Just curious as to what specifications the sphericals in a-arms from other teams are - i.e materials for ball and race. I've found that on disassembly of previous cars, the bearings are always really stiff - do any other teams find this to be a problem? We use Aurora AWC-5T's.

Hi,

we've used the Aurora HAB-4T PTFE lined bearings so far, which we've found to be excellent but, as you say, a little stiff. One trick is to spin the balls in the races using a drill for a while to loosen them up. Can't remember where I read this, but it was in a book somewhere.

Cheers

Thanks Alastair. We have used the PFTE lined bearings but found them to be quite stiff after even relatively few testing hours. I was really surprised at the difference in freedom of movement between 2 different types of bearings we took delivery of today... it really got me thinking about our bearing choices.

I'm assuming most teams simply press in their bearings to housings. Anyone stake the housings, or use any other methods to fix them in place (besides loctite)? I think the pressure from the housings might contribute substantially to this problem.

This last year was our first year experimenting with spherical bearings in our A-arms. Wish I could tell you which brand bearing we used but I have no idea.

Pressed them into 4130 rings on the ends of our a-arms. They were originally machined well under tolerance, welded to the A-arm, heat treated, and secondary machined.

The trick becomes the tolerancing on the press fit, if you do press them in. For a .750 outer diameter bearing your tolerance limits we decided on for the piece you're pressing it into were roughly .7490 to .7495. Probably wiser is .7495 to .7498 - Doesn't need to be a heavy press since you have such a light axial load and you can use other stuff to beef up the hold.

Couple ways of nailing that. One is an undersize reamer, either specific to that size or a .7500 reamer that's been heavily used and is a bit worn down. Or, get a good boring head with .0001 increments (not a Criterion POS) and dial that in, bore it out with a couple spring passes. Or finally you could use a 3/8 or 1/2 endmill, preferably carbide, 4 flute, light feed and again a couple spring passes. Mic it up and measure the tool dia to within +/- .0001. What's sketchy about that is unless your school has a really top dollar machine (Mori Seiki, Makino, etc...Haas and Fadal are really not that good in terms of accuracy and repeatability) the circular interpolation the machine runs isn't going to be all that great and your piece may be out of round or out of spec on the diameter.

Since we were fairly unsure on how all that would work we also used a set screw from the side and loctite to hold the thing in. With the tolerances we held our bearings didn't slip out of the race at all, and with the exception of 1 stiff one, spun pretty freely.

Another issue is measuring what diameter you're machining accurately. Good dial calipers (Starrett) I'd only trust to within .0005-.0010, and a lot of people don't even use them right. For something like this a pair of ID micrometers is really what you need. We of course didn't have any so what we did was a best effort.

Worst part was our rear lower a-arms, the bearing race sits at a compound angle relative to the plane the rest of the A-arm defines. Spent a couple nights up to 2am dicking around with an adjustable reamer and all sorts of crazy setups just to get ONE to spec. Somehow the other one came out perfectly without us touching it.

We're doing things differently this year, I guarantee you that.

We have pressed ours in against a shoulder and then used a snap ring on the other side. One thing to think about is which direction the axial force will be in and make sure you orient the bearing such that it is pushing against the sholder side, not the snap ring side. I had one pop out on our 05 car... I pressed another one in with some serious locktite. I imagine locktite should be used on all them. Also I machined all of the housings first to spec and then welded them to the a arms with a brass insert in place of the sperical bearing. Worked out pretty good. It sound s alot easier than trying to machine it after the fact on a mill.

I was speaking with a CART engineer two weekends ago about stiff spherical bearings. Apparently the stiction is greatly reduced in dynamic conditions due to the load on the bearing. He had even run an experiment on the rear a-arms between the spherical bearings and some very expensive needle bearings. The conclusion was that there was no measurable difference to the performance of the car.

