Drive-train shaft size
 

I did a search but didn't see what I am looking for at first glance.
I just recieved some samples from IGUS that are plastic bearings and such. Their website lists two FIRST teams who used them or other products in the last two years.

My question is two fold:

1. What size of shaft do you use on your drive-train if it is a roller chain sprocket type? We have a lot of stock in 5/8 with a keyway since the SkyWay Beadlock wheels have that bore type. I am not an ME so I don't feel solid on doing calculations but that seems to be extra beefy for FIRST class robots. Is 3/8 or 7/16 too weak?

2. If a robot only competes in one regional and then the championships, would the IGUS plastic bearings hold up to the abuse? Pillow block sets of steel are a big weight user. We are not at a fabrication level to be able to use flanged bearing set into the frame.

Re: Drive-train shaft size
 

In the past, our team has used either 5/8" or 1/2" keyed shaft with ball bearing pillow blocks. Pillow blocks are available with a lighter alloy frame instead of cast iron or steel. This year we used the kit of parts method which eliminates the pillow blocks. The 3/8" bolts worked fine. We did use grade 8 bolts though. Allot depends on the hub and bore options that are available for the wheels you chose. This year's game wasn't as abusive as some years games. Don't know if the bolts would have survived some of the massive impacts we have taken in past years .

Re: Drive-train shaft size
 

It all depends on the material. In 2004 we used a 5/8" aluminum keyed axle (3/16" key) , turned down to 1/2" on the ends to go in bearings. Over time, the keyway got a little sloppy and it got some play. But keep in mind there were large 12" wheels on this axle.

In 2005, we used a 1045 carbon steel 1/2" keyed axle (1/8" key). We had the wheels external (cantilevered out) and we bent the actual wheels but not the axles.

The problem when you go smaller than 1/2" is that your key size goes down (less than 1/8") unless you do an oversize key.

If you had a dead axle, or a hex axle, or a woodruff key maybe, then you could probably go down to 7/16 or 3/8.

As for calculations, I have no idea. In the short six weeks of FIRST Robotics build, I'm not even sure that something like that needs to be or should be calculated. Go with what you know will work, and try a little less if you are feeling brave. That is my rule of thumb. Of course it would probably never apply in the real world of engineering, but it FIRST it works beautifully.

Also, can you describe a little bit more exactly what your fabrication abilities/resources/capabilites are? With that info we can give you some more ideas for bearings and axles and stuff.

Re: Drive-train shaft size
 

100 has used 3/8" and 1/2" shaft. Never have had problems with either. Had plenty of problems with poorly secured sprockets, ie: set screws only.

Re: Drive-train shaft size
 

We have used nothing bigger then 3/8 for the last 4 years and haven't had any problems. Now for arms we have use 1/2 inch.

Re: Drive-train shaft size
 

Thanks for the input so far. We went back to the KOP wheels and shaft this season after a three years. The bolt on sprocket to the wheel eliminates the keyway issue of which was not in my thought process. Downsizing the shaft diameter would then require such a drive mechanism. We bent an axle this year but I am certian it was due to an over-zealous mechanic with the chain tensioner.


So 5/8 or 1/2 if using a keyway works, otherwise 7/16 or 3/8 can work if not using a keyway.

How about the IGUS plastic bearings? I'm looking through their catalog now and see uses in conveyor systems and the like.

Our team has no on-site milling or lathe capability. Our partners at Kelly USA, Lockheed Martin, have made some custom bearing blocks for mechanisms they help fabricate but that is it.

Re: Drive-train shaft size
 

If you're using 3/8" shaft, you definitely want to key it. NEVER rely on only set screws unless it's absolutely the only way you can do it.

Re: Drive-train shaft size
 

We used 5/8 aluminum for our stationary axles with pillow blocks/clamps on both ends. We directly attached the sprocket to the custom made wheels/hubs. We just used regular ball bearings inside the hubs and custom made wheels.

Re: Drive-train shaft size
 

I think he meant having a dead axle with the wheel driven directly byt he sprocket.

Re: Drive-train shaft size
 

It all depends on how you're driving your wheels. If youre driving them with the bolt on sprocket style, the wheel just rests on the shaft and has its own bearings. A 3/8" steel shaft works fine for this, even 5/16" has been used with no problems.

