1862 question

[jbollis]

I said in my post that the numbers may not be exact but would be close. Here are some specs I found on cast aluminum. http://www.substech.com/dokuwiki/dok...um_alloy_443.0

there are others but this one had the least shear strength at 13,200 psi. My figures were in psi because of the way I figured the contact area of the bolt. Using this formula ,http://www.engineersedge.com/thread_...olt_stress.htm , is how I got my numbers, witch will be different with the new number of 13,200. The amount of contact area per bolt is .38725 sq inch. Using .3331 as the Max, .3643 for the Min, .500 for the length, and 16 threads per inch.

13,200 psi x .38725 sq inch = 5,111.7 lbs x 5 bolts = 25,558.5 lbs

Wow these new numbers are working in my favor.

Now being according to the chart the yield strength is 10,150 psi we only need to reduce the 25,558.5 lbs by about 30% (not 50 - 60 ) we are at 19,660.38 lbs or lets say 20,000 lbs to keep it easy. Then figure in the safety factor of 2 you say, we are still at near 10,000 lbs.

Now one must keep in mind that the whole rearend is only held on with 6 bolts. And they are in aluminum. I have heard of a few having problems when the bolts work loose on the tubes but that is pretty few compared to the total amount of them out there. Some of the these tractors have been working for what around 30 years now. Just think of all the twisting and jarring and banging they have gone through just mowing. Then how about all the snow/dirt plowing, cart/tree/whatever pulling they have seen.

I am not an engineer and don't claim to be one, and these are garden tractors not Airplanes.

[Matt G.]

Originally Posted by jbollis

I said in my post that the numbers may not be exact but would be close. Here are some specs I found on cast aluminum. http://www.substech.com/dokuwiki/dok...um_alloy_443.0

there are others but this one had the least shear strength at 13,200 psi. My figures were in psi because of the way I figured the contact area of the bolt. Using this formula ,http://www.engineersedge.com/thread_...olt_stress.htm , is how I got my numbers, witch will be different with the new number of 13,200. The amount of contact area per bolt is .38725 sq inch. Using .3331 as the Max, .3643 for the Min, .500 for the length, and 16 threads per inch.

13,200 psi x .38725 sq inch = 5,111.7 lbs x 5 bolts = 25,558.5 lbs

Wow these new numbers are working in my favor.

Now being according to the chart the yield strength is 10,150 psi we only need to reduce the 25,558.5 lbs by about 30% (not 50 - 60 ) we are at 19,660.38 lbs or lets say 20,000 lbs to keep it easy. Then figure in the safety factor of 2 you say, we are still at near 10,000 lbs.

Now one must keep in mind that the whole rearend is only held on with 6 bolts. And they are in aluminum. I have heard of a few having problems when the bolts work loose on the tubes but that is pretty few compared to the total amount of them out there. Some of the these tractors have been working for what around 30 years now. Just think of all the twisting and jarring and banging they have gone through just mowing. Then how about all the snow/dirt plowing, cart/tree/whatever pulling they have seen.

I am not an engineer and don't claim to be one, and these are garden tractors not Airplanes.

Ultimate strength (or yield strength, shear strength, etc) has units of psi or pounds/sq. in. You are multiplying a value in psi by a value in sq. in. (area) so you will get a FORCE in lbs, NOT a stress in psi. And once again, you are using the yield tensile strength that is given in the table, which you'll notice is actually 43% of the ultimate tensile strength. The yield shear strength is not given in that table, but like I said...50-60% (maybe even less) of the ultimate shear strength. You are still overestimating the numbers, and still aren't accounting for the stress that is already there due to bolt preload (more than you think). You aren't an engineer, but you're second-guessing the ones that originally designed this? I know we don't have any of them to consult, but you aren't in a good position to second-guess them.

Also, the six bolts that hold the rear to the frame are in shear, not tension, so the bearing strength of the aluminum (MUCH higher than the shear or tensile strength) is what is important there, not the shear strength. Why would it matter that it isn't an airplane? The engineering principles used are the same regardless of whether it is a tractor, airplane, chair, etc...

[jbollis]

If it takes 10 pounds per square inch (psi) to (sheer the aluminum out of thread), and there is 1 inch of contact area on the thread pitch, then theoretically, would it not take 10 pounds of force to pull that thread ?

Then take that 10 pounds of force per square inch (1) and multiply that by the 5 bolts, would that not equal 50 pounds of force needed on those 5 bolts with a total thread pitch contact area of 5 square inches to sheer the threads ?