We also use a shoulder to retain the bearing in its primary force direction. Loctite and a press fit were used, however the press fit often ended up a slip fit. We pretty much machined heim joints that fit into an A-arm tube which meant the bearing bore was done on a mill with a boring bar, not super accurate, more like guess and check. Only one bearing has removed itself from its home, and in that case the ball popped out to, we're still not sure which came first, the chicken or the egg.

Further to what I said previously - we've now used the staking method of retaining the bearings for 2 years. In our first year, we had bearings come loose. We tracked this down to the plate into which the bearings were staked being under thickness by about 0.015", which meant that the staking wasn't effective. A bit of loctite cured that....

This year, we got the plate thickness right, and followed the instructions in the Aurora catalogue for the bore tolerances and the staking procedure. One thing - stake the bearing, then rotate the whole assembly 90 degrees and repeat. Keep doing this till you're back where you started. Apparently this gives a much more even staked joint. We've had no trouble this year with bearings coming loose.

Hope this helps

The easiest way I found to attached them was to machine a sleeve with a shoulder in the lathe that the bearing would have a light press fit into, then weld the sleeve to your a arm, press the bearing in and make a jig that will fit in the press and allow you to crimp the back edge over in three or four places around the edge of the sleeve. This hold the bearing on one side by shoulder and on the other by the crimped over metal. This eliminates any chance of the bearing coming loose, but it also means if you have to replace one it has to be cut out. In the two years we did this we never needed to replace one though.

You have to understand them to break them in correctly. The steel ball, over time, will burnish some of the teflon into the pores of the metal. When this happens the joint will work correctly. Just like any other teflon lined anything. I used .5" rod ends on our baja car in the past and they will never loosen up if you dont do it before you run them. I was told to put a bolt through it and tighten the nut down. Put the excess bolt into a drillpress at low rpm and turn it on. Slowly move the shaft(or whatever the rod end is connected to) through the rod ends angular range until it moves smooth. The goal is to burnish the wroking surface of the ball. Do not, under any circumstances use any kind of lubricant,dry lube,grease or anything with a telfon/ptfe lined bearing. That stuff will keep the telfon from impregnating permenantly. No amount of cleaning will resolve this situation. I was told this years ago and it works perfect. Just go slow and steady. Rush it and you will get the teflon too hot and it will deform permanently. The rod end shouldnt get more than slightly warm when you do this. Youll feel it when it the rod end is working properly. Also, staking is the way to go with sphericals. Call frank @ 610-515-0550. He sells staking tools for these bearings.

I think we'll definitely be burnishing the teflon in the bearings this year, might have had a lot to do with the problems. Mike - I'm assuming you weld a pre-machined housing to your A-arms - do you find the groove for the snap ring deforms with the heat of welding? Is there much movement of the snap ring in the groove axially?

Out of about 16 a-arms we welded up only one snap ring groove got damaged and I cleaned it up with a dremel. I cut the snap ring grooves to whatever it was suppose to be, so no there was not much play there. There is some gap between the bearing and the snap ring (which might be waht you were actually asking...) but really the snap ring is just there to keep the bearing from falling out, not to hold it in during normal operation.

Speaking of spherical bearings, how many hours do you all get out of them? I'm using the aurora com-5 bearings and just had to replace two after excessive play. We probably have 50-100 hours on the car. Also for rod ends i'm using Am-5's and they seem to need replacing about now. Aurora makes a few series of bearings/rodends that are stronger, has anyone used these? I'm thinking about the COM-KH or COM-E. Thanks

Honestly if you are running anything but a 3 piece bearing you are asking for trouble. The 2 piece bearings arent meant for precision work and are suspect in general. The manufacuring process for them is inferior. I think the am series is entry level at the least. I would use a aerospace grade or performance racing version from aurora. I would also like to note that the hab-t versions are rediculously strong for the size compared to the other joints. Look at the dimensions and you will see that the ref. ball diameters are bigger because of the necks that protrude off of them. They are only 2 dollars more a joint that the aerospace ones. So in all of my designs i eliminated all spacers and just use the width of the hab4-t as the spacer itself. It is worth thew 2 bucks a joint to save the hassle of making and not losing the damn things during assembly and repair. Also my entire car uses those things in every place. All of my joints use .25 bolts with those hab-t bearings. I found that they are stronger than what i need in most places and i can keep the same bolt size, which is already small, throught the car. Also cheaper to buy in bulk in real life and the cost report. Something to consider for all of you.

