Banebot 56mm gearbox - double D - RELOADED

[cbpetrovic]

Originally Posted by ZZII 527

By some twist of luck, my steel came early this morning AND I was able to get 20 minutes on the waterjet. So I now have four A2 tool steel 56mm carriers, which are about the same hardness as the originals but can be treated to pretty much anything from C20 to C40. I will probably shoot for C23 for now to match the current shaft.

My dimensions were as the drawings say, except:
a) I went tighter on the double-D: 0.470in (11.94mm) radius and 0.350in (8.89mm) flats. With the taper from the jet, this seems to be a good press fit, but may require some filing.

b) The pins will be press fit in the other direction, against the taper of the jet, so I just made their holes extra small, 0.150in (3.81mm), and I'll drill them out later.

The stock I used is McMaster part #9019K147, 18"x2"x5/16" A2 tool steel.

In the attached picture, the two carriers at the left are the originals (1CIM, 12:1) with almost no visible damage after "mild" usage. The four at the right are the new A2 tool steel ones that I will try to harden to C23 for testing.

The 4140 prehardened stock has arrived for us as well, and should be in the hands of our model shop along with the drawing package.

Our plan is to remanufacture the 3:1 DoubleD plates and we should have some test results this weekend. We're running a 4 transmission/motor chassis.

C. B. Petrovic - Team 166

[ZZII 527]

Update:

I tried reassembling my 12:1 gearboxes today with the new press-fit A2 tool steel plates I cut and discovered a few issues with press fitting that I didn't think of:

1. I had to file out some to get it on at all, which is my fault because I went way under on the hole to be safe. Doing this, though, caused significant alignment problems. It was tough to get the press to go on straight and if it is even a little bit off, the gearbox will be tend to be stiff at certain positions.

2. Assembly was generally more difficult because of the extra constraint. I couldn't, for example, assemble the rest of it and slip the shaft in later.

3. Luckily this didn't happen because I over-filed a bit, but if you press it on real tight, it will be tough to get back off. I can't think of a good way to pull it back off since the plate comes so close to the bearing block.

So in conclusion, I think I'm going to file these out to be close fits, but not press fits. Loose enough that they will slip on and self-align during operation. If I cut more on the waterjet, I'll just make them 12mm x 9mm exactly.

Also, I was mistaken in my previous post: A2 tool steel will temper to C40-C65, not C20-C40. It comes already just below a C20. I will shoot for the low end of the tempering range, so as to avoid brittle failure.

[MrForbes]

Originally Posted by ZZII 527

3. Luckily this didn't happen because I over-filed a bit, but if you press it on real tight, it will be tough to get back off. I can't think of a good way to pull it back off since the plate comes so close to the bearing block.

one could use a bearing separator, maybe?

[ZZII 527]

Originally Posted by squirrel

one could use a bearing separator, maybe?

Probably would work. I'm sitting here watching American Chopper and filing carrier plates...it's mildly relaxing. The next set I put on I hope never to have to take off anyway. I think after I finish with this gearbox stuff, I'm taking the rest of build season off.

[eugenebrooks]

At this point, we have machined carrier plates and axles out of the 4130 steel that we have on hand, using a square broach for the carrier plate. We have hardened at 1600F with an oil quench, and tempered at 800F.

A link to a table showing the strength characteristics of 4130 as a function of the temperature for temper is shown below:

http://www.eng-tips.com/viewthread.c...=116052&page=4

<<<<<
Following is a listing for tempered 4130 steel:

UTS YS

C F MPa psi MPa psi El RA HB

205 400 1627 236 1462 212 10 41 467
315 600 1496 217 1379 200 11 43 435
425 800 1282 186 1193 173 13 49 380
540 1000 1034 150 910 132 17 57 315
650 1200 814 118 703 102 22 64 245
>>>>>



The yield strength at this temper is 173,000 psi and the elongation at break is 13%, not too brittle.

We have assembled and broken in the gear boxes using no load and 8 volt power, running both ways until the gear box "sped up" from the polish established on all the parts. This was about a half hour in each direction. We then inspected for any evidence of metal chips, and installed them in the robot.

We should get a day or two of testing by Monday. After some agressive testing, we will report the results.

