Looking at the Technokats gearbox, and working on a gearbox of my own, I noticed that there were accomodations made for “shock loading” the system with a Flexible coupling.
My question is, how necesary is this to a geartrain design (we have always run with chains and sprokets before and have not encountered any need to shock load the system.) to shock load the system. Would for example an impact of a robot running into a wall or another robot pose a risk factor to a geartrain that was not shockloaded?
Thanks in advance for anyone who still reads the boards over the summer
I’ve actually never heard of the method you’re describing
There is a risk factor of having a drivetrain without the shock loading system but as long as it is built robustly, it should be okay. We have never had problems with sprockets and chains. I don’t thinks you could really break a chain link or sprocket tooth wery easily. However, we ahve had problems in a couple other areas. We mounted our drill motors with 6-32 threaded rod and connected them with the SPI drive couplers. Under extreme abuse the couplers came loose and the threaded rod flexed. However, this was the least of our concerns. We were using 32P Brass gears from SPI and those stripped and broke like crazy. As long as you use Steel 20P gears from Boston Gear or sprockets and chains, you should be alright. Also, keep the chains straight and tight and maybe a little slippage with the tires isn’t such a bad thing. It will be easier to turn and won’t be suck a shock when going instantly from full speed to stopped such as when runing into something. Hope this helps.
*Originally posted by Patrick Wang *
**My question is, how necesary is this to a geartrain design (we have always run with chains and sprokets before and have not encountered any need to shock load the system.) to shock load the system.
**
Patrick,
Excellent question. My general answer is that I think that we did need it, and the only proof that I have is that even with these shock loaded couplings in use, we still stipped some gears (gears were replaced 4 different times).
There were two main reasons that we put on the shock loaded couplers: contact of the game and gear shifting on-the-fly.
From looking at the game, it was obvious that there was going to be some heavy contact between robots and from ramming the goals. That was pretty obvious.
The second reason of protecting ourselves from shifting on the fly was probably more important. Sure, it was pretty safe to slam the transmission into high gear when we were in low… but the real test was downshifting from high to low gear. This is what we were most worried about.
I remember the first time we shifted on the fly from high to low… I had to look away, in fear that the gearbox would blow, but to our relief, it didn’t. Sure, we didn’t really push the gearbox in this area, but at least it was possible if we needed it.
The negative thing about these special Lovejoy couplings is that they were heavy… weighing over 1 lb. each. We whittled them down to about 2/3 of their original weight, but they were still heavy. And worth it.
I always thought that flexible couplings were for misaligned shafts not for shock loading. Of course it is generally a good idea to always shockload anything you think will experience lots of stress.
For those of you who said you use chains and sprockets with no means for absorbing shock … The chains are your shock absorber. They stretch and flex to absorb a lot of shock load. If you go from a chain design to a gear design, you really should investigate how to absorb the shock.
*Originally posted by wysiswyg *
**I always thought that flexible couplings were for misaligned shafts not for shock loading. **
95% of flexible couplings simply handle misaligned shafts. The coupling used on this system not only handled misaligned shafts, but also handled the shock. It’s one of LoveJoy’s new couplings, called something like a “pin-in-shear” coupling.
I will definately look into shock loading the new geartrain. I had a speculation that the chains were the shock loader in the system, but was not too sure.
I hope you don’t mind if I borrowed your gearbox’s dog teeth Mr. Baker, I figured using them was better than reinventing the wheel
Well, we didn’t have shock isolation in our gear box, but…
The reason we missed our first couple practice matches at nationals was that we were rebuilding the gearboxes to beef up the shear pins. We sheared one in the semi’s at KSC and to make it through the finals we had to use a drill bit because we didn’t have a spare with tight enough fit. We built new shafts during the allowable 2 days after regionals and spent all day thursday at Disney reboring, drilling and fitting the gears.
We spent the entire time between Division Finals and Final Four dressing the gear teeth with a Dremel tool since they had taken so much abuse. We were having trouble shifting gears (Almost cost us the last match).
We ran full blast at the goal and into Beattie on a wing and a prayer. I’m afraid of what will happen at MiM - I don’t know if the gearbox has many more matches left in it.
Don’t use shear pins because you’ll shear the pins. We used to use those but have graduated to keyed shaft. We used 3/4" keyway shaft for our axles turned down on the ends to fit in 1/2" bearings. Can you spell BEEFY.
