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Tube Frame Mower Deck Gearbox Roll Pin Modification


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#1 29 Chev OFFLINE  

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Posted May 27, 2017 - 05:57 PM

Ever since I have used the tube frame mower deck I have come to realize that the roll pin that secures the lower gear to the vertical shaft is a weak spot as I have broken quite a few of them over the may years that I have owned it.  The strange part is that the gear on the horizontal shaft is retained the same way and I have never had a problem with it breaking  I am not sure if the roll pins were designed to act as a shear pin or whether Bolens just used the roll pin as a convenient way to secure the gear to the shaft in a very confined area but either way I broke the one inside the gear box again today.  When I first got the deck and rebuilt it I found that the original gear had been spot welded to the shaft and when I quizzed my brother about this he said that he had gotten tired of replacing the roll pin and had spot welded the gear to the shaft as a solution.  I had to grind the weld off to get the gear box apart and decided to put a roll pin back in as that was the way it had been designed to work.  Over the years I have always thought that the hole in the original shaft was worn a little bit and that the roller bearings used in the spindles probably require a bit of power to drive the blades and as a result that is why the roll pins broke on me plus I will be the first to admit that sometimes the grass gets a little longer than it should before I get a change to mow it.  I replaced the shaft with a new one last year and at the same time I replaced the tensioner spring with a NOS one and last season I had to replace the roll pin about four times over the mowing season.  Since the spindle bearings were worn I reasoned that they could be part of the problem and also the summer was a dry one last year which seemed to make the grass tougher to cut so I did not think a whole lot about them breaking plus with the new spring the belt probably did not slip much.  This year I replaced the spindle bearings with new ball bearings and got the lawn mowed once and was cutting the grass for the second time when the roll pin broke about two weeks ago so I assumed that the pin had been partially broken from last year and replaced it for the first time this year.  I have tried a regular roll pin, a spiral style that is shown on MTD's website (- http://www.mtdparts....&productClass=1) and even pressed a smaller diameter roll pin inside of the larger 1/4" roll pin but they have all broken on me on a regular basis.  Before doing the modification I looked at the way the gears are retained on the large frame decks and they are both keyed.  When the roll pin broke today I decided to replace it with a solid 1/4" diameter pin.  I had thought about using a 1/4" diameter bolt and lock nut but there is not much clearance inside the gear box where the gear is so I decided to take a 3/8" diameter bolt and turn it down to .251" diameter so it would be a snug fit in the holes in the gear and shaft.  I turned the head down to .310" diameter and turned a snap ring groove in the other end and used an "E" style snap ring to hold it in place.  The small head and snap ring clears the case and should keep the pin in place and it can be removed for disassembly.  Not sure I would recommend this to anyone if there pin is not giving problems but if someone is having problems with theirs breaking on a regular basis it may help someone.  Given the fact that the roll pin is still available from MTD makes me think they do give problems as they must still sell a few of them.   Sorry I did not take any pictures while I was making the pin but here are pictures of it installed and it will definitely be easier to remove than the original style roll pin as all that will be required is to remove the snap ring, rotate the shaft 1/2 turn and pull the pin out of the hole.  Since the other gear still has a roll pin it should still break if the intention was to have the roll pins act as a shear pin - will see how it works out.

Attached Thumbnails

  • 1 Solid Pin Modification.jpg
  • 2 Snap Ring On End Of Pin.jpg
  • 3 Head Of Bolt Turned Down And Made Round.jpg
  • 4 Snap Ring Clears Inside Of Case.jpg

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#2 LilysDad ONLINE  

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Posted May 27, 2017 - 07:12 PM

From all you describe, it sounds as though the roll pin is marginally suited to it's function at best. Does everyone have this same issue?


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#3 KennyP ONLINE  

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Posted May 27, 2017 - 07:31 PM

Nice fix! Hope it continues to hold!


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#4 637Yeoman OFFLINE  

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Posted May 27, 2017 - 08:21 PM

From all you describe, it sounds as though the roll pin is marginally suited to it's function at best. Does everyone have this same issue?

I despise roll pins. Im a simplicity guy, im sure you know. Simplicity has to have the dumbest drive train design i know of. 

The trannys are great, bgb's hold up good, but still  dumb design. Anyway, back to the roll pins. In the differential, roll pins drive all 700 pounds of the tractor. Just 6 measely roll pins. They are strong, and i only broke them, because someone replaced them with the wrong pins. We bought triple coil roll pins and havent had a problem. 



#5 Bruce Dorsi ONLINE  

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Posted May 27, 2017 - 08:32 PM

I can say with certainty that something is wrong if roll pins keep shearing.  ....They are there to act as shear pins to protect the gears.

 

The earliest 32" decks had the center blade attached to the gearbox shaft.  ....Hitting objects caused the pins to shear, and sometimes causing greater damage.  ....Bolens learned quickly, and changed to belt-driven center blades. 

 

While I have seen broken roll pins in Bolens gear boxes, it was never a common occurrence unless the gears were worn or broken, lack of lubricant, worn or broken bearings or thrust washers, or something getting jammed in gear teeth..

 

I have seen the bore of the gears scored from running with broken roll pins.  ....I believe any significant gap between the shaft and gear at the location of the roll pin places increases the probability of shearing a pin.

 

Looking at your pics, the gear teeth look worn considerably.  ....I wonder if increased backlash would tend to break pins.

 

The quality of roll pins available today may be questionable.  ....It seems many of these small items are from overseas and quality is suspect.


Edited by Bruce Dorsi, May 27, 2017 - 08:37 PM.

