Typical Force on front LBJ Stud...

[entx]

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...

 

[entx]

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...

 

[entx]

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?

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[entx]

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...

 

[BobM3]

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.

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[entx]

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...

 

[GregLocock]

"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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[entx]

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...

 

[BobM3]

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.

 

[entx]

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...

 

[BobM3]

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.