but the size of the ball joint...is that typically referred to as the diameter of the ball???  If so these are then 1-1/16" that I am reversing...

RE: Typical Force on front LBJ Stud...

(OP)
I use Aurora already...but they are just rod ends and spherical bearings...but as a matter of fact...I am using their .75" spherical bearing for this application staked into my own housing...but what is hanging me up is the stress on the attached bolt...that is where my concern is...

What I'm doing is replacing the outer lower ball joint with the above bearing and using a pin, that hopefully will still use the taper fit, so I am not drilling out the taper for a thru bolt...and this is where I'm am at a stopping point until I can feel confident with the loads I have...  this new arrangement will allow me to drop the bearing in relation to the spindle to restore the factory layout when the car is dropped...without this..the camber gain is lost on compression...

RE: Typical Force on front LBJ Stud...

(OP)
I guess in my thinking...I look it as SF as under normal use...the lateral loads are not going to be as high at 3G as the tires will slip well before you see 3g and the same for longitudinal of 4G's...so I have always looked at as SF's, for those instance of impact loads...obviously not smashing into a curb, but rough roads, potholes...etc..

If this doesn't account for a SF, what do you typically see as the added SF...the typical 2 or what?

RE: Typical Force on front LBJ Stud...

A 3/4" ball is completely underspec for this application unless you are racing and replacing it after each race.

A safety factor is a factor of ignorance, 3/4/5 is based on real road events, not once in a car's lifetime things like square edged pothole, as such there is no SF built in. That is to say in normal driving your car WILL see those loads, so you had better not be breaking steel bits at those loads.

An additional factor over and above the maximum measured loads is required for fatigue.  




 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

(OP)
The 3/4" bearing has a static load rating of 53,770lbs and Aurora will tell you this is suitable for dynamic loads of 13,443 lbs, way more than needed.  

Like I mentioned taking the original Ball joint stud and running calculations on it with the taper constrained and applying a load on the ball of 6250 lbf produces a stress of 270ksi...  So there is no way the actual loads are up there like you mentioned...they have to be much lower...and the only way to determine that is attach strain gages to measure them in real world situations....the 2000-3000 lb loads as BobM3 mentioned seem more realistic for actual loads...as I have previously mentioned that for the factory ball joint to have any reasonable margin of failure it has to have typical loads in the 2000 lbf range and lower...this was in no way taking what BobM3 said and applied it to mine...because you can't do that as the suspension geometry will alter the loads..this was reversing the factory ball joint and calculating bending stresses for various load until the stresses reach reasonable values..

Greg, where did you get this load rating that you mention...who's the manufacturer...

"A 1.125" ball joint with a 5/8" thread is designed for a typical service load of 7364 lbf"

I'm trying to figure out if you are referring to a ball joint or a rod end or maybe a spherical bearing...  Cause I see you mentioning Aurora and they do not make ball joints such as an OEM ball joint which is different than what either Aurora or NHB manufactures...

RE: Typical Force on front LBJ Stud...

Greg -

2000-3000 lbf lateral at the tire CP would be about 4000-6000 lbf at the ball joint stud.  I probably didn't explain our calibration procedure very well.  We refered all loads to the static load at the tire patch.

The 3/4/5 is based on body accels, isn't it?  If each tire hit a pot hole and generated equal lateral inward loads, the body wouldn't accelerate would it?  

I think any part that is safety critical would have a significant factor of safety applied to it.

RE: Typical Force on front LBJ Stud...

I find it hard to believe that Aurora make a 3/4 dia ball with a 5/8 thread. That would leave 1/16 of an inch of material for an axial pull.

Bob - 3/4/5 is based on measured wheel force transducer data, not accelerations.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

(OP)
Greg,

Aurora P/N HAB-12T  it's page 61 of their catalog...

I figured we are not talking about the same thing....I'm not sure where you got 3/4"dia and 5/8 thread combined, because there is no such thing...  a size 12(3/4") would at min. have 3/4 threads or the heavy duty ones would have 7/8 threads...beside this type of product would be a rod end/hemi joint.  What I was trying to get was load ratings on ball joints, which is not the same as a rod end or spherical bearing...  but I can only assume you mis-understood the dimension that I gave previously of Ø.654", which was the diameter of the ball joints stud...a completely different product from what Aurora manufacturers..  If you have ever worked on your cars front end you will see ball joints, which is what connects the LCA to the steering knuckle....it's got the rubber boot sealing it off from the elements...also they are used for tie-rods...

