View Full Version : I'm doing a favor for a friend
02-29-2004, 01:56 PM
who brought me a sealed Ball Bearing and a shaft for a Bush Hog that had been snapped in two and that he had welded back together. He wants me to turn down the weld so he can fit the bearing and return the borrowed machine to its owner.
Question: How much difference do I make on the shaft as compared to the I.D. of the Bearing? I'm more accustomed to fitting Bronze Bushings than Ball Bearings is why I ask.
02-29-2004, 02:35 PM
I would use .0015" to .002". If the bearing O.D. fits inside of a tight housing
remember that the roller or ball clearance
will be reduced by the amount of the interference fit on the I.D. In actual use
a slight interference fit of .001" would work but for the use its being used for
.0015" to .002' would be better.
02-29-2004, 02:45 PM
Who broke it?
02-29-2004, 04:21 PM
I suggest that you make a new shaft. There is no way for you to tell how good the weld is. The last man working on it is likely the one who will be blamed when it snaps again. If someone is injured when it flies apart there will be hell to pay.
02-29-2004, 04:41 PM
I would agree with G.A. I have a bush hog at work and you should see the beating that thing goes through. I won't even make the knives for them as they are tempered. I would suggest making a new shaft. Just to be safe.
02-29-2004, 04:43 PM
I agree with G.A. Ewen. Welding a shaft is chancy at best, especially one that may see some of the stresses of a brush hog with the possiblity of damage to life and property in the event of future failure.
03-01-2004, 01:44 PM
Normally, I would agree about just making a new shaft, especially since I did not have anything to do with the shaft breaking, but, the guys responsible for it do not want to pay for a new shaft!! Thanks for the info though.
03-01-2004, 02:12 PM
If this is the main shaft that takes the cutter head make a new one of good material not just mild steel.
Look at the shaft fracture. Chances are it's a classic fatigue failure and the fault of no-one but the equipment designer.
Maybe it looks like this:
http://images.google.com/imgres?imgurl=www.structint.com/analytical/metal.jpg&imgrefurl=http://www.structint.com/analytical/metal.htm&h=261&w=327&sz=22&tbnid=1h2MvOD5aRcJ:&tbnh=90& tbnw=112&prev=/images%3Fq%3DMetal%2Bfatigue%2B%26hl%3Den%26lr%3D% 26ie%3DUTF-8%26sa%3DG
The fracture likely propagated from a set-screw mark and looks silvery gray like it was brittle to part way across the shaft. Ultimate failure will look ductile.
Help your buddy explain it to the machine's owner so he won't feel like his equipment has been abused. Ask him to spring for a piece of 4130 (how big is this shaft?). Explain the busted shaft cannot be safely welded and you'll take no part in a weld repair or the subsequent machining.
If the bearing are pillow blocks the shaft is a slip fit (0.0005" to 0.0010" clearance). If the bearings were pressed on look up the press fit for bearings in that size and number for that service.
03-01-2004, 02:13 PM
They may not want to pay, but you can bet the lawyers will want to make YOU pay if someone gets hurt.
03-01-2004, 07:30 PM
Years ago I was supervising a couple of Hydro Axe operators clearing a powerline ROW when one of the machines broke the main shaft. It was a short, thick, VERY EXPENSIVE piece of steel. The owner wasn't too concerned about it because they had a lifespan and he had gotten more time from it than he expected. Point here is, maybe the shaft that broke on the Bush Hog was at the end of it's lifespan and fatigue was the problem. Harsh work enviroment, eh?
03-01-2004, 07:44 PM
I think that the deal here is that the two "borrowers" do not want the "owner" to know that they busted his machinery. Some of these Tennessee farmers can be pretty touchy about their equiptment. Since the "borrowers" have done all the welding, why should I be responsible for anything other than turning it down to a correct tolerance?
