-Shock Loading How important is it?

[Patrick Wang]

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

[sanddrag]

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.

[Patrick Wang]

Thanks! was looking to use 20 pitch gears anyhow but knowing that steel makes a big difference helps.

Still looking for an answer on shock loading however...

[Andy Baker]

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.

Andy B.

[Adam Y.]

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.

[Paul Copioli]

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.

-Paul

[Andy Baker]

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.

AB

[Patrick Wang]

Thanks for all the feedback.

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

[Gary Dillard]

Well, we didn't have shock isolation in our gear box, but.......

1) 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.

2) 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).

3) 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.

[sanddrag]

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)

Check out McMaster Carr.

[Gary Dillard]

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).

[Patrick Wang]

I'm curious to know what size shear pins you used and what type they were.

Did you use split end roll pins or dowel pins?

I was considering the key idea for a while then I looked at the price

The key would also make replacing gears a wee bit simpler, but again is it worth forking out the extra dough?

[sanddrag]

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.

I hate pins

[Jim Giacchi]

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.

[Matt Reiland]

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.

Keep it in mind as an alternative