normde2001
Elite Member
- Joined
- Mar 10, 2002
- Messages
- 4,163
How did you arrive at that safety margin of 2?
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YM 2220D too narrow
- Thread starter Zoster
- Start date
- Views: 10073
htiek126
Silver Member
- Joined
- Mar 11, 2002
- Messages
- 236
- Location
- Southern Maryland
- Tractor
- Iseki TA270F, Yanmar YM1510D, Mitsubishi MT2201D
For steel, it is common to use a safety margin of two (this is application dependent). You do not want to design to the breaking point of steel. This is a rather simplistic model for the forces encountered, so the safety margin is also a way to account for some of the more complicated or unexpected stresses that may be encountered.
Also, the previous calculations do not account for dynamic forces. These tractors move rather slow and have large tires to absorb some of the shock, so the dynamic forces should not be extreme.
Also, the previous calculations do not account for dynamic forces. These tractors move rather slow and have large tires to absorb some of the shock, so the dynamic forces should not be extreme.
rlee6
Silver Member
Stress from bending is somewhat different from stress from pulling. The previous calculation was for the latter. For bending, max stress on the ansle is (bending moment)x(radius)/(moment of inertia). Using 2" diameter, 8" from outer bearing to wheel, 1000 pound load on the wheel, I get a number like 10200 psi. Axle is most likely a hardened steel with yield stress of 50,000 psi or greater. I doubt that the axle will snap even with dynamic load. But you are increasing the chance the axle may fail by small crack growth such as fatigue. (I really don't think it is likely)
But increase in bending moment (as previously said, 33% increase) will increase the load on the bearing. It may fail way before the axle fails by fatigue type cracking. In my uneducated opinion (I don't even have a tractor yet), safety improvement from wider wheel span (if it is real) outweighs the safety compromise due to premature failure.
That's my 2 cents.
But increase in bending moment (as previously said, 33% increase) will increase the load on the bearing. It may fail way before the axle fails by fatigue type cracking. In my uneducated opinion (I don't even have a tractor yet), safety improvement from wider wheel span (if it is real) outweighs the safety compromise due to premature failure.
That's my 2 cents.
rlee6
Silver Member
Zoster,
I re-read the whole thread and noticed HoyeTractor's comment on some engineer estimating doubling stress for every 1/2 in extension. THIS COMMENT BOTHERS ME. Since I don't have a tractor and don't know how a wheel is mounted, I cannot say for sure if his comment can be safely rejected.
So, please tell me someone how a wheel is mounted on the axle. I was relying on another post that mentioned 2" diameter axle, 6" distance between the outer bearing and wheel center. In other words, the outer bearing is the last support of the axle. From then on, the axle is just sticking out freely. If this is the correct description of the axle/wheel, then the other engineer was wrong. There are 2 components of stress on the axle. One is torsional stress which should remain same. Only bending stress increases in proportion to extension. Combination of the two stresses does not magically increase total stress.
Zoster,
Before taking off with the idea, can you talk to an engineer? Or, can you post some pictures for me?
HoyeTractor,
What kind of engineer was he? I am not challenging him. I just don't want to extend beyond my capability.
I re-read the whole thread and noticed HoyeTractor's comment on some engineer estimating doubling stress for every 1/2 in extension. THIS COMMENT BOTHERS ME. Since I don't have a tractor and don't know how a wheel is mounted, I cannot say for sure if his comment can be safely rejected.
So, please tell me someone how a wheel is mounted on the axle. I was relying on another post that mentioned 2" diameter axle, 6" distance between the outer bearing and wheel center. In other words, the outer bearing is the last support of the axle. From then on, the axle is just sticking out freely. If this is the correct description of the axle/wheel, then the other engineer was wrong. There are 2 components of stress on the axle. One is torsional stress which should remain same. Only bending stress increases in proportion to extension. Combination of the two stresses does not magically increase total stress.
Zoster,
Before taking off with the idea, can you talk to an engineer? Or, can you post some pictures for me?
HoyeTractor,
What kind of engineer was he? I am not challenging him. I just don't want to extend beyond my capability.
jdmar
Gold Member
Maybe an engineer...hmmm maybe a good physics professor at your local college/high school. They may take this on as a project/problem for the class.
We know the mass of the tractor stays relatively constant. The vertical force exerted on the drive shaft (not rotational force)certainly changes with load (bouncing, bumps, etc).
If each outer bearing sees a 600 pounds of force statically (only a rough estimate), that force will increase with bumps etc. Also if you exert that same 600 pounds at a farther distance shouldn't you exert a greater force at the bearing. Think torque wrench or when you slip a pipe over your socket wrench handle to get extra power.
One question is does all the increased force act at one bearing point or is it divide force at two bearing points. Also what total force can the bearing take (might call Timkin bearing in Canton Ohio on that one).
Generally I think an engineer would be the right person to answer this. I have only farmboy questions and gut feelings...no real answers /forums/images/graemlins/grin.gif I can't imagine small spacers make any significant change. Maybe someone with a similar "domestic" tractor of similar size (eg a John deer 650 or one of the Kubotas) could call the factory rep and ask what the think. I imagine manufacturing specs are similar with these machines.
OK, so I ramble with no real help/goal/info...I just find these type of questions interesting and love learning from everyones knowledge. Hopefully we get an answer. I think Htiak126 is the star so far. I am just not sure if all the increased stress is applied to the inner bearing surface (i.e. the outer bearing only acts as a pivot point without risk of increased force).
I think we needs wun of dem der twaktor enginears to cypher dis wun. /forums/images/graemlins/confused.gif
Peter
We know the mass of the tractor stays relatively constant. The vertical force exerted on the drive shaft (not rotational force)certainly changes with load (bouncing, bumps, etc).