Because you may not be doing a favor for the friend or the owner of the machinery...and it seems like you may know a bit more than your friend about machinery, fractures, heat affected zones, etc. etc. And hence, have an obligation as a friend to teach these guys a thing or two. There also seems to be an ethical issue with these gentlemen hiding the fracture from the owner...anyway you're all adults and I'm not there to see the shaft so...
I'd mic an undamaged portion of the shaft and match that...or go 1~1.5 thousands max on the bearing fit.
<font face="Verdana, Arial" size="2">Originally posted by Al Messer:
I think that the deal here is that the two "borrowers" do not want the "owner" to know that they busted his machinery. Some of these Tennessee farmers can be pretty touchy about their equiptment. Since the "borrowers" have done all the welding, why should I be responsible for anything other than turning it down to a correct tolerance? </font>
[This message has been edited by abn (edited 03-01-2004).]
03-02-2004, 07:57 AM
Ok here is a question. If you were to machine down a ways, and make a coller to go on the shaft. Maybe make it a shrink fit. Weld it on, and then machine the the new sleeve to the correct tolerances. Wouldn't that make the shaft stronger? You would have the initial weld, underneath the coller and the coller would help spread the load over both sections. I figure that would help.
What do you all think?
03-02-2004, 08:24 AM
I wouldn't touch that with your ten foot pole.
A guy wanted to have me re-machine welded dump truck front end spindles once.
I asked him who would pay when it broke and killed a young mother and her two kids?
I told him to get bent.
03-02-2004, 11:00 AM
I would question the quality of the weld. I doubt that it was beveled very far below the surface before welding. Are these guys certified welders or just farmer welders?
I use bush hogs and those shafts take a beating. I don't see how any repair could be safe. When the blades coming flying out the next time, some one could be seriously hurt or killed; probably the person driving the tractor.
03-02-2004, 12:03 PM
I sounds as though you are being asked to help perpetuate a lie, As other have said I also suggest you not do it. A good weld is unlikely to break, but as this tube already broke once it will likely break again, if so the owner will then realize that it had been repaired before. I am sure you do not want to be introduced to someone as the one who repaired it.
03-02-2004, 12:56 PM
I'm going to jump in on this one since i'm in the farm equipment business. We sell Bush Hogs or rotary cutters, whichever term you want to use but, we DO NOT, WILL NOT, or even THINK ABOUT welding a PTO shaft. When life or limb is at risk replace with OEM parts only. This isnot just a personal opinion but is is a company policy, rather be safe than sorry anyday.
03-03-2004, 10:44 PM
I have bush hogs. Even the five footer has enough energy stored in the blades at 550 rpm to move the tractor 15 feet or more with engine killed (no over running clutch back then.
If the shaft breaks, all that energy is turned loose under the mower. It would not be pretty. buy (or at least make) a new shaft. THe lower bearing fit is not critical but tight is better. I have seen the shafts so worn that the bearing just "rattled". So long as the oil seal will keep the oil in the bearing is probably tight enough. I suspect some severe abuse took place. The blades should collapse when hitting logs and stumps. I can think of no way to bend the shaft except snagging it on an obstacle while moving. If break occurred outside the transmission case, do not trust it at all. if inside, shrink a collar on the shaft so it cannot drop out if the break re-occurs.
Seems to me the owner should be involved in the decision too. More ways to lie than with your mouth and a hidden repair is one of the worst ways.
To answer the question you asked- how much clearance? Sloppy will work, but PTO and shafts should be an interference on retainers/ Hot bearing (boiling water at a minimum, and shaft in freezer. Be very sure every thing is ready to go because it will take a press to remove the bearing if you position it wrong- Very embarrassing story here.
03-04-2004, 12:06 AM
I am interested in your story http://bbs.homeshopmachinist.net//wink.gif
03-05-2004, 08:13 PM
Rob: I have a Massey Ferguson tractor, looks like a Fordson. Many parts are interchangeable. Massey wanted an obscene price for a new PTO shaft (splines were worn on the original). Checked with ford, sure looked the same to me. Bought shaft, had already bought a collar and bearing from Massey.