However if you are using a shaft driven wheel, I would definitely use a 1/2" steel shaft with a 1/8" keyway. Just for security. If you want to use aluminum, then you should use a 5/8" shaft with a 1/8" keyway. Of course there are always variations. If you have a small 3" wheel, you can easily run it on a 3/8" shaft because it isnt taking so much torque, especially if the shaft is supported on both sides of the wheel.

Just use your common sense and ask yourself, what forces is this taking, and how big? If The wheel is run by bolt on sprocket, then the shaft only takes weight. If the wheel is shaft driven, then it takes torque and weight, so its gotta be stronger. If the wheel is smaller (high speed, low torque) then the shaft can be smaller than if the wheel were bigger (low speed, high torque). Just take all these things into account, and figure out the best shaft size.

A few notes: keyways are superior to set screws and roll pins in transmitting torque. I had to learn this the hard way freshman year. Also, the plastic skyway wheels can be stronger than the shaft they run on. You can break the shaft without breaking the wheel.

As for transmissions, it all depends on speed again. Your motors can only give a maximum power rating, so a high RPM shaft will take less torque than an low RPM shaft. Thus, my own transmissions have always had high RPM outputs, just to keep the shafts small (lowest RPM I've geared to was 1000 rpm at no load, on two CIMs). If you keep relatively high gearbox shaft speeds, you can easily have your entire transmission running 3/8" or 5/16" shafts. For shifters, I have always used 1/2" shafts on the shifter itself, for safety, since shifting can be rough.

As for your bearing question, I've always used steel bearings on everything. I dont know what type of bearings you have, whether they are thermoplastic or ceramic based plastic will change their load and rpm ratings significantly. It's best if you can get the bearings specs, just to be sure.

Hope that helps.

Re: Drive-train shaft size
 

I have always used 3/8 inch shaft supported on both sides, both with keyways driving a wheel and having a wheel ride on it with a sprocket bolted to the wheel. Never had one failure.

Re: Drive-train shaft size
 

What size key and what diameter wheel?

YAY! This is my 3500th post!

Re: Drive-train shaft size
 

The 3/8" bolts did work fine. We, however, were told during inspection that after the wear of two regionals, it would be best that we replace the bolts with bolts of stronger steel before championships because the ones the kit provided did have a tendency to warp after a while.

As for 3/8" shafts in general, we've never had a problem with them.

Re: Drive-train shaft size
 

My team 1251 has always used 5/8 in steel for our drive axles and usually miller bearings and or a plastic material with a high oil content called UHMW for bearings. Generally for transmissions miller bearings and not the plastic. We have never had a problem with the 5/8in shaft. We are currently working on a gearbox for our arm from the past years bot which uses 3/8 steel internal shafts and a 1/2 in output.I know the gears will have a set screw and a key way however I am not sure of the size as of yet.

Re: Drive-train shaft size
 

For our arm joints we used shafts that were 1" or more diameter. I can't remember exactly. I know they took a 1/4" key. You can see here Massive torque when you take a van door motor and gear it down another 20:1 We were able to bore it out a bit inside to save some weight.

Re: Drive-train shaft size
 

The IGUS bearings can stand quite a bit of abuse. We use them in many applications in my facility. The majority are used for linear motion in vertical and horizontal slides.

We use the G300 series with little wear on equipment that runs 24/7. We do not lube them as this picks up particles that then contribute to drag.

We have also experimented with the L1 in a vertical slide mechanism. Currently we have three years with no significant wear.

It is important that the bores for these bearings match the catalog. This provides the proper clearance for the shaft. This one of the reasons we have not installed any in our robots. You cannot produce those tolerances on a drill press : )

Read the recommendations carefully. Our experience concurs that a shaft without a high polish is best. We have shifted from Thomson ground shafting to stainless shafting with standard grind. The bearing has an initial wear that embeds its material into the shaft and that this increases longevity and reduces friction.

Re: Drive-train shaft size
 

I've had great success in the past attaching sprockets to shafts using a keyless locking device known as a TranTorque Keyless Bushing. I've found them in MSC and they are produced by a company called Fenner Drives (www.FennerIndustrial.com)

The cool thing about them is that you don't need a keyway or setscrew and they come in a whole range of sizes in both english and metric. I think the smallest I saw they make fits an 1/8" shaft, but they go all the way up to something like a 3" shaft or so.

Just a thought...???

Re: Drive-train shaft size
 

Anyone have a good source for pillow block bearings with aluminum housings? Especially an economical one. Seem that most common ones have cast iron housings. Hopefully now that I'm setting up a lathe I can make my own bearing blocks, but sometimes it's less hassle to buy.

