30-12-2006 01:34

Rafi Ahmed

Re: pic: Shaft Drive Bearing Blocks

Why cant you use chain instead of having a shaft all the way down your drive system?

30-12-2006 01:50

MAteo9944

Re: pic: Shaft Drive Bearing Blocks

I believe(haven't actually calculated) that the shafting will end up weighing about the same as the chain required to drive a similar system.

Last year I got annoyed with tensioning chain.

If I do it properly I won't have to worry about it breaking(though the same goes for a properly designed chain system).

It is different. Its cool.

Why not(seriously I would like to be critiqued. Though I probably don't have to ask on CD)?

I just checked out the weight issue a little bit. #35 rollerchain is about .275 lbs/foot
If you use 6 feet that is about 1.25 lbs
the shafting in this system only weighs about .25 lbs
If you went with #25 like a lot of teams the the chain is lighter

I still havn't looked into sprockets vs miter gear weight

30-12-2006 01:56

JoelP

Re: pic: Shaft Drive Bearing Blocks

I'm curious as to the advantages this method provides over more traditional chain drive systems, other than being really different and cool.

30-12-2006 02:31

Tom Bottiglieri

Re: pic: Shaft Drive Bearing Blocks

I would not want to have to fix that thing...

Dropping a chain is one thing. Swapping out a whole shafting system might turn out to be another.

30-12-2006 04:03

Gabe

Re: pic: Shaft Drive Bearing Blocks

The point is to come up with a design so durable that failure will not be a factor to worry about in competition. I like this design

30-12-2006 04:09

Salik Syed

Re: pic: Shaft Drive Bearing Blocks

I know Team 1097 did something very similar to this in 2005. It took them almost the entire 6 weeks getting the base running perfectly, but it was one heck of a bot, they won the Davis Regional.
Even with the flexible shaft coupling your frame needs to be welded very straight, and should be sturdy enough not to bend. 1097 had everything very precisely machined and put together.
Is the transmissions efficiency as good as a well tensioned/aligned chain drive? I take it there are several mitre gears, as well as shaft misalignments that all add up.

30-12-2006 10:44

Stuart

Re: pic: Shaft Drive Bearing Blocks

the only thing that scares me about this is those little bitty gears direct driving those moderately sized wheels., but what do I know Im not a meche. good luck on that drive system though

30-12-2006 11:17

Ben Piecuch

Re: pic: Shaft Drive Bearing Blocks

It looks like you are machining your own wheels. If that's the case, why don't you just make a center wheel that is slightly bigger that the two outer ones. The speed of that wheel will be negligibly higher than the outer two, but it will allow you to have a standard pillow block in there, as opposed to the IGUS ones. This could make for a possibly more robust drive setup, as some teams had problems with those plastic IGUS bearings last year.

BEN

30-12-2006 11:23

MAteo9944

Re: pic: Shaft Drive Bearing Blocks

In the case of a failure I plan on having an extra side module pre-assembled to swap out.

30-12-2006 11:33

TubaMorg

Re: pic: Shaft Drive Bearing Blocks

I always like the idea of a shaft drive. It can be quieter and requires less space and can be more reliable when put together well. I like the bearing support at the drive wheels. Some additional considerations: The drive pinion is ok, but should probably go to a larger gear, use the final gearing as a factor for the transmission. Also, you should consider incorporating a thrust bearing to absorb axial thrust. One other consideration. long shafts need more bearings for support. Have you looked into using a hollow shaft? There is always a certain amount of torque involved which a hollow shaft can absorb better without breaking. Or if you want to get really fancy you can use a quill shaft (you'll have to look it up). A quill shaft is essentially a shaft within a shaft that extends the effective length of a shaft. Length is good because that, too, increases torque flexion which reduces breakage.

30-12-2006 11:37

MAteo9944

Re: pic: Shaft Drive Bearing Blocks

This is actually something that I considered for a while, but one of my team's ME mentors talked me out of it. I can't figure out if it would be a problem or not. I probably need to test it out and see. If anyone has experience with this could you please help me out?

As for the IGUS bearings, they should be fine. They are not under too much load from the miter gears. We are using conventional ball bearings where there are higher loads.

30-12-2006 11:52

MAteo9944

Re: pic: Shaft Drive Bearing Blocks

I went with the solid shafts so that I could use shaft keys instead of set screws. I do like the idea of quill shafts. I will have to look into them.
Hopefully one day soon I can get hold of an ME for long enough to go over all of the calculations for the strength of the system. As of now the design is still without the support of the calculations, so the sizes and types of components are subject to change.

30-12-2006 13:01

Greg Needel

Re: pic: Shaft Drive Bearing Blocks

Efficiency might be your worst nightmare with this system.

Chain & Belt Efficiency ~ 95% - 98%
Spur Gears Efficiency ~ 95% - 98%
Bevel Gears Efficiency ~ 90% - 95%
Planetary Gears Efficiency ~ 80% - 90%


Efficiencies multiply for every stage in your gearbox, so for your drive you have spur>planetary>bevel(x2) assuming that your gearboxes are that the top of efficiency you would be getting .98*.95*.90*.90= ~75.4% efficient at transferring torque. This also does not take into account the effects of torsion of your drive shaft on the efficiency. I like the idea and how different it is in application, but sometimes being innovative for no reasons can have adverse effects. That being said I would love to see this drive work, but I would be hesitant to build this for the competition season without building a prototype drive in the off season.

