Screw threads

[Mikey41]

I am new to this group and have a problem to discuss.

I have a 25 ton machine screw jack, used as an actuator, which has failed in service. The jack in this application is normally in tension,
up to 28,000 lbs. This structure is 40 feet in the air. A crane is not available nor usable as this is inside a building.

In the past, we have built extensive cribbing beneath the structure, then positioned hydraulic jacks, jacked the structure up, installed additional cribbing, then released the hydraulic jacks so the structure rested on the cribbing. We then are able to remove and replace the machine screw jack/actuator. This has been a dangerous operation and I hate when we have to do it. I don't even like being on the same planet!
I have struggled with trying to find a safer way to do this.

I had this idea the other day: Remove the 1-1/4 -7 x 4 inch long Gr5 bolts , one at a time, and replace each one in turn with a 48 inch long stud (threaded 1-1/4 x 48 inch long). I'd put double nuts on the bottom of the jack/ I'd then run another nut down from the top, and snug it on top of a couple of well greased hardened washers or a bearing. Then I'd repeat this until all 4 bolts have been replaced by threaded rod stock 48 inches long.
Then begin to unscrew the nuts on top of the jack base evenly. This will allow the machine screw jack to rise up, allowing the structure
to be lowered safely to the lower resting point.

The current failure will require unscrewing the nuts 31 inches until the jack is fully unloaded. Strength is not the issue as any single stud will be able to support the load. Torque to unscrew this thing is my concern. We will be on a very small work platform up in the air. I can not establish a credible number to lower the structure using well grease 1-1/4 -7 screw threads! I am also trying to decide if I need to put a thrust bearing under the upper nut to reduce friction torque.
What do we think of this approach?
With all the unknowns w/r coefficient of friction, any practical estimate of unscrewing torque (we will never have to lift with the screws). Thanks, Mikey41

 

[Mikey41]

Sorry, the 4 bolts I allude to hold the jack down.
Mikey41

 

[HowDidYouBreakThat]

Is there any way you can post a picture or diagram? It sounds like it could work, but I'm not sure if I'm visualizing what you're doing correctly.

Terry
KI6FCI

 

[Mikey41]

/Users/michaelmoore/Pictures/iPhoto Library/Masters/2012/Mar 23, 2012_5/Image 12.jpg

 

[Mikey41]

I don't understand how to post photos yet. But imagine this: A machine screw jack with an acme 4 inch dia screw thread coming out of the top. The base of the jack is bolted to a pivoting plate by 4 1-1/4 inch bolts. The upper end of the screw is attached, vi a clevis and blade, to the load, which is over center and applies a 28,000 tension on the 4 inch ACME thread screw jack, and also to the 4 bolts on the base.

Normally, the screw extends from 4 inches to 88 inches out of the jack body, and that allows the load to move (which is the objective). But ours is seized, so I have to replace the jack, which I can't move. If the jack could extend for another 31 inches or so the jack would be unloaded and I could remove it. My theory is, I can let the whole jack body go up 31 inches, and rest the load so I can replace the jack. Mike

 

[Mikey41]

I think I've attached a couple of pictures.
Mikey41

 

[HowDidYouBreakThat]

If you intend for the gray painted part to separate from the white painted part in a controlled manner, I don't see a problem as long as the studs are at least the equivalent of the hold down bolts.

Terry
KI6FCI

 

[Mikey41]

Thanks Terry.That is correct, This is the new jack which just failed after 8 months. I also think its much safer than what we have been doing. My next big question is how much torque it will take to turn a well greased nut on the threaded studs. I have tried different methods for calculating torque, and can get numbers ranging from 77 ft-lbs up to 3300 ft-lbs. I see in one torque table I have that a 1-1/4 7 bolt torque torqed to 380 in-lbs lubed has a clamp force of 24000 lbs. If this were true, I'd be home free. I only need to lower this structure, not raise it.
How much torque do you estimate it will take, assuming all 28000 lbs is on one stud? (They are sized to have lots of margin with just one stud installed. Thanks, Mike
ps-Here is another pic.

 

[HowDidYouBreakThat]

I'm not a mechanical guy (though I've spent most of my life solving mechanical problems), so I'm not sure of the calculations, but I'd guess the lowering torque would be less than the loosening torque of a fully tightened nut.

Terry
KI6FCI