MMM RC60-72H mower gearbox bearings

[spanner48]

I did a search and didn't find information on this, so forgive me if this has been covered in other posts. I have an old B7100 with a RC60-72H mid-mount mower (It is an RC60 for sure by the label but the rest of the label is missing, I identified it as the RC60-72H by comparing parts diagrams).

Last time I mowed it seemed to vibrate more than usual and today when I mowed I saw lubricant thrown about the deck. Parked it and saw fluid dripping from the input shaft of the mower. Ran it and watched, and there seemed to be more wobble in the shaft than expected. Turned it off and verified side play in the shaft (maybe 1/8", didn't measure it). My guess is that I need to replace the bearing and seal on the input shaft.

I've done a lot of basic repairs on various machines from lawnmowers to outboard motors, and I have a lot of tools. Is this a reasonable job for the owner or does it need to be done by the dealer? I see Messick's stocks the bearing.

Thanks for any advice and assistance!

Hi Bluelick, The gearbox on RC60 decks is poorly-designed. It's a recognised weakness of all these decks. The input shaft has two SKF 6205 ball bearings virtually back-to-back [there's only about 3/4" between them], so there is very little strength against sideways forces from the PTO shaft. And the sideways forces are high, because the shaft operates at an angle. So the bearings wear out quickly.

And the whole PTO shaft develops play:
. The output shaft from the tractor gearbox
. The joint between that and the PTO shaft collar
. The PTO shaft Universal Joints [they're undersized, for the duty]
. The sliding plunge joint between the PTO shaft and the tube
. The PTO collar to the gearbox input shaft
. The gearbox input shaft bearings [probably the worst bit]

So the shaft ends up thrashing around off-centre, causing massive vibration, noise and general wear and tear.

I'm working on upgrades.
The PTO shaft can be replaced with a North American Series 6 standard
that includes increasing the UJ caps from 20.5 to 24.6mm
Replacing the single-row ball bearings on the gearbox input shaft with either:
. double-row bearings [involves remachining either the shaft or the housing]
. taper-roller bearings [ ditto ]

 

[Bluelick]

I finally had to replace the old deck, it was just falling apart. I was lucky to find a great deal on an RC60-71B locally. It took some adapting since I have the older style B7100. Fortunately so did the previous owner so he had made really nice adapter brackets. I still needed to do some drilling to make the front mount work but it all came together. I might be open to selling the rebuilt gearbox which has probably only about 10 hours on it since the rebuild or the whole deck, as is where is!

 

[spanner48]

I did a search and didn't find information on this, so forgive me if this has been covered in other posts. I have an old B7100 with a RC60-72H mid-mount mower (It is an RC60 for sure by the label but the rest of the label is missing, I identified it as the RC60-72H by comparing parts diagrams).

Last time I mowed it seemed to vibrate more than usual and today when I mowed I saw lubricant thrown about the deck. Parked it and saw fluid dripping from the input shaft of the mower. Ran it and watched, and there seemed to be more wobble in the shaft than expected. Turned it off and verified side play in the shaft (maybe 1/8", didn't measure it). My guess is that I need to replace the bearing and seal on the input shaft.

I've done a lot of basic repairs on various machines from lawnmowers to outboard motors, and I have a lot of tools. Is this a reasonable job for the owner or does it need to be done by the dealer? I see Messick's stocks the bearing.

Thanks for any advice and assistance!

AFAIK all the RC60 gearbox casings and input shafts are common; but the gears and the output shafts vary. Mine is an RC60-F20R [Rear discharge, on an F2400 out-front mower].

The gearbox input shaft is poorly-designed. It's supported by two SKF 6205 ball bearings [25x52x15]. But they are virtually back-to-back [there's only about 18mm between them], so they have very little resistance to sideways loading. And the sideways loadings are high, because the PTO shaft is operating at quite an acute angle. So the bearings wear out and develop slop, which then wrecks the oil seal [25x52x10].

Add to this the play that develops in the PTO shaft itself, and between the shaft couplings and the drive and deck shafts, and it is soon thrashing about causing massive vibration, noise and general wear-and-tear.

New 6205 bearings [and seal] will help - for a time. But they don't last that long.

Upgrades are:
1: Replace the 6205 ball bearings with NJ205 cylindrical roller bearings. They are also 25x52x15, so a straight drop-in replacement.
2: Replace them with 30205 taper roller bearings. These are 25x52x16.25, so some machining [either of the bearing itself, or of the shaft or casing] will be needed.

The output/mower blade shaft by contrast is well-supported. It has s 6303 [17x47x14] upper bearing and a 6206 [30x62x16] lower bearing with a 30x62x10 lower oilseal.

 

[spanner48]

Just replacing the bearings and seals will correct SOME of the out-of-balance play and vibration; but not all of it. The gearbox input shaft is very poorly-designed: supported by two 6205 [25x52x15] ball bearings, that are mounted virtually back-to-back. There is only 9/16" separation between them; so their resistance to sideways forces is minimal.

And those sideways forces can be considerable, since the PTO shaft typically operated at an angle about 15 degrees, in the case of my mower. So the bearings wear out quickly, and the resultant play wrecks the seals. Then the oil runs out; and the gearbox seizes. LOVELY . . . .

UPGRADES
1: Make sure the bearings you fit are close-tolerance: at least ABEC-3; preferably ABEC-5. And close fit as well: CM or CN, rather than the usual C3 [loose fit]. That will help - for a time.
2: Replace the ball bearings with cylindrical roller bearings: flanged NJ205 bearings are 25x52x15, so will fit right in. Limited resistance to axial thrust; but much higher sideways resistance.
3: Do away with the oil seal, and replace it with ANOTHER bearing, with its own oil seals. That's a 6205 2RS. That means leaving out the internal circlip, drilling and tapping 5/16" holes around the end face of the gearbox housing casting [I drill 6; you could go to 8]. Making up a clamp ring-plate to bolt round the shaft and onto the casting end. Using that to clamp and hold in the outer of the TWO bearings. If you want, you could then make the inner of the two bearings an NJ205 roller, for more strength - since the two ball bearings will take care of end thrust.
4: Carry out the same gearbox housing modification as [3] above. But, in place of an extra bearing replace BOTH ball bearings with 30205 Taper Roller bearings. They are 25x52x16.25 so, to get the gear wheel properly positioned, you have to machine 1.25mm off the inner and outer shells of the inside bearing. If you take a wider 25x52x22 taper roller for the outer bearing, and add a 10mm oilseal, that will protrude from the housing, and you use the ring clamp frame to apply the necessary preload - through the oilseal - for the taper rollers.

Four solutions, in rough order of effort needed, and resulting strength.

But God knows why Kubota made such a crappy design in the first place . . . .

 

[spanner48]

Add to my previous comment:

1: The play and out-of-balance may not just be worn bearings. The bearing outer races may be loose in the gearbox housing. Mine certainly were; I had to use one thou brass shim round them, followed by Bearing Mount adhesive, to get them properly located. Not sure whether that is because the races fretted material off the [cast iron] housing, or whether the housing was bored 'loose' from new.

2: The PTO shaft itself will contribute to the play. Both in the plunge splines, the UJs and the spline end fittings onto the gearbox shaft. A new shaft will run you $500-$600 at Kubota prices. But you can make up one from Weasler or Neapco parts, using standard Domestic Series 6 UJs, in place of the special Kubota ones, for $250.

3: The deck drive belt also contributes to the shaking, if it's worn in parts. New belts are more even; make things smoother.