pic: New serve mk.1-2517

[T^2]

Originally Posted by asid61

I thought screws were meant to be in shear load? 10-32 screws have ratings in the hundreds of pounds range IIRC.

Load rating =/= intended usage. Screws will misalign little by little when used as the sole locating feature.

[DampRobot]

Originally Posted by EricH

I broke out my machine-design textbook--it's really handy for a lot of things with robotics, including a general screw-bolt-nut overview. (Also one of the few chapters I didn't get into college.)

Would a machine design textbook be a good buy/read for a prospective ME student with little to do this summer? I've heard a lot about this mythical textbook from mentors and on CD, and it sounds like it contains a lot of "real" engineering information. Any suggestions on where to buy it, or what edition (assuming there are multiple editions)?

[aldaeron]

The "old one" I have is Kent's Mechanical Engineers Handbook. The copy gifted to me by my first boss is the 12th Edition (1950). It is a good reference, but not an exiting read cover to cover. It has a section on gearing stress and fasteners and many other mechanical things.

A "more modern" design book would be Mechanical Engineering Design by Shigley and Mischke. Those two have a ton of editions of mechanical design. Thanks to indecisive profs I have both the 4th and 6th editions. They are now on the 9th edition according to Amazon. Again, not a cover to cover read, but this book has a section on gearing stress and fasteners.

I caution you that these are 3rd or 4th year collegiate level texts. There are a lot of support classes that will help these equations make a lot more sense (statics, strength of materials, etc). Reading them cold could be very difficult.

-matto-

[T^2]

Originally Posted by DampRobot

Would a machine design textbook be a good buy/read for a prospective ME student with little to do this summer? I've heard a lot about this mythical textbook from mentors and on CD, and it sounds like it contains a lot of "real" engineering information. Any suggestions on where to buy it, or what edition (assuming there are multiple editions)?

A former mentor gave this version to our team a few years ago. I have no idea how it compares to others, but it's been handy as a reference and learning resource for new designers. All of the math is understandable with the equivalent of Calc BC and Physics C Mechanics knowledge.

[nathannfm]

Originally Posted by T^2

Are you sure you want to be loading screws in shear at all? Why not replace half of them with spring pins?

Aren't all of the extremely common dead axle (sprocket/pulley bolted directly to the wheel) drives in FRC putting 6 bolts in shear on each wheel? (we found out what happens when you accidently forget 3 of the bolts...ignore the EI metal )

[Gdeaver]

This design effort seams to be focused on mechanical losses experienced in the 2014 swerve module. What specific problems did you have? Have you identified where the losses on the current module are? Have you quantified the total losses? I see some potential problem areas in the current Module. Before I say more, what have you found so far? Know what the 1st iteration problems are before you design the 2nd iteration.

[Chris is me]

Originally Posted by nathannfm

Aren't all of the extremely common dead axle (sprocket/pulley bolted directly to the wheel) drives in FRC putting 6 bolts in shear on each wheel? (we found out what happens when you accidently forget 3 of the bolts...ignore the EI metal )

While this is common and may work, it is not optimal. Screws aren't great in shear due to the stress risers of the threads and the effectively smaller load carrying diameter of the bolt. A better solution is to use 3 bolts for fastening and three shear pins to take the load. Shear pins are better simply because they aren't threaded and thus have a larger effective diameter.

[Lil' Lavery]

Why transmit torque through the screws at all? Do something similar to what Vex does with their versahubs.

[autox_vette]

Not all of the force is transmitted through the bolts (in an ideal world, none of it would be).

Think about the wheels on a car. They are held on with 4-6 studs in single sheer. Some are taking 500+ HP. The bolts/studs only hold the wheel onto the hub and create a HUGE clamping load between the wheel and the hub. All of the power is transmitted through the friction between the wheel and hub.