The material is from McMaster and you can make these parts easily if you have access to a lathe, a mill, and a heat treating furnace. This approach to the problem is best described as Dr Joe's plan C. If you don't have this equipment, I would suggest that you head to a local machine shop that can heat treat with suitable material and the existing parts to copy with the modified square hole setup.

If I bought material just for this task, I would use 4340 steel and suitably adjusted hardening and tempering temperatures in order to get a little more strength while maintaining toughness.

Well, I would really use maraging steel from http://www.onlinemetals.com if I ordered material just for this, but I won't get into that.

Eugene

[ZZII 527]

Update on the A2 tool steel carrier plates:

After cutting them on the waterjet, finishing the pin holes with a 5/32" drill bit, and filing the double-D to be a close fit, but not a press fit, I tempered the steel today. Here was my process:

1. Heat at 950C (1742F) for 20 min.
2. Air cool (A2 is air-hardening tool steel) until warm enough to touch.
3. Temper immediately at 650C (1202F) for 2 hours. Air cool. Repeat temper for two hours.

The high tempering temperature brought the hardness down to a C40 on one plate and a C39 on the other. (Tensile strength around 182 ksi.) I am hoping that this won't be too brittle.

A2 holds tolerance well in heat treatment, so the press fit pin holes worked fine. The double-D was a bit tight, but after a bit of sanding they seemed to go on easy. One gearbox assembly went smoothly. Putting the two stages in first and then fitting in the shaft later is much easier than messing around with a press fit shaft and carrier. It ran smoothly with the motor, drawing about 2.5A. (Is this normal? Good? Bad?)

The second gearbox is still having alignment issues, binding slightly in particular positions. I think the carrier plate may need some more filing to get it on straight. Maybe a fresh look at it tomorrow morning will do the trick.

So it is indeed possible to make a working HRC40 carrier plate out of A2 tool steel. It's a nice steel, because it can cool slowly, not deforming much, and still harden. McMaster also carries 5/32" (4mm) stock, saving one machining step. But it is somewhat expensive and needs to be heat-treated to get to the right hardness, so it probably isn't the ultimate solution.

The most important thing I learned so far is that a press fit makes assembly (disassembly) and alignment VERY difficult, and so I wouldn't recommend it. If I cut more, I'll cut them exactly 9mmx12mm. If the plate can self-align in the motor, it will run much more smoothly. But your results may vary, of course.

For now, I think I'm done. As long as the second gearbox goes together okay and nothing breaks this weekend when we drive the heck out of it, I think I'll be happy. We are using 12:1, 1 CIM per side, so I don't think I need to make a sqare-hole version in anticipation of needing the harder shafts. But I might cut some just in case..

[eugenebrooks]

We got a chance to test our 56 mm banebots transmission solution
today, using 12:1 gearing and two cim motors on each transmission.
As noted in the post earlier in this thread, we made new plates and
axle shafts out of 4130 steel, using a square hole, and hardened in
an oil quench at 1600F followed by a temper at 800F. The 4130 is
what we have on hand for heat treated welded parts, we would have
used 4340 for this application (or maraging steel considering the risk
to the function of the robot) if we had bought the material specifically
for it.

Our test was a half hour of ramming the robot into a old tire positioned
against a wall at full power from a distance of a foot or two, back and
forth. The test was stopped when everyone got bored of it. The
transmissions are doing fine with no apparent extra lash, but we have
not yet pulled them apart for inspection. We will do that when time
permits, there isn't all that much time available in the last two weeks
of the build period as we have our share of other problems to solve.

Have fun,
Eugene

[RichardCMongler]

One possible method of ensuring that the motors are started in a controlled way, would be to have the programming team put a limit on how fast the PWM outputs to the drive motor speed controllers increase. If they allow only a small change per unit time, then the gearboxes cannot be damaged by sudden direction reversals, even if the driver gets a bit exuberant (and I do realize how exciting the matches are!). Look into this, hopefully some of the programmers here can come up with some code to control motor acceleration rate.

http://www.chiefdelphi.com/forums/sh...ad.php?t=53488

[eugenebrooks]

The following table provides comparison data for 1040, 4140 and 4340 steel, that makes it clear what the best choice is if you are concerned about part failure.

http://www.auto-ware.com/techref/materials.html