Quick Question on RodEnds/Sphericals

[Aaron M]

Hi, Apologies if this has been covered before, I tried searching for it but couldnt find an answer:

To avoid rod ends in bending, it is advised to use encapsulated spherical bearings, however does this mean taking a plain spherical bearing and welding the race/housing to the a-arm?

Or will a male/female rod end with a "spherical bearing" housed in the rod end be sufficient?

Many Thanks and Regards
Aaron Mortimer

[Aaron M]

Hi, Apologies if this has been covered before, I tried searching for it but couldnt find an answer:

To avoid rod ends in bending, it is advised to use encapsulated spherical bearings, however does this mean taking a plain spherical bearing and welding the race/housing to the a-arm?

Or will a male/female rod end with a "spherical bearing" housed in the rod end be sufficient?

Many Thanks and Regards
Aaron Mortimer

[Drew Price]

Aaron,

The threaded portion of the threaded rod end is what gets bent. To completely avoid putting threaded rod ends in bending (in the a-arms) you will have to substitute them for spherical bearings mounted in housings.

I will say this once, and in bolded italics:

DO NOT WELD ON THE BEARINGS!

Bearing races are heat treated, and then ground to exact size - welding them directly into a housing will make them both weak, and warped, which will make them not bearings.

To fit the spherical bearings into a housing you will need some sort of mechanical retention, either in the form of:

1) A press-fit, which must be sized according to the bearing manufacturers specs to make it tight enough to retain the bearing, but no so tight as to cause the bearing to bind. Press fits are often used in conjunction with either two snap rings on either side of the bearing to locate and retain it further, or a shoulder on one side and a snap ring on one side.

2) A staked edge in the housing - this is where you leave some extra material around the edge of the sherical bearing's housing which you then crush down onto the housing using a special staking tool that you make or buy.

There is lots of info on this topic, and the bearing manufacturers are good sources of info as well. Aurora bearing has their whole catalogue online, and the end contains info about mountin schemes, and dimensions for housings.

Read these:

Rod End Search (http://fsae.com/eve/forums?a=search&reqWords=rod+ends&use_forum_scope=on&forum_scope=125607348)

Best,
Drew

[fade]

i remember texas a&m ran welded rod ends one year i think 04 or 05

[Aaron M]

Thanks for the reply,

Does this therfore mean that Rod ends (male or female) cannot be used for the outer A-arms? (especailly the lower/upper with respect to push/pullrod location)

Thanks
Aaron Mortimer

[Composites Guy]

Aaron... that is correct. Many teams do it, and provided that the rod end shank (threaded portion) is thick enough you can get away with it. In general though, this is looked down on as bad engineering.

The push/pull rod location would certainly add to the magnitude of bending on an outboard rod end. However, bending exists on the outboard rod ends due to braking acceleration also. Inboard rod ends are generally accepted as ok, because the a-arm rougly approximates two 2-force members (thus the forces are primarily axial along inboard rod ends).

[screwdriver]

Indeed. These rod-ends are only supposed to be loaded with forces in the direction, the threaded bolt part points. If you load it with forces partially or completely normal to that direction, you'll bend the rod-end.
With female rod ends, it's the same situation. Just that not the rod-end part will be bent, but the rod part.

[RStory]

We used to run female rod ends welded into the A Arm tubing. I would advise against this.

[Davo]

I agree with Drew. If you're going to press fit spherical bearings make sure you have a mechanical retention as well. We once had them just press fit in and it doesn't take much to pop them out. Our last minute fix was to find some washers that were the right ID and weld them either side of the bearing to hold them in. Having a shoulder and a circlip is a much better idea.

[flavorPacket]

and staking them is even better than that!

[duckei]

I would definitely recommend pressing spherical bearings into your A-Arms. Research some of the bearings that Aurora makes. Then explore the craftsman in you with a mallet and some tubing... Obviously you'll need some suspension points from your CAD or kinematics program. But it's a start...

[Chuckster]

Bad design?
Gee, Swift has used threaded rod ends in the outer lower BJ position for years and years. And that was on some of their downforce cars too. (DB-3)

If you bore a hole for a spherical bearing, you better be ready to make a new lower A-arm each time you make a large change in Caster or Camber during your suspension testing.
Running out of travel in a spherical bearing is probably the bigger engineering sin.

On the other hand, if you are exploring the limits of material strengths to the exclusion of most other considerations, like ease of chassis development, one would not use a threaded rod end.