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#6 nsengineer OFFLINE  

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Posted May 27, 2017 - 08:56 PM

I have only replaced a roll pin in the gearbox maybe 2 times over many years of cutting.  Yes they are a weak link in the box but you should not be breaking that many.  When I would turn on the deck with the pto lever I would very slowly move the lever to engage the deck and not let it instantly engage.  If there is any slop between the gear and shaft or in the diameter of the hole where the pin goes through it could cause premature breakage.    As stated by Bruce in the above post worn gears will cause backlash and play.  Once I switched over to the spiral cut gears I can say I never had a problem again.  Also the spiral are much quieter.  I like the new pin you fabbed up.  Hope it works.


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#7 tractorskipper ONLINE  

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Posted May 27, 2017 - 09:05 PM

Not directly related, but I have a well used Case deck that had a very worn woodruff key slot in the center (drive) spindle (belt drive). I drilled the pulley hub & shaft & installed a roll pin. Not too many hours of mowing & the roll pin sheared. Next step will be to weld the sheave to the shaft. Also have a Bolens Duratrac snow blower with a gear box similar to the mower box. Swallowed a rock & broke the input shaft clean off right next to the bevel gear. Skip
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#8 Doug E. ONLINE  

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Posted May 28, 2017 - 05:51 AM

I have not had any problem with the roll pins breaking on my tube frame deck, but my grass is not particularly thick either.  If your gear is loose on the shaft, it will work the pin and break it.

 

Regards,

 

Doug


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#9 29 Chev OFFLINE  

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Posted May 28, 2017 - 08:56 AM

The gear is not a press fit on the shaft but it is not sloppy either - I replaced both the gears with new ones that I was able to purchase from a MTD dealer back about 2002.  The gear on the horizontal shaft is not a press fit either as I can push the shaft into it by hand the same way.  When the roll pin was installed in the gear and vertical shaft the roll pins needed to be driven in to both the gear and the shaft as I checked to make sure the pin was a tight fit in both holes before installing new pins.  There is a bit of in and out movement on the shafts which does create a bit of back lash in the gears but it is not excessive.  Since the gear on the horizontal shaft still uses a regular roll pin it should still act as a shear pin - it will be interesting to see if it starts to break now or if things stay working.  Will have to keep my eyes open for a good used gearbox to have as a back up - I do have one with a snow caster but I think they use different gear ratios. 


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#10 SimplyRad OFFLINE  

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Posted May 28, 2017 - 10:12 AM

If you are worried about that pin shearing you might step up to a grade 5 or 8 bolt and they won't shear.



#11 nsengineer OFFLINE  

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Posted May 28, 2017 - 10:33 AM

If your looking for a spare gearbox look for a black box off newer G-H series tractor. They were the spiral cut ones and have almost no backlash as the straight cut gears do. Also much quieter
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#12 Sawdust OFFLINE  

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Posted May 28, 2017 - 11:34 PM

I've had my H16 for three years & every year twice last year the same thing happens. The last time part of the pin meshed with the gears & destroyed them. I bought another drive unit for parts but haven't fixed it yet. I followed your repair thread awhile back & I agree it looked like you had it rebuilt very well. I'm disgusted with mine. I've already spent more on this deck than it's worth. I hope your new design works well for you. You think a little loc-tite on the clips might be a little more security.
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#13 29 Chev OFFLINE  

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Posted May 29, 2017 - 07:56 AM

I've had my H16 for three years & every year twice last year the same thing happens. The last time part of the pin meshed with the gears & destroyed them. I bought another drive unit for parts but haven't fixed it yet. I followed your repair thread awhile back & I agree it looked like you had it rebuilt very well. I'm disgusted with mine. I've already spent more on this deck than it's worth. I hope your new design works well for you. You think a little loc-tite on the clips might be a little more security.

The clip should work ok by itself as there is no side load on the pin other than centrifugal force from the shaft turning.  I got the majority of the lawn cut yesterday as they are talking rain for the rest of the week - cut grass for five hours steady yesterday and so far things are still turning. 


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#14 WrenchinOnIt OFFLINE  

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Posted May 29, 2017 - 09:23 AM

I've had my H16 for three years & every year twice last year the same thing happens. The last time part of the pin meshed with the gears & destroyed them. I bought another drive unit for parts but haven't fixed it yet. I followed your repair thread awhile back & I agree it looked like you had it rebuilt very well. I'm disgusted with mine. I've already spent more on this deck than it's worth. I hope your new design works well for you. You think a little loc-tite on the clips might be a little more security.

On my favorite H16 back in the day when it was my go to machine, EXACTLY the same experience, it would break the freaking pin all the time, I found out the hard way on a seal replacement (initial go around) you don't drive an unbroken pin all the way thru and out the bottom :(
You could still get all the parts at the time, I TOTALLY rebuilt the damn thing with all new parts,(and went on to totally rebuild the deck) the vertical shaft for the gearbox alone was $150 from the dealer (only option other than a machine shop then) no BS,I won't tell you what the helical gears cost! I had more money in the friggin gear box and deck than I had in the tractor and it came with three implements! BUT I was determined, committed, and in retrospect foolish to prevail over this SOB ! In the end something gave again UNBELIEVABLE , INCOMPREHENSIBLE BS, I gave up, I stripped the gearbox and deck of all the new parts. I bought the largest rosebud torch tip I could find and vaporized the aluminum gearbox, and scattered the ashes. Met Rick Show years later and bought several gearboxes off him, just in case. I've since retired the H16 of belly mower duties. The memory of my folly lives on , I hung the empty deck shell at my storage facility, after relaying the story to my daughter as she asked why it was hanging there, she asked me to get it down, went to the house, retrieved a Mr Yuck sticker, anointed the deck with it and I re-hung it. I smile now when I look at it.
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