Keyway shaft, shaft collars, keyed sprockets/wheel hubs, and keys. Stick with these and you’ll have no problems (other than empty pockets. That keyway shaft is kinda pricey)
we would have preferred keyed shafts, but we were manufacturing resource limited. It’s in the plan for next year. The shear pins were plenty beefy once we up-sized them but are still life limited (keys are too if you don’t size them correctly).
It is very worth it. With keys you just have to loosen set screws. With pins, you have to tap them out - and if they’re broken it makes the job much more difficult. Or if you have pins that just slide in, they will be epoxied in so they won’t just slide out.
We used keys for the first time this year and it was SOOOOOO much nicer. We wanted to put a smaller gear on the motor to get a little more power and with the keys we just loosened the set screw popped them off and then put the new gears on. Also I’ve had pins break several times but have never broken sheared or ripped apart a key.
Whats better than pins or keys? Welding of course,
Advantage: Its never going to slip or come apart
Disadvantage: Its never going to slip or come apart
If you are going to weld, better get it right the first time, we build the pegasus drivetrain first with set screws to try the ratio out then welded everything into place. To mate aluminum to steel we welded a ‘yoke’ to the shaft and bolted the wheel to the yoke. YOu can also bolt the gears to welded yokes. Keep in mind the yoke will most likely deform under the heat so we turned it back to normal after the welding before bolting the wheels to it. No failures with the same drivetrain for two reigonals and nationals.
I am a apt TIG welder myself, I have mostly had my experience welding aluminum frames together.
Knowing that the heat from the welder is going to bend the shaft if you keep the heat on too long, does anyone have advise as to the proper way of welding a gear to a shaft and how much weldment you actually need.
Two years ago, we tried to build a ball grabbing device with welded sprokets onto drill rod. (This is probably a bad basis for judgement because the design was flawed from the start had, 16 in of unsupported rod) but we had the drill rod shear. Now, I don’t believe that a shear would be because of the weakening from the weld, if anything I’d think it was bend out if the drill rod was warped from heat.
Basically what I am looking for is someone to tell me that there is a way to weld a gear/sproket to a shaft without damaging the shaft. And how great are the benifits? (it seems that the little weldment just intuitively would offer a lesser degree of maximum torque than a key or pin)
The UAW welder that helps our team out has a few tricks that make his welds clean and very strong.
I would think (this comes from an electrical guy) that if you are shearing a shaft that the shaft size needs to go up some? For our boom this year the final stage was 1/2" shaft with 20pitch Boston gears that took huge abuse and never had any problems, of course it was also supported well with bearings.
In the past when we used keyways and pins, cutting the keyway into the shaft took away a substantial amount of material and if cut incorrectly could put extra stress on the shaft according to one of our mechainical engineers.
For our gears, we usually chamfered the inside bore of the gear to allow the weld to make a nice contact at that point, also we sized everything for a very tight fit without the weld. The welder has a little tool kindof like a drill facing upward with a foot pedal that spins the shaft while he welds to make a nice continous bead around the shaft. Also he has a tub of some purple stuff that looks like playdoh that absorbs heat off the materials so it doesn’t deform as much. Properly welded gearing makes for a very strong geartrain.
Also we lightened all of the gears to a great degree (some as much as 80%) as a note to all teams: lighten the steel first its a much bigger bang for the buck!
Looks like I’ll have to be resourceful if we want to weld the gears We’ve got a TIG welder, a table and a 90 degree jig.
Thanks anyways for the information, we may just try and experiment with welding gears/sprokets, see how it turns out.
I’ve been trying to stay away from that 1/2 in shaft, the bearings start to take up a lot of room and the shaft becomes very heavy very quickly. (too bad you can’t lighten the drill rod)
Thanks again for all the replies, and good luck with all your teams next year.
Well, there just might be a resourcefulness award next season. As for the 1/2 inch shaft, were you refering to when I said 3/4 keyway shaft turned down to 1/2 inch on the ends to fit 1/2 in. bearings? An alternative would be to do smaller skyway wheels with 1/2in. bore keyway hubs and use 1/2 keyway shaft for the axles turned down to 3/8 on the ends for 3/8 bearings. The only real downside is that I don’t think skyway has the really big wheels with the 1/2 inch hubs. Our 10x3 beadlocks came with 3/4 inch keyway hubs.
I hope this wasn’t too confusing.
I’m not sure what you mean by “drill rod” but you can lighten any shaft/rod by using a lathe to turn it down or drill out the center.
Of course it is generally a good idea to always shockload anything you think will experience lots of stress.