30-12-2006 14:00

John C

Re: pic: Shaft Drive Bearing Blocks

My team has also designed a similar system. There is one thing you may want to take into consideration. In our design we use spider couplings in the driveshaft to allow for slight misalignments in the bearing position. This was important for us because if the robot frame becomes bent slightly in any way it will bind the shaft. Ill post the drawings after our next meeting on Tuesday so you can see where they are located.

30-12-2006 21:18

Jeremiah H

Re: pic: Shaft Drive Bearing Blocks

Cool idea. I like it.
On the keys/set screw thing, I suggest looking into using hex stock rather than round. I know that it will make the build phase harder, but I believe that the advantages gained will be well worth it. The hex gives you six corners to bear the load, and I would think that you could drill quite a bit out of the center without losing much strength. I know hex is used in John Deere hay balers for the main driving shaft, and I have never heard of there being problems with them (not there, at least). JH

30-12-2006 21:22

amos229

Re: pic: Shaft Drive Bearing Blocks

Great design. but if the frame flexes chances are something is gonna skip or break. plus under the tremendous forces on that shaft i doubt the bearing blocks will stay in place. If its a flat game this year then it might work but even then the robots still take alot of beating.

31-12-2006 01:23

Dan Richardson

Re: pic: Shaft Drive Bearing Blocks

Yeah this is a neat design something you don't see very often, and there may be some good reasons, the efficieny is one thing, another is what you've addressed above.

It will be significantly important for you to go over the stress strain differentials with an ME. There will be many things you'll have to worry about, one is axial and torsional stresses, make sure the shaft is strong enough so that it can handled the axial stresses over the distance and that your degree of rotation is not very significant, as to where it could cause failure.

Make sure the shock, or impulse, of the robot flying full speed into a wall, or another robot is not transfered through the shaft. And if it is at all transfered make sure that the shaft is robust enough so that absorb the impact plastically without significant deflection and no deformation.

Also important, and quite possibly the easiest thing to overlook. is the frame. Frames over the course of competition have a tendancy to get all bent out of shape, it is often minute, but It does happen. In a system where allignment is paramount for optimum effciency its important that frame flexure is limited. A rigid frame will be important make sure that frame is designed in such a way that deflection is minimum and even slight deformation is avoided.

Looks good tho, I'm excited to see the newer drive style, will love to see how it works out.

31-12-2006 08:27

TVan

Re: pic: Shaft Drive Bearing Blocks

Looks great, I would consider a flexible shaft coupling on each side. If it looks like your frame will flex, you would not like teeth on the field

Tim

31-12-2006 11:37

Al Skierkiewicz

Re: pic: Shaft Drive Bearing Blocks

Matt,
I know that the drawing may not show the components in correct scale but with this kind of drive there are some significant forces that need to be addressed. The rotational forces on each shaft when pushing or in stall will produce a side force if you think about it. Because of those forces, larger bearings and stiffer blocks are needed relative to other types of drive. Without the extra beef, misalignment and high frictional forces will occur in the bearings and shafts. Think of the construction of a car differential and you will get an idea of the kind of strength you will need.

31-12-2006 15:20

Not2B

Re: pic: Shaft Drive Bearing Blocks

There's been alot of good engineering concerns (side forces, stress on the shaft, etc...)

Here's one more.

Imagine hitting it with a sledge hammer. Can you get it working again in 5 minutes? Can you get a chain drive working again in 5 minutes?

31-12-2006 17:17

MrForbes

Re: pic: Shaft Drive Bearing Blocks

How about mounting both bearings on either side of one support, and get rid of the othe support? it would save some weight, and should give adequate strength to hold the gear in alignment.

31-12-2006 19:29

RyanN

Re: pic: Shaft Drive Bearing Blocks

Great work on your drive train system! One thing to think about that I thought of... How about making a worm gear system. I think this would be more reliable than the one that you currently have, and you will have much greater control of your robot. The only thing is that you probably would want a fairly small gear on the axle that goes to the wheel.

Just my $0.02...

31-12-2006 20:10

MAteo9944

Re: pic: Shaft Drive Bearing Blocks

The problem with worm gears is in the efficiency. They are usually about 60-70% efficient as opposed to the 95% efficiency of a miter gear. Some worm gears are even as low as 20% though I have also seen 98% reported.

31-12-2006 22:27

Jeff Pahl

Re: pic: Shaft Drive Bearing Blocks

Depending on the game and strategy, the resistance of worm gears to being back driven may be advantageous, or a significant disadvantage. However, I think it would take a lot of strategy advantage to make up for the loss in efficiency.

And it would make it about impossible to push the robot around without turning it on and having the controls. Since it usually has to go in the back of my truck to go to demos, etc. I would really hate that

01-01-2007 11:22

RyanN

Re: pic: Shaft Drive Bearing Blocks

I was not sure of the efficiency of a worm gear system. I did not analyze how a worm gear system would be disadvantageous to a team and only looked at the advantages. I saw that the robot would be very responsive (Like when the driver would let go of the joysticks, it would immediately stop) and that other robots could not push my team's robot around, but then you said that it would be a "significant disadvantage," which I can see happening very easily. It would be almost impossible to push a locked robot across the field in the time frame that FIRST gives us during a match. Another disadvantage I found, and I'm not sure if you guys said or not, is that when pushing a robot, the efficiency of the system decreases as friction between the gears increases... I remember this now from my Erector Set when I was a kid. Brings back some memories... Now back to reality; now that I see the disadvantages, I would not use a worm gear system. I guess another advantage that I thought of is that this system would probably be more reliable than our traditional chain method.