Inherent design flaw in the Kubota Grass Catcher

[Junkman]

Today, while assembling my new to me Kubota grass catcher (GCK60-BX) I learned of an inherent design flaw in the attachment of the Top Link Support Bracket (Pg.24 #4). I purchased this catcher used from a fellow TBN member on the West Coast and he dissembled it and shipped it to me. While putting the bolts(#7) into the Blower Drive Assembly(#3) I found that they weren't going in to the tapped holes easily. I removed the two bolts that I had just installed and found that the threads contained aluminum from the gear box (Pg.22#9) which they screwed into. This left me with no alternative but to remove the gear box and decide how to fix the problem. I had already come to the conclusion that it was going to be a helicoil repair. Upon getting the gear box out, I had to remove the bolts from the other gear box (#10) and found that those also showed signs of problems. I realized then that it was a case of the steel bolts going into the aluminum gear housings that is the problem. After examining the gear box attachment bolt holes, I found that they were tapped all the way to the bottom and that a longer bolt would make this repair a lot easier. I also went to the gear box manufacturers web site and learned that the holes are tapped 1 1/4" deep 5/16"x18 threads per inch. The top of the gear box mount is 3/16" thick and the top link support bracket is also 3/16" thick. The bolts that Kubota uses to attach all this is 1". That leaves only 1/4" of threads to hold this whole attachment together. The bolt is an overall 1" long and has 5/8" of threads. On Monday, I am going to purchase 8 bolts that have at least 1 1/4" of threads to replace these shorter bolts. I ran a tap down the tapped holes and if everything goes correctly, this will be a permanent repair. I also looked over the gear box and realize that I can flip the top for the bottom of the gear box housing and have 4 new holes to use if the damage was more severe, since the top and bottom are identical. The only thing that I would need to purchase to make this swap is a new expansion plug (Pg.28#8). When I reinstall all the parts, I intend to use never sieze on all the bolts to stop this corrosive action of dissimilar metals.
While researching the manufacturers site, I learned that the original factor fill of lubricants is 5 OZ. AGMA 5EP GEAR LUBRICANT. The Original Junk.....

 

[btolle]

</font><font color="blueclass=small">( I intend to use never sieze on all the bolts to stop this corrosive action of dissimilar metals.)</font>

I think the problem is not corrosion, but the bolts are a lot harder than the aluminum and cause the aluminum to "gall".

When the previous installer of the bolts tightened the bolts he deformed the aluminum threads ever so slightly, then when he backed them out the aluminum stuck to the steel and was ripped out by the bolt.

And yes, I would not even think of putting it back together without anti sieze compound.

EDITED to change "previous owner" to "previous installer of the bolts".

Bill Tolle

 

[aardvark]

Junk, who is the gearbox manufacturer, and what is thier website? Is the gearbox 1:1, or is the ratio a step up ratio? I'm looking for a similar gearbox. Thanks.

 

[Tom_H]

</font><font color="blue" class="small">( I think the problem is not corrosion, but the bolts are a lot harder than the aluminum and cause the aluminum to "gall".

When the previous owner tightened the bolts he deformed the aluminum threads ever so slightly, then when he backed them out the aluminum stuck to the steel and was ripped out by the bolt.

And yes, I would not even think of putting it back together without anti sieze compound.

Bill Tolle )</font>

Bill,

Not to be disrespectful, but I think your post assumes a lot of opinion. Let me give some facts to help clarify the situation.

Firstly, I'm not the one who originally assembled the unit. It came factory assembled. This is not to say that the bolts were or were not overtorqued, but simply that it was not I who torqued them.

Secondly, even if I had assembled it, I fully understand torque and torque tolerances; I do not overtorque bolts.

Thirdly, neither you nor I are there to examine the bolts. I used to teach chemistry however, and the effect that Junkman describes about dissimilar metals certainly does occur. What you have described also does sometimes occur as well.

I have observed through reading his numerous posts that Junkman is a very intelligent person and I would not so easily dismiss his observations.

Tom

 

[Junkman]

There are two different gear boxes in the unit that are connected by a steel tube. To see a breakdown of the workings of the grass catcher, please go to this web page. The gear box that connects to the tractor PTO is a model #200M part #938522 with a ratio of 1.92:1. There is also a designation of "BC" on the part label. The manufacturer is Curtis Machine Company, Inc., P.O. Box 700, Dodge City, Kansas 67801, Phone#: 1-620-227-7164 Fax#:1-620-227-2971 Toll Free in the USA: 1-800-835-9166 . The website is http://www.curtismachine.com/. The direct link to the mechanical specifications for this gear box is.... http://www.curtismachine.com/200.html I haven't gotten the information off the other gear box, but it appears to be the same as this but with a different shaft on the output side. I also noticed last night while working on this, that the gear ratio of the other box was also different than this one.

 

[btolle]

</font><font color="blue" class="small">( Bill,

Not to be disrespectful, but I think your post assumes a lot of opinion. Let me give some facts to help clarify the situation.
------------<SNIPPED>---------------
Tom )</font>

Sorry, I didn't mean to offend anyone and certainly did not wish to demean Junkman in any way. I edited the post to say "previous installer of the bolts" to protect the innocent.