Re: Drive-train shaft size
 

One thing to take into consideration when looking at shaft selection is the support of the shaft. If your shaft-wheel is out there with no protection you'd better beef it right up. When you use a "protected" shaft(has two bearings/bushings/etc. on the two opposite sides of the wheel) you have to consider two things, the lenght of the shaft and the weight of the robot. The shorter the shaft the less flexx theyre will be and the smaller diameter you can get away. The longer that shaft gets the larger diameter you need to use to prevent the shaft from flexing/bending.

Another thing to consider is the material that the shaft is made out of. Aluminum would need to be a considerably thicker shaft than a steel shaft.

-Hope thats helpful and good luck
Pat

Re: Drive-train shaft size
 

Here's the only ones I've found.
http://www.cncbotparts.com/bearing_blocks.htm
http://www.teamdelta.com/products/prod5.htm

Have you considered stamped steel ones like this http://robotcombat.com/images/drives...wblock2_lg.jpg instead of cast iron ones?

Also, I don't believe pillow blocks are made on a lathe, a mill would be more like it, and a CNC one if you would like to get fancy.

EDIT, omg CD censors your username when I quote you. lol

Re: Drive-train shaft size
 

Grainger and McMaster-Carr. Allot of times they are listed as die cast frame. Bronze bearings are more common but there are die cast ball bearing units. Stamped steel frames and flanges are also light. There are composite units that are extremely strong and light but the price is high.

In 2004 I bought some fiberglass rods in 1/2", 5/8", and 3/4" dia. to evaluate as shafts. Very strong and light. Just a slight deflection under impact. Our team decided against them but, I am certain they would have worked fine and the weight savings is substantial.

Re: Drive-train shaft size
 

One problem with fiberglass shafts is that you really shouldnt make any cuts into them, only cut them lengthwise. If you are just running a wheel with bearings in the wheel and pillow blocks, sure. But if you want to cut a keyway into the fiberglass, you seriously weaken its strength. Fiberglass relies on the continuous glass fibers in it for its strength. If you cut the fibers, they lose their strength. If you can get a company to form the fiberglass into a keyed shaft, that would work very well. But... no cutting!

Re: Drive-train shaft size
 

How big should a drive shaft be on a FIRST Robot?

I think that this is an excellent question, since every robot I can think of is using wheels of some sort to carry it’s load. Unfortunately, of all the answers I’ve seen, I haven’t seen any math involved. Some students get inspired by seeing numbers (I know I did) so... here are some numbers.

Assumptions

To first quantify this question, you need to make a number of assumptions. I’ll try to go ahead and make some assumptions, and hopefully we’ll see if we can understand why some of these sizes that have been mentioned are okay, why some are too large, and why some might be a bit too small.

There are a number of considerations that need to be addressed, some that immediately come to mind are the following:

• Will fatigue be a factor?
• What is the typical load on the shaft?
• What is the maximum torque transmitted by the shaft?
• What will the maximum load on the shaft be?

The above questions all need to be addressed to answer the quetsion.

Will fatigue be a factor?

This is an interesting question. I’m going to say that from experience, a FIRST robot won’t see more than 10 hours of continuous run time. Assuming that the robot runs around on average at 2 ft/s with 6” diameter wheels, that gives us a life time of 72,000 feet traveled, or about 46,000 cycles (revolutions). In the scheme of steel shaft, I’m personally not too terribly worried about that, but it’s not negiligble. Whereas you should begin to start designing for infinite life around a million completely reversed bending cycles, depending on the assumptions you make above, you could possibly reach 100,000 cycles pretty quickly. Since there is actually a logarithmic relationship, 100,000 cycles is a lot closer to a million than to a static loading case. Whereas for inifinite life (million cycles+) you’d treat the allowable bending strength around 50.4% of the ultimate strength, I’d say in a situation like this, you’re going to not give a lot more leverage than say 70% the ultimate strength. What this means, in short, is that fatigue should probably be somewhat considered in situations where robots are heavily practiced with and demonstrated.

What are the forces on the shaft?

One should probably consider the average load of a full capacity robot, (read: holding maximum number of playing field objects).

Let’s assume that on the average game this is a nice round 150 pounds, and there’s a slight bias to one particular wheel of 33% of the weight of the robot. This would put a load of roughly 50 pounds on one shaft.