If each outer bearing sees a 600 pounds of force statically (only a rough estimate), that force will increase with bumps etc. Also if you exert that same 600 pounds at a farther distance shouldn't you exert a greater force at the bearing. Think torque wrench or when you slip a pipe over your socket wrench handle to get extra power.
One question is does all the increased force act at one bearing point or is it divide force at two bearing points. Also what total force can the bearing take (might call Timkin bearing in Canton Ohio on that one).
Generally I think an engineer would be the right person to answer this. I have only farmboy questions and gut feelings...no real answers /forums/images/graemlins/grin.gif I can't imagine small spacers make any significant change. Maybe someone with a similar "domestic" tractor of similar size (eg a John deer 650 or one of the Kubotas) could call the factory rep and ask what the think. I imagine manufacturing specs are similar with these machines.
OK, so I ramble with no real help/goal/info...I just find these type of questions interesting and love learning from everyones knowledge. Hopefully we get an answer. I think Htiak126 is the star so far. I am just not sure if all the increased stress is applied to the inner bearing surface (i.e. the outer bearing only acts as a pivot point without risk of increased force).
I think we needs wun of dem der twaktor enginears to cypher dis wun. /forums/images/graemlins/confused.gif
Peter
Philip_L
Member
Hello all
I hve been reading this post, and it peaked my interest. As a mechanical engineer by profession, I was curious about how adding a spacer to widen your track width affects the stress on the axle and the load on the bearing. Well being the nerd that I am, I plopped myself down in Excel and banged through the calculations required to determine the axle stress and deflection of the axle vs/ spacer length. I was surpirsed to see not a a huge increase as spacer length is increased ( it definitely is not 2x per 0.5in increase in spacer length as someone earlier mentioned). I have attached an Excel spreadsheet which you can download and play with for yourself. To summarize the data:
Assuming a 6 in spacer per side (worst case)
You have a safety factor of at least 3 for torsional stress, torsional deflection, bending stress, axle deflection (sag), shear stress, and radial load on the bearing.
In the engineering world, something with a safety factor creeping up on 4 is considered "tractor engineered" no pun intended. Most tractors are way overdesigned mainly b/c weight is not a negative but a positive, and you never know what someone will try to do with a tractor.
Note:
my calculations are for static loads only and do not incorporate fatigue - this is why a safety factor is used - to account for these things.
I think you will be fine with the spacer, whenever I get a tractor I will make me some.
PS - if anyone needs spacers, I can design some in Solidworks for you and provide you with prints to take to your local machine shop (free of charge of course) or machine them for you myself.
I hve been reading this post, and it peaked my interest. As a mechanical engineer by profession, I was curious about how adding a spacer to widen your track width affects the stress on the axle and the load on the bearing. Well being the nerd that I am, I plopped myself down in Excel and banged through the calculations required to determine the axle stress and deflection of the axle vs/ spacer length. I was surpirsed to see not a a huge increase as spacer length is increased ( it definitely is not 2x per 0.5in increase in spacer length as someone earlier mentioned). I have attached an Excel spreadsheet which you can download and play with for yourself. To summarize the data:
Assuming a 6 in spacer per side (worst case)
You have a safety factor of at least 3 for torsional stress, torsional deflection, bending stress, axle deflection (sag), shear stress, and radial load on the bearing.
In the engineering world, something with a safety factor creeping up on 4 is considered "tractor engineered" no pun intended. Most tractors are way overdesigned mainly b/c weight is not a negative but a positive, and you never know what someone will try to do with a tractor.
Note:
my calculations are for static loads only and do not incorporate fatigue - this is why a safety factor is used - to account for these things.
I think you will be fine with the spacer, whenever I get a tractor I will make me some.
PS - if anyone needs spacers, I can design some in Solidworks for you and provide you with prints to take to your local machine shop (free of charge of course) or machine them for you myself.
rlee6
Silver Member
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PS - if anyone needs spacers, I can design some in Solidworks for you and provide you with prints to take to your local machine shop (free of charge of course) or machine them for you myself.
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I will take a rain check. I don't have a tractor yet but soon I will need a spacer myself.
Thanks for the calculations and numbers.
PS - if anyone needs spacers, I can design some in Solidworks for you and provide you with prints to take to your local machine shop (free of charge of course) or machine them for you myself.
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I will take a rain check. I don't have a tractor yet but soon I will need a spacer myself.
Thanks for the calculations and numbers.
mark777
Veteran Member
- Joined
- May 16, 2004
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- 1,300
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- S.E Texas
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- Ym1300d-1401D-1601D , 1610D & Massey Ferguson 1020
Philip,
Welcome to TBN. You will find that members on this site are very creative and make many of their own parts and attachments with amazing results.
It is always nice to have shared ideas backed up with a gracious offer like yours. I will bookmark your name with others for my future plans.
Thank You and regards, Mark
Welcome to TBN. You will find that members on this site are very creative and make many of their own parts and attachments with amazing results.
It is always nice to have shared ideas backed up with a gracious offer like yours. I will bookmark your name with others for my future plans.
Thank You and regards, Mark
gbirky
Platinum Member
- Joined
- Mar 25, 2002
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- Near Hagerstown, MD
- Tractor
- Yanmar models: YM1110D, YM1301D
Philip,
Nicely done spreadsheet. My only comment is that the change in stress from adding a specific thickness spacer will be a strong function of the separation between the inner bearing and the outer bearing. You have shown this separation as 6", which may be a bit high for the smaller tractors.
Nicely done spreadsheet. My only comment is that the change in stress from adding a specific thickness spacer will be a strong function of the separation between the inner bearing and the outer bearing. You have shown this separation as 6", which may be a bit high for the smaller tractors.