So, it depends upon what your wheel and spacer materials are. If you use metal, then less sheer force is transmitted to the bolts because you can get enough clamping force to transmit the power. If you use plastic/nylon, you probably can't get enough clamping force, so some power will be transferred as sheer force on the bolts.

Kenton

[T^2]

Originally Posted by nathannfm

Aren't all of the extremely common dead axle (sprocket/pulley bolted directly to the wheel) drives in FRC putting 6 bolts in shear on each wheel? (we found out what happens when you accidently forget 3 of the bolts...ignore the EI metal )

This used to be the case, and still is for AM products. As mentioned above, VEXPro products rely on the VersaHub to transmit torque; the screws hold purely in tension. Guess which system is more reliable?

[Chris Endres]

Originally Posted by Tyler2517

We do not want 2 of the same material gears so we don't have to grease them so carpet cant get stuck in the grease.

I understand where you come from with this idea, although the power that the gears with transfer will need grease, no matter what type of material you use. I know that carpet fibers and other junk gets stuck in normal grease, but you can also use dry grease. Machining tools (boring bars/milling disks) use dry grease so chips don't get stuck between important seals. I would look into using dry grease because of the low friction index, such as Dry Graphite Lube (I once used it to win a pine-wood derby).

[Tyler2517]

Starting from the top.

Gears- Use steal on steal unless we can prove that aluminum would hold up which i personals will hold reliably.

Screws- switch to having 6 screws going all the way through exerting a clamping force on the spacer in the middle and the gear. This way the screws are not taking a large amount of shear load or no shear load. Would a softer material like nylon work better or would AL6061 work the best?

Thrust bearing on the top bevel gear and not just a thrust washer due to the end conditions of the transmission

On the same manner of thought the reasons for changing designs from last years module. Is primarily to improve year to year in as many ways as possible.

Increasing mechanical efficiency by removing a chain reduction. And maybe reducing weight in the moving parts of the drive transmission(we might end up adding weight not sure on that yet)

Slightly improved traction by increasing suffice area the tread and more importantly 2 leading edges per wheel. This will make our passive defense stronger.

Smaller foot print with in the robot. Alowing the wheels to get closer to the corner still and increasing foot print slightly.(not much but i am a firm believer that a large amount of small changes for the better will eventually make a better product then competition)

Price reduction no longer buying sprockets/gears/bearings. (yes the gears used now might be more spendy but still should be under the price of the old system)

Improved senser feed back adding in primary drive encoders for pid loops. And using a limit switch to auto home the module(things i have not mentioned in this post yet)

[artdutra04]

Originally Posted by kk052

also the brass on titanium is to make the brass take all the wear, so we dont need to replace the titanium

I've used brass gears before in FRC, and I wouldn't ever use them again in torque carrying situations. They were way too soft and wore too easily when interfaced with steel gears - they needed to be replaced fairly often as they quickly turned to dust even when well greased.

There are some alloys of bronze that would be a lot better, but I suggest that you use two of the same material gears and lubricate them well. You do not need to use the gooey grease, there are some good lubricants that form a fine film like this one that won't attract carpet dust:
http://www.mcmaster.com/#8710t35

[EricH]

Originally Posted by DampRobot

Would a machine design textbook be a good buy/read for a prospective ME student with little to do this summer? I've heard a lot about this mythical textbook from mentors and on CD, and it sounds like it contains a lot of "real" engineering information. Any suggestions on where to buy it, or what edition (assuming there are multiple editions)?

I'll give that a Yes. And, like any college-level course, there are multiple authors and editions, which may be used by various colleges.

I use the same one that T^2 linked, down to edition. If you happen to have some ideas as to where you want to go to college, I'd suggest trying to get one from one of the college bookstores.

[AdamHeard]

Originally Posted by EricH

=I'd suggest trying to get one from one of the college bookstores.

I'd suggest avoiding purchasing anything at a college bookstore. Much better deals on amazon (especially used), ebay (used/internationl), and smaller companies that have used books.