One often neglected factor is the fact that something is driving the shaft, and often times, this is a sprocket. For the sake of this sample calculation, we can probably assume that the sprocket is pulling in the orthogonal direction. This may actually be slightly less than conservative, but I’ll hope you’ll forgive me.

In a full fledged pushing match, assuming some super sticky tires with a coefficient of friction around 1.2, the chain will be pulling with roughly 60 lbs of force.

Also, you’ll have some torque about the shaft, equal to the 60 lbs times the radius of the wheel, and assuming a 6 inch diameter wheel, that’s 180 inch lbs of torque.

To sort of simplify some things, the critical elements, because the weight and chain pulling forces are orthogonal, are at the extreme fibers in perpendicular directions. Hence, you wouldn’t want to combine these force vectors, or you would get a critical element with bending stress that is higher than it actually is.

Because these stresses are in directions that are orthogonal (that is, the torsion is causing stress in the direction perpendicular to that of the robot weight on the shaft and the force the sprocket is pulling on the shaft) you need to combine these forces to get the total stress. Fortunately, you can combine these using von Mises or von Mises-Hencky theory, which is essentially just using distortion-energy theory.

It should be noted that this is not a particularly conservative estimate, so it should bring some answers to our question that can be somewhat reliable.

For those of you unfamiliar with von Mises, for a plane stress, it is essentially:

TotalStress = SQRT(stressX² + stressX*stressY + stressY² + 3*Torsion²)

In our case, we only have 1 stress in the bending direction, it simplifies to:

TotalStress = SQRT(stressX² + 3*Torsion²)

Now there are a few more questions to ask:

Where is the shaft loaded?
How long is the shaft?
How is the shaft supported?
Is the shaft notched?
What is the shaft material?
How does that whole fatigue thing come into play?


I’ll answer these with the following assumptions:

I’ll assume that the shaft is loaded in the center, which is a conservative assumption.

I’m going to assume a typical FIRST robot has an inside shafts length of 5” long.

I’ll assume the shaft is supported by fixed bearings on either end.

I’ll also say that the ends will be turned down to fit into some bearing blocks, but with some pretty sharp edges because a lot of teams don’t have the tooling to put good round corners in. (Stress concentration factor ~2)

Let’s do a steel shaft, say just like McMaster-Carr keyed shaft, of ANSI 1045 with a Rockwell Hardness (B Scale) of about 90. That hardness translates over to about 91 kpsi from a chart that I found here: http://www.gordonengland.co.uk/hardn...version_1c.htm

In a textbook I have (Mechanical Engineering Design, 8th edition, Shigley et al) it actually places the ultimate tensile strength of 1045 steel at exactly 91 kpsi, so I feel comfortable also using it’s yield strength of 77 kpsi for calculations involving static (or nearly static) loading.

Up above I said that we ought to use a working strength of about 70% of the ultimate strength when we do design for fatigue, which means I’d use about 63 kpsi.

What I haven’t done very well so far is explain that I’m going to be determining basically 3 calculations:

• The size of the shaft to support maximum static loads
• The size of the shaft to withstand the fatigue of reversible bending cycles.
• The size of the keyway that is needed in the shaft to ensure the wheel or sprocket stays in place.

The first calculation for a maximum total stress due to chain pull (60 lbs) involves the following calculations:

Bending Stress:

D = Diameter of shaft
Z = Section Modulus (Moment of inertia I over extreme fiber distance y (y = D/2)
L = length of shaft
F = force on shaft

My Machinery’s handbook tells me:

Z for a round shaft = pi * D³ / 32

Stress at extremes ends, center = (F * L) / (8 * Z) = (4 * F * L) / (pi * D³)

Torsional Stress

T = Torque applied
R= Radius of shaft
J = polar moment of inertia (for a round shaft, D^4/32)

Torsion = T * R / J

Now, I started to cheat and did some work in excel.