I was going from my experience with aluminum parts of various objects from aluminum pipe to the dreaded Cadillac aluminum block engines that could not be rebuilt because all the steel bolts galled the aluminum threads. That "uh-oh" cost a lot of Cadillac owners a lot of money.

My experience was that virtually all aluminum thread failures I have seen were caused by galling of the aluminum, not corrosion.

I have seen corrosion cause problems when, say, aluminum sheets were riveted to steel framing but the damage I have seen was an erosion (disappearance) of the aluminum around the rivet.

As for Junkman, anyone who steals a 1983 Ford Club Wagon for a mere $5,200 is obviously an extremely knowledgable individual /forums/images/graemlins/grin.gif

Bill Tolle

 

[Junkman]

Bill........ Tom........ you are both wrong.... It must have been the 1" impact gun that I was using that destroyed the threads. I read the instructions on the side of the gun and they said to hold the trigger until the fastener turned freely in the bore......... /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif In my opinion, you are both correct..... too short a bolt, over tightened by the factory, removed and reinstalled twice, lead to a failure of the threads. After examining it more closely today and trying to run a 5/16 - 18 tap down the holes, I have come to the conclusion that the manufacture of the gear box might also have some culpability in the problem area. The tap wouldn't go down the holes by hand and I don't want to chance it until I talk to them tomorrow. It seems that the hole were tapped with a undersized tap, better known as a worn out one. If you go to the manufacturers site, they make reference to the size as 5/16-18 (M8x1 1/4) type 8. I need to learn exactly what this designation means before I go any further. The thread chaser went down the holes easily and cleanly, but a new bottom tap won't. I am open to all suggestions at this time. A friend that I was talking with today, suggested that I use Loctite on the holes to make sure that they don't work loose in the future. Another said that the anti seize will just make it easier to come apart unintentionally during use. Another said just like Bill did, that you can't take a steel bolt out of aluminum without destroying the threads and that the best thing to do is helicoil it now. At this point, my head is so filled with conflicting information that I am going to wait to hear all the other opinions as to what is the correct procedure. Thanks for all the help and commentary... a /forums/images/graemlins/confused.gif junk.... /forums/images/graemlins/grin.gif

 

[Tom_H]

Now this is just a thought, pure speculation. I have nothing solid on which to base this. The bolts of which Junkman is speaking hold three plates of metal together. The support bracket is welded to a base plate. This base plate fits on the driveline housing. Inside the driveline housing is the gearbox. Several bolts hold the support bracket base plates to the driveline housing yet only two go through to the gearbox. These draw the bracket base plate snug to the housing from the outside and the gearbox snug from the inside. As I try to envision the blower in operation, it seems that an unusually high degree of vibration could possibly be concentrated (in terms of the physics of the oscillations, a virtual focusing of the vibrational waves) at the points where these two bolts hold the gearbox snug to the drive housing. Aluminum is an easily fatiguable metal; repeated pressurization and depressurization and vibration in aircraft require them to be X-rayed ever so often to be recertified. I just have to wonder if vibrational fatigue could have anything to do with this. I could be totally wrong, but it was a thought.

 

[Junkman]

Tom... there are 4 bolts that go through 2 pieces of steel into the aluminum housing of the gear box. The more that I think about this, the more convinced that I am that the gear box tapped holes were undersized and when the bolts were "forced" into them at the factory, this led to the destruction of the threads. In the morning, I am going to speak to the manufacturer of the gear box and see if this is a possibility. I tried a 5/16-18 tap in the holes, but it wouldn't thread into any of them easilly....... even those that are factory fresh on the bottom of the gear box, although my thread chaser that is quite worn had no problems. An interference of 2 or 3 thousands might be all that makes the differance...

 

[Tollster]

I'm hoping I can help, believe it or not I am a certified bolt inspector.
I would suggest not using the loctite, neither red, blue or green. Use the antisieze and this will lubricate the treads during reinstall and in the event you ever go back. Check to get the proper torque values and apply them. What holds the bolt is the tension place between the treads and head of the bolt at final torque. They actually stretch. It sounds as if you do not have insufficient thread engagement on the lower portion, do to the design. It will work though but all bolts need to be the same torque, I'm thinking inch pounds. I also think I saw they are course thread, at least I hope I did, as fine thread with the mild steel and aluminum will be more apt to strip in the softer malible materal, particularly with the thread ingagement issue.
Is it possible to drill all the way through the body, then tap it to the next size. With out getting into a seating surface. You could then torque the longer bolt and put a nut on the backside, this would help take the stress off the threads. I'm not sure about the design. But its a viable option if you have the space.
Upon closer visual inspection /forums/images/graemlins/grin.gifThat bolt appears to be a metric 8 mm. As the washer is part of it, non removable, and the threads appear to be of a metric tolerance. Also a 5/16" wrench will fit an 8mm bolt. Those may be the incorrect bolts which would be the root cause of the failure.