# Mud Motor U-Joint



## gun410 (Oct 8, 2019)

A couple months ago I built a mud motor from scratch and It was working well up until last week. When I initially built the mud motor I cranked down the set screws on the u-joints but after running the motor for 5 mins the prop shaft came out of the u joint. So I did some weld tacks on the u-joint and that held up until last week when the prop shaft yet again broke off the u-joint. The u-joint is cast I believe so I shouldn’t have trusted welds to a mild steel shaft but I thought a couple racks would be strong enough. Any suggestions on a good way to keep the u joint attached to the prop shaft?
	

	
	
		
		

		
			






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## Latinrascalrg1 (Oct 8, 2019)

Um....whats a Mud Motor?


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## MontanaLon (Oct 9, 2019)

Run a bolt into the end of the shaft that won't let it go through the yoke of the joint.


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## just old al (Oct 9, 2019)

If you're not running them in pairs it's probably the changes in velocity that are breaking it loose over time and breaking the welds.  Weree it me I'd consider (as a rough and ready fix) inserting the shaft and through-drilling it to bolt the whole damn thing together. No guarantees it won't just shear the shaft at the bolt hole, though.

Might want to re-examine the design and see if you can ease the angle from the shaft to whatever is being driven. 

And I have to know...what is a mud motor? I am assuming some sort of off-road vehicle...


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## fixit (Oct 9, 2019)

I would like to see more of the total set up. I would guess that the shaft to U-joint is not a true or solid fit (undersized shaft and/or oversized hole in joint) and causes vibration. Can't see the size of the shaft but if there is enough "meat" cross drill it & put a bolt through it. With the thrust being toward the motor I don't understand how it is "popping" out, but then all the mud motors I have seen use a straight shaft and no u-joint. I would like to total set up. Interesting.


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## Cadillac (Oct 9, 2019)

Looks like the prop side comes in at a angle. I would treat this like a pto shaft which all have a locking collar to lock u joint to shaft. Some use a through pin some use a through bolt. Because the prop is angled the prop shaft or hole on u joint can wobble out eventually. A pillow block support on prop side would help the situation.


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## homebrewed (Oct 9, 2019)

It looks like a mud motor is used to drive boats in shallow water (mud???).  Take a look here.


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## pontiac428 (Oct 9, 2019)

That U-joint yoke is forged, not cast.  Look at the tell-tale wide parting line.  You should be able to weld mild steel to it and have it hold.

My thought is to cut a recessed groove into the end of the splined shaft and fix it with a lock ring.  Looks like you have room to do so.  Else, try to find and eliminate any movement along the length of the shaft.


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## Superburban (Oct 9, 2019)

Ia there a key way? or something besides the setscrew & weld to take the rotational force? If not, I think that is the first place to look.

Can the shaft on the left move in and out? Looks like it has moved out of reach of the U-joint. If so, that is the next thing I would look at, a set collar, thrust bearing, something to keep the end inside the u-joint yoke, even if the setscrew loosens. A bolt, shear pin, would also be worth considering.

Does the U-joint angle change in use? is there a partner u-joint? If it is a single U-joint, I would try to find a way to add another, to smooth out the rotations, or swap to a CV joint.


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## gun410 (Oct 14, 2019)

The prop shaft comes in at a 12° ish angle and both sides of the u-joint are keyed. I like the idea of putting a through bolt in the prop side but then the force would go to the motor shaft-prop connection unless I drilled a thru hole in both sides. But I don’t know how I would drill a holes in the shaft as they are in the mud motor and I could only get access with a hand drill. There isn’t anywhere I could put a pillow block, the prop shaft is housed in a pipe with 3 bronze bushings (held in place with a set screw and nut welded on outside of pipe) and filled with grease (also have an auto grease site lubricator attached to shaft). The shaft can slide in the bushings if not connected to the u joint and both 3/4” shafts are a tight fit into the u-joint. There is only a single u-joint from the prop shaft to the motor shaft and it doesn’t change angles.


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## pontiac428 (Oct 15, 2019)

Elementary problem.  You need a double cardan or CV type of joint.  U-joints need to be run in phased pairs to cancel out the vibes.  If you really want to run a single u-joint, you might need to weld that slip yoke solid, but I don't think you'll get good performance out of a single joint no matter what you do.  I'd consider splicing in a CV to replace that u-joint for best results.


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## matthewsx (Oct 15, 2019)

Or, just zap a couple more welds on an go huntin'


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## gun410 (Oct 15, 2019)

pontiac428 said:


> Elementary problem. You need a double cardan or CV type of joint. U-joints need to be run in phased pairs to cancel out the vibes. If you really want to run a single u-joint, you might need to weld that slip yoke solid, but I don't think you'll get good performance out of a single joint no matter what you do. I'd consider splicing in a CV to replace that u-joint for best results.



Well I guess I got bad mud motor plans (called for a single u-joint), I don’t think there would be room for another u joint, maybe this?  https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/132180194926 I could find any 3/4” to 3/4” cv joints. If I could find a applicable cv joint that works I would install it but I’m considering welding the u joint but the downside would be not being able to take the motor off if the need arises.


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## matthewsx (Oct 15, 2019)

Ok, on our kart motors we use a bolt to hold on the clutch, I seem to remember that hole in the end of the PTO being 3/8 fine thread (maybe 5/16). If you weld a thick washer inside the CV joint you could run a bolt up into the PTO shaft on your motor then re-assemble the CV joint. You'll have to take the CV joint apart to do it but that shouldn't be a big deal.

John


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