Here’s the numbers:


Wheel Diameter 6.00 in
Shaft Length 5.00 in
Allowable Stress 77,000 lbs / in²
Weight of Robot (max) 150 lbs
% Weight on Wheel (max) 33.3%
Coefficient of Friction 1.2
Shaft Load 60.0 lbs
Moment of Inertia 0.000192 in^4
Section Modulus 0.00153 in³
Modulus of Elasticity 30,000,000 psi

Shaft Estimated Max Stress 24,444 psi
Shaft Estimated Max Torsion 58,665 psi
von Mises 177,684 psi

Shaft Diameter 1/4 in
Shaft Deflection 0.007 in
Deflection as percent of length 0.14%
Factor of Safety 0.43


Key Width 3/32 in
Key Length 1/2 in
Key Allowable Torsion 45,000 psi
Key Applied Stress 30,717 psi
Key Factor of Safety 1.46

Wheel Diameter 6.00 in
Shaft Length 5.00 in
Allowable Stress 77,000 lbs / in²
Weight of Robot (max) 150 lbs
% Weight on Wheel (max) 33.3%
Coefficient of Friction 1.2
Shaft Load 60.0 lbs
Moment of Inertia 0.000971 in^4
Section Modulus 0.00518 in³
Modulus of Elasticity 30,000,000 psi

Shaft Estimated Max Stress 7,243 psi
Shaft Estimated Max Torsion 17,382 psi
von Mises 52,647 psi
Shaft Diameter 3/8 in
Shaft Deflection 0.001 in
Deflection as percent of length 0.03%
Factor of Safety 1.46

Key Width 3/32 in
Key Length 1/2 in
Key Allowable Torsion 45,000 psi
Key Applied Stress 20,478 psi
Key Factor of Safety 2.20

Wheel Diameter 6.00 in
Shaft Length 5.00 in
Allowable Stress 77,000 lbs / in²
Weight of Robot (max) 150 lbs
% Weight on Wheel (max) 33.3%
Coefficient of Friction 1.2
Shaft Load 60.0 lbs
Moment of Inertia 0.003068 in^4
Section Modulus 0.01227 in³
Modulus of Elasticity 30,000,000 psi

Shaft Estimated Max Stress 3,055 psi
Shaft Estimated Max Torsion 7,333 psi
von Mises 22,211 psi
Shaft Diameter 1/2 in
Shaft Deflection 0.000 in
Deflection as percent of length 0.01%
Factor of Safety 3.47

Key Width 1/8 in
Key Length 1/2 in
Key Allowable Torsion 45,000 psi
Key Applied Stress 11,519 psi
Key Factor of Safety 3.91


Now, there were a lot of assumptions made, but I think that this sort of tells the story pretty well in terms of what the shaft diameter needs to be in order to be reliable.

I’m not going to paste the numbers for fatigue, but the short story is that when you be more realistic about the loading for fatigue conditions (less weight and more evenly distributed) it sort of cancels out the slightly lower allowable stress, .

When I look at these numbers,and then consider the real world factors of roughness that tends to occur (setting the robot down too rough, flipping, etc) my humble opinion is that half inch shaft is probably a pretty safe bet, and three eights isn’t terribly risky either.

Keyway Calculations:

It should also be noted that if you choose to use a keyway, calculating their strength is important as well. If you decide to use some super strong keys, they’ll have an ultimate tensile strength of around 90,000 psi, (and a shear strength of roughly half that). If you assume that you have a 3/32 keyway for less than a half inch shaft and a 1/8 inch keyway for over a half inch, you can do some calculations in this respect as well, which I did in the colored text above.

Basically, you find the torque on the wheel, divide it by the radius of the shaft and divide that force by the area of your keyway. That will be the shear stress on your key. If the shear stress is less than the shear strength, you’re good to go.

I hope that this helps answer your questions. I've attached the quick spreadsheet for those who want to play along or check my quick math.

Matt

Re: Drive-train shaft size
 

yeah Matt busted out some crazy ME skills there and back in January during the begining stages of our drivetrain. It really did inspire me to get more involved knowing that i had numerical data and formulas there to back those numbers up.

Re: Drive-train shaft size
 

Matt: What about the movement of the bearing blocks and the subsequent additional "cantilever" effect (that's EE speak for something that seems to be mechanically bad)?

Re: Drive-train shaft size
 

Could you describe in more detail what you're refering to? What do you mean by movement of the bearing blocks? I've assumed that they are fixed in my calculations here.

Mayhaps some mad MS paint skillz to clarify?

Matt

Re: Drive-train shaft size
 

I'm thinking about when a bearing block shifts in its channel, causing the shaft to no longer be perpendicular to the bearings, etc.

Re: Drive-train shaft size
 

To cut a long story short, this won't affect the shaft and it's life nearly as much as it will affect the life of the bearings. Bearings are much more sensitive to misalignment, and will cause your bearings to fail well before you'd worry about your shaft life.

Sorry it took me so long to reply,

Matt