Took them home, damn collar was a little small. AH!!! A shrink fit says I. Shaft into deep freeze, then soused it into propane (wished i had liquid nitrogen). Heated the collar while the shaft was cooling. Dropped the collar on, hit a mighty swipe with a pipe sleeve. All went almost well, maybe half the collar width to go when it froze. NO SWEAT, I supervised a fine machine shop, full of men who would bail me out, including a 100 ton press if need be.
Asked one of the men to "press this collar the rest of the way on for me". About noon man says how did you get it on in the first place. I mentioned the freezer and pipe. Neglected to mention the propane (My reputation for doing unsafe things was in no jeopardy anyway). Collar was still shiny so they knew i had not made it red hot.
Old boy looked at me kind of anti-godlin, like maybe I had not told ALL the story. When they could not press the collar off, they ground it off, intending to machine a proper fitting collar. The shaft had a ring compressed under the shaft.. You could snag a finger nail on the ridge. A smart ass young engineer (one of my men too) calculated the temp difference required to make the ring, I don't think they allowed for the collar stretching http://bbs.homeshopmachinist.net//smile.gif. Any way, They made my collar, shaft is still in service. Long time before I admitted using propane, they insisted I had at least dry ice, LOX, or Nitrogen. Claimed no one could do what i did otherwise. I bet a fifth of Old Grand Dad that I could duplicate the feat. No one was willing to witness after I told how to do it. So we split the fifth, told more lies about how we got into and out of troubles. That bunch was bad, one would lie, the others swear to it. I was new man on the job, later discovered most of what they told for lies was really true. Man, I do miss those men (hung it up in 1984).
03-05-2004, 09:03 PM
Some of you guys scare me. I would be hard press to even think about repairing that item for myself, much less like this story is unfolding. My liability would be on the line for one thing, but more than that, the fact that I knew they were trying to hide it from the owner would tell on my reputation. That should never be sold and especially not at the price of a bushhog...
David from Jax
03-10-2004, 09:30 PM
I un-covered some new facts last night and I have learned that the owner of the machine got tired of replacing shear pins in the PTO shaft, so he substituted a 1/2" Grade 8 hardened bolt and nut for the shear pin and when it hit someting out of the ordinary, the PTO shaft went but the bolt held. My friend says that if it was up to him, he would buy a new shaft, but the machine is so old they cannot find a name or a maker's plate on it.
03-10-2004, 11:27 PM
I think that might shift the blame a little.I am just glad they aren't fixing it behind the owners back. Still wouldn't fix it, unless you could make a new shaft. Welding it just ain't where it's at. Might be able to locate one on ebay if you keep looking.
good luck, David from jax
03-11-2004, 04:48 PM
Can you send me a pic of the bush hog. I will help track down the maker of it. I don't mind. I have a bush hog with a chevette motor on it. I think it is custom (snicker) http://bbs.homeshopmachinist.net//smile.gif.
Why do people do such things anyways. Switch out the grade 5 bolts for Grade 8. If people only knew...
03-11-2004, 08:01 PM
Rob, I don't even know where the bushhog is located!! And I'm beginning to wish I had never heard of it either!!
03-11-2004, 10:09 PM
well if you can get your hands on a picture of it, post it here and we will all try to remember to find you one.
David from jax
03-12-2004, 07:41 AM
Hey Al i feel your pain. Just want to help.
03-12-2004, 08:33 AM
Thanks, Guys! I need all the "support" I can get!!
08-14-2004, 01:22 PM
so what ever happened?
08-14-2004, 08:49 PM
Well, since the weld had made a "hard spot" on the shaft and my HSS bits would not cut it, I just gave it back to him and told him he would have to have it centerless ground. Since then, I have not heard a word, not a single word, about it. And hope I never do!!
08-15-2004, 10:29 PM
David from jax