I am not second guessing the engineers that built the rear end. I am trying to prove that it takes more than people want to admit to pull those threads.

And it does matter what the product is in the end. I have made parts in the tool trade for 20 years. I can show you documentation and over engineering that does happen in the Aerospace and other industries. If I were to make that rear end for the military I would know exactly how much it takes to pull those threads, because we would have to test it and document it. Then it would get X-rayed, thermal tested, stress tested, magna fluxed, and possibly some others on request. That would have to done every X number of parts, not just once. Believe me , I am the person who wrote the programs for and ran the CNC Wires/Rams that made the generator housings for the f-16 , so I know how it works. I seriously doubt CCC had those tests performed.

[Matt G.]

That there is correct; what you had initially posted had a unit error.

You still are ignoring the fact that the bolts holding that bracket on are torqued (to 360 in-lb as per the manual), and consequently, there is preload in the bolts that will create a large shear stress in the threads, just from reacting the axial preload in the bolt, even when no other load is applied. For a lubricated 3/8" bolt torqued to 360 in-lb, there is an axial load of about 4800 lb. The 5100 lb ultimate shear strength of the bolt holes you calculated is probably reasonable, but you must subtract out the preload, so you're down to 300 lb per fastener or 1500 lb total, and that's taking it all the way to failure. Like I said before, we don't want yield either, so using the yield shear stress instead, and a factor of safety, there isn't much of an allowable load left.

This is supported by the CCC service manual for the 1811, 1872, etc. On page 3-28: "The drawbar bracket is designed to pull only light loads such as utility and garden carts not exceeding a steady pull of 75 lbs." Toss a tiller or 3-point bracket on there and you've got the same problems.

You missed my point before...it does not matter what industry this is in because the engineering principles are the same across the board. You can't ignore the fastener preload because this is a tractor and not an airplane. Things get over-engineered because it is not economically feasible to design every component of everything with the minimum allowable margin of safety.

[jbollis]

I was not intentionally ignoring the preload. I just haven't found a workable formula to figure it out yet.

And not to argue what the manual says, but we all know that the hitch bolts will pull more than 75 lbs. I am just guessing but I would say I have easily had a pull load of 700 -800 lbs before. Just your normal small lawn roller would easily be a couple hundred on non level ground. I have personally seen whole 82 series tractor hanging on end by the hitch plate several occasions. That is how a guy I know strips them to part them out.

[Matt G.]

You didn't account for it...knowing it's there and ignoring it because you don't know how to estimate it isn't a good excuse.

An 800 lb trailer will not have a pull force of 800 lbs. We put wheels on things so they move easier. Under a steady pull that 800 lb trailer probably has a pull force of about 20 lbs. You are comparing apples to oranges- Hanging the tractor by its drawbar is a steady load (and only applied once if the tractor is being parted) and that's not the same as the loads in question. And just because somebody does something doesn't make it a good idea...there's plenty of people that pick up tractors with a forklift and a chain around the steering column, but that's a really bad idea because it ruins the steering column.

[jbollis]

That is funny there, I completely understand the difference between what something weighs and it's rolling resistance. I would like to see you pull my lawn cart with 800 pounds in it around my yard with your hands. I bet within the first 5 seconds you will agree that it takes more than 20 pounds of force to move it. And no were did I say the pull force would be equal or even close to what it weighs. Please read my post before telling me what I am comparing.


True hanging a tractor by its hitch is a 1 time load, but then again it only takes once to pull a thread. And according to you the manual states a steady load. Well that is pretty steady if you ask me. And that is an example that we all can relate to. Plus going by the manual recommendations of 75 lbs, hanging one by the hitch would be what at least 7 times the recommended limit.

[fire2509]

I have been successful putting a #2 on an 1862. However, it took some time and fabrication. I was able to find the proper size pulley for the gearbox at tractor supply (they had 2 in stock), and the belt from NAPA. I will try to get some pictures and post later.


Jon

[cheelo8668]

fire2509, pics would be great!
jbollis, thanks for the easily understandable info.
matt, thanks for breaking down the atom

didn't mean to start an argument. i was just trying to get some info.
from the looks of it, it can be done. just might be a little more work than im willing to do. i will not be using the tractor for mowing, as i have a 0 turn. so, the time and effort would be for the tiller only. i would still like to see the pics of fires tiller. might change my mind. i have an opportunity to buy a red 782 diesel, is this the same rear end as the 1862? thanks for the welcome and info.

[aagitch]

From the factory it's pretty much the same rear end. You could always put a cast rear in it. They're pretty easy to come by.