For a staked spherical bearing, which is more likely to fail first under axial load, the ball pressing out of the race or the staked race pressing out of the housing? So far we've used HAB-4T auroras also and had no problems, but the housings have been way over engineered and take far too long to make, so i'm after a simple staked bearing design.

Mike, do you have any pics of your a-arms? I'm just wondering how close you can get the welds to the housing bore without serious distortion and how little material can be used.

BTW, slighly off topic but does anyone have experience using oval/non-circular tubing for their a-arms?

quote:

Originally posted by Rob Woods:
...Also cheaper to buy in bulk in real life and the cost report...

Real life - Yes
Cost Report - No
4.3.4.2(B)

We used aerofoil (teardrop) cross sectioned tubing in our A-arms last year. They looked damn hot, but we went back to round tubing this year after one bent in the endurance. The main problem was the forces in each arm member were calculated wrong (they must have neglected pullrod force or something), and the mounting of the pullrods onto the A-arms caused a substantial bending moment. I think they used the wrong moment of area (i.e horizontal, not vertical) to calculate their buckling load capabilities. Also found we saved a considerable amount of weight in the component from last year. Still, asthetically VERY pleasing, and not written off for consideration on future cars.

From reccolection, you might be fairly limited due to size availability too. Also consider you need to explain WHY you went for non-round tubing to the judges - I wouldn't try passing it off as an aerodynamic choice if you have nothing to prove it, and don't know what they'll say if you did it "'cos it looks good"...

quote:

Originally posted by James Waltman:

quote:

Originally posted by Rob Woods:
...Also cheaper to buy in bulk in real life and the cost report...

Real life - Yes
Cost Report - No
4.3.4.2(B)

This is why (we at least) have used the Aurora C-series rod-ends and spherical bearings. They're almost $3 cheaper per part, and with 8 pullrod rod ends, 20 a-arm rod ends, and 4 a-arm spherical bearings, that's almost $100 (cost report price). i'd kill for those extra points.

w.r.t. non-round a-arm tubing, auburn? had round tubing they ran through a sheet metal roller to make semi-rectangular tubes. we're looking into this for next year, reason being we have lots of cone impacts and are worried about bending an a-arm around one.

One advantage of aero / oval section tubing is that it has a lower buckling strength than round tubing of the same stiffness / area / weight.

So, you can design your control arms to have a certain stiffness, and if you hit a wall, the aero section members will buckle at a fairly low load, saving the uprights and frame from damage.

Not that we have many walls to hit. But that's really the only argument for aero tubes I've heard in FSAE. We don't run aero tubes because it's not worth the expense. Also, they are more likely to be damaged by people lifting the car, or a well-placed cone lodged under the arm which lifts the car up. The cones get nice and hard around here in the winter

I'm not worried about the capablity of the oval tubes under compression. For a pushrod design, tensile strength is more of an issue. Aero efficiancy isn't a good reason for oval tubes, packaging is my main goal and as Mike pointed out, an oval tube is less likely to bend around a cone. I thought availabiliy may be a problem, so for manufacture, i was thinking about folding 1mm(ish) sheet, welding and grinding the seam.

Back to bearings.

Sorry to nag about this. Is a staked spherical alone strong enough to take the axial loads on the pushrod (or pullrod) loaded a-arm, or is a shoulder required. Obviously, it's dependent on the quality of the bearing race and the staking process, but i'm assuming it's done correctly here.

Can someone explian the staking process a little more clearly?