PTO shaft- key or bolt

gspig · #1 10/12/06, 11:35 AM

I got an old bush hog at an auction. The driveshaft coupler at the gear box was bad. The old coupler used a shear bolt, the new coupler uses a shear key. So the question is: Do I drill a bolt hole in the new coupler or do I grind a key slot in the gear box shaft?

dagwood · #2 10/12/06, 11:36 AM

Without looking at it I'd say drill a bolt hole..........

ovendoctor · #3 10/12/06, 12:20 PM

go for the bolt vs the key way

unless you have a machine shop handy

Up North · #4 10/12/06, 12:48 PM

I'd take the new coupler back and get the right one with the bolt hole in it.

**Ross** · #5 10/12/06, 01:16 PM

I agree take it back they're too expensive to fool with. I doubt its a shear key it'll be a solid key to drive and the bolt on direct drive U joints aren't really shear bolts either. If that bolt lets go you could be wearing the spinning shaft or snapping off the tractor PTO. Generally a shear bolt equiped machine uses two slip plates that bolt together and the powered side free wheels under control on the shaft if the bolt shears. Maybe adding that set up would be a good idea, but it's going to be expensive.

gspig · #6 10/12/06, 01:17 PM

I wish I could have purchased the correct coupler, but the local TSC never seems to have the perfect item. The reason the old coupler is bad is that the bolt hole for the shear bolt is elongated.

agmantoo · #7 10/12/06, 04:15 PM

Drill a new set of holes in the old coupler and continue to use it. Return the new one for a refund.

Bearfootfarm · #8 10/12/06, 10:42 PM

Redrill the old holes for a larger bolt

rambler · #9 10/13/06, 09:08 AM

Seems we are unclear if this is a shear bolt. Sounds like it ends up working as one.

I would not go to a keyway, and I would not drill out for a bigger bolt. You may wreck something else (tractor?) if this doesn't shear - if it is a shear pin.

I would turn the old coupler 1/4 turn, drill a new proper size hole in it, & return the purchased wrong coupler to TSC.

Other options are available - find the proper new joint, install a slip clutch, etc. But, with what you have, reworking the old joint with a new hole is the best (cheapest...) option without changing the funtion of the parts.

--->Paul

farminghandyman · #10 10/13/06, 02:32 PM

If it is a shear bolt, the coupling should have a snap ring on the end of the shaft so that when it does shear the PTO shaft does not get lose and go wild, and kill you or something,

many smaller mowers used shear bolts, but a slip clutch is a better set up,

in most instances a grade two bolt is used,

rather than the farm store for generic parts, contact the manufacture or dealer of the mower, and see if you can get the correct item, if re-drilling the old will not work for you,

and if it is a "bush hog" brand, getting a correct part for it should not be a problem,

gspig · #11 10/13/06, 03:16 PM

Thanks for all the advice. Farminghandyman sounds correct for a shear bolt set-up, this mower does not have that set-up. The mower is ancient, no markings what so ever. The u-joint in the coupling in question was also bad. I managed to mangle the old coupling just removing the u-joint. I ended up grinding a keyway in the gear box shaft and it seems to work well. There are no keepers on any shaft to keep the PTO shaft from flailing about when the shear bolt breaks, and it did indead do that, which lead to my discovery of the oblong holes in the coupling. I did buy an overrunning clutch for the tractor.

**Ross** · #12 10/13/06, 05:02 PM

So how does the U joint stay on the shaft with just a key? Is there a set screw or something to lock it to the shaft? I mean well by saying, it sounds a little dangerous if it's not locked on. The over running clutch won't save your driveline from a sudden stop like a stump or rock, and if you've been shearing the drive bolt you've been running on luck IMO. A key may not shear as well! I know you're kinda stuck with this repair but I'd hate to see you hurt.

rambler · #13 10/13/06, 10:41 PM

Quote:

Originally Posted by Ross

So how does the U joint stay on the shaft with just a key? Is there a set screw or something to lock it to the shaft? I mean well by saying, it sounds a little dangerous if it's not locked on. The over running clutch won't save your driveline from a sudden stop like a stump or rock, and if you've been shearing the drive bolt you've been running on luck IMO. A key may not shear as well! I know you're kinda stuck with this repair but I'd hate to see you hurt.

All things I was thinking - ditto.

--->Paul