Please describe "install U-joints in compression"

[JStieger]

Topic says it all! I'm getting ready to replace u-joints in the next week. I have done these before on my rigs with non-greasable joints, but this time I will be installing greasable ones. What does it exactly mean to install u-joints so they are under compression? Does this mean with the zerk fittings pointing outwards since the driveshaft is in effect extended with the joints being compressed towards their respective transfer case or pinion yokes?



-Thanks-!

 

[Woodenhead]

If you picture the way the load is transferred through the U joint 2 sections of material between 2 lobes of the joint will be in compression and 2 will be loaded in tension. You want the section with the zerk to be loaded in compression while your standing on the go peddle. The hole for the zerk is a stress riser meaning this area will see higher stress because there is less material. Stress in compression won't make your U joint bust but stress in tension will so you want the zerk to be in the section that's in compression. So how do you know what part is in compression and what's in tension? First look which way the joint/driveshaft turns while going forward. If the joint was made out of rubber you would see it compress 2 sections of the material and stretch/tear the other 2. You want the zerk to be in the material that's getting compressed.



I hope this makes sense, it's late. I'm sure someone will explain a little better than I did.



Mike

 

[tgbol]

When installing u-joints with zerks they mean to have the zerk ahead of the driving yoke. So when the yoke at the trans is pushing the zerk. The the next u-jointthe zerk is being pushed by the driveshaft and so on. The u-joint is stronger when the hole for the zerk is being compressed than when it is being pulled.

 

[Rman]

I can give this a try, those were both very good explainations...



Draw a big O on paper, now draw a big + through it. Visualize the drive shaft turning this +. 2 of the sections between the + will be forced together while the other 2 will be forced apart. You want your zerk fitting inside the section being forced together.



Please remember that when you unhook the driveshaft to have the parking brake on and the tires chalked! I've made that mistake before and it's quite frightening! Good luck!

 

[JStieger]

I think I got it!



I see where I got confused! - I was looking at it from the perspective of the driveshaft axis.



So if I understand it correctly - I need to look at it in the sense of the driveshaft rotation when the truck is going forward. The driver yoke causes the "downturn" side of the cross to be under compression, whereas the "upturn" side of the cross is under tension.



If the u-joint cross was rubber then the zerk hole in the cross would be getting squished flat under compression as the driveshaft rotates. Conversely, if the zerk hole was under tension then the rubber u-joint would cause the hole to be stretched oval-shaped.

 

[Woodenhead]

You've got it.



Mike

 

[Greenleaf]

Is this new technology or information that applies only to a particular brand/application?



I have replaced many u joints over the years and have not had one fail. I install them anyway I can so as long I have access to the zerk fitting. My trucks as well as the school bus fleet have my replacement joints and no premature failures. (I have seen some fail due to lack of grease)



I have been in this industry since the early 90's and graduated from a two year tech college prior to my employment with no mention of this in the past.



Sounds odd to me.



Come to think of it... ... the former fleet tech whom I replaced when he turned 73 years of age, watched and helped me R&R joints with no mention of this procedure.



Just thought I'd ask.



Scott

 

[RustyJC]

Under normal usage, this is pretty much a non-issue. Whether the Zerk fitting and passage is in tension or compression, normal usage keeps the U-joint well within its capabilities. Where this procedure comes into play is under extreme conditions such as serious off-roading, drag racing, etc. where the last bit of strength is required - since the U-joint is stronger with the Zerk fitting and passage in compression rather than tension, this installation method gives that last bit of available strength and endurance that might just mean the difference between driving home and walking home. :{



Rusty

 

[Greenleaf]

That helps! Thank you.



-S

 

[Rman]

I'm an avid rock crawling fan, and believe me a broken Ujoint is definitly better then an axle shaft or pinion. I prefer the sealed U Joint's for my application. If you have zerked joints you better like crawling under your truck OFTEN, because road salt and water seaps into the seals rapidly once you've pushed grease out of it. Having had both types, I'll always prefer sealed joints. Yup they fail, but when replaced at intervals they are convenient and maintenance free.

 

[Todd T]

That helps! Thank you.



-S

just think Greenleaf, now you can bring this little exercise up the next time one of your piers is replacing a u joint and make them curse "that little whiper snapper" under thier breath, but also impress them at the same time.

 

[Greenleaf]

It will be some time untill I have someone to work with... ... like, retirement time. I have the ENTIRE bus fleet to myself.



Scott

 

[fbaurley]

Ive never really heard of this before, but it makes sense. I was always a believer, for extreme use to buy the sealed ones, because they are stronger, and for most people theyll last just as long