But I did finally get the technical data from Moog, after talking with one of the engineers...though I'm still waiting to here back from TRW...

RE: Typical Force on front LBJ Stud...

"If you have ever worked on your cars front end "

Mate I quite possibly DESIGNED your car's front end.

You wrote

".75" spherical bearing "

"3/4" bearing "

"The ball joint's diameter where the highest stresses are is Ø.654", which is the point right below the tapered fit of the steering knuckle"

So I assumed you had a 5/8 thread on a 3/4 dia ball.

HAB 12T has a ball diameter of 1.375" and should be adequate for your intended function.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Typical Force on front LBJ Stud...

(OP)
haha...I figured there was some type of confusion....a picture is worth a thousand words...then there would have been no confusion...

The comments referring to 3/4" was for the spherical bearing that I am replacing the ball joint with, but for rod ends and spherical bearings they are sized by the bore not the bearing diameter...

I reversed the ball joint that I am replacing to target in on what the max permissible loads are till it fails, at that point I can at least see what the ceiling limit is for loads on the ball joint...and with the lower ball joint load of 6250 lbf that I calculated...  The factory ball joint stud is seeing 270ksi...that's pretty high...so any additional load on the ball joint, then you start running out of materials to use...because the OE manufacturers, would not be using exotic material to mass produce their ball joint with....  Most likely it's a forged unit, well it is forged, machined and then induction hardened on the region that is loaded..that's why I mentioned getting it PMI tested and load test to know what I have to work with...and make sure my assumption are correct...

RE: Typical Force on front LBJ Stud...

entx -

The 30 to 40 g's was vertical acceleration measured during pot hole strikes.  The accelerometers were usually located on the lower control arm as outboard as possible or on the steering knuckle.  The loading is so dynamic during that event.  I can't say for sure if the strains on lateral and longitudinal gages on the ball joints were caused by external loads applied to the tire/wheel or if they were caused by internal reactions due to the vertical loads.  At the time I was convinced most of the lateral strain was caused by the vertical loads and accelerations.

I wish I had kept all the calculations when I was working on designing the transducers.  I don't remember the actual strain values corresponding to the mesured outputs (volts or my "calibrated static loads").  I remember using half bridges to pick up the bending of the stud (one half bridge oriented to sense lateral loads the other oriented to pick up longitudinal loads).  I also drilled holes to act as strain concentrations to help with the output of the gages.  I do remember that I didn't need anywhere near as much amplifier gain on the ball joints as on other gaged components (control arms, stearing knuckles) to get good signal/noise outputs for lateral and longitudinal loading.  

I remember being impressed with the design of automotive ball joints.  You could expose them to 100s of pot hole strikes during a 6 week prove out of the road simulator and not see any evidence of yielding (even with the stress concentration holes) but they had enough stress/strain to make a good transdcucer.

RE: Typical Force on front LBJ Stud...

(OP)
Thanks Bob...

I was a bit curious how the drilled holes affected the measured strain...but I see you what you did now...

Let me ask you this...did you have to drill the studs, because you found the strain gages were not providing any information so you drilled them out for concentration points.  I know you mentioned you milled flats to mount them and then drilled holes radially and one axial hole, I assume to communicate with the radial drilled holes to route the wires up and out of the ball joint...

I am taking the ball joint studs today to get PMI tested as well as hardness testing at various point through the cross section to see where the parts was post heat treated...the neck above the ball and below the taper was heat treated, so I want to see by how much...and then tensile testing....  I want to know what the material is and what the properties are...as I've mentioned based on the factory ball joint, with all the combined loads from all three vectors this ball joint stud is seeing about 270ksi of stress with a load of 6250....So I need to know what material I am working with...because I have never heard or seen a ball joint break from normal use, they wear out, but never break...unless of course you slam into a curb or something...I really hope the the material properties are at least up here or greater...because if not...I will have to ask myself...how is this ball joint lasting so long...

But yes...I will be putting strain gages on a test car to measure the strain for this part...

RE: Typical Force on front LBJ Stud...

We drilled the studs right from the beginning.  It had been done before (my time in the company) so we felt comfortable continuing to do it.  The signals were so nice that we probably didn't need the stress concentration factors.  But the holes also were very handy for getting the wires out.

 

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