[screwdriver]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chuckster:
If you bore a hole for a spherical bearing, you better be ready to make a new lower A-arm each time you make a large change in Caster or Camber during your suspension testing.
Running out of travel in a spherical bearing is probably the bigger engineering sin. </div></BLOCKQUOTE>

Indeed. But then again, how does this engineering sin emerge in the first place? Isn't it due to a bad construction?
I mean you have your suspension calculations, so you know how much you'll have to adjust. Then you construct it in a CAD programme of your choice. In most programmes, you can add kinematics for to your parts. You can easily check out if your mechanism works or if it doesn't.
I think an even bigger engineering sin is not to use the engineering tools properly or ignoring them altogether.

[duckei]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chuckster:
Bad design?
Gee, Swift has used threaded rod ends in the outer lower BJ position for years and years. And that was on some of their downforce cars too. (DB-3)

If you bore a hole for a spherical bearing, you better be ready to make a new lower A-arm each time you make a large change in Caster or Camber during your suspension testing.
Running out of travel in a spherical bearing is probably the bigger engineering sin.

On the other hand, if you are exploring the limits of material strengths to the exclusion of most other considerations, like ease of chassis development, one would not use a threaded rod end. </div></BLOCKQUOTE>

Agreed, but with the little suspension travel we design for, our spherical bearings won't reach their 23 deg misalignment angle. Although the right transient situation may put a spherical bearing in its place, Rod End Spherical bearings are no different than your standard spherical bearing as far as misalignment angles (looking at the high mis-alignment series).

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by screwdriver:
I think an even bigger engineering sin is not to use the engineering tools properly or ignoring them altogether. </div></BLOCKQUOTE>

Agreed. Supposing you do your HW beforehand, you won't have to be remaking A-Arms.

Oh and to Aaron, DON'T WELD THE BEARINGS! I think we can all attest to that.

Respectfully,

-Ian

[Chuckster]

High misalignment series?

You can have NO misalignment at all using a threaded rod end when making large caster changes.

That's the difference-since you can rotate the threaded bearing to center it perfectly in its travel.



I have 30 years of engineering behind me now and calculations and simulations only get you so far.

The ability to adjust the physical apparatus beyond the spot that your sims told you was right is actually how one learns to refine his simulations, understand concepts better, and occasionally make significant breakthroughs.

If a team really needs to save a tiny bit of weight or is trying hard to impress a particular judge, then do a fixed bearing A-arm after you spend part of a season developing the car with fully adjustable suspension.

[enudd83]

Has anyone looked into running the push rod directly onto the upright like the option on the new dallara F3? would eliminate most of the bending issue.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by enudd83:
Has anyone looked into running the push rod directly onto the upright like the option on the new dallara F3? would eliminate most of the bending issue. </div></BLOCKQUOTE>
Only from the vertical forces, not from braking or acceleration.

Regards, Ian

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chuckster:
If a team really needs to save a tiny bit of weight or is trying hard to impress a particular judge, then do a fixed bearing A-arm after you spend part of a season developing the car with fully adjustable suspension. </div></BLOCKQUOTE>
Rod ends in bending is one subject where FSAE competitions transition from pure engineering into politics or possibly a branch of sociology...

Yes, one can build an easier to adjust car with rod ends in bending. No, they won't break if you know your loads accurately. Maybe, the weight penalty is insignificant.

But, FSAE doctrine is that 'no car with rod ends in bending will make the design finals'. Argue all you want...

Personally I think they look heavy, ugly, lazy and there's always the possibility they'll be adjusted incorrectly by uninformed personnel and fail (even if they are calculated to be strong enough).

Regards, Ian

[duckei]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chuckster:
High misalignment series?

You can have NO misalignment at all using a threaded rod end when making large caster changes.
</div></BLOCKQUOTE>

Please explain to me why the Aurora Bearings "HXAM-T & HXAB-T High Misalignment Series Male Rod Ends" can have NO misalignment when they are spec'd for M.A.'s of ~20 deg each. In full droop and bump they WILL be misaligned. Isn't that the point of a spherical bearing?

http://aurora.thomasnet.com/viewitems/ll-products-comme...72|3001110&forward=1 (http://aurora.thomasnet.com/viewitems/ll-products-commercial-male-rod-ends-english-units/am-t-hxab-t-high-misalignment-series-male-rod-ends?&bc=100%7C1064%7C1066%7C1067%7C1072%7C3001110&forward=1)

I just need clarification. Please don't make this a flame war.

Oh, I do agree with you that fully adjustable suspension is key to dialing in a car. But if a team knows what characteristics they want in a car (i.e. straight line stability, better turn in, etc) and are on a tight schedule with limited resources, design work can go a long way. We're not professional race teams with buckets o' hands, resources, and time to tune. Your 30 years of engineering experience tells me you were never on a Formula SAE team.

Respectfully

-Ian