Drive shaft with single uni joint

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Will
Posts: 76
Joined: 21 Nov 2015, 14:04

Drive shaft with single uni joint

Postby Will » 09 Dec 2015, 13:39

Hi All Im new to the forum and looking for a boat! Have looked extensively thru the posts and noticed some boats running single universal on the drive shaft. Im surprised these boats actually work satisfactorily because uni joints should ONLY be used in pairs with the yolks parallel. The reason being there is an angular velocity change with shaft angle change and this sets up a massive internal harmonic vibration. The worst part about setting up a drive shaft like this is it 'looks' good - and I must admit usually if it looks good it is good- but not in this case. The obvious excuse is "the prop is running in water so she'll be right"
If you do have a single uni drive shaft - yeh I know its been like this for 30yrs - doesn't make it right, you probably have one or more of the following;

- badly worn splines for no apparent reason
- engine bearing failures - could be 1 or more location depending on engine and revs used
- broken crankshaft/s
- V or Z drive - gear / bearing failures
- prop shaft support failures.

CV joints were invented to overcome this problem with uni joints tho most automotive CV joints probably WONT handle the torque / speed of a boat drive shaft.
Short shaft pair of U Joints probably the best solution

The Spook
Posts: 198
Joined: 20 Jun 2009, 15:49

Re: Drive shaft with single uni joint

Postby The Spook » 09 Dec 2015, 15:10

Will

I couldn't agree more. The jack shaft in The Spook is only short but it does have 2 uni's. I have seen a single uni used but I agree with you that it doesn't allow for any misalignment. I have seen a set of two uni's bolted back to back in a situation where the engine and gearbox were very close together. Mine are in a "W" configuration.

I know of a boat that used a single uni and it was forever blowing the engine up and no one could explain why. In fact I've seen that boat at a race day with a new engine and go home with a stuffed engine.

Bob

Wildwoody1
Posts: 105
Joined: 24 Dec 2013, 18:47

Re: Drive shaft with single uni joint

Postby Wildwoody1 » 09 Dec 2015, 16:29

Single universals can work well if angles are not to severe.

As for "engine blowups" more likely to be caused by drive line Harmonics being transmitted into the crank shaft due to insufficient thrust control.

Wild Woody.

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bootlegger
Posts: 1446
Joined: 09 Mar 2009, 20:18

Re: Drive shaft with single uni joint

Postby bootlegger » 11 Dec 2015, 10:41

My hydro runs a single uni joint. Ive had several blow ups which could be attributed to the mentioned problem. I think more likely the engines were simply hand grenades.
I have put together a decent engine now and have run the engine hard and touchwood after several events it is till holding together.
I dont notice any vibration.
I simply cant get another joint in the space to solve the problem

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loco1
Posts: 45
Joined: 22 Jan 2013, 22:58

Re: Drive shaft with single uni joint

Postby loco1 » 11 Dec 2015, 14:11

My hydro has run a single uni for 45 years and never suffered any of the problems mentioned. It might be because the angle is only 2 degrees. However it is doing 7500 rpm or more depending on which engine was fitted at the time.

My Formula Ford hydro Loco ran a single uni as well and the angle on that was 15 degrees. For the 5 years I raced it never had any problems whereas an identical boat with a constant velocity joint had several constant velocity joint failures. I believe the cv joints were from a Mini and had to transmit around 115 BHP.

All of these setups have been on the prop shaft and not to a gearbox.

Back to back uni joints have been responsible for several drive line failures that I know of on a skiff.

Will
Posts: 76
Joined: 21 Nov 2015, 14:04

Re: Drive shaft with single uni joint

Postby Will » 11 Dec 2015, 17:12

Hmmm the replies are pretty much as I expected. I alluded to the fact that there would be examples of many years of use without problems - doesnt mean the principle is wrong just means the drive line adsorbs the harmonics. But 15deg is a bit scary maybe only 115hp helps but you wouldnt get me straddling that drive shaft! I dont quite understand why a pair of uni's wouldnt work tho - if the yokes are parallel and its properly balanced with a slip joint it should work ( obviously this takes up space which often isnt there ) We used to run this set up in sports cars (very short shafts) without probs.
I assume the single UJ's used dont see the prop thrust? - go on now tell me they do and really shock me
I was about to ask about experiences with CV joints because I suspected they would not work - for long. Physically CV joint can do the job - constant velocity very little harmonics BUT there are high internal acceleration forces which the balls and cage have to manage - the greater the angle the higher the forces. This creates lube / temp / cage issues which the cv joint wont handle.
Any others out there that tried the CV joint? outcomes?

Wildwoody1
Posts: 105
Joined: 24 Dec 2013, 18:47

Re: Drive shaft with single uni joint

Postby Wildwoody1 » 11 Dec 2015, 19:07

The old system on most setups was to have a Thrust Bearing bearing fitted between the Log and the Universal fitting on the drive end of Prop shaft when the connection was to go directly to a flange fitted to the end of the Crank Shaft. Shafts that were connected to some Commercial drives had a Thrust Bearing built into the Alloy drive housing enclosing the Crank Shaft.

The use of Slip Joints on double universals works fine in all cases as long as there is adequate space available. In some cases the single Universal was purely to take up any small amounts of variation the same as a Chain Coupling can.

Wild Woody.

Will
Posts: 76
Joined: 21 Nov 2015, 14:04

Re: Drive shaft with single uni joint

Postby Will » 15 Dec 2015, 13:40

Bit more info I came across yesterday the H joint basically back to back uni apparently used on jet drives but?
http://www.boatdesigns.com/H-Joint/products/69/

The Spook
Posts: 198
Joined: 20 Jun 2009, 15:49

Re: Drive shaft with single uni joint

Postby The Spook » 15 Dec 2015, 21:01

Bootlegger

I wasn't referring to your boat actually, but I was aware that you have had some engine issues over the years.

The one that I have in The Spook now has given me the most dramas of any engine that I have built. Almost got it worked out now though.

Bob

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bootlegger
Posts: 1446
Joined: 09 Mar 2009, 20:18

Re: Drive shaft with single uni joint

Postby bootlegger » 16 Dec 2015, 21:40

All good Bob. I did explore all the uni joint stuff when I was hurting engines.
Decided to stick with it. The only other way for me was to mount the box and engine together and have it all leaning. I didn't have room for that either.


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