Tormenta 2 - Build Diary

[Ellis]

Well there is a slight issue in that the boss of the sprocket is the same diameter as the shaft on the drill.

(already been round that loop and came to what we have been using until now as the solution. I had no idea the motor could twist bolts in half, that's insane!)

Edit: and even if the boss were big enough, we have no reverse thread screws of the right size and type. Everything on this motor is 2x bigger than a standard drill motor.

Now leave in 3 days. Argh!

[PJ-27]

If the bolt sheering is the only issue then use a high tensile bolt instead, a bolt that doesn't have a thread all the way to the head. The threading creates a weakness.

Like this:




I know a lot of people who have to attach sprockets to shafts (chain driven HWs, FW spinning weapons) will put a pin through sprocket and shaft and then weld it in place. Just a tack on the end and where the pin sits.

[robotwars]

Theres a potential lesson here that we've all been through before and I see it time and time again...

If you cant drop your robot off a table or throw it accross a room with confidence it cant take it, then normally its not strong enough.

Watching your videos of testing the lifter, you are very gentle with it... when in reality in a battle you are not going to be gentle, im not sure if it has mimit switches fitted now, but if it hasnt its going to get some grief in the arena when your being flipped all over the place !

Always plan for the unexpected, and a robot is only ever as strang as its weakest component.

I hope you get it sorted and keep us updated with progress, as i think its a cool little robot and im looking forward to seeing how it does in battle!

John
http://www.RoamingRobots.co.uk

[Ellis]

Yeah, solid advice, John, and we have been much more rough with it off camera. We broke it in starting to more heavily test it, so I suppose better in testing than in the arena after travelling thousands of miles!

PJ, we're thinking of drilling out the threads on the drill shaft as we'll never get the right bolt again, or certainly not for use in this machine (would break in the same way). Remember it's a weirdly sized reverse thread bolt, which we can't find anyway, let alone a high tensile version. And certainly not within 3 days along with everything else.

We're going to drill the threads out and stick a 6.5mm steel tent peg in there and bodge something up. We don't have time for anything more clever. As for welding, it's on a minuscule scale and we can't weld with any confidence anyway. We risk wasting time and/or ruining the tiny sprocket. We could *potentially* find someone to weld it for us, but again, with a day or two to do so... meh.

We'll work something out by the end of today and hopefully by tonight we'll know if we'll be able to fix it in time.

[kane]

Have you got a picture of the current setup and what broke?

[Ellis]

Will take one in a bit, we're about to nip down to the post office to use their scales. Fingers crossed our bathroom scales are good and it's not actually 15kg or something!

Edit: not all bad news hoorah! Weighed in at 13.125kg. Our scales reckoned 13.4, so definitely a result! If we're clever with it, 4-500grams that should be enough.

[PJ-27]

I'm confused as to how it's currently held on, pictures please!

Are you just sliding the boss over the gearbox shaft thread, and then using the reverse thread screw to hold a washer against it?

If you want a quick fix I'd drill down through the boss and shaft (when in situ) as big as you can, and then drop through a bolt (same size as the hole). That way it's pinned in place laterally and radially. (It's not going to fall off or turn)

If you have the time/tools, I'd use a high tensile bolt, thread lock on the other end of the nut and tack weld points.

If you can't do that, take lots of bolts with you!

[Ellis]

The sprocket can't be put around the drill shaft. The boss of the sprocket is literally the same size as the drill shaft. Physically impossible to put the sprocket over it!

The sprocket has a bore the same size as the reverse screw. We clamped it between the shaft (with the nut pulled from the chuck) and the reverse thread bolt. Really tight. With loctite. We then cotter-pinned the nut, shaft, and bolt (not sprocket) so that they couldn't loosen. The sprocket had a setscrew which went onto the bolt. It was seriously solid.

That worked well for a while, but as the setscrew loosened a little the reverse bolt got mashed (the sprocket could turn with the torque despite being compressed to hell between the bolt and the shaft as well), and so it didn't grip. We decided to cotter the sprocket through the bolt too. This worked well, until our first pin sheared completely. Out second stronger pin should have been the final fix, but after lifting about 6kg of Tormenta 1 the reverse thread bolt twisted and broke in two. We have strengthened parts and so moved the weak point from the setscrew, to the cotter pin, to the bolt.

That is where we are at now. I'll try and get a pic together but it's in pieces now! This is a very old picture but you can sort of see it here: http://s1088.photobucket.com/albums/i32 ... 012433.jpg

[Liftoff]

I think your problem is the locking bolt diameter is too small, it's length too long and the material weak, specially if it's got no plain shank on the bolt.
I would drill the motor shaft and sprocket to a larger diameter, then tap a new thread to the motor shaft and use a proper bolt as already mentioned, high tensile or similar.
If you were not so much pressed with time I'd have sent you a couple of titanium bolts, it would be difficult to damage those :wink:

[Ellis]

A bolt isn't the way to go, really. By their nature they aren't as strong as solid rod, and the ability to tighten and twist isn't necessarily useful. Plus (as I have said), we'd need a reverse thread tap and a matching reverse thread screw. For us, at least, that's really out of the question, and more so before Monday.

Well we fixed it. For a while. We drilled out the drill shaft threads and hammer-fitted a 6.5mm thick solid steel rod. We made a fresh cotter-pin going through the chuck nut, the drill shaft and the rod. Very solid. We then also hammer-fitted the sprocket into place and cotter-pinned that and the rod. Took us two rod attempts, and two broken drill bits (they're cheap bits, admittedly, but that rod is seriously tough stuff. We believe its original function was to be a big tent-peg). Put it all back together, put Tormenta 1's remains on it, and all looked perfect for a minute or two after some rough use. Then, snap.

The thing has snapped the steel rod too. It's crazy. We have built something we can't handle.

At this point we're so stuck for time that we may have to bash out another peg, build it just as we did, and put the torque limiter down. At least we'd have a robot that won't destroy itself. It may well be unable to lift anything much heavier than itself, though.

This is pretty disastrous, really. We never predicted this to be the weak point. Mike, if you read this, man I didn't believe you when you said you were snapping shafts but by-God, I do now!

Show is over for tonight folks, bedtime now.