If it was me, I would be looking at a system using 3 or 4 threaded bars, rather than one big one. High strength threaded bars (150ksi tensile strength) with all the hardware are readily commercially available (primarily for post-tensioning) in diameters up to 3". I would be very surprised if they could not be adapted for your application.

Hi HotRod10,

Thanks for the comment. What are your thoughts on balancing the load though during a jacking motion? Adjusting them evenly at the same time? Just coordinated 3/4 people coordinated turning nuts at the same time / risk of overloading one more than the other etc.... ?

Regards,
Reuben

My latest idea is I'm going to include a spot for a 50t hydraulic pancake jack directly under the threaded bar. That way we can measure the exact compression load we want to put into the system and the threaded collar will be able to be moved easily nearly by hand (no friction) in 10mm increments.

If the bearing surface has to remain perfectly level, having multiple screws might not work. If you can tolerate the bearing plate being temporarily out of level during the adjustment process, you would have to look at the eccentricity that can be tolerated at the interface and calculate how much adjustment that is for each screw and see if the adjustments can be made with a reasonable number passes adjusting each screw. For instance, if you can only accommodate a 1/4 turn on a screw before having to turn the other 2 to get it back to level, and a typical adjustment would take 5 turns, that's 20 passes (20 times of getting the wrench on each nut and turning it a 1/4 turn). That might be too labor intensive (or not). It'll have to be your judgment call on what will be acceptable, since you know the project specifics and the people involved.

I'm not able to visualize how you unload the single threaded bar with a jack underneath it.

Sorry if I'm being a pest about this, but it seems like the pancake jack idea would work well with the multiple threaded bars, though - 3 bars at the perimeter of a bearing plate with the pancake jack centered under the bearing plate.

Quote (reubz)

I am in the process of designing a heavy duty screw jack... We are designing it for approximately 400kN compression.

Looking at thread options for a 6" dia threaded round stock it looked like 6-16UN, Class 2A was the most applicable thread.

...when attempting to convert a determine the torque required to achieve a certain vertical force it appears it may become problematic to turn (torques potentially too large!).

Threads like 6-16UN are a very poor choice for screw jack type loading... way too much friction, as your calculations indicate.
Screw jacks use more efficient threads, square thread is normally first choice with trapezoidal thread (like Acme) being second choice.
Check with a mechanical engineer experienced in this type design to keep from coming up with a jack that won't work with the anticipated loading.

www.SlideRuleEra.net

Thankyou! I had seen that style threads before for post tensioning equipment, great idea. I’ll look into thread options further.

Forgive me, I just now converted the compression load to English units (90kips). Now I'm wondering why you're wanting to use such a large bar. That load could easily be carried by a 2" bar (or 1 3/8" high strength bar).

Reubz:
400kN in compression is about 90 kips, is that per frame leg or for the entire frame? You really need to provide some sketches of the entire load and support frame, and of your concept of the lifting and leveling scheme you are imagining, because we can’t see it from here. Include plans, side views, dimensions, loads, member sizes, etc. etc.; to some reasonable proportions so as not deceive as to the overall arrangement; include a good sketch of how your 4” or 6” threaded rod mates with a collar and a 300mm pipe. Are you trying to lift or level the 6m tall support frame, or are you trying to determine its total weight, since you keep talking about a force/torque relationship, over time? Why does this need to be adjusted over time? If I were designing a screw jack, I would be using acme threads or some thread form near an acme thread form. And also, screw torque is a really poor way to measure the actual force being applied by the thread or jack. It is just not very reliable because of the system variables which you can not determine/control accurately in this type of application, and over time. You would design the jack to be strong enough to lift the desired load (plus a FoS) and then use lift distance as your measurement means, since you really already know the load you are lifting. In terms of coordinating a number of guys at the different legs, I would use a turn-of-the-nut method, where each guy applies one half turn or .75 turn at a time and then stops.

Your idea of a 50t hydraulic pancake jack directly under a hard/fixed lifting point on each leg (but not under a/the threaded rod) sounds like it has potential. 5 or 10mm of lift may not be too much temporary out of level or out of plumb on a sufficiently large (or flexible) frame. You lift it, uniformly +/-, to where you want it and put added shims under its true bearing points, +/- a couple mm. You might need some lateral guides at each leg base.

Sure, A few sketches for you are attached for your interest. Was aiming to design the leg of the frame for 400kN. The load is not equal per leg.
The frost layer comes into play for one foundation we can't get down below the frost level with that foundation due to how tight it is with existing equipment.
The idea was to allow the compression load to be monitored and adjusted every few months to mitigate this risk.
The arrangement has full lateral restraint for each leg. and lateral restraints at the load application area.
Yes agreed regarding accuracy and the pancake jack. I think lifting under the threaded rod should be fine though (refer to attached sketches) for reference.

https://files.engineering.com/getfile.aspx?folder=...

dhengr's suggestion to use a hydraulic jack under each corner is one that I have used to vertically align a machine component (fairly stiff iron casting) weighing about 75 tons . There was a jack screw (probably a UNC or UNF thread) at each of the 4 corners with a spot for a hydraulic jack (enerpac in this case) next to the screw. We would measure the required displacement for the alignment, lift the machine with the hydraulic jacks (unloading the jack screws), turn the jacking screws enough to achieve the required displacement and then set the machine back down onto the jack screws. Check the alignment and repeat as required. When everything was within spec, we locked the jam nuts on the jacking screws (the "jam nuts" were actually Superbolt nuts so they could be tightened with hand tools). I think we were able to get within about 0.010" of our target fairly easily. I'm not sure how that would work on your structure though, someone with structural experience